Re: Exposure Value
Re: Exposure Value
- Subject: Re: Exposure Value
- From: Ben Goren via colorsync-users <email@hidden>
- Date: Mon, 24 Jun 2019 10:15:48 -0700
On Jun 22, 2019, at 10:31 AM, Robin Myers via colorsync-users
<email@hidden> wrote:
> Perfect reflecting diffuser 100
> Fluorilon FWS-99 2009 99.9
> Spectralon SCS-99 99.7
> Kodak Gray Card 1977 (3 specimen average) 95.0
> ColorChecker Passport 2013-12 Neutral 9.5 52.2
> ColorChecker Passport 2013-12 Neutral 5.0 95.2
Spectralon is great stuff, but damned expensive. Expect to pay as much for a
Spectralon target as a Canon L series lens.
There’re two homebrew-style options that are typically plenty “good enough” in
photographic settings.
The first, if you want 99%+ reflectance, as close to “pure white” as you’re
going to get without spending lots of money: PFTE (Teflon) thread seal tape.
Not very durable, and it’s got basically an eggshell surface that’s a bit more
specular than ideal (Spectralon is close to ideally Lambertian)...but it’s dirt
cheap and you can get it anywhere. Use a bit of creativity and you can figure
out how to wrap several layers around a card to make a target. If it gets dirty
or damaged or lost...no biggie; all you’ve invested is your time, so just make
another.
The second is a styrofoam coffee cup (or packing material, etc.) Its total
reflectance is a lot less...in the 80% range. But it’s much more spectrally
flat than the photographic tools (ColorChecker, gray card, etc.). Outdoors or
in mixed lighting, its near-cylindrical shape lets you sample the illuminant
from all angles, which can be creatively useful. And, with the right
combination of lens and cup, you can fit the cup over the lens and get a
near-ideal uniform sampling of all the light in the scene — much like those
hundreds-of-dollars custom white balance tools that fit over the lenses do,
but, again, with near-perfect spectral uniformity. (Put the cup over the lens,
take a picture, and use it for your in-camera custom white balance. It’s
probably pretty close to where you want to meter, too; experiment some and you
could get that dialed in perfectly and consistently.) Coffee cups are pretty
reliable spectrally; the more creative shapes you might find in a craft store
(spheres, cubes, etc.) are more hit-or-miss. Visually compare with some thread
seal tape (if you don’t have a spectrometer) if you’re unsure.
Last note: read everything Iliah has on the Raw Photo Processor / RawDigger Web
site and accompanying blog. You couldn’t ask for a better crash course on
camera exposure, metering, development, etc., etc., etc.
Cheers,
b&
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