Red filter compensation

[gmed]

Thank you all.

[Drew Wiley]

To get an objective answer you need to start with something like a neutral gray card and
make a series of exposures using your preferred film and filter. Do not meter thru the filter.
Then read the resulting negatives with a densitometer and compare them to your normal
exposure value without any filter in place. This gives you your Zone V midpoint and how
filtration will affect it. Shadow values are a supplementary subject, and can be affected
by many things, including just how much blue light is in the shadows. At high altitude in
can be substantially more than at lower elevations, and require more compensation to the
red. TMax100 can render these deep shadows wonderfully, but if you overexpose, it will
be a lot harder to keep the highlights crisply separated. Believe me, I know this game.
With less contrasty lighting, or a film with a more gradual toe, you can get away with more.

[Heroique]

...At high altitude blue light can be substantially more than at lower elevations, and require more compensation to the red filter...

Here’s an example for this oft-overlooked situation.

If a shadowed scene at 500 feet w/ a red filter requires – let’s say – 2.5 stops of compensation, then a similar scene at 4,500 feet can reasonably require 3.5 or 4 stops, sometimes even more.

Yes, sounds funny, esp. if your (unfiltered) meter reads the two scenes identically.

Here’s why:

The high-altitude scene contains a higher proportion of blue light, the kind of light that a red filter takes away. This means that at 4,500 feet, the red filter is taking away a higher proportion of your total light – that is, less total light is reaching your film.

So you need more compensation at 4,500 feet, often significantly more.

But never mind all this. Just take notes. Examine your results. Adjust future shots to taste.

[Drew Wiley]

4500 ft?? That's LOW elevation. Try the color of light at 12000 ft. Oh yeah ... you guys
get more rain in the NW, so the sky is bluer due to the precipitation, so factor that little
dial or button on your light meter too. And while you're at it, note the pollen count and
how many campfires, forest fires, and jet contrails are in the vicinity, along any potential
reflections off approaching alien spacecraft. So when Bigfoot shows up, ask him to hold
his pose and say "cheese" a little longer, due to the slower red filter exposure, and hope
that he has a sense of humor.

[John Koehrer]

Best would be to test with different compensation values used. develop your film and choose the one you prefer.

As above this value can vary with different film. The meter does NOT likely have the same spectral sensitivity any of your films do.

[Heroique]

...When Bigfoot shows up, ask him to hold his pose and say “cheese” a little longer, due to the slower red filter exposure, and hope that he has a sense of humor.

Sounds like the age-old problem, How do you photograph a black cat in white snow?

BTW, 12,000 feet? I envy you Californians. Thanks to our heavy mountain snowfalls, it’s tough to get above 4,500 feet on wheels right now (forest roads), and here we are, mid-June!

[aporodagon]

Here's a test of filter factors and meters I posted a while ago. I think it gives an idea of how variable reading exposures through filters can be.

http://www.largeformatphotography.in...hrough-filters

[Corran]

My red filter calculations are usually in the neighborhood of 1 or 1.5 stops of adjustment.

Interesting idea about elevation. Where I live the highest hill is about 100ft above sea level...

[mdm]

Throw it away, its probably just red glass. I have a cheap set of ebay filters for a cokin p system that are completely useless, maybe made for dodgy digital people. Nothing like a proper filter.

[Corran]

Mine are Nikon and Hoya filters. 25A I should mention.