B/W filter compensation:

[phenix]

Hello everybody! There are 4-5 years I’m here almost daily, reading only. Great community, great forums! Now, here I am with my first post.

Compensation for yellow filters is claimed to be between 1 and 1.5 stops (factors 2 to 3). I have only 2 cameras asking manually compensation of this filter, and 5 or 6 others reading the light through the filter, so not necessitating a manual compensation. But no matter the camera I use, when shooting indoors in low light I always take the yellow filter of the lens, expecting to win the claimed 1-1.5 compensation stops.

I never checked the real gain until this evening, when I had a big surprise: 3 of my cameras reading the light through the filter did automatically compensate for the yellow filter with… 1/4-1/5 of a stop! Not more than a skylight! This is nothing in 35mm and MF, and is far from the claimed 1-1.5 stops! Why than taking the yellow filter of the lens when indoors? And why manually compensating 1-1.5 stops (on cameras needing such an adjustment) when outdoors?

I also checked an orange and a red filter: 0.5 stops for the orange, and 1 stop for the red (read TTL). This is also far from the claimed compensations: 2 stops for orange and 3 for red.

What should I do in the future? Why would I manually compensate the claimed factors on one camera, when another camera automatically compensate far less? What’s your opinion, experience, and advise? Thanks.

 

[Nick Zentena]

Do your meters read the same way the film reacts? Meters don't always react just like film. The filter factors are intended to keep the film well exposed not to make the meter happy. Why not try a series of exposures? Start with what the meter thinks is right with the filte. Then keep adding until you've got one more stop then the filter factor. Easy enough with 35mm film.

 

[Ole]

Indoors, you have artificial lighting?

This contains far more yellow and less blue light than daylight does. The filter factors are stated for daylight.

Check again outdoors.

 

[phenix]

Ole, thanks! This should be the explanation! Indeed, as I mentioned in my post, I first checked the readings this evening, indoors, so at artificial light. I should have been patient until tomorrow morning to check it outdoors too. However, a conclusion issues from this experience: indoors, with artificial yellowish light, there’s no need to take the yellow filter of the lens. This is excellent for the lens’ protection especially in places with lot of people like restaurants, bars, etc. Thanks again!

Nick, if what you say about light meters and films (other than IR) is true, than TTL or any other through the filter metering should be trashed. My films shot with SLR come out very good, not underexposed as they should in respect with your hypothesis. The same good results I got with films shot with manual compensation for filters. Therefore, the explanation should be that of Ole: the ambient color of the light. Thanks.

 

[CBG]

Ole's exactly right. If you are shooting indoors under incandescent / quartz lighting, anything with a bright tungsten filament as the light source, the light tends to have a disproportionate amount of yellow. A yellow filter passes yellow light at closer to 100%, and so the filter factor drops under incandescent lighting.

Conversely, if you were shooting with a 47 blue filter or similar, the factor goes way up since the amount of blue light decreases under incandescent / quartz lighting.

C

 

[phenix]

Thanks CBG! I admitted Ole’s explanation from the first moment I read it – perfectly logical. I even was jealous I didn’t thought of it first, it was so obvious but I haven’t seen it.

Now, today in the afternoon, by about 4pm, the sky became beautifully blue after a day of raining, and the air was clear too. The light wasn’t yet reddish, just the well known afternoon light, emphasizing colors and increasing contrast – the perfect light for color shooting. Seeing this blue sky, I immediately took one of my SLR and checked the readings with different B/W filters as well as without any filter. I used two Y filters (Hoya and Nikon), one O filter (Nikon), and one R filter (not a sound brand, still coated glass). I went out, on the balcony on the north side of the house (to avoid any influence from the +/-direct sun, and to have also the street in front of me in full light. I made readings with each filter, and without filters: one reading with 50% sky in the viewfinder, and another with 100% sky. Here are the results (the stop increment measures the difference between the reading with filter and the reading without filter):

Y filter (both Y filters provided the same readings): 50% blue sky = 1/3 stop; 100% blue sky = 1/2 stop; recommended compensation = 1 stop (or, factor 2x).

O filter: 50% blue sky = 1 stop; 100% blue sky = 1+1/2 stops; recommended compensation = 1+1/2 stops (or, factor 3x).

R filter: 50% blue sky = 2 stops; 100% blue sky = 2+1/2 stops; recommended compensation = 3 stops (or, factor 8x).

Well, as you can see, only the O filter reached the recommended compensation, but it did it only when I pointed the camera to the sky.

Anyway, this experience doesn’t contradict Ole’s explanation, because I could see clear differences between the daylight readings and the indoor readings under tungsten light. But I still don’t understand why filter manufacturers claim bigger compensations for their filters, than the cameras are reading through the same filters in daylight. This means that exposures with cameras reading the light through the filter, and exposures with cameras not reading the light through the filter, are different by 1/2 to 1 stop. It is true that the films where I compensate manually the filter, according to the claimed factors, have somehow more contrast, which means they might have been overexposed by 1/2 to 1 stop. The results don’t quite bother me. What bothers is the fact I feel like losing control over the outputs and the process, if I use the claimed compensation factors. Sincerely, the outputs that came out just like I previsualized them, are those made with automatic readings through the filters, so different from the claimed compensations.

OK, I don’t want to write here a book about this issue, so I’m waiting to hear about your opinions. Thanks.

 

[John Koehrer]

The spectral sensitivity of the film is different than the sensivity of the meter cell. When you compensate manually you take the cell out of the equation.

 

[phenix]

Turning down almost all my cameras into “cold” exposure mode, use instead a handheld light meter, and manually compensate for filters, simply doesn’t make sense to me. Should I do that even with panchromatic films whom larger spectral sensitivity should be closer to the specter of the light meters? It sounds to me like an impeded technology. Handheld light meter I use only to expose by the zone system in MF, never with 35mm.

And I repeat: best results I got when I left the cameras to automatically compensate, ignoring the claimed compensation factors. So, I don’t think I’ll change to soon this modus operandi. If I asked here the above question, was only to understand the phenomenon. And in part, I already did thanks to Ole.

Other opinions?

 

[Rich Ullsmith]

Whatever the filter is, their basic function is to restrict one part of the spectrum and allow another portion to pass.

Example: 18% gray wall in sunlight. Meter TTL with no filter, then meter with 25 red filter. Probably see about 2 1/2 stop difference in recommended exposure.

Now you have a red brick wall in sunlight. Meter TTL with no filter, then meter with 25 red filter. The red filter allows more of the spectrum reflected by your subject to pass. Maybe see a 1/2 stop difference (at this I am guessing, but hope you get the point).

I know what you mean, getting good results by letting the camera do it's thing with TTL. It will get you in the ballpark. But by going by what some of the folks above say, and thinking about what spectrum is being reflected and what you want to bring out of a scene and then deciding on a filter, you will see the possibilities.

 

[phenix]

Thanks Rich, now it makes sense to me. This explains why the TTL reading I made with the camera pointed 100% to the blue sky matched the claimed filter compensation ONLY with the ORANGE filter: blue is complementary to orange only, neither to yellow nor to red.

While Ole’s answer treated about the incident light, your answer came to complete the picture with the reflected light.

I knew it had nothing to do with suspected differences between the spectral sensitivities of the film and of the camera’s light meter. If it was so, TTL metering would have been a trash from its beginnings – or, such an idea is difficult to digest.

I consider that the thread reached its goal now, and want to thanks again to its main contributors: Ole and Rich.

 

[CBG]

Both John and Rich are right.

John is right since various panchromatic emulsions (and various color emulsions to a lesser degree) have differing spectral sensitivities, neither a camera's meter cell nor a hand held meter can match all of them. At best it is an approximation of some chosen average. At worst, it is a poor match. Rich is right too. That's just the physics of it.

In metering through the filter, you've got a solution that works for you, so don't stop.

C

 

[timbo10ca]

Thanks CBG! I admitted Ole’s explanation from the first moment I read it – perfectly logical. I even was jealous I didn’t thought of it first, it was so obvious but I haven’t seen it.

Now, today in the afternoon, by about 4pm, the sky became beautifully blue after a day of raining, and the air was clear too. The light wasn’t yet reddish, just the well known afternoon light, emphasizing colors and increasing contrast – the perfect light for color shooting. Seeing this blue sky, I immediately took one of my SLR and checked the readings with different B/W filters as well as without any filter. I used two Y filters (Hoya and Nikon), one O filter (Nikon), and one R filter (not a sound brand, still coated glass). I went out, on the balcony on the north side of the house (to avoid any influence from the +/-direct sun, and to have also the street in front of me in full light. I made readings with each filter, and without filters: one reading with 50% sky in the viewfinder, and another with 100% sky. Here are the results (the stop increment measures the difference between the reading with filter and the reading without filter):

Y filter (both Y filters provided the same readings): 50% blue sky = 1/3 stop; 100% blue sky = 1/2 stop; recommended compensation = 1 stop (or, factor 2x).

O filter: 50% blue sky = 1 stop; 100% blue sky = 1+1/2 stops; recommended compensation = 1+1/2 stops (or, factor 3x).

R filter: 50% blue sky = 2 stops; 100% blue sky = 2+1/2 stops; recommended compensation = 3 stops (or, factor 8x).

Well, as you can see, only the O filter reached the recommended compensation, but it did it only when I pointed the camera to the sky.

Anyway, this experience doesn’t contradict Ole’s explanation, because I could see clear differences between the daylight readings and the indoor readings under tungsten light. But I still don’t understand why filter manufacturers claim bigger compensations for their filters, than the cameras are reading through the same filters in daylight. This means that exposures with cameras reading the light through the filter, and exposures with cameras not reading the light through the filter, are different by 1/2 to 1 stop. It is true that the films where I compensate manually the filter, according to the claimed factors, have somehow more contrast, which means they might have been overexposed by 1/2 to 1 stop. The results don’t quite bother me. What bothers is the fact I feel like losing control over the outputs and the process, if I use the claimed compensation factors. Sincerely, the outputs that came out just like I previsualized them, are those made with automatic readings through the filters, so different from the claimed compensations.

OK, I don’t want to write here a book about this issue, so I’m waiting to hear about your opinions. Thanks.

I believe we're missing something here though.... I don't think you can compare a metered scene with 50% sky to one with 100% sky when assessing filter factors, because the 100% sky scene is much brighter, thus throwing off your camera's meter (which is trying to make everything 18% grey). You would need an average scene using the camera's center weighted metering mode, no?

As I understand this, from reading a number of posts:
1) you have to take into consideration not only the color of the light, but also that of the subject when deciding on exposure through a filter. So one must
2) test their filters under different situations, or have a very good understanding of how colored filters affect incoming light on B&W film.
and
3)The most reasonable way to get into the exposure "ballpark" is to meter through the filter and "adjust" from there, using past testing/experience/knowledge of wave physics.

Tim

 

[Doremus Scudder]

Just a few observations advocating metering through the filter instead of applying filter factors:

Yes, the spectral response of different films and different meter cells are different. However, metering through a filter often (in my experience, almost always) gives a more accurate exposure than applying filter factors.

One must remember, filter factors are just average "approximations" that are most valid for low-color-saturation subjects and specific color temperatures of light. Applying the full filter factor for an orange filter at sunset, when the light is predominantly orange will result in general overexposure. The opposite is true when the light is predominantly blue (e.g. twilight, heavily overcast days, shadows on clear days lit only by blue skylight). In cases like these, the meter-through-the-filter method will be more accurate.

Furthermore, for us Zone System users, the ability to spot meter a specific area of the scene through the filter gives a much closer idea of what the final print value for that particular area will be than just guessing from the filter color and factor. I would much rather read through an orange filter and see from my meter that a dark blue sky falls in Zone VI and an orange house in Zone VI than meter them without the filter (maybe they would both be the same value) and then "guesstimate" from experience how many zones an orange filter will raise/lower a given subject area.

The big problem with metering through filters is not the spectral response of the different materials/meter cells. These are usually "close enough" that the differences are small to negligible. The real catch is that different films, due to their sensitizing, often react differently in terms of contrast and even exposure when exposed to light of predominantly one color. This is something that filter factors cannot compensate for either.

My advice is to meter through the filter, and then develop some "fudge factors" for specific film/filter combinations. One can do thorough tests, or simply keep good field records, and note which combinations result in large changes in contrast and exposure from the expected results.

I have found that Tri-X with a #25 red filter is about 2/3 stop underexposed and a zone more contrasty than my meter tells me when I read through the filter. (I am not sure why, but I suspect that the "red-sensitive" component of the film is more contrasty and less sensitive in comparison to the whole.) I simply indicate a minus development and give appropriately more exposure with this combination. I thus retain the ability to meter specific areas of the scene and "place" them in zones.

Metering through filters, especially once one has a system of compensations in place, also allows one to better select the right filter, or to see if a filter has the desired effect or not. Often, I find that the color saturation in the areas I wish to alter with a filter are simply not high enough for the filter to make any appreciable difference. For example, I might meter a low-saturation blue sky behind snow-covered peaks with several filters to see which one gives me the desired separation. Sometimes, no filter gives any more separation than no filter at all. So, I can shoot without a filter, take advantage the higher shutter speed/smaller aperture available without the filter and use development controls, etc. to deal with the contrast. (Or, realize right away that the shot won't give me what I want and pack up...) This would probably not be immediately apparent if the metering had not been done through the filters.

When using a polarizing filter, one must realize that, in addition to the neutral density effect, other parts of the scene are darkened selectively according to how much polarized light there is and the orientation of the filter. This is not predictable at all with filter factors. Metering through the filter is the only way to get an accurate idea of how to orient the filter and how much the polarization will affect the values in the final print. Also, when combining filters, metering through the stack is much faster, simpler and more accurate than multiplying factors. Usually, when filters are combined, a polarizer is part of the mix, so the advantages of metering though the filter are combined as well.

I believe that through-the-filter metering allows more control, is more accurate and gives the photographer more information about the effect the filter will have on specific areas of the scene than making estimates about these things based on experience, "guestimates" or obediently following manufacturers instructions.

Best to all,

Doremus Scudder

www.DoremusScudder.com