TTL Metering with Red Filter for B&W contrast?

Discussion in 'Film and Processing' started by johncarvill, May 6, 2010.

  1. HI folks
    I've been reading around this subject, but only getting more confused. If I use a red filter when shooting black and white, this should help boost contrast, right? Particularly with blue skies, I should get an almost black effect, yeah? But even leaving nice blue sky/white cloud scenes out of the equation, shouldn't I get more contrast in general? (Will the red filter do any good at all indoors, by the way?)
    In any case, how do I meter this? I'm guessing that when I put the red filter on my camera (Nikon F3), and let's say I'm in Aperture Priority mode, well, I expect the camera will meter through the filter and therefore adjust the shutter speed downwards. So, do I meter with the filter off, then put it on and switch to manual, ie. retain the unfiltered metering? Or do I let the camera meter through the filter? Or do I just overexpose by a stop or so?
    And how does this relate to using a polariser? Won't a polariser give me a similar effect (in B&W)? When shooting colour, I don't think the general practice is to second-guess the TTL metering, is it? I don't set exposure compensation when using a polariser in colour. Same goes for B&W?
    Cheers
    John
     
  2. Meant to say: my current guess in this is:
    - When using Aperture Priority, with a red filter on, I don't need to adjust exposure compensation, because the TTL metering will do this for me, and the red filter will still have its effect (on certain colours).
    - When using Manual mode, I should set the exposure compensation to +3 to allow for the darkening effects of the red filter.
     
  3. Ansel Adams (and others) used a red filter to help visualize a scene in black and white by removing the sensibility of the eye to color. It's hard to predict how TTL metering will react to a strongly colored filter. Nikon detectors with over a 1000 RGB sites work very well, giving reliable exposures even with gelled stage lighting. My old F100 did poorly in this environment. A red filter is far more extreme.
    Give this a little thought. Filters remove something. Red filters remove blue light, which causes blue sky to darken and grass to appear lighter, when compared to the same scene photographed in B&W without a red filter. Yellow filters remove some blue without affecting greens, so scenes appear to have a tonality closer to that of the eye than with unfiltered B&W.
    A polarizer removes cross-polarized light (variable with angle). Since part of the blue sky is polarized (and clouds not), the sky is darkened, increasing contrast with the clouds. It has the same effect with B&W film - darkening blue sky with little effect on other objects. Polarizers (good ones, anyway) remove all colors equally, and TTL metering is unaffected. If the TTL meter uses a beam splitter, which polarizes light, you must use a circular polarizer for accurate metering.
    It is common practice to "second guess" TTL metering, or any kind of metering. The "correct" exposure depends on a lot of things, and all meters have peculiarities you learn with experience. I'm amused by people who keep diaries on the shutter speed, aperture, and which tripod they used. It's more important to know how you arrived at an exposure decision, what you measured (not just the results) and why you did it that way. To a large extent, that applies to which lens you used too.
     
  4. There are a lot of variables in using filters. First of all, the filter factor for a given filter isn't the same for all film types. Kodak's data sheets are still pretty good at providing filter factors, but they are still only a starting point.
    TTL metering through a strong filter is a hit-or-miss operation. Depends a lot on your camera's metering system. See if the change in the meter reading corresponds to the filter factor you want to use with that filter with that film.
    As for the effect of a strong red filter, view it as a minus-blue minus-green filter -- only red passes. So skies are made much darker, as is grass.
    But color filters do not inherently change contrast. They just make certain colors darker.
    The old Kodak handbooks on filters explain this all very well.
     
  5. It is easy enough to meter with both filter on and off, and look at the difference. This will tell you if your meter works with the red filter.
    A medium red filter has a factor of 8, which equals three F stops. With a red filter in daylight, most meters will do just fine. Most people find that red filters are a bit too harsh for everyday use. And they can make paper choice difficult. This is why the med yellow, became the everyday, walk around filter, for B&W shooters.
    Filters lighten their own color, and darken their "complimentary" colors. Polarizing filters, act on light that has been polarized. The light is vibrating on more than a single plane. Think of it as a sort of venetian blind, that only lets light pass in on a single plane. This is why eliminating this "scattered" light increases contrast, and color saturation, and of course it removes many unwanted reflections.
     
  6. I think there needs to be some color theory reviewed here. The sharp cut filters, like the heavy red 29/25 just eliminate all color but red--that means it cuts both blue and green. The reason sometimes trees look lighter is because they reflect great amounts of infrared, which will also get captured to some extent with a red filter and normal films.
    Filter factors are the same for most films unless the film has an unusual spectral sensitivity, like the old orthochromatic films. Using a red filter on these would be pretty futile! In general, however, most films available will have the same filter factor for a given filter.
    I never had an issue with exposure and using TTL metering, but learned the filter factors when shooting LF. The red filter will increase contrast in some cases--blue sky and water, but you may lose a great deal of contrast shooting a normal caucasian face (lips go white, blemishes and ruddiness are diminished) or even shooting in the red rock country of the southwestern US. Most advanced shooters would carry several different filters of varying colors and then use them sparsely--over use is a pretty quick sign of a novice.
    Seriously, a good study of color theory can be extremely valuable when looking to use contrast filters for photography. And the lessons learned are indispensable when shooting and working with color as well.
     
  7. Well, ok, thanks for the info.
    The only reason I thought of using a red filter at all was becuase a lot ofthe time, my 'black and white' photos actually come out looking 'grey and white'!
     
  8. How are you printing them? Most labs do a very bad job with black & white film prints--consumer labs. What you describe is very common. They don't make adjustments for contrast and use pretty soft paper because most folks want to see the detail in the image.
    If you aren't processing it yourself, it could also be in the processing as well. There is another thread running around here about the quality of film processing these days and I think you could be seeing that issue. Black & white is one of those things you pretty much have to take into your own hands, even when there wasn't digital or find a custom lab that understands and cares about the image.
     
  9. "It is easy enough to meter with both filter on and off, and look at the difference. This will tell you if your meter works with the red filter".
    Well, yes. Actually the meter will "work", but the question is how well. Back when I was shooting an FE, I found I needed to make about a one-stop adjustment when metering through a red filter because the meter did not respond well to this wavelength and the camera over-exposed. About half a stop was needed for orange. More modern meters seem to be better at this, but I never shot an F3. So meter a fairly neutral subject with and without the filter and see if the reading changes by the published filter factor. That will give you a starting point, but as others have said it now becomes a matter of your intent with such a strong filter.
     
  10. James G. Dainis

    James G. Dainis Moderator

    An increase in contrast involves the subject. The filter lightens colors of the same color as the filter and darkens colors on the opposite side of the color wheel. Shooting an apple tree with a red filter will cause the apples to appear lighter and the leaves darker. That would have more contrast. An unfiltered shot would have apples and leaves of the same gray tone. If you just shot the leaves, they would appear darker against the dark shadows so the contrast would seem to be less.
     
  11. Filters are frequently used in black-and-white photography to darken a blue sky. First the problem: Black-and-white films and digital when used in black-and-white mode often fail to deliver tones as anticipated.
    First consider, what shade of gray should result when the subject is bright red? How about light red objects ? If a blue object is adjacent to a green object, which object will reproduce darker? These are difficult questions because we all have preconceived ideas about how shade should be rendered on the finished print or screen and we are often wrong. We use colored filters in many applications to alter how colored objects reproduce.
    We use colored filters when objects do not reproduce exactly as we would like. These are called contrast altering filters. In general, a filter lightens objects that are nearly the same color as the filter and darkens objects that are its complement (opposite). A red filter lightens a red apple. A green filter lightens green foliage.
    A polarizing screen filter darkens blue sky and also mitigates reflections when the subject is side lighted
    We use colored filters to change the rendering of the sky when using panchromatic film
    No Filter - The sky will be rendered lighter than expected. This is true because black-and-white films are highly sensitive to shorter wavelengths of light as they are more energetic rays. Blue and violet being the shortest cause more exposure action on the film. This translates to lots of exposure, which means the film will be very dark in areas receiving blue light. Since we are dealing with a negative - positive system, dark film areas are rendered quite light, maybe light gray on the finished picture which is a positive meaning it is the opposite of the negative.
    Since shots with no filter are often disappointing, we can:
    Mount a No. 8 (K2) Yellow-Green
    This yields a positive image with a sky that most people think is a correct rendering.
    Mount a No. 15 (G) Deep Yellow
    Yields a positive image with the sky darker than correct
    Mount a No. 25 (A) Red renders sky very dark
    Mount a No. 29 (F) Deep Red renders sky almost black.
    When mounting a filter, the filter blocks light so we must somehow compensate for this light loss or underexposure will result. Each filter comes with a data sheet that tells us what the filter factor is. The filter factor is a multiply that we use on the shutter speed. Example the exposure without filter is 1/250 second @ f/11. We mount a filter with a filter factor of 8. We are asked to multiply 1/250 by 8. Sorry this is not the math most people are accustomed to. Anyway 1/250 x 8 = 1/30. Since multiply a fractions is almost a lost art, we resort to another way. We count on our fingers in powers of 2. Thus 2 - 4 -8 - 16. Wherer 2 = one finger 4 = two fingers 8 = three fingers 16 = four fingers. Each finger is a f/stop. So a filter factor of 8 is three fingers or 3 f/stops compensation. In other words, mount a filter factor of 8 we must open up the lens three f/stops. Now modern cameras meter and set exposure by reading the light that transverses the lens. When you mount a filter, likely the camera's metering system will do all the math and the resulting exposure will be spot on.
    Allow me to add that all theses Hollywood made cowboy movies shot in black-and-white with night scenes were filmed in the daytime with a red No. 25 (A) filter mounted and then internally slightly underexposed. This simulates night with a dark sky.
    The filter nomenclatures are from the filter catalog of the master filter maker Frederick Wratten (England 1840 - 1926). Kodak purchased his company in 1912. The filter nomenclature continues per his catalog to honor his memory example Wratten No. 25 (A).
     
  12. Just "Coles Notes" summary: red filter is gonna make blue things really dark, green things kinda dark, and meter through the filter.
     
  13. There are times when it's easier to use a hand-held meter . . .
     
  14. I think Nikon's matrix metering doesn't work well with a red filter, use CW or spot instead
     
  15. "The filter nomenclatures are from the filter catalog of the master filter maker Frederick Wratten (England 1840 - 1926). Kodak purchased his company in 1912. The filter nomenclature continues per his catalog to honor his memory example Wratten No. 25 (A)."
    Did he have some system or he simply put aleatoric numbers to the filters?
    Eugen
     
  16. Here's a process that I've used. It should compensate for any oddities in the color sensitivity of your camera's meter.
    Most filters' instruction sheets include an exposure adjustment amount, like "1 stop", or "2x".
    Take the camera outdoors and position it (without the filter) so the entire frame is filled with something of neutral color. 18% gray would be good, white is OK too.
    Note the exposure reading given by the camera. Now put the filter on and repeat the reading. Note this exposure reading.
    Now, if the filter factor was 2x (1 stop) and the difference between the readings was 1 stop, you're all set. The meter is correctly compensating for the reduced light coming through the filter. Just set the film speed normally.
    What if the difference was 2 stops? The meter is telling you that it is a little less sensitive to (for instance) red light than to multi-colored light. So you have to change the meter setting such that the meter reading is one stop less that the unfiltered setting, in this case, by lowering the film speed by one stop. (say from 400 to 200).
    May be a little confusing to read, but it's really pretty simple if you step through it.
    Of course, all this gives you is a starting point. There are other variables, including the color sensitivity of the film. (They differ) There's no getting around it, you're going to have to shoot a roll of film, develop it, see if you like the results, and adjust again if necessary. Take notes. Tag the film so you know which notes go with which roll.
     
  17. On second thought, ignore what I wrote above. It'll work, but it's unnecessary.
    Set your F3 to the box speed of the film (or whatever you normally use). Set the camera to aperture priority, put the filter on, and shoot away just as you would without a filter.
    This will probably give an acceptable result. If the negs are a little thin, then next time set the meter a little slower (say, 200 instead of 400). If they're a little too dense, set the meter higher.
    The only reason you would not just meter through the filter would be if the exposures were inconsistent from one frame to the next.
    This is unlikely.
     

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