Measuring stops of light output?

sk66

sk66

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Ok, this might seem like an odd question... but I was testing the AD200 and a homemade modifier (fresnel spot) and I wanted to start with a base knowledge of "how much light can the AD200 add?" The idea is that if I know the amount of light the strobe can add at 10ft unmodified I can quickly SWAG distance/power settings to get the results I want.
But now I'm not sure I'm doing it right technically. I started with a base black frame exposure w/o flash and where the flash at minimum power (1/128) was visible, and at 1/2 power the white wall began to clip.

So, is that 7 stops to clipping (8 stops total)? I don't thinks so as that's really just the adjustment range of the flash. Or maybe that's just coincidence... using the same method I got approx. 9 stops for the AD360 w/ standard reflector (the AD200 was w/ the fresnel head and I think my "black frame' might not have been quite dark enough).

Is it the 10 (12) stops of the zone system? I don't think so... I think the zone system is more "theoretical."

I was using the D810, so is that 14stops at 1/2 power, the DR of the sensor? Maybe, but how would that relate/convert to other cameras?
Or is there some other relevant measure/standard?

BTW, the fresnel spot was a flop, I actually lost .3 stop of light...
 
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Sorry Steven, you've lost me with whatever it is you're trying to do, but 1/128 to 1/2 power is six stops. Seven stops range, difference of six (in bold): 128, 64, 32, 16, 8, 4, 2.
 
I'm glad that I'm not the only one who's a little confused. On a more positive note....it has me rethinking whether my own attempts actually gave valid results.

Bob
 
HoppyUK said:
Sorry Steven, you've lost me with whatever it is you're trying to do, but 1/128 to 1/2 power is six stops. Seven stops range, difference of six (in bold): 128, 64, 32, 16, 8, 4, 2.
Click to expand...
I was starting from "zero" (black frame). The idea is pretty simple... how much light does it put out at full power from 10ft. I.e. lumens converted to "stops," but I'm just measuring stops.

Measuring up from black (off) to white (just clipping) at 1/2 power indicates it added 7stops of light... 8 at full power (which seems too convenient). And measuring up is a lot easier than trying to go down (unless you have a lens that goes to f/1.8-f/44)... when you hit "white" you know how many steps are left in order to determine max output... then you just work backwards to see what minimum power is worth (min may well be more than 1 stop with a lot of heads).

In this scenario... it seems the AD200 adds 8 stops of light at full power which means 1 stop at min power. Say I have a situation where I want to fill in shadows about 2 stops with the light in a softbox that costs 1 stop. The 1 stop from the softbox means that 1/128 at 10 ft is effectively "zero." So I can use 1/32, or 1/64 @ 5 ft, or 1/128 at 2.5 ft as a starting point.
My AD360 is approx. 9stops, 2 at min power with the reflector or 1 in a softbox (from 10ft). At least it seems to work that way...

But, is "white" the stops of light added from zero, or is it the DR of the camera sensor in use? If it is basically "both," then that would mean that my "black frame" actually had significant data in it that could be recovered (quite possible, not sure it matters)... either that, or the "usable DR" of the D810 is actually only about 7 stops (without recovery, also possible I guess).
 
Canon Bob said:
I'm glad that I'm not the only one who's a little confused. On a more positive note....it has me rethinking whether my own attempts actually gave valid results.

Bob
Click to expand...

With a little thought and care, it's quite easy to get within say a third of a stop accurate figures. Brightness estimates you read about on the web, without knowing precise details of the set-up, I wouldn't trust them to be any better than that and quite possibly worse. I know, because I've spent long hours testing every method I can think of and it takes a fair amount of effort to do better than that, especially when comparing different types and designs of flash head.

My current rig is very good, and not that hard to set up. It basically uses two softboxes joined face to face, though with quite a few important modifications inside. Happy to share and explain if anyone's interested :)
 
HoppyUK said:
With a little thought and care, it's quite easy to get within say a third of a stop accurate figures. Brightness estimates you read about on the web, without knowing precise details of the set-up, I wouldn't trust them to be any better than that and quite possibly worse. I know, because I've spent long hours testing every method I can think of and it takes a fair amount of effort to do better than that, especially when comparing different types and designs of flash head.

My current rig is very good, and not that hard to set up. It basically uses two softboxes joined face to face, though with quite a few important modifications inside. Happy to share and explain if anyone's interested :)
Click to expand...
Please do
 
Canon Bob said:
I'm glad that I'm not the only one who's a little confused. On a more positive note....it has me rethinking whether my own attempts actually gave valid results.
Click to expand...

HoppyUK said:
With a little thought and care, it's quite easy to get within say a third of a stop accurate figures. Brightness estimates you read about on the web, without knowing precise details of the set-up, I wouldn't trust them to be any better than that and quite possibly worse. I know, because I've spent long hours testing every method I can think of and it takes a fair amount of effort to do better than that, especially when comparing different types and designs of flash head.
My current rig is very good, and not that hard to set up. It basically uses two softboxes joined face to face, though with quite a few important modifications inside. Happy to share and explain if anyone's interested :)
Click to expand...
I'm always willing to learn, Richard, and others might find the methodology interesting too.

My comment goes back to a thread I started about "high intensity" reflectors which are more aligned with Steven's fresnel than softboxes. Garry made a comment which I accepted unquestioningly and have since realised that I didn't quite understand it.
From Garry "I found a 2.5 stop increase in effective power at 10', and from memory that goes up to a 3.5 stop increase at 15', compared to the Lencarta standard reflector". This would seem to imply, at first glance, that one or other of the reflectors isn't obeying the inverse square law? If the figures are correct then something else needs to be factored into the testing if a distance/intensity calculation is going to be made based on a specific reading at a fixed distance.

soeren said:
:thinking:
I may ofcource be wrong but it sounds to me like your reinventing guidenumber measurement/calculation.
May I suggest one of these?
http://m.sekonic.com/denmark/products/exposuremeters/l-308s.aspx
Click to expand...
I'm actually using a Sekonic meter....albeit the 758D.
 
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There is a simple confusion here, people are trusting that 1/2 power setting is 50% of the power etc. - sometimes it is, sometimes it ain't, if it was then really easy to work out the stops, for everybody else there is meter and test

@HoppyUK would be good to see included in flash tests

Mike
 
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Canon Bob said:
.
>SNIP

I'm actually using a Sekonic meter....albeit the 758D.
Click to expand...
Though not always good at practicing what I preach myself I have and use an old 308B for both analog photography and flash. Wondering whether an upgrade to the 308S is worth the cost.
 
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soeren said:
:thinking:
I may ofcource be wrong but it sounds to me like your reinventing guidenumber measurement/calculation.
Click to expand...
Kind of... for the AD200 with a GN of 60M it would equate to (roughly) a proper exposure at ISO100, f/1, 3m (12ft), 1/16th power.
But, how many stops of light is "correct exposure?" 1/2 of the sensor's DR? (not likely). Half of the zone system? (I don't think so) Half of the histogram? (how many is that?) And how do I use "proper exposure" to estimate a lighting ratio?

Interestingly, if "proper exposure" is 4 stops and mid way between the 7stops I got as a result, then 1/128 would be 1 stop... the same result I got.
 
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mike weeks said:
There is a simple confusion here, people are trusting that 1/2 power setting is 50% of the power etc. - sometimes it is, sometimes it ain't, if it was then really easy to work out the stops, for everybody else there is meter and test

Mike
Click to expand...
We don't really have anything else to go by... regardless of the method it's always "meter and test."
 
Canon Bob said:
My comment goes back to a thread I started about "high intensity" reflectors which are more aligned with Steven's fresnel than softboxes. Garry made a comment which I accepted unquestioningly and have since realised that I didn't quite understand it.
From Garry "I found a 2.5 stop increase in effective power at 10', and from memory that goes up to a 3.5 stop increase at 15', compared to the Lencarta standard reflector". This would seem to imply, at first glance, that one or other of the reflectors isn't obeying the inverse square law? If the figures are correct then something else needs to be factored into the testing if a distance/intensity calculation is going to be made based on a specific reading at a fixed distance.
Click to expand...
Light doesn't loose a lot of energy over distance through air. So anything that concentrates the light and prevents it from spreading over distance will not follow the ISL at all, and it will have a significant power advantage compared to something that does... that advantage will increase as the distances become greater.
 
mike weeks said:
There is a simple confusion here, people are trusting that 1/2 power setting is 50% of the power etc. - sometimes it is, sometimes it ain't, if it was then really easy to work out the stops, for everybody else there is meter and test

@HoppyUK would be good to see included in flash tests

Mike
Click to expand...

Yes, sometimes it is, sometimes it isn't - is a pretty good summary. As a very rough guide, I've found IGBT controlled flash units to be either dead accurate or within a tenth or two as power is changed. Elinchrom heads are always very accurate and with exactly proportional modelling lamps too, ditto Profoto. Some other brands are frankly hopeless with power adjustment that is neither linear nor consistent. Easy to sort out though, with a few tests to calibrate so you know what's what but rather less than ideal. Almost all flash units can be inconsistent flash to flash, the better ones only at low or lowest power though, maybe a tenth or two - noticeable, but rarely an issue.

Along the same lines, I try to avoid using lens apertures in tests like this. Lens diaphragms are not always exactly accurate or consistent especially at high f/numbers, T/stops vs f/stops, and vignetting can also throw things out a lot. ISO and shutter speeds are more accurate.
 
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sk66 said:
Light doesn't loose a lot of energy over distance through air. So anything that concentrates the light and prevents it from spreading over distance will not follow the ISL at all,
Click to expand...

I'm failing to grasp this.....for example; a 25 degree reflector will follow the ISL but a 45 degree one won't?
 
sk66 said:
Kind of... for the AD200 with a GN of 60M it would equate to (roughly) a proper exposure at ISO100, f/1, 3m (12ft), 1/16th power.
But, how many stops of light is "correct exposure?" 1/2 of the sensor's DR? (not likely). Half of the zone system? (I don't think so) Half of the histogram? (how many is that?) And how do I use "proper exposure" to estimate a lighting ratio?

Interestingly, if "proper exposure" is 4 stops and mid way between the 7stops I got as a result, then 1/128 would be 1 stop... the same result I got.
Click to expand...

'Correct' exposure (as opposed to 'optimum' exposure, that can be anything you like) puts 18% mid-grey* right in the middle of the histogram (standard JPEG). On a Macbeth colour chart, it's the third grey box from the right here http://gb.colorconfidence.com/colorchecker-chart.html

Also, the default background tone in Lightroom is exactly mid-grey and I often find that is a very handy visual reference for both exposure and colour.

*Let's not get into 12% or 15% as we'll never get to the bottom of it!
 
The following might be useful....

The inverse square law of light defines the relationship between the irradiance from a point source and distance. It states that the intensity per unit area varies in inverse proportion to the square of the distance. ... Example: You measure 10.0 lm/m² from a light bulb at 1.0 meter.
Its my understanding that for every 1 meter, the light falls off at 70% so Light Source to 1 meter 70% of the light reaches the subject, at 2 meters 70% of that 70% reaches the subject etc. I might be wrong though as Math is a bit of a challenge for me.

How that translates in to the OP question I dont know, but thought it might help :)
 
As above.
Let's not trivialise the importance of the Inverse Square Law, it's absolutely basic to photography and everyone is affected by it, whether they are an expert physist or whether they haven't even heard of it, but it is often misunderstood, resulting in misconceptions.
We all know that a point source of light in free space loses intensity to the square of its distance, i.e. if the light is twice as far away then 3/4 of its intensity fails to reach its target.
The inverse square law applies to sources of radiated energy, e.g.. light, sound, heat, gravity, electrical forces.
BUT we start with the premise that the energy (light in our case) is a point source of light, and in fact it isn't. Nobody knows what a point source actually is, and for all practical purposes even a large light source can become a point source if the distance is great enough, but for our purposes we're talking very short distances, so the size does matter. Although I don't know what a point source is, I do know that things like softboxes or large reflectors are not a point source, they are millions or billions of point sources, very close together but still individual, and in different places to each other.
And, we do know that a point source is a source that can travel equally in all directions, which doesn't even begin to apply to artificial light sources that are directed in a certain direction
And, the test conditions are free space, which is a vacuum. There is no light in a vacuum, no gravity, no pollution - try firing your flash into fog, or through a haze of tobacco smoke and see how far the light goes:)

So, a standard reflector pushes light forward but, over distance, that light is spread over a large area and unless the camera is using a fisheye lens, most of that light is wasted.
A high intensity reflector, although by no means a true parabola, does focus the light pretty well, basically it's just efficient, and as the distance increases (within reason) it is less inefficient than many other lighting tools, which means that less light is wasted.
 
sk66 said:
Ok, this might seem like an odd question... but I was testing the AD200 and a homemade modifier (fresnel spot) and I wanted to start with a base knowledge of "how much light can the AD200 add?" The idea is that if I know the amount of light the strobe can add at 10ft unmodified I can quickly SWAG distance/power settings to get the results I want.
But now I'm not sure I'm doing it right technically. I started with a base black frame exposure w/o flash and where the flash at minimum power (1/128) was visible, and at 1/2 power the white wall began to clip.

So, is that 7 stops to clipping (8 stops total)? I don't thinks so as that's really just the adjustment range of the flash. Or maybe that's just coincidence... using the same method I got approx. 9 stops for the AD360 w/ standard reflector (the AD200 was w/ the fresnel head and I think my "black frame' might not have been quite dark enough).

Is it the 10 (12) stops of the zone system? I don't think so... I think the zone system is more "theoretical."

I was using the D810, so is that 14stops at 1/2 power, the DR of the sensor? Maybe, but how would that relate/convert to other cameras?
Or is there some other relevant measure/standard?

BTW, the fresnel spot was a flop, I actually lost .3 stop of light...
Click to expand...
The zone system was theoretical, but with very useful practical applications.. Basically, what that clever Mr. Adams did was to work out a way of getting wide dynamic range from silver bromide plates that only had a very narrow dynamic range. But I'm not at all sure how that helps your quest...
I entirely agree with those who have pointed out that you seem to be finding stops of adjustment that don't even exist, i.e. regardless of what Godox and some re-sellers tell you, full power to 1/128th power is in fact 7 stops, not 8. And, as has also been pointed out, adjustments are not always either linear or accurate.
So, accurate measurement (in free space:) ) are needed to get a true figure.

Having said all that, why does it actually matter to you?
 
Garry Edwards said:
As above.
Let's not trivialise the importance of the Inverse Square Law, it's absolutely basic to photography and everyone is affected by it, whether they are an expert physist or whether they haven't even heard of it, but it is often misunderstood, resulting in misconceptions.
We all know that a point source of light in free space loses intensity to the square of its distance, i.e. if the light is twice as far away then 3/4 of its intensity fails to reach its target.
The inverse square law applies to sources of radiated energy, e.g.. light, sound, heat, gravity, electrical forces.
BUT we start with the premise that the energy (light in our case) is a point source of light, and in fact it isn't. Nobody knows what a point source actually is, and for all practical purposes even a large light source can become a point source if the distance is great enough, but for our purposes we're talking very short distances, so the size does matter. Although I don't know what a point source is, I do know that things like softboxes or large reflectors are not a point source, they are millions or billions of point sources, very close together but still individual, and in different places to each other.
And, we do know that a point source is a source that can travel equally in all directions, which doesn't even begin to apply to artificial light sources that are directed in a certain direction
And, the test conditions are free space, which is a vacuum. There is no light in a vacuum, no gravity, no pollution - try firing your flash into fog, or through a haze of tobacco smoke and see how far the light goes:)

So, a standard reflector pushes light forward but, over distance, that light is spread over a large area and unless the camera is using a fisheye lens, most of that light is wasted.
A high intensity reflector, although by no means a true parabola, does focus the light pretty well, basically it's just efficient, and as the distance increases (within reason) it is less inefficient than many other lighting tools, which means that less light is wasted.
Click to expand...

As Garry says, the photographic lights we use are a million miles from strict inverse square law theory, but it's still a damn good rule of thumb. It's used as the basis for Guide Numbers and that has served us pretty well for decades. The main exceptions are softboxes that fall-off less than the ISL suggests, eg if you move a softbox back from 1m to 2m, you'll lose something like 1.5 stops rather than 2. And at the other extreme, high-intensity reflectors (eg those tulip-shaped jobbies) have a mild collimating effect on the light and fall-off less over distance. Ditto true parabolics.

Hand in hand with this is the environment that can have a very big influence. Eg, take a portrait with a shoot-through umbrella in a smallish light-toned room, then take it outdoors at night and set up exactkly the same at the and distance and effective exposure will drop by one stop or more (the light will also be harder). The environment plays and increasing role as distance is increased, so I use 1.0m for this kind of test - at 2m or 3m the environment will skew things.
 
From Garry and Richard's post above I deduce that the area of the front surface of the reflector (be it softbox, parabola or whatever) is the missing link in the calculation as it deviates from the "point source" on which the physics is based.
 
Canon Bob said:
I'm failing to grasp this.....for example; a 25 degree reflector will follow the ISL but a 45 degree one won't?
Click to expand...
soeren said:
Though not always good at practicing what I preach myself I have and use an old 308B for both analog photography and flash. Wondering whether an upgrade to the 308S is worth the cost.
Click to expand...

I had a mild panic recently and began to doubt my trusty Sekonic 308S. I've cross-checked it a few times and I know it to be very accurate, but this time... So I rushed out and bought a Sekonic 478D.

And there were some differences, but it wasn't actually a fault with the meter as such - it was the different fields of view taken by the LightSphere dome and the flat LumiDisc that had caught me unawares. The LumiDisc sees 180 degrees, the LumiSphere more like 270 degrees on the 478 and in some cases that makes a significant difference. I now use the LumiDisc (the LumiSphere retracted with the 478) as it's easier and more consistent.

So I wasted a few quid, but money well spent actually - the 478 is much easier to use for certain things and has a few handy features (y)
 
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Canon Bob said:
From Garry and Richard's post above I deduce that the area of the front surface of the reflector (be it softbox, parabola or whatever) is the missing link in the calculation as it deviates from the "point source" on which the physics is based.
Click to expand...
The area is relevant but there's more to it than that.
For example, if we take a softbox that measures say 4' x 4'. If we place this say 100' from the subject then it becomes relatively small and although the size is still relevant it won't make a real-world difference, but if we place it say 1' from the subject then the light from the centre will travel just 1' but the light from each edge will travel 3', and the light from all of the intermediate parts will travel between 1 and 3' - so, although the immutable laws of physics must still work, they apply to each and every one of the millions or billions of point sources of light that are travelling varying and different distances, and this will affect the overall exposure.

When it comes to an efficient reflector, the shape forces more of the light to go in a straight(ish) line than a less efficient reflector that has less depth. With the shorter reflector, much more of the light will bounce off of the rim and go all over the place.
 
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Just to cover something that's appeared in the opening thread, is repeated later on (with specific reference to converting lumens to stops) and I don't see being addressed subsequently, and I think is fundamental to the confusion..

A stop is a relative measure of difference between two exposures, it is not an absolute measure of exposure. You cannot say "This exposure is x stops", but you can say "The difference between these two exposures is y stops".

The closest absolute measure is Exposure Value (EV) - which is relatively obscure, very rarely mentioned on this forum and I've never seen mentioned in discussions of flash. The interval between units of EV is 1 stop (which is convenient) and the easiest reference point is that EV15 is equal to Sunny-16.
 
mike weeks said:
There is a simple confusion here, people are trusting that 1/2 power setting is 50% of the power etc. - sometimes it is, sometimes it ain't, if it was then really easy to work out the stops, for everybody else there is meter and test

@HoppyUK would be good to see included in flash tests

Mike
Click to expand...
I started using off-camera flashes when the Vivitar 283 (and its accessories) was invented way back in the last century with film SLRs. When I converted to digital photography a decade ago I was still using that excellent old Vivitar 283. The new young digital enthusiasts didn't seem to trust old "analogue" flashguns from this digital era so I picked up several old film era flashguns very cheaply. I operate all these old flashguns in manual mode, setting their power levels up with the aid of a flashmeter. I've always found their fractional powers to be pretty accurate.
 
I'm sure @sk66 knows this but increasing by one stop halves the amount of light reaching the film/sensor/subject/whatever. So perhaps the way to go about finding out the effect a modifier has in front of a light source (which is what I think the original question was) is simply to measure the light intensity on at a fixed distance with and without the modifier and then compute the number of stops. Easy to say but I suspect hard to do without an accurate lux meter
 
soeren said:
Please do
Click to expand...

Okay then. Eliminate all environmental factors, all distance and field coverage and brightness fall-off variables, ensure all the light is distributed then scrambled and diffused to eliminate hot-spots and cold-corners, then collect every drop and funnel it down to a perfectly even brightness for measurement with a meter. Results must be consistent with both studio heads and speedlights, Fresnel heads and bare-bulb tubes.

I use two identical double-diffuser 90cm square softboxes, with recessed fronts. They're not the push-up type with a mechanism inside - not a problem at all in normal photography, but might affect things slightly for this, particularly speedlights. The softboxes are joined and sealed face to face, the flash fires at one end and is metered at the other.

Care is taken to position the flash tube at a) the same distance, b) free of all obstructions around the mount so the light can radiate freely, and c) any gaps around the flash head are filled with reflective material so that nothing escapes from the back (mostly only speedlights). This was quite tricky to achieve with all types of flash unit and I have a collection of adapters and brackets and DIY bits to custom fit different flash units. All these things don't make much difference individually, but together they're worth about 0.2 stops, sometimes 0.3, compared to just shoving the flash head in as you normally would. BTW if you remove the modelling bulb (the common, larger ones) that's worth a tenth, and if you then blank off the socket hole with reflective material, that's worth a bit more too - but I always leave the modelling bulb in place, as per normal use.

I take out the inner-diffuser panel from the first 'transmitter' softbox, to give the light more space to bounce around and scramble together. Then there's a thin wire separator hanging between the two softboxes, to put more distance between the front diffuser panels that would otherwise be only about 8cm apart. The gap is now 18cm for better diffusion. The second 'receiver' softbox has both diffusers in place, so that's three in total.

Then the light is measured exactly on the exit of the second softbox, using the flat LumiDisc on the meter. I moderate (reduce) the readings by 2.3 stops so they tie in more closely with the brightness you'd get with a typical medium size softbox (or white umbrella) at 1.0m and ISO100, and also with my previous test rigs. Very roughly, 100Ws delivers f/8 d5 or thereabouts (at 1.0m, ISO100). It varies quite a bit of course but that'll usually be within about half a stop.
 
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Alastair said:
Just to cover something that's appeared in the opening thread, is repeated later on (with specific reference to converting lumens to stops) and I don't see being addressed subsequently, and I think is fundamental to the confusion..

A stop is a relative measure of difference between two exposures, it is not an absolute measure of exposure. You cannot say "This exposure is x stops", but you can say "The difference between these two exposures is y stops".

The closest absolute measure is Exposure Value (EV) - which is relatively obscure, very rarely mentioned on this forum and I've never seen mentioned in discussions of flash. The interval between units of EV is 1 stop (which is convenient) and the easiest reference point is that EV15 is equal to Sunny-16.
Click to expand...

I use EV (ISO100) for measuring continuous lights, but since EV includes shutter speeds it's not applicable to flash. Lumens are becoming common parlance nowadays, with LED bulbs etc, so I might change to Lumens instead of EV.

I express flash output with f/numbers (ISO100). Guide Numbers are highly variable, depending on the environment, and they also only apply to the centre of the field, taking no account of how rapidly the light falls off around the edges (and that's always a lot!). Watt-seconds (Ws) are only a measure of energy stored in the capacitors, not necessarily how much light is produced.
 
HoppyUK said:
I use EV (ISO100) for measuring continuous lights, but since EV includes shutter speeds it's not applicable to flash
Click to expand...
Surely EV still works if you treat the shutter speed as a constant? and there should be a relationship between lumens (as an incident measure, not as an output measure) and EV but I wouldn't like to try and calculate it.

I've just taken a quick look at BS ISO 2827:1988 and BS ISO 1230:2007 on BSOL. Para 7.2 of BS ISO 2827:1988 gives a methodology for determining fall-off.
 
Canon Bob said:
I'm failing to grasp this.....for example; a 25 degree reflector will follow the ISL but a 45 degree one won't?
Click to expand...
This quote from Gary answers your question... "So, a standard reflector pushes light forward but, over distance, that light is spread over a large area and unless the camera is using a fisheye lens, most of that light is wasted.
A high intensity reflector, although by no means a true parabola, does focus the light pretty well, basically it's just efficient, and as the distance increases (within reason) it is less inefficient than many other lighting tools, which means that less light is wasted."

The ISL is about the spread of light over distance/ light per area. So a light source that allows less spread over distance is less affected by the ISL when compared to another that allows more spread over distance (i.e. "less inefficient").
 
Alastair said:
Surely EV still works if you treat the shutter speed as a constant? and there should be a relationship between lumens (as an incident measure, not as an output measure) and EV but I wouldn't like to try and calculate it.

I've just taken a quick look at BS ISO 2827:1988 and BS ISO 1230:2007 on BSOL. Para 7.2 of BS ISO 2827:1988 gives a methodology for determining fall-off.
Click to expand...

If you use EV with a fixed shutter speed and fixed ISO (EV doesn't technically include ISO) then you might just as well use f/numbers - that everyone can relate to.

Yes, there are testing standards laid down, but some manufacturers appear to take no notice of them. For example, I could name at least three flash guns with the same Guide Number, but they all give different total light outputs, maybe a third to half a stop different. Mostly it's the amount of fall-off that makes the difference, but sometimes they're just lies.
 
Garry Edwards said:
The zone system was theoretical, but with very useful practical applications.. Basically, what that clever Mr. Adams did was to work out a way of getting wide dynamic range from silver bromide plates that only had a very narrow dynamic range. But I'm not at all sure how that helps your quest...
I entirely agree with those who have pointed out that you seem to be finding stops of adjustment that don't even exist, i.e. regardless of what Godox and some re-sellers tell you, full power to 1/128th power is in fact 7 stops, not 8. And, as has also been pointed out, adjustments are not always either linear or accurate.
So, accurate measurement (in free space:) ) are needed to get a true figure.

Having said all that, why does it actually matter to you?
Click to expand...

I was trying to shortcut experience w/ a new light... You know, with experience you can grab a light and say "I'm going to put it about here at x power" and be pretty close right from the start.
 
Alastair said:
Just to cover something that's appeared in the opening thread, is repeated later on (with specific reference to converting lumens to stops) and I don't see being addressed subsequently, and I think is fundamental to the confusion..

A stop is a relative measure of difference between two exposures, it is not an absolute measure of exposure. You cannot say "This exposure is x stops", but you can say "The difference between these two exposures is y stops".
Click to expand...
Excellent point... one I had overlooked.
But, if you start with a black "no light" image/environment there is a certain amount of light required to put mid grey or white at the correct exposure level. So correct exposure must be "stops of light above black."
 
Alastair said:
Surely EV still works if you treat the shutter speed as a constant? and there should be a relationship between lumens (as an incident measure, not as an output measure) and EV but I wouldn't like to try and calculate it.

I've just taken a quick look at BS ISO 2827:1988 and BS ISO 1230:2007 on BSOL. Para 7.2 of BS ISO 2827:1988 gives a methodology for determining fall-off.
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Conversion is easy enough http://www.sekonic.com/united-states/support/evluxfootcandleconversionchart.aspx Lux is Lumens per sq m.
 
HoppyUK said:
If you use EV with a fixed shutter speed and fixed ISO (EV doesn't technically include ISO) then you might just as well use f/numbers - that everyone can relate to.

Yes, there are testing standards laid down, but some manufacturers appear to take no notice of them. For example, I could name at least three flash guns with the same Guide Number, but they all give different total light outputs, maybe a third to half a stop different. Mostly it's the amount of fall-off that makes the difference, but sometimes they're just lies.
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Then if we take the flash-subject distance into account we have the.......... (n) :LOL:
 
sk66 said:
So correct exposure must be "stops of light above black."
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Stops of light above only just black. And it has to be incident.
 
sk66 said:
I was trying to shortcut experience w/ a new light... You know, with experience you can grab a light and say "I'm going to put it about here at x power" and be pretty close right from the start.
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I think that if you use mid-grey (centre of histogram) as a starting point, then it'll all come good - and that's what you actually want to know without any further conversion or variables to account for. The problem with both black and white is that in practise they're dependent on the sensor's dynamic range.

mike weeks said:
Nah, thats what meters are for

Mike
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Yes! And they'll give you mid-grey ;)
 
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