Help with filter factors!

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jljohn

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I recently picked up a set of basic filters for the Leica (yellow, orange, red, green, & ND) and tonight I decided to do a little testing of the listed filter factors. Now, these are all B+W filters with filter factors written on the filter. The medium yellow is listed as a 1 stop (factor=2), the yellow-orange is listed as a 2 stop (factor=4), the dark red is listed as a 3 stop (factor=8), the dark green is listed as a 1.5 stop (factor=3), and the ND is a 3 stop.

So, in my living room, which is lit by two 100 watt incandescent bulbs, I set up a grey card and pull out my Pentax Spotmeter. I meter the grey card through the ND and it show 3 stops difference. I meter the green--1 1/3 stop difference. So far so good. I meter the red, and it shows only 1 stop difference on the grey card and on other items around the room. This is 2 stops off! I try the orange which shows 1/3 - 2/3 stops difference--also way off. The yellow produces no difference in the spot metering. In other words, it meters as if it were clear! What the heck?

Is this simply because my light was incandescent and therefore must have a yellow cast, thereby impacting all filters that fall along the yellow-orange-red spectrum? Or is something else going on? I thought I could slap a yellow filter on, adjust my meter one stop and shoot all day, but if light color cast makes that much of a difference, moving from outdoor to indoor could be very tricky!

I have a modest amount of experience with filters for black and white photography, but all my filter use to date has been for medium format landscape photography. Outside, my filter factors always seemed pretty straight forward and consistent.

Any thoughts or insight would be appreciated!
 
Its common knowledge, or at least I've always thought so, that you cannot rely on metering through filters to give accurate readings. Especially red filters which orange is fairly close to.
Go with the filter factors and do some trials to find out what effect they have on different colours. Using a macbeth colour chart or equivalent (you can print your own) you can try out all your filters with different factors. Keep notes.
 
what tiltody said. not to say filter factors are always right, many times i've had them be anywhere from 1/4 to 1/2 stop wrong. Just place the filter over the lumisphere on an incident meter and take a reading then you'll be sure.
 
I would add that its not an exact science because it depends on how much effect you want and the colour temperature of the light falling on what you are photographing. So optimum factor for any filter is variable. i.e. all filter factors are ball park figures.
 
Filter factors, stops, or the way for me is to change the EI/ISO on my handheld meter. I figure the stops that I reduce for every filter by experience using the filter factor as a start. And then I determine what the new ISO will be for my handheld meter. I know it sounds all the same but if I don't like the way a yellow filter looks on my final product when using the filter factor or stops, I then adjust for what I like by just changing the EI/ISO.

That way I never meter though the camera and I continue to meter my normal way. But don't forget that every filter (except an ND) changes your shadow detail and possibly other tones. Sometimes, but not often, I like them but other times it isn't worth the trouble (for me).
 
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Filter factors for colored filters are in practice affected by the color of the light. Since your incandescent light is strongly yellow, it makes sense that the yellow filter would have little or no effect on exposure... the light is already yellow! The ND, since it's neutral in color, is unaffected by the color of the light.

Another issue to consider is that different meters have their own spectral sensitivities... that may not match the color sensitivity of the human eye, and will not necessarily match the color response of various films. A film with low red sensitivity will need more exposure in reddish light than one with more red sensitivity. Same with a yellow/orange/red filter, where the actual exposure factor will not only depend on the light but also the particular film.

My Gossen meters tend to have more red sensitivity than most B&W films, so will usually give me a reading that results in underexposure in incandescent light. OTOH, Ilford XP2 for instance has more red sensitivity than Tri-X for instance, and so the meter reading is closer to right for XP2 in that situation.
 
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Thank you all. This is very helpful. The common knowledge re: spotmeters had eluded me. It's particularly interesting because I've known folk to by two of each filter-one for the camera and one for the spotmeter.

I've been in the practice of reducing the iso setting on my incident meter by the amount of the filter factor and shooting away, and this has worked, but most of my photos have been outside. So, if I bring this setup into incandescent light with the yellow filter on and my meter adjusted down one stop, will the images be exposed well or overexposed a stop?
 
jljohn said:
Thank you all. This is very helpful. The common knowledge re: spotmeters had eluded me. It's particularly interesting because I've known folk to by two of each filter-one for the camera and one for the spotmeter.

I've been in the practice of reducing the iso setting on my incident meter by the amount of the filter factor and shooting away, and this has worked, but most of my photos have been outside. So, if I bring this setup into incandescent light with the yellow filter on and my meter adjusted down one stop, will the images be exposed well or overexposed a stop?
Click to expand...

i think what you're saying is you first determine the amount of loss using an incident meter outside, and you are asking whether this loss would apply indoors as well. Yes, the same loss would apply in any situation, indoors or outdoors doesn't matter.
 
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Don't forget that it also matters if the filter is coated or uncoated. This will effect how much light goes through once they are put on the lens and/or stacked together.
 
Both calculating with filter factors and measuring through the filter are only a coarse approximation. Even if spectral responses of filters and film/sensor are better known than usual, the spectral distribution of the light is a unknown factor unless you are working in controlled studio conditions.

Even dramatic sunsets or cloud conditions can already throw calculations or measurements off by a stop or two - filter exposures with neon lights or technical fluorescents can cause meters or calculations to be off quite dramatically, if their spectral range happens to match one of filter or film but not the other.
 
A modern silicon meter will work better through filters than an old CdS meter, which is oversensitive to red. However, you need to do your own tests because b/w films vary a lot in their sensitivity to different colours. Generally all of them are oversensitive to blue (which is why people use yellow filters on sunny days) but some have more red sensitivity than others and may only need 2 stops with a 25 filter instead of the usual 3 stops.
 
rogerzilla said:
A modern silicon meter will work better through filters than an old CdS meter, which is oversensitive to red.
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Strictly speaking, silicon cells are worst when it comes to spectral response, with their red sensitivity massively out of proportion due to a inherently linear (by frequency) response, and corresponding IR sensitivity.

They only tend to turn out better than CdS in practice because they are so bad that they require filtration to work at all, while cheap CdS meters often omitted using a filter. On old cameras or meters these spectral response correction filters may have faded - that should be considered whenever trough the filter metering gives bad results.

A naturally good spectral response can be had from selenium cells.
 
Maybe true, but selenium cells usually stop working after a period of time; you normally need a hand meter for those 1960s and 1970s rangefinders with supoposedly built-in metering.
 
rogerzilla said:
Maybe true, but selenium cells usually stop working after a period of time; you normally need a hand meter for those 1960s and 1970s rangefinders with supoposedly built-in metering.
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That is mostly a issue of production quality (or maybe designed-in self destruction?) or of poor storage. If we look at better made and better cared for professional meters, many still do work after 40-50 years. Selenium cells do not "deplete" regardless of how much light they are exposed to, but they will oxidise as soon as their seals fail.
 
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