Re: Primer on photographic exposure, etc.
Re: Primer on photographic exposure, etc.
- Subject: Re: Primer on photographic exposure, etc.
- From: Iliah Borg <email@hidden>
- Date: Mon, 20 May 2013 23:02:48 -0400
Hi Lars,
>
> Do you have a ballpark dE from unit to unit of same model for pro gear? 5, 10, 20?
One can't really measure dE from raw data I guess, but if I do it through profiles, centres of the sensors on some pro models are closer than 3 dE2000, serial numbers of sensors far apart. But on some cameras it is up to 6. I can't go into more details of which make/models are that way or the other way. BTW, shooting targets filling the frame with the target may be not the best approach.
> So you're not calibrating the entire camera system (including actual light, lens and polarizing filters) then? I'm surprised you don't.
With polarizing filters bets are off because the colour rendition depends on the angle of rotation and the angle of light, to put it loosely. If in studio and polarazing filters are used for reflections the one-time profiles from a target shot can be accurate enough.
Profiles (especially matrix) can be multiplied. So a separate spectral measurement from each lens multiplied by the sensor measurement (and by the light spectral data if in studio) is a simpler approach. My favorite lenses are too many to profile each camera with each lens I think.
Actual light is an extremely interesting question, I do not feel like I can do better with actual light than just measuring it with some portable spectrophotometer.
Even with artificial light, stable halogen, the power in the blue portion of the spectrum can change dramatically during the session. There is a reason why optical grade light sources are so expensive, and why in precise setups the monochromator output is constantly monitored.
>> The greatest and often overlooked problem is that profiles depend on exposure, especially for saturated darker colours. It is a good idea to make "bracketed" profiles, like for white patch exposed to 248 for normal exposure and -2 EV, -3 EV, -4 EV profiles.
>
> Can you explain why that is? Non linear sensor? Wrong black level?
Noise. In-camera flare and lens flare. Another yet connected thing is that RGB about 30 (on 0..255 scale, gamma 2.2) is too "light" for what cameras can do now.
--
Best regards,
Iliah Borg
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