kar (03/24/83)
(NOTE: a temporary local failure may have prevented this from going out when
I first tried. I apologize to those who have seen this already.)
This article follows up on a request for information on how various
filters affect photographs. The requestor did not specifically mention B/W vs
color, but from the phrase "blue filters give a blue tint and green filters
give a green tint", I infer that color is the main point of interest. I in-
tend to state some general information about several filters and then give my
personal preferences. While now a computer science professor, my undergradu-
ate education began with 2 years of Professional Photography at the Rochester
Institute of Technology, Rochester NY, followed by a year of Photographic Sci-
ence. I shoot both BW and color (transparencies) in 35mm, and have a BW dark-
room. Now that you know "where I am coming from", here goes.
Skylight and UV Filters
Skylight and UV filters are supposed to filter out ultraviolet radiation
to which the film is sensitive but our eyes are not. This invisible radiation
could affect the color of the photograph even though we can't see it. The UV
filter looks clear, but the skylight filter has a very pale pink color, as it
is also supposed to make photographs of scenes lit mainly by the sky (rather
than the sun) appear more natural.
Salespersons in photo stores frequently advise folks to buy a UV filter
for each of their lenses and leave it on all of the time. The reason is that
the filter does the good things described above, and also adds some protection
for the lens. It is cheaper to replace a scratched filter than a scratched
lens, they say, and of course they are right.
I don't use these filters myself. Not only do I notice no difference in
the color of my slides, I find that the filter degrades the image quality in
some situations. Adding another layer of glass (especially a cheap one)
between the scene and the film gives the light another surface to bounce off
of. The increased flare reduces the contrast of the picture and can add an
overall tint to it. This is more noticeable as you aim your camera closer to
the sun, (not right at it, which is where most people first notice flare) as
the sky is much brighter there.
Polarizers
You see objects by light reflected from them. Some light, however,
bounces off of the surface of the object in the same way that light bounces
off of a shiny surface. This light is not colored by the object because the
various colored components it contains have never had a chance to be absorbed
by the pigment in the object; it therefore tends to hide the true color of the
object.
Fortunately, this glare is often polarized. The polarizing filter
reduces the amount of glare the camera sees, so the colors in the photograph
more accurately reproduce the real colors of the objects. Note that the
"realism" of the picture is worse: what you get is not what you saw, but many
people find the result pleasing. Colors tend to be deeper and more pure when
the glare is removed. Also, much of the light reflected out of the portions
of the sky 90 degrees away from the sun is also polarized, and removing it
results in dynamite-looking deep blue skies. This contrasts dramatically with
any fluffy white clouds that happen to be around.
The polarizing filter is usually more costly than ordinary filters be-
cause it is mechanically more intricate. It is held by a ring that rotates on
the threaded portion, so that when the filter is screwed onto the lens, the
filter can still be rotated. To obtain the maximum effect, one simply looks
thru the camera and rotates the filter while observing the results. Stop when
you like what you see. Owners of non-SLR cameras can accomplish the same
thing by looking thru the filter at the scene and rotating it to the desired
position before screwing it onto the lens. Note the orientation of the filter
by the position of the writing on it, and after attaching it to the lens ro-
tate it to the same orientation.
Some SLR cameras with thru-the-lens metering can be fooled by a polariz-
ing filter. In some cameras (my old Canon FT, for example), light is diverted
to the meter cell by a partially-silvered mirror which reflected only light
polarized in one particular direction. Rotating the filter on the lens had
the effect of increasing and decreasing the percentage of the incoming light
that went to the meter, which compensated by specifying the wrong settings.
Does your camera work like this? To find out, put on the polarizing filter
and point the camera at a scene that contains as little polarized light as
possible. Rotate the filter and see if the camera changes its mind about the
exposure. If not, you're probably OK, but I would make sure by trying it on
some typical subjects before using it for any critical work.
If rotating the filter changes the exposure significantly, you can still
use the filter, but it is harder to do. You can't just meter the scene
without the filter and apply a filter factor to the exposure because the
filter removes an amount of light that varies with the scene (and even the
time of day). Try this instead. First, while looking at a non-polarized
light source (try directly opposite the sun), rotate the filter to whatever
position gives you the minimum exposure reading. At this point the filter is
not removing any of the light that ought to be going to the meter, and your
exposure should be correct. Leaving the filter alone, now look at your scene
and rotate THE ENTIRE CAMERA to achieve the desired effect. You must rotate
the entire camera so that the position of the filter in relation to the camera
doesn't change, which would confuse up the meter. Read the exposure that the
meter indicates and remember it. Now you can hold the camera the way you want
to for the picture, rotate the filter for the desired effect, and manually set
it for the reading you remembered from before. This worked fine on my camera,
and I see no reason why it shouldn't work for other brands/models as well.
Although it's sort of a nuisance, I found the results to be worth it.
Colored filters
Except for special effects and correction, colored filters have little
use for color photography. If you are using color film that is balanced for
daylight and are shooting inside with tungsten light (NOT flourescent, and
without a flash), then your pictures will come out with an overall reddish
cast due to the difference in color balance between daylight and tungsten.
You can get a filter to make the correction, but the penalty for shooting this
way is that you lose most of the speed of your film.
On the other hand, the filter that converts daylight to the proper color
temperature for indoor-type film has a considerably lower filter factor. If
you are mixing indoor and outdoor shots on the same roll of film it is better
to get indoor film and the filter to convert daylight to tungsten light.
There aren't a whole bunch of types of film around that are balanced for
indoor light, however, unless you want to get into the professional stuff
(harder to find, less tolerant of mistakes than "consumer" films). This is
because most consumers use flash indoors, and electronic flashes have color
composition similar to that of daylight, so no conversion is required. Also,
nearly all of the flashbulbs sold now have a blue coating, which converts
their light to be similar to daylight. If you really wanted to do available
light work inside, however, you need a film balanced for tungsten illumina-
tion. Kodak used to offer Kodachrome (and I think Ektachrome also) balanced
to 3200 degrees Kelvin (which means "indoor"). I've not looked for that stuff
in years, though, so I'm not sure it is still around.
How filters affect B&W film is an entirely different story. In a nut-
shell, filters of different colors are used to change the lightness of various
colors in the photograph relative to one another. To understand how this
works, one must really look at a color wheel (for which I have no key on my
terminal), and accept the fact that the sky is not blue, it is cyan (which is
Lesson #2 in Color Photography). For these and other fun diversions, I refer
the reader to any of several photography books one can buy. The Koday techni-
cal books are good sources of information, and are also cheaper than real
books.
- Ken Reek, Rochester Institute of Technology
ucbvax!allegra!rochester!ritcv!kardmmartindale (03/30/83)
A comment in support of "leave a UV or skylight filter on your lens all the time": A friend of mine back in high school had his camera slide off of a desk and hit the floor lens-first, after falling about 3 feet. The filter was broken and its mounting ring badly bent, but the lens itself was fine. They are also nice to have on if there is any water spray in the air or if it's dusty. It takes a long time to carefully clean the front element of the lens if it's dirty, but I don't feel quite so badly about cleaning my $7 filter with a Kleenex if that's all that's handy. I take my skylight filter off only when I'm really worried about getting the maximum possible quality.
wunder (04/01/83)
I used to keep a UV filter on my lenses, but I quit that a while
back. I broke more filters by dropping them during a filter change
than by dropping my camera. So I'm a klutz.
a) As has been stated, an extra layer of glass costs you contrast.
It doesn't matter how multi-coated your filter is, it makes
some difference.
b) If you do drop your lens, do you want pointy pieces of broken
filter in the immediate vicinty? How likely is it to fall so
that something would poke in there? (not very) How likely is
it to fall so that the filter would break? (very)
c) Most lens scratches, pits, bubbles, etc. make *no* measurable
difference in the performance of a lens. The area lost is so
small a percentage of the total, that it really doesn't matter.
Several years back, some nut fired six .45 caliber rounds into
and astronomical telescope (120", I think). There were six
nasty-looking pits in the mirror. After they swept out the
glass chips, they reran the acceptance tests -- no change.
Weird, huh?
d) Most films are more sensitive to UV than the human eye. High
energy photons really make the reactions go. If you are at
high altitude and don't want the sky washed out, go ahead and
use the filter. I often use a light yellow (K1) filter with
B&W film just to balance the blue sensitivity.
Suggestion -- instead of a filter, buy a rigid lenshood, and use it all
the time. This might even increase contrast, it helps in the rain, and
most of all, it keeps your fingers off the front element. My Canon
lenshoods are made of a stiff, slightly flexy plastic. I don't know how
many times I've dragged them against a brick wall, or backed into something
pointy (no '59 Caddys yet). My lenses would be worn to a nub by now except
for the hoods. And I'd be awful poor from buying new filters.
wunderwood
Ps: I always did wonder whether the UV filter trick was just a camera
salesman scam to sell add-ons.
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