Three filter questions for B&W film photography

Question

I understand why B&W filters (e.g. Wratten #25) diminish light, but why do I need to compensate manually? I work with an (admittedly ancient) Pentax K-1000, and I thought it measures light through the lens, i.e. whatever reduction is caused by the filter is already factored in, so that when I expose such that the needle is in the zone I already compensate for whatever light my filter eliminated...what am I missing?

Answer 1

As you know, a filter stops some of the exposing light while passing some. The Wratten #25 is the red tricolor used with the Wratten # 58 Green and the Wratten # 47B for color separation work and for haze penetration in aerial black & white photography. It was used to simulate night in black & white movies. It adds contrast and brings out clouds when used with black & white films. OK, its published filter factor is 8.

We handle a filter factor by multiplying the shutter speed without filter by the filter factor, in this case 8. Likewise, since each f-stop = a 2x change, a filter factor of 8 = 2x2x2 =8 meaning 3 f-stops compensation. We can also divide the ISO by 8, and reset the ISO to this calculated value and take an exposure reading at this reduced ISO.

Your camera sports a thru-the-lens metering system. The Wratten #25 will takes its toll, so the measurement will be about 3 f-stops less. This should be an OK reading and likely you will not need to take any additional action. In other words, the exposure system will compensate for the mounted Wratten #25.

Now comes the “but if”: A filter factor is a compensation derived based on a specific light source, usually sunlight. The filter factor will be slightly different for other light sources like tungsten. As a general rule, the differences are negligible. After all, the best we can likely set our camera is + / - 1/3 f-stop. I also want to tell you that the filter factor is also dependent on a particular film emulsion. This is because different emulsions have different spectral responses.

Now comes the rest of the story: Light meters also differ in their spectral response. Therefore, they will give different readings when tasked to measure daylight or tungsten etc. They surely will give different readings when tasked to measure the exposure through strong filters. The good news is, likely your readings will be in the ballpark -- meaning no special action required. However, be forewarned, the Pentax K-1000 might be fooled by the Wratten #25. Your approach should be to bracket your exposures. Making a test exposure series is also a wise approach.

Answer 2

A lot of the instructions for black & white photography in general and color filters in particular were developed and written at a time when cameras did not have built in light meters. The light was metered using a handheld unit that didn't measure light coming through the lens of the camera. If the meter read that for 64 ASA film at f/16 the proper shutter time was 1/60 seconds then placing a filter on the camera's lens did not change the amount of light being measured by the handheld meter.

If you measured the scene with such a handheld meter and then set your camera manually you would need to take the reduction into account. In the above scenario if your #25 filter came with specifications of a filter factor of 8 (1/8 the light is allowed to pass), you would need to increase exposure by three stops. Filter factor = 2 raised to the power of n where "n" is the number of stops. Since 8=2³, a filter factor of 8 is 3 stops. The math is just like the NDx system used by Hoya, B&W, and Cokin except that colored filters aren't neutral in density because they affect some colors more than others.

So you would increase your shutter time by three stops, open your aperture by three stops, or use a combination of both that equaled three stops. Let's say you decided to go with f/8 (two stops higher exposure than f/16) and 1/30 seconds (1 stop higher exposure than 1/60). Remember, we're using a large view camera mounted on a sturdy tripod!

Now fast forward to your trusty Pentax K1000. There's a light meter built into the camera that measures light coming through the lens.

Suppose you set the camera's exposure settings so that the needle is centered without the filter in front of the lens. The you put the filter in front of the lens. What happens? The needle shows 3 stops underexposure.

When you change your exposure settings to move the needle back to the correct zone, you have just compensated for the reduction due to the filter.

But wait! There's more!

Remember up above when we said that color filters are not neutral density filters because they don't affect all colors the same? Well because of this we also have to compensate for a particular film's spectral response. That's why we have to pay attention to the instructions that come with each specific type of film.

Your camera's meter is based on a neutral response to daylight colored light of around 5600K. When shooting under different sources of light the light meter can be fooled, especially if there is a color filter in front of the lens. So it is usually best to meter without the filter and then manually compensate the exposure using calculated values based upon the instructions that are provided by the maker of the particular film you are using. You'll likely get more accurate exposures.