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I have a few problems with exposition with my Canon 70D: with same camera settings I get random EXIF data for focal lenght and aperture.

This is the setup

  • 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": ISO 100, 1/160, f9.0, Metering Mode: Evaluative metering.
  • I use a 18% grey card to get proper white balance, set the WB to "custom" and get the WB data from the picture of the grey card.
  • I also use an exposimeter to check that I have the best metering and it is all fine.
  • Subjects are bags or similar leather products.

After I take pictures, that should all be the same exposition, I notice that there are a few difference between the photos. In fact I took two consecutive photos, not touching the camera or anything, of the same subject on the table, and one looks darker than the other (actually I can't show the products because they are trademarks).

Surprisingly, when I went to check the EXIF data, I found that they are quite different than the camera setup, and that seems explaining why some photos are darker/brighter than others.

Why that happens? Is my camera malfunctioning?

Please check the two screenshots (sorry the software is not in english but you can guess...=.

enter image description here

enter image description here

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    \$\begingroup\$ Did you try to move it slightly from 35mm? Did you try to remove the lens and clear the contacts (on camera and lens)? Did you try to check EXIF data with different tool? \$\endgroup\$
    – Romeo Ninov
    Jun 5, 2020 at 17:31
  • \$\begingroup\$ @RomeoNinov 1. I have not moved the zoom from the original position between two shots. Even if wrong it sould remain the same value. 2. I have tried to check EXIF data with different tool and the results are the same as in Adobe Bridge. 3. I will try to clean contacts with a specific product I have. Thanks. \$\endgroup\$
    – Mario
    Jun 8, 2020 at 13:45

2 Answers 2

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It looks like you've probably got 'Safety Shift' enabled and set to option 1: 'Shutter speed/Aperture' in your 70D's menu. It's found under the "Custom Functions" menu at 'Menu → Custom Functions (Orange Tab) → C.Fn I: Exposure → C.Fn I -6 Safety Shift'.

It also looks like you're using a manual flash triggering method that the camera does not detect.

Thus your camera is not taking into account any of the light from the flashes and is assuming only the ambient light is there to illuminate your subjects.

It's all covered on page 367 of the EOS 70D Instruction Manual

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    \$\begingroup\$ Interesting. But I can't imagine Safety Shift changing the recorded focal length between 37mm in the first file to 135mm in the second file. What's going on there? \$\endgroup\$
    – scottbb ♦
    Jun 5, 2020 at 19:46
  • \$\begingroup\$ @scottbb Occum's razor says the lens was zoomed. If that was not the case, then who knows? I don't have a 70D to play with to see if Safety Shift is even applied in Manual exposure mode, either. But it's the simplest explanation I can think of without more information from the OP. \$\endgroup\$
    – Michael C
    Jun 6, 2020 at 7:55
  • \$\begingroup\$ There are some third party lenses which "spoof" the lens ID of Canon lenses. Most of the time they are not the most similar lenses with regard to focal length(s) and maximum apertures. If the OP is using a third party lens, who knows? There's also the possibility that whatever application the OP is using to view the EXIF data is not interpreting Canon's maker notes correctly. Notice how the lens listed near the bottom shows 18-135mm f/0 for the 'Specifica lente' field? \$\endgroup\$
    – Michael C
    Jun 6, 2020 at 7:57
  • \$\begingroup\$ The lens serial number is certainly "garbled" in a way that looks like the EXIF viewer doesn't know the "secret" way that Canon encodes serial numbers in the EXIF maker notes.. \$\endgroup\$
    – Michael C
    Jun 6, 2020 at 8:05
  • \$\begingroup\$ Good point about Occam's razor, but I'm taking OP at their word that they took the pictures back to back without touching anything else on the camera. I'm not a Canon person, but I think the EF-S 18-135 f/3.5-5.6 IS is the (one of the?) kit lenses with the 70D. So this probably a combo kit. However, it's certainly possible it was gray market, and if so, who knows what's going on? Good eye on the "f/0". That certainly raises eyebrows. \$\endgroup\$
    – scottbb ♦
    Jun 6, 2020 at 17:57
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I could finally verify that the problem was 100% caused by the lens. It was internally unstable and in certain position the internal connections were not communicating the right values to the camera body (or they were completely isolated).

This problem seems at first random happening, then after more shootings it got worse and in the end the lens got stuck and would not zoom anymore. If you shake the lens, you can hear some little thing (maybe a little screw) going around.

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