Flash power?

[Garry Edwards]

This thread goes a bit further than the post I made a long time ago the Lencarta Lighting Centre about how much flash power is needed, but to save repeating what I wrote there, It's aimed squarely at home studio users with digital cameras, you may want to read it.

Let's move on a bit from there, and address the query raised by @Cannyekerslike here
He wants to know how much power he needs for his 5"x4" camera - which I can't answer in absolute terms.

But regardless of how much power is needed for a 5x4, power requirements do vary a lot, depending on a lot of different factors. Take popular cropped-sensor cameras. I think it's fair to say that these cameras are normally used in the studio at something like f/8 - f/11,which gives pretty adequate depth of field, and because of image quality loss caused by diffraction limitation, it really isn't a good idea to go smaller than about f/11 anyway. Now, with the ISO set at 100, even a low powered hotshoe flashgun is good enough (in terms of power) for that, with a cheap softbox fitted to it, at a distance of 9' from the subject, you'll be at around f/5.6 - so, unless you have sunlight streaming through the window that you can't or won't block out, just increase the ISO to 400 and you'll get f/11

With a low-powered studio flash head, around 200Ws, you'll get around f/11 at 100 ISO.
But if you're using a so-called full frame camera, you'll be shooting at around f/16 to get a similar depth of field, so you'll need 400Ws
Move up to 6 x 6 or 6x7 medium format and you'll be shooting at around f/22, so you'll need 800WS
Back now to Cannyekerslike, he's using 5x4, so will probably be shooting at around f/45, which means 1600Ws. And there's no workaround, I00 is the normal ISO for this size.
But that's a bit simplistic, for a number of reasons, some of which are specific to large format cameras and some of which aren't.
Large format cameras are often used for true macro photography, rack out the bellows 100% to produce life size and the power requirement goes up with a jump.
But large format cameras also usually have full front and rear standard movements, and the people who use them are usually knowlegeable enough to be good friends (figuratively) with Scheimpflug, and although tilt and swing cannot actually change the depth of field, it does shift the plane of sharp focus, and so much larger apertures can be used than on a camera that doesn't have tilt and swing.

So, can you rely on the example figures I've given above? Yes, more or less, but there are a LOT of variables.

I started off with the assumption that we're using a cheap softbox at a distance of 9'. Cheap softboxes tend to have "efficient" reflective walls and to have flimsy diffusers that are diffusion inefficient but light efficient. Well-designed, more expensive softboxes produce less light, but the light is diffused much better.
A lot of people on forums (and on youcraptutorialtube) state that softboxes lose 2 stops of light, or maybe 3 (compared to what?) but that's just nonsense. Good quality softboxes do lose a bit, but not much. The real losses aren't actually losses at all, they just spread the light over a large area, so that a lot of it can be wasted. 9' is actually a hell of a long way away from a portrait subject, 3' is much more like it, if the photographer has any understanding of light, but a lot of people who don't know any better DO place their lights a long way away, it's easier for them - and the closer the lights are to the subject, the less power is needed. But if we're photographing a really large subject, lights often have to be a long way away, and that's when very powerful flashes are needed. Some years ago I did an enormous amount of room set photography. I also did a lot of industrial photography, photographing the inside of factories. At that time I had an Elinchrom 6000Ws flash generator, 6 x 2400Ws ones and a lot of mono heads too, and sometimes they just weren't enough. I remember trying to photograph an enormous boiler once, I was shooting on 5x4 and making full use of Scheimpflug to run the plane of sharp focus along the length of the boiler, but even so I could only manage enough light for f/8, and I really wanted at least f/32 and preferably f/45 - needs must.

And of course, different lighting modifiers produce very different results from each other and also greatly change the amount of light that hits the subject at any given distance. And so does the reflectivity of the subject, and the angle at which the light is striking it

Note that I've referrred throughout to watt-seconds, the standard measurement of flash power - Joules (j) are the same thing. I haven't mentioned guide numbers, because they can't be relied upon. I think that most people now recognise that the guide numbers of hotshoe flashguns give no real indication of actual power, the makers use a tiny, mirror-like fixed reflector that zooms, and the guide number published, even if true, can only be true at the flash's maximum zoom setting, which means that the flashguns with the longest zoom setting claim to have the highest guide numbers. Unfortunately though, the makers of hotshoe flashguns don't want us to know their watt-second figure.

Moving on to studio flash, Elinchrom and Lencarta publish true guide numbers, this doesn't mean that nobody else does, what it does mean is that none of the others that I've tested are accurate. Not only are they not accurate, they are also always wrong in the same direction - exaggeration - which makes me wonder whether the false figures are due to accident or design... Mentioning no names, but I know of 2 retailers who are selling the same flash head with a claimed guide number of 49, in metres. I've tested this model and the TRUE guide number is in fact 32.8. Whatever they may claim to the contrary, neither of these retailers know anything at all about the products they sell, and a bit more digging took me back to the manufacturer's website, where they quote the figure of 49, which is a ridiculous claim.

 

[Cannyekerslike]

Thanks for the above Garry.

I was starting to get a feeling in the back of my head that working indoors with 5x4 was going to be a step too far and this confirms it. The weight goes up including stands etc. to support the weight of the head and of course the price, all things that have to be taken into consideration.

I will have to give the matter a rethink and hope that I can remain ambitious enough to go forward with medium format 6x7 stuff.

In the meantime I will concentrate on 35mm film and digital and once Christmas and the New Year are past. I will start looking for a lighting kit to satisfy this. I get really fed up of changing and recharging batteries and also waiting for the flash to recharge between shots.

On the power rating of various flash units this is something that really confused me. Its like a moving target with just hot shoe units, it seems manufacturers move the goals to quote larger figures. I now recalculate their figures for 100asa at a zoom of 50mm. This gives me a better idea of the supposed power but as you say it is probably still ambitious because using the guide numbers they give for manual flash I always have to open up the iris to get a decent exposure when trying them out.

I have read the article on How much Power and found it useful and straight forward.

Many thanks for your input and the answer to my question.

 

[Steve Smith]

Surely the format size has nothing to do with it... unless you are using higher f No's to get similar depth of field I suppose.


Steve.

 

[Garry Edwards]

Surely the format size has nothing to do with it... unless you are using higher f No's to get similar depth of field I suppose.


Steve.

It's very largely to do with DOF, but there's another factor too - LF lenses aren't designed to be used at anywhere near their max aperture. A 150 or 180mm lens for a 5x4 may have a max aperture of around f/5.6, but that's just as a focussing aid, the maximium usable aperture is typically no larger than f/16 and the best performance is typically around f/45.

Thanks for the above Garry.

I was starting to get a feeling in the back of my head that working indoors with 5x4 was going to be a step too far and this confirms it. The weight goes up including stands etc. to support the weight of the head and of course the price, all things that have to be taken into consideration.

I will have to give the matter a rethink and hope that I can remain ambitious enough to go forward with medium format 6x7 stuff.

In the meantime I will concentrate on 35mm film and digital and once Christmas and the New Year are past. I will start looking for a lighting kit to satisfy this. I get really fed up of changing and recharging batteries and also waiting for the flash to recharge between shots.

On the power rating of various flash units this is something that really confused me. Its like a moving target with just hot shoe units, it seems manufacturers move the goals to quote larger figures. I now recalculate their figures for 100asa at a zoom of 50mm. This gives me a better idea of the supposed power but as you say it is probably still ambitious because using the guide numbers they give for manual flash I always have to open up the iris to get a decent exposure when trying them out.

I have read the article on How much Power and found it useful and straight forward.

Many thanks for your input and the answer to my question.

Well, hotshoe flashguns are a waste of time with LF and, to a lesser extent, with MF too.
I don't know what kind of shooting you're doing, but please remember that if you're shooting static subjects then you can use multiple flash pops to build up the power, e.g. 8 x pops = 8x the power, or 3 stops.

 

[Steve Smith]

Also, when LF was the 'normal format, the lighting would've continuous rather than flash so it would have the advantage of a longer shutter speed allowing for more exposure.


Steve.

 

[PPPPP]

As I always understood it, reviewing based on wattage is misleading because different designs of flash head are more or less efficient in terms of how much of the wattage drawn is converted into light vs heat and other items. Guide numbers are more accurate in the sense that they are based on the output of light. It is true that guide number can be effected by a flash guns zoom setting, and of course you don't know if the figures any manufacturer quote are accurate, however if you want a technically accurate measure of power, neither wattage or guide number as quoted on websites will give you that. The only thing you can do is test with a light meter.

 

[Garry Edwards]

Also, when LF was the 'normal format, the lighting would've continuous rather than flash so it would have the advantage of a longer shutter speed allowing for more exposure.


Steve.

Yes. But for very many years, LF shutters have been fitted with a sync socket for flash. And although LF is no longer popular with amateurs, there are still a lot of very skilled amateurs using it, especially for still life and landscape work, in the USA it's very popular for landscapes, in this country it's fairly heavily used by high end pros for food and other product photography too. And in China it's used a lot for large group shots - cameras up to 24"x20" film size are common.

As I always understood it, reviewing based on wattage is misleading because different designs of flash head are more or less efficient in terms of how much of the wattage drawn is converted into light vs heat and other items. Guide numbers are more accurate in the sense that they are based on the output of light. It is true that guide number can be effected by a flash guns zoom setting, and of course you don't know if the figures any manufacturer quote are accurate, however if you want a technically accurate measure of power, neither wattage or guide number as quoted on websites will give you that. The only thing you can do is test with a light meter.

It's watt-seconds (or joules) not watts, but you're right in the sense that different levels of system efficiency can affect actual output, but only up to a point and measurement in watt-seconds is the most objective data we have.
Guide numbers ARE arrived at by testing with a flash meter. But the problem with guide numbers isn't just that some sellers are either incompetent or dishonest, the largest single factor that affects the guide number is the modifier fitted to the flash head - the benchmark standard is a standard reflector, but standard reflectors vary a lot, some have a highly efficient mirror-like surface that produces very high theoretical readings, others have a matt finish. The only way of levelling the field would be to test with no reflector fitted, but that will never happen because it won't suit the industry to do so.When we do our tests, we exclude reflected light as far as possible, which means that we shoot in a large blacked out studio, but some other people (if they actually test at all) must be testing in a white painted broom cupboard
Richard Hopkins @HoppyUK has his own way of testing for photography magazines, this involves using the same umbrella softbox (I think) for every single flash that he tests, regardless of its make or design, and it seems to work - but the industry will never adopt this method because it doesn't suit them.

 

[Steve Smith]

Yes. But for very many years, LF shutters have been fitted with a sync socket for flash.

True. However, I have some folding cameras and some large format shutters with sync sockets which predate electronic flash so they would have been intended for use with flash bulbs.


Steve.

 

[HoppyUK]

It's a can of worms. If you want to know how much total light you'll get from different flash heads or guns, the only 100% reliable way is to test them side by side in an identical manner. As Garry mentioned, for the work I do for various magazines and websites, I use a Lastolite 90cm Umbrellabox because, a) it's a reverse-firing umbrella-softbox, with a matt-white interior that scrambles the light very effectively for a level playing field, and b) it can be fitted to any head or gun, c) results are consistent and repeatable. Test distance is 1.0m. It's not perfect, and I've often thought about building some kind of custom rig/diffusion box or whatever, but I know it wouldn't make a significant difference (maybe a tenth of a stop, anything more would be exceptional) and one of the things I like about the Lastolite Umbrellabox is it's real-world and produces results that people can relate to. I've cross-checked it every which way and it's consistently close to an 85-100cm white umbrella or softbox, within 0.3-0.5 stops pretty much regardless of make or model - so not bad as a ball-park. The Umbrellabox is very tolerant of different zoom head settings with speedlites too, which suggests it's doing a good job of scrambling the light without creating hot-spots.

I've done about 50-60 different heads and guns now over the last maybe three years or so, something like that. I probably know more about how much light these things really put out than most manufacturers. FWIW, I mostly use Ws as a shorthand reference for studio lights, but there are some big names that are a long way off on that basis. I also know that, generally speaking, a speedlite with a guide number of 58-60 will give close to a 100Ws equivalent output, sometimes a tad more, sometimes a tad less, when tested in the same way.

I won't mention flash durations then

 

[sk66]

A lot of people on forums (and on youcraptutorialtube) state that softboxes lose 2 stops of light, or maybe 3 (compared to what?) but that's just nonsense. Good quality softboxes do lose a bit, but not much. The real losses aren't actually losses at all, they just spread the light over a large area, so that a lot of it can be wasted.

I don't think I can agree with this...
A light modifier only uses the existing spread of light, which is why the type/size of light/modifier matters in terms of "filling" the diffuser. And all modifiers of the same type have the same spread regardless of size (with a fixed head position). That's because to fill a larger modifier it has to be farther from the source... i.e. the same spread, just "collected" a little later.
The only way a modifier makes it spread "larger" is if: a) the diffusion is not recessed from the front then there is a slightly larger/softer edge (fairly negligible). or b) the "modifier" is some form of reflector first designed to scatter the light (i.e. "backwards facing" head).

Diffusion does use some power as compared to the same modifier/reflector w/o diffusion. And additional layers of diffusion use additional power. I wouldn't say 2 or 3 stops is "typical;" I don't actually know what I would call typical... maybe 1 stop for a modifier designed for bare bulb.

 

[Garry Edwards]

True. However, I have some folding cameras and some large format shutters with sync sockets which predate electronic flash so they would have been intended for use with flash bulbs.


Steve.

The ones built before about 1960 yes, but there are plenty marked with both 'M' (normal bulb flash) and 'X' (electronic flash). Flash bulbs came in many different sizes, the only thing that they all had in common was that they were very inefficient compared to modern electronic flash, and very expensive. The smallest ones were pretty pathetic, and the large professional ones were far less powerful than the average studio flash head, and were very expensive. But I digress, the point is that flash was used with LF

I don't think I can agree with this...
A light modifier only uses the existing spread of light, which is why the type/size of light/modifier matters in terms of "filling" the diffuser. And all modifiers of the same type have the same spread regardless of size (with a fixed head position). That's because to fill a larger modifier it has to be farther from the source... i.e. the same spread, just "collected" a little later.
The only way a modifier makes it spread "larger" is if: a) the diffusion is not recessed from the front then there is a slightly larger/softer edge (fairly negligible). or b) the "modifier" is some form of reflector first designed to scatter the light (i.e. "backwards facing" head).

Diffusion does use some power as compared to the same modifier/reflector w/o diffusion. And additional layers of diffusion use additional power. I wouldn't say 2 or 3 stops is "typical;" I don't actually know what I would call typical... maybe 1 stop for a modifier designed for bare bulb.

I don't disagree with you, although I would say that having the front diffuser recessed makes almost no difference in terms of exposure. The point I was making is that the internet is full of false statements about the amount of light "lost" due to using softboxes

 

[Steve Smith]

I probably know more about how much light these things really put out than most manufacturers

I think most manufacturers are a bit optimistic with their ratings. My father told me that in the late 1970s they tested a range of flashes for wedding use and determined that of all those they tried, only the Vivitar 283 and 285 put out the amount of light claimed.


Steve.

 

[Cannyekerslike]

I don't know what kind of shooting you're doing,

Mostly still life but messing with some portraits as well, lack of subjects willing to have ago are a drawback with the latter. Nothing serious, just as pastime, I am not looking to go into business of anything.

Well, hotshoe flashguns are a waste of time with LF and, to a lesser extent, with MF too.

Yes I was haveing some difficulty with the MF thats what prompted the thought about LF. I have now got two Nikon flashes mounted in a softbox and that is a help, I am working on 4 flashes but will need a larger sofbox to do it comfortably and a way to trigger them all. I am not short of shoe mounted flashes but it is a real mishmash of makes, Canon, Nikon, Metz, Vivitar and others all going back years but they will all work in manual mode.

please remember that if you're shooting static subjects then you can use multiple flash pops to build up the power, e.g. 8 x pops = 8x the power, or 3 stops.

Thanks for the above I am going to give this a go, I should have thought of it myself but got hung up on power, sideways thinking, great stuff.