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I occasionally use gels to match my flash to ambient lighting. It’s easy to find green gels (CTG) to match fluorescent lights and orange gels (CTO) to match incandescent. However, in my experience, the common gels provide too much coloration when shooting with DSLR. In particular, to match fluorescent, I usually need to shoot ½CTG, which is much harder to find than full CTG.

I have heard that the full CTG works well with film SLR, but something about the color sensitivity of DSLR means that they require ½CTG instead, and likewise for ½CTO. Is that correct? What’s the physical reason for it?

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I tried hard to find a source for that information, but no luck. However, it doesn't sound right to me because the idea behind putting a gel on a flash is to alter the light to match the ambient, be it tungsten, fluorescent, etc. That activity is independent of the sensor or the film in and of itself, it's simply about making the light outputs match each other.

Now, what I think you may be encountering is that not all fluorescent lights are full CTG (CC30) these days, in fact there are a lot of variations, and that may result in needing ½CTG (CC15) or even weaker.

In any event, if you're having a hard time finding them, try an online source like B&H or Adorama.

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    +1 flash must be matched in color to ambient light. Then you either set WB on digital or put necessary filter on the lens of film camera – aaaaa says reinstate Monica Apr 11 '15 at 1:11
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    I also found it fishy that there's a color difference between and digital, but it's not totally implausible. I suspect that you are right that fluorescent bulbs simply vary in their greenness, but some more solid evidence would be nice! – Bradd Szonye Apr 12 '15 at 21:30
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Why do SLRs and DSLRs use different flash gels?

They don't.

I have heard that the full CTG works well with film SLR, but something about the color sensitivity of DSLR means that they require ½CTG instead, and likewise for ½CTO. Is that correct?

No it's not about the sensor vs film. Different film did however had some slight colour renditions differences, but it was film dependent not camera.

However, in my experience, the common gels provide too much coloration when shooting with DSLR. In particular, to match fluorescent, I usually need to shoot ½CTG, which is much harder to find than full CTG... What’s the physical reason for it?

It's about incandescent light not having identical temperature, meaning you won't have 100% colour match if you have more than one ambient light source. So too much colouration can be the result.

Even with only one ambient light source you may not find a gel to match it 100%, so the point is to avoid major colour cast and match the light as close as possible. The smaller differences can't be matched, however hitting a temperature averaging the sources are you best option to get a neutral image temperature.

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    While this is all true, it doesn't answer the question which is about the difference between film and digital SLRs. – Philip Kendall Apr 5 '16 at 12:29
  • @Philip Kendall I believe I explicitly answer's the question, I have written my answer in a more explicit way now. Do you still not see how I answered the question? – Goat Apr 5 '16 at 12:52
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    No, I don't. You're talking about more than one light source when nobody has mentioned that at all. – Philip Kendall Apr 5 '16 at 12:56
  • @Philip Kendall How about now, I have copies the full question so it's clear what I am answering? – Goat Apr 5 '16 at 12:58
  • The environments I was shooting had only one ambient light source, fluorescent tubing in a large, windowless room. Typically they required both a 1/2 CTO and a 1/2 CTG to match the ambient color. – Bradd Szonye Apr 9 '16 at 0:09
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Sensitivity curves of negative film are different from the permissivity curves of the filters used on digital camera sensors. For example, film was decidedly sensitive to UV light, making the use of UV filters (and also skylight filters) a good idea for certain film situations. With digital, UV filters are basically a wash and don't really cause additional changes (in theory, they might affect purple fringing but I haven't been able to see a noticeable effect: probably they don't do more than the sensor filters already do).

Now fluorescent lights tend to have very pronounced lines in their visible light spectrum. While for the broad spectrums of incandescent light sources and typical reflective colored surfaces the responses of film, human eye, and digital sensor filters can be pretty well dealt with by simple linear mappings, the narrow lines of fluorescent lights are much more problematic. If the relative sensitivities at those wavelengths are not similar to the relative sensitivities of what the sensors broadly cover, you get a problem.

A similar problem is matching paint swatches: if you perfectly match a car's color in sunshine for a paint repair job, that is no guarantee that the result will not stick out like a sore thumb under sodium vapor or fluorescent lighting. Incandescent, in contrast, will usually look fine.

Good correction filters will be a good compromise for various forms of light, but for line spectra there just is not much room for compromise.

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