Measuring stops of light output?

soeren said:
Then if we take the flash-subject distance into account we have the.......... (n) :LOL:
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Haha yes. The two most critical aspects of exposure testing any artificial light source is distance and field coverage (area of light pool, plus the amount of fall-off within that). They're both closely related, and at the kind of distances we tend to work at, just a few inches or degrees of coverage can make a big difference.
 
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No, incident lightmeter readings will only give you mid-grey if the subject is mid-grey. They don't measure the reflected light.
 
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Alastair said:
Stops of light above only just black. And it has to be incident.
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Yes. But if you start from a point of "correct exposure" which is very easy to do in a variety of ways, then all light reaching the sensor is also relative to black. I.e. something is recording as mid grey so I need to add 4 stops of light to push it to white (not clipping).

HoppyUK said:
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.
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I think that is what was throwing me... w/ the D810 I find I need about 6 stops to push mid grey (or correct exposure) to black or white (clipping).
 
sk66 said:
Yes. But if you start from a point of "correct exposure" which is very easy to do in a variety of ways, then all light reaching the sensor is also relative to black.
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Only issue is that Nikon use some fine tricks to open up shadows etc. so shooting a mid grey target and measuring a mid grey target eliminates any of that

Mike
 
mike weeks said:
Only issue is that Nikon use some fine tricks to open up shadows etc. so shooting a mid grey target and measuring a mid grey target eliminates any of that

Mike
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Also true. Any exposure tinkering going on (and it's common, though rarely specified) is always at the extreme ends of black and white. Mid-grey should be unadulterated.
 
soeren said:
No, incident lightmeter readings will only give you mid-grey if the subject is mid-grey. They don't measure the reflected light.
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Yes, but they measure "for" mid grey. Additionally, they are based upon standard ISO's (which are not at all standard in practice), and there is no universal standard for meter calibration either... luckily, you can usually calibrate the meter to your camera and what you feel "correct exposure" is (but I don't think most do). I also don't understand why people don't set them to display full stops in 1/3 stop increments (i.e. f/10 instead of f/8.0 7).
 
sk66 said:
Yes. But if you start from a point of "correct exposure" which is very easy to do in a variety of ways, then all light reaching the sensor is also relative to black. I.e. something is recording as mid grey so I need to add 4 stops of light to push it to white (not clipping).
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It's only relative to only just black (which is purely theoretical).. because once you hit black you can keep on lowering the exposure but the result stays the same.

It's why the baseline relative measure of exposure has to be a tone of grey. Pure black and pure white aren't fixed points and are artefacts of the sensitivity of the method of measurement.
 
Alastair said:
It's only relative to only just black (which is purely theoretical).. because once you hit black you can keep on lowering the exposure but the result stays the same.

It's why the baseline relative measure of exposure has to be a tone of grey. Pure black and pure white aren't fixed points and are artefacts of the sensitivity of the method of measurement.
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Got it... I missed the emphasis on "just" the first time around.
 
sk66 said:
Yes, but they measure "for" mid grey. Additionally, they are based upon standard ISO's (which are not at all standard in practice), and there is no universal standard for meter calibration either... luckily, you can usually calibrate the meter to your camera and what you feel "correct exposure" is (but I don't think most do). I also don't understand why people don't set them to display full stops in 1/3 stop increments (i.e. f/10 instead of f/8.0 7).
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But they are not affected by subject/scene reflectance
 
sk66 said:
True, but honestly I think too much is made of that fact... because if you have black packed up in shadow, or white blowing out due to angle, that's also what you will get from an incidence reading.
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That's where knowledge and experience kicks in. :)
 
Garry Edwards said:
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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.
.......
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That's the part that had escaped me. With my high intensity reflectors (28cm dia') the centre to subject and edge to subject distances are essentially the same at any reasonable distance and remain so as the distance increases.....thanks Garry.
 
sk66 said:
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").
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Since the area of the beam target at whatever distance is still an area (such as square meters) and the distance to it is still a distance (such as meters), the light fall is still affected by the ISL, it's just that the narrower the beam the larger the factor by which the almost completely spherical coverage of a bare bulb is divided. That factor remains constant for all distances, such as (1/6th)^2 for a 60 degree square spread.
 
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