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I have a scenario that requires manually setting the aperture and focus on a lens attached to a machine vision camera on an outdoor scene (mirrorless camera with global shutter). I need to manually set the aperture and focus to best handle the outdoor scene during very low light conditions (i.e. just after sunset). However, because of security concerns I do not have access to the camera during these low light time periods, making it impossible to set the aperture and focus during the lowest lit conditions.

One theory I had was to use the relationship between LUX and EV to simulate the low light conditions during the day. Basically I would purposefully be under exposing daytime images at a level that simulates my twilight lighting conditions and then use those settings to set the aperture and focus during the time I have access to the camera. I am able to get LUX measurements of the scene for my night time lighting conditions.

Would this work? If so, how?

Any other suggestions for how to overcome this issue?

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If you know the daytime brightness for a properly exposed scene, and the nighttime brightness, you can just calculate how many stops difference between the two you have and set the night aperture accordingly. Say, 2048lux day, 64lux darkness = 5 stops difference. (log2(2048)-log2(64))

I don't know how focus comes into this, though, that wouldn't change dependent on lighting.

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  • \$\begingroup\$ With this technique, wouldn't I need to rely on the hatch marks on the lens to make the calculated f-stop adjustment? I fear that this wouldn't provide the accuracy I'm hoping to achieve in the time allotted. The aperture setting on the lenses I have are very sensitive. This might require checking the images at night, then making a small adjustment the next day, then checking again at night. \$\endgroup\$
    – BigMacAttack
    Commented Mar 31, 2015 at 21:25
  • \$\begingroup\$ Well if you don't trust the settings of your lens, all you can do is trial and error. To underexpose images like you wrote, you'd have to close the aperture, then apply the same adjustment in the other direction, which can't be more precise. Especially since you'd have to eyeball the exposure of the underexposed image, you can't take a brightness reading of a photo. \$\endgroup\$
    – ths
    Commented Mar 31, 2015 at 21:38
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    \$\begingroup\$ Try a 5 stop ND filter to simulate night conditions. \$\endgroup\$
    – Wirewrap
    Commented Jun 1, 2015 at 7:36

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