I had to read about compensating exposure for my new red filter :
Nearly every filter decreases the amount of light by filtering some of it.
This has to be compensated by exposure increase.
The needed increase is called the filter factor (Verlängerungswert in german) , Usually the filter factor is imprinted on the filter itself.
The number of f-stops required to correct the exposure with my 091 red filter may be calculated using the formula:
Filter factor = 2X
where the exponent „X“ is the number of f-stop increases required.
My red filter with a filter factor of 8
8 = 23
The normal exposure will be increased by three stops with this
By the way :
I compensate changing the ISO on my lightmeter, e.g. with a TRI X 400 and red filter my lightmeter is set to 100 ISO.
Any red filtermakes the negative more grainy. Thats not a film problem, but the contrast enhancement of the filter itself. So – Maybe you would like to take a higher ISO film (considering the filter factor), but beware of the grain !
When developing – beware of the grain.
ISO 400 Films are good because they are more sensibilized for the longer (red) wavelenghts, some old and experienced guys told me to pull a 400 ISO film to 200 ISO, should give a nice result.
OK, and when do I use the filter ?
I need red light, so if the sun is near the horizon, sunrise and sunset, are the best times.