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I have this situation while shooting still life photos of e-commerce products, lets say for example leather shoes or leather bags:

  • Table with white paper as white background.
  • Two strobe flash lights with umbrellas at the each side about 45° angle (triggered by hot shoe trigger on camera) manually set power, no TTL.
  • Canon 70D camera on tripod.
  • Camera settings in "M": 35mm, ISO 100, 1/160, f8.0, Metering Mode: Evaluative metering. AWB Auto white balance enabled.

Given these fixed settings, all the pictures should show the same exposure no matter the object that is in the picture.

Instead I get different exposure depending on the object that I am shooting:

  • If the object is big (like 50% of the frame) and let's say brown or dark blue, the general exposure appears darker.
  • If the object is small (like 20% of the frame) and let's say yellow or pink, the general exposure appears more enlighted.
  • I have some cases where there is a big exposure difference despite fixed settings for all.

I am facing this problem since a long time and I fix it in post production with Camera Raw. I have made research and tests on the camera with different settings to understand why it acts like that, but I can't figure it out

It looks like there is some sort of auto-exposure going on that I can't control or that I can't find on the camera settings.

Unfortunately I can't attach any example since I cannot publish the products photos.

I would like to know if is possible to have same exact exposure results giving fixed camera settings, without the camera making any auto-exposure (or similar).

EDIT : I cropped the same area on two different images, with same settings in all, but as you can see, the white background, who should be same grey, is darker in one and lighter in one. It is not a shadow since the more or less brightness affects also in the object itself. The image looks overall brigter or darker. The black areas are the objects that I had to erase because I can't show or publish them.

EDIT2 : Investigating in Adobe Bridge I found a very weird thing, even if in the metadata the settings are f8,0 1/160 ISO 100 35mm for both images, in the Exif data it says: 24mm max value f/3.8 for the first image, 135mm max value f/5.7 for the second image. That's pretty strange... Is it possible that would be a hardware error of the lens sensors?

enter image description here

  • "Given these fixed settings, all the pictures should show the same exposure no matter the object that is in the picture" If all the settings and the light source do not change from photo to photo then the exposure is the same. ( IN MANUAL MODE ) Does your camera have auto ISO ? Objects of different colors reflect light differently, Does the white background stay constant ? If you are in manual mode then the light meter should not be changing the settings, Are you sure the settings are staying constant ? – Alaska Man Mar 10 at 18:30
  • @AlaskaMan ISO is fixed to 100 (as mentioned in my question). The white background stays the same as tone color but looks is bighter or darker. – Mario Mar 10 at 18:47
  • Are the flashes also in manual, or TTL mode? If TTL, the flashes may be compensating. – Tetsujin Mar 10 at 19:55
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    The "max aperture" field in the EXIF tells you what the lens' maximum available aperture at that focal length is. It has nothing to do with the actual aperture used (well, except if you set aperture to f/4 when the les is zoomed out and the max aperture zoomed in to 135mm is f/5.7, you're only going to get f/5.7 if you zoom in to 135mm). – Michael C Mar 11 at 1:52
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    Have you considered the interactions of the objects in the scene with respect to reflections? Even if the objects aren't shiny, a darker object is going to make nearby objects appear somewhat darker because it doesn't reflect as much light onto the other objects, while a lighter object will reflect more light onto other objects, making the scene somewhat brighter... – twalberg Mar 11 at 14:13
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If you're holding all exposure parameters constant, then the next suspect for me would be your white-balance setting, which I don't see mentioned above; e.g., do you have auto-white-balance (AWB) enabled, or set to a fixed temp?

Remember that most cameras are designed to interpret any scene you throw at them as 18% gray and compensate for that.

If that's not it, and if having a similar/constant exposure between shoots is a big time-saver for your work — have you considered picking up a gray card or similar? That would help take the guesswork out of adjusting in post-processing, at least.

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  • Yes it has AWB enanled. Do you think it can affect global brightness of the image from a shoot to another? I'll add to the OP as extra info. – Mario Mar 10 at 22:23
  • It won't affect the data available in your RAW files, but it will influence your metering; JPG renders; and how software interprets the RAW files. It's also why if you meter a correct exposure against a snow-filled landscape, you'll end up with gray snow. There's some great discussion in this post as well re: the relationship between exposure and white balance. Should be easy to test; what happens if you set a constant WB in-camera w/ the same experiment? – Jesse Stuart Mar 10 at 23:15
  • Thank you very much for the links, I have read them and they are all very interesting. I can't make a test in this moment, but I will. I am convinced now that the problem is the AWB, seems that it will influence metering in some way. I will get a grey card and experiment by the next days. If I can replicate and fix the problem I'll post the solution here. – Mario Mar 11 at 8:03
  • You bet :) Keep us posted on how it goes; I'm curious to hear your results! – Jesse Stuart Mar 11 at 11:37
  • This is clearly the problem. The camera is adjusting white balance based on some internal algorithm. Do a manual balance and then the camera won't do anything. – Tiger Guy Mar 11 at 17:58
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How repeatable is it? A few ideas:

I've managed to significantly change light levels by standing between the light source and the subject :-)

Are these always-on studio strobes or are they Speedlites that might have dropped into sleep mode? Are they Speedlites that might have dropped into sleep mode then awakened reset to default settings?

Are your ambient levels low enough all of your lighting is coming from the strobes? Could be a change in ambient levels. Could even be a change in ambient levels caused by blocking from the larger subject.

Do you have to move the lights to get the larger subjects in?

You mentioned a difference in EXIF, that's apparently recording current focal length and current max aperture for different zoom positions. It's not impossible that a variable aperture zoom isn't really at f8 when you set f8 -- that the non-wide-open apertures vary too. I'd assume they'd design in a compensation factor--just a lookup table--but eh, maybe they didn't. Wouldn't cause problems in TTL metering situations.

Is anything firing a TTL pre-flash? I had a strobe that wasn't properly turning off the built-in slave cell when I had a radio cable plugged in. For my usual no-Speedlite, radios-everywhere setup it worked fine--it was working differently than I'd intended, but it still worked fine. But I try to introduce a little on-camera fill and suddenly that strobe wasn't contributing to the exposure. Took me a while to figure out I'd left the on-camera flash in TTL and the pre-flash was firing the defective strobe early.

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  • Even if your guessings are all reasonable (and sometimes I have experienced them myself too) none of them are the cause of the the problem in my case. – Mario Mar 11 at 8:00
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You mentioned there's an exif data incoherence with the parameters you shot. Although this could be an error of internal parts of the lens (dirt or moisture in the metal tracks of the barrel, whatever...) It won't affect exposure and colours or white balance. I've tried right now inhibiting the connectors and shooting manual.

What I don't know very well is the studio light settings, height position of the strobes, distance to object, distance to background etc... This may in fact interact with the exposure of the object to photograph. Given a way bigger object it may throw shadows to the white background, absorb/reflect light to/from the background/light or even trigger dynamic range corrector some cameras have. You should try a different light settings with height and distance that affect the less given two different object sizes if it happens to interact. Of course, as other mates have stated, turn off AWB and stablish a parameter by yourself. Let us know if you find the causes of this issue.

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