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How can I maximize the amount of detail captured in both dark and light objects in the same photo? I don't have the option of post-processing, so "traditional" HDR is not possible.

If I reduce exposure, close aperture, etc, detail in the darker object is lost, while increasing/opening causes the bright object to get washed out.

Are there any adjustments I can make to improve the amount of detail in both, once I've found a reasonable middle ground?

Context:

The actual issue is taking images with fixed settings - so being able to photograph dark objects and bright objects in separate images, with the same aperture, exposure, gain, and no automatic compensation.

I have an OCR-like system that reads a hole-pattern in a metal tag (a Pepperl+Fuchs OIT500, if you're interested).

It relies on contrast between the surface of the tag and the dark hole.

However, as the metal tags become dirtier, and reflect less light, the contrast becomes lower.

As the camera does not include any form of automatic compensation for this, I want to set the camera up so it can read the widest possible range of tags, producing usable contrast on dirty tags, without washing out the image of cleaner, more reflective tags.

The camera has adjustable, but fixed aperture, exposure time, and gain. Currently, we have the aperture wide open, shutter short as possible (as the tag is on a moving object), and gain set to give the best results in the range we've seen, although we still have errors.

Is there settings we can adjust to increase the readable range?

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  • \$\begingroup\$ Is the resulting image file processed automatically via proprietary software, or is there room to introduce a post-capture processing step before reading? \$\endgroup\$
    – HamishKL
    Apr 12, 2016 at 21:08
  • \$\begingroup\$ It's all internal: push button, receive number. \$\endgroup\$
    – RoadieRich
    Apr 12, 2016 at 21:46
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    \$\begingroup\$ Can you rephrase this in a way which might be generally applicable to photography outside of your non-photographic use case? \$\endgroup\$
    – mattdm
    Apr 27, 2016 at 16:15
  • \$\begingroup\$ @mattdm I suppose it's about maximizing available dynamic range - so could also apply to photographing dark and light objects in the same shot - except without the luxury of HDR. \$\endgroup\$
    – RoadieRich
    Apr 27, 2016 at 16:21
  • \$\begingroup\$ Looking at the device's manual, it doesn't seem that any traditional photographic techniques are going to be applicable -- the device has its own lighting grid and it doesn't seem to be producing an image as much as a code, as you say. \$\endgroup\$
    – David Rouse
    Apr 27, 2016 at 16:58

1 Answer 1

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You can try using polarizers on the light source and/or lens. You will need to determine experimentally if using crossed polarizers increases the contrast sufficiently.

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    \$\begingroup\$ Yeah, modifying (increasing the output of) your light source is probably the best/simplest way to increase contrast, given the automated nature of the system. \$\endgroup\$
    – HamishKL
    Apr 12, 2016 at 21:48

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