I recently developed a very small application to help a new photographer that was having trouble understanding making exposure adjustments for filters, primarily because he wasn't used to thinking in f/stops. This application converts filter factors for up to three filters to f/stops expressed in decimals and fractions (rounded to nearest 1/4 stop). It also will show the exposure options (f/stop shutter -- shutter speed combinations) before and after filtration. See screen shot and information
If you are already familiar with filters and are looking for specific information, you might want to use one of the links below:
Download an Excel File with Filter Factor to F/stop Calculator
Filter Factors to F/stops Table
Filter Factors to F/stops Calculator
Filters Commonly Used in B&W Photography
Purpose of Using Filters
Color Filters allow the black and white photographer to exercise some selective control over tone values. To this extent, they can be an important tool in helping photographers realize their creative vision; to put on film what they see in their minds eye. At times, the use of filters are almost mandated by the limitations of the medium. For example, film users quickly learn that, despite being called "panchromatic", film is extra sensitive to (i.e., overexposes) the blue and ultraviolet (UV) light in skies resulting in the dreaded "white sky" effect. A filter may be needed to compensate for this bias.
Another problem in using the gray scale pallet is that, with the exception of the blue bias mentioned above, objects of similar reflectance may have similar tone values in the resulting print. For instance, if we were photographing an apple tree and found that the green foliage and red apples had similar reflectance (similar light meter readings) they might be almost indistinguishable in tone. In this case, the photographer might choose to use a red filter to lighten the apples and darken the foliage or use a green filter to darken the apples and lighten the foliage.
Color filters are pretty much exclusively used by b & W photographers. However, we share some other filters with color photographers. These might be used to reduce haze, reflections or glare. We might also use neutral density filters to reduce the light reaching all or part of the film. The use of these filters are discussed below.
How Filters Affect Tone Values
In a nutshell, a colored filter used with b & w film will lighten similar colors and darken opposite colors. The color wheel below provides a visual example of what is meant by similar and opposite colors. A red filter, for instance will darken the other two primary colors (green and blue) and will especially darken its complementary or opposite color (cyan) that is formed by combining green and blue. On the other hand, it will lighten red objects and, to some extent, will lighten colors that contain red such as yellow, orange and magenta. Yellow will do a particularly good job of darkening blue objects but tends to lighten red and green objects and so on.
A filter lightens and darkens because it transmits some colors and absorbs (or filters) others. Obviously, because it absorbs light , using a filter will necessitate an increase in exposure. (The UV filter is an exception.) Logically, darker filters require more exposure compensation. Filter makers will suggest an amount of exposure compensation and this is discussed below. However, think of this as just a suggested starting point. The actual effect of a filter and the amount of exposure compensation it needs will depend on:The amount of exposure compensation is often expressed as a "filter factor". A filter factor of 2x means that you should multiply the (unfiltered) exposure by 2, a 2.5x means you should multiply it by 2.5 and so on. However, filter factors can be confusing. How in the heck do you multiply f/11 @ 1/60 by 2.5? For this reason, most of us want to think of exposure in terms of f/stops instead. Below is a table that shows the f/stop equivalent for various filter factors and also, a filter factor to f/stop calculator that I created which will calculate the f/stop compensation for any filter factor. (See Filter FAQs for a discussion of using filters with and TTL metering.)
The film being used. I think we can expect some differences in color sensivity among different films and particularly among different emulsions such as the older emulsions, (Tri-x, Plus-x), the new T-grain emulsions (T-max) and chromogenic emulsions such as Ilford's XP2. The color of the ambient light. The color temperature of light varies throughout the day and could be thought of as changing the exact color of objects and how much of the light is transmitted or absorbed. The predominant color of the subject. For example, making an image using a yellow filter and filling the frame with yellow sand dunes or "amber waves of grain" might require significantly less exposure compensation since less light will be absorbed by the filter.
Converting Filter Factors
Filter Factor f/stops 1x --
4x +2 1.2x +1/4
4.8x +2 1/4 1.25x +1/3
5x +2 1/3 1.4x +1/2
5.7x +2 1/2 1.6x +2/3
6.4x +2 2/3 1.7x +3/4
6.8x +2 3/4 2x +1
8x +3 2.4x +1 1/4
9.5x +3 1/4 2.5x +1 1/3
10x +3 1/3 2.8x +1 1/2
11.4x +3 1/2 3.2x +1 2/3
13.5x +3 3/4 3.4x +1 3/4
or use this calculator:
Convert Filter Factor to f/stop
Most photographers I know write the exposure compensation in f/stops on the plastic filter case so that they don't have to think about it in the field. Should you get stuck with just the filter factor information and it is some odd number such as 5.7x, you can divide the film's ISO (or your personal exposure index) by 5.7 and use this as your new ISO.
Suggestions Regarding the Use of Color Filters
Here are a few things you might consider, especially if you are relatively new to using color filters:
What Filters Do You Really Need?
Nothing beats experience. Trying filters with your choice of film is the best way to get a feel for the effect they will have and the exposure compensation needed. Experiment. If possible, try making the image with and without a filter, bracket exposures and try different filters. Think it through. Look carefully at what you are photographing and remember that a filter may affect several of the objects in the image, not just the one that you are concentrating on. Even then, you can expect some unintended or unanticipated results. Don't overfilter. Filters come in different strengths and it is a good idea to use the least amount filtration that will accomplish your goals. For example, it is easy to get carried away with trying for dramatic skies by using, say, a red filter and, in the process, creating a negative that is overly harsh and difficult to print, perhaps, partially due to the unanticipated results mentioned above.
Many photographers, myself included, wind up with more filters than they really need and certainly more filters than they can easily carry into the field. We have a tendency to put together a set of filters piecemeal by buying them for a special situation or as we need new sizes for new lens. Step-up rings can be a real pain, but you can possibly save yourself some money as well as bag space by using them. A step-up ring is an adapter that allows you to use a larger filter than is required by your lens. For example, you might get a 49mm to 58mm step-up ring. (There are step-down rings but I wouldn't recommend them except for the smallest of increments and even then I would avoid using them with wide angle lenses.) The trick is to anticipate the largest filter you might need by looking at the lenses you might add to your system. And, as long as that this lens does not require an exceptionally large filter, buy filters of this size and step- rings. Filter costs increase as size increases and only so much step-up is really feasible.
Because of all the different colors and strengths, there is a fairly large number of color filters available. It would be nice to have all of them available for use, but, in my opinion, there would be many that you just wouldn't use that much and you certainly wouldn't be able to carry them all with you all the time. I think it is best to think hard about what the filters do and the kind of photography you do and set some priorities. Just as an example, I have listed the filters I usually carry below. The color filters are for b & w work and the other, the UV, polarizer, and graduated neutral density, are used for color work as well. These are discussed below.
*The correct title probably should be "The Filters I Usually Carry in the Field".
Filters Commonly Used in B&W Photography* Filter** Filter
Filter Effects Lightens Darkens Medium
2x +1 Probably the most widely used. Offers an accurate tone range in compensating for the blue sensitivity of panchromatic films. Will slightly darken sky and increase contrast between blue sky and clouds. Also may help reduce haze. Yellow, Chartreuse, Olive, Red, Pink, Orange, Lime Green Blue, Violet, Purple, Lilacs Deep Yellow 15
2.5x +1 1/3 Stronger effect than medium yellow. May darken sky considerably Yellow, Chartreuse, Olive, Red, Pink, Orange, Lime Green Blue, Violet, Purple Red 25
8x +3 Produces very dramatic skies. Effects may border on the surreal. Darkens foliage. Reduces haze. Reds, pinks, magentas, some browns, yellow, orange Blues, greens, cyan Green 11*** 4x +2 Lightens foliage and will darken skies somewhat. Sometimes used to produce pleasing skin tones in portraits Yellow, yellow-green, olive, greens Blue, violet, magenta, red, maroon UV 1x 0 Absorbs UV radiation and will reduce distant haze or fogginess
Polarizer 2.5x +1 1/3 Helps remove reflections and glare. May cut pollution haze. Darkens sky.
Grad ND n/a n/a Reduces the amount of light reaching a part of the image -- usually used to darken the sky.
**The numbers such as "Red 25" are Kodak designations. These filters may be assigned different numbers by other manufacturers. I have included some other common designations such as "K2".
***For some lenses I have a darker version called Green 58 instead of the Green 11. It requires 3 stops extra exposure.
Other Filters for Black and White
UV Filter.--The Ultraviolet or UV filter helps block ultraviolet light that can cause a hazy look in distant parts of an outdoor image. In color work it can help reduce the blue cast due to UV rays. Some leave it on all of the time to protect the lens. (See FAQ below).
Polarizer or Polarizing Filter.--I consider a polarizer to be essential for color work and sometimes useful for b & w work. One use of a polarizer is to darken skies although you are limited to that portion of the sky which is at a right angle to the sun. For example, if the sun is in the far east or west, you can darken the southern or northern sky. The effect can easily be seen by rotating the polarizer. You do, however, have to watch out for uneven effects when using a wide angle lens.
The polarizer can be effective in reducing specular highlights (glare) and reflections on nonmetallic surfaces. Again, the effect can be seen though the camera when the filter is rotated and is strongest at about a 30 or 35 degree angle. Eliminating these can increase color saturation. Glare or specular highlights are always the color of the light source and, of course, the source is usually whitish. If you are photographing a lawn, for example, each shinny blade of grass could have a small white highlight and, while the lawn still looks green, it will look even greener without the white highlights. The same is true of other surfaces.
This has always seemed to me to be one of the biggest differences between b&w and color photography. Glare and reflections are sometimes be annoying distractions in a color photo and removing them can result in color saturation that is more pleasing. On the other hand, using a polarizer with black and white can result in an image that seems to me to lack sparkle and life.
If needed, a polarizer can be used as a neutral density filter in a pinch. Even when it is not rotated for its polarizing effects, it requires about a 1 1/3 stop increase in exposure.
Dinosaurs like myself that use older equipment can usually get away with using a linear polarizer. Those of you using newer autofocus and autoexposure equipment most likely will have the privilege of paying more for a circular polarizer. If in doubt, consult your cameras manual or a dealer, but if you have a relatively new 35mm camera the odds are you'll need the circular variety.
Graduated Neutral Density Filter.--Part of this filter is clear and part of it is a color neutral gray with a gradual transition between the two. The most common type is square and fits in a special holder that allows the filter to be rotated and shifted up and down so as to place the dense portion where exposure needs to be reduced. These are commonly used to reduce the exposure the sky gets and thus darken it. Obviously you need a fairly straight horizon for this to work and, even then, it requires a little practice to keep its use from being too obvious. My technique is to put the gray portion well above where it needs to be and gradually move the filter down in the holder until I can see the effect . At this point I know from experience I have gone too far, so I then move it up approximately one quarter of an inch. Works for me!
Some of these I know a good answer to and some I don't. In all cases it's just my opinion.
1. Should I leave a UV filter on at all times to protect the lens?
Some, especially the people that sell filters, think this is a good idea. Others think they can have a degrading effect and, if you are going to do this then why bother with expensive multi-coated lenses designed to reduce flare. Frankly, I don't know if you could tell any difference in the majority of images. One drawback to leaving them on with a wide angle lens would be that you would most likely have to remove them in order to use another filter. Stacking another filter on top would probably cause a vignetting problem.
I don't leave them on, but I do keep a lens cap on unless I'm making an image. If I would subject the lens to damage by simply removing the cap, I probably wouldn't have my equipment there in the first place. Also, I'm a big fan of lens hoods. This also offer a little bit of protection.
2. What is the difference between a UV filter and a skylight filter?
These are often confused. Both help reduce UV rays, but the UV filter is clear and the skylight filter is a very light pink.
3. Will TTL metering give me the correct exposure when using filters?
It would probably be wrong to assume that a camera meter's sensitivity to a color would be the same as any particular film's. However, I would guess that it would get you very close. Try it and see.
4. Can two filters be used at the same time and, if so, how do you calculate exposure compensation?
You can "stack" filters but, with the exception of using a polarizing or UV filter along with another filter there is very little reason to do so in black and white photography. If you were to use two reds, say a Red 23 and a Red 25, the effect would be the same as just using the strongest, the Red 25, except that more exposure compensation would be needed. If two filters are used, you can multiply their filter factors or add their respective compensation in f/stops.
All Text and Images © Joe Miller, 2004