The camera accessory market
offers an abundance of filters that we can screw on, slide on or otherwise
attach to our lenses. Along with it
there is the never-ending discussion about their necessity. Filters certainly are not some frivolous item
that sinister accessory manufacturers have dreamed up to get their hands onto
more of our photography budgets.
For instance, there are color
correction filters. These have lost a
lot of their importance with the advent of digital photography where white
balancing has virtually eliminated their need.
But especially among Leica users, film and film cameras are still widely
used (Leica still makes several film camera models) and so are color correction
filters. Anyone who has ever shot under
fluorescent lighting appreciates the FLD and FLB filters that get rid of the
ugly green cast common under those lighting conditions. We have the choice of daylight and tungsten
film, but have the wrong film in your camera, and you will appreciate a proper
color balancing filter to be able to keep on shooting without ending up with
overly red or blue images. Excessive
amounts of blue also occur when shooting during winter with snow covered ground
on bright, sunny days. The blue of the
sky reflecting off the snow will generally cause an excess amount of blue,
something easily corrected with a skylight filter.
Infrared photography, with
film or digital, cannot be done without a proper infrared filter. No digital camera makes it as easy as the
Leica M8 to use infrared light to create surreal pictures that stand out in
radiantly bright green leafs in extreme contrast of a dark sky, for
example. As is known, the barrier filter
of the sensor cover glass is very weak for design-related reasons. Therefore an additional blocking filter is
always recommended for normal photography to avoid a magenta cast of skin tones
or black textiles. The M9, ME and M monochrome are also ideal to create the
effects of long light waves.
For this nothing more is
necessary than an infrared filter like the B + W IR 093 Black Red 830 F
PRO. All B + W filters are made from
high quality optically ground and finely polished, plane-parallel and
streak-free glass.
These filters block out all
visible light, which gives these filters a totally black appearance. However, the results are photographs taken with pure infrared light. Light transmission starts at a wavelength of
800 nm with just 1 percent and rises at 900 nm to 88 percent.
Since infrared filters have a
very large exposure factor and therefore require long exposure times, it is
always recommended to use a stable tripod.
Automatic exposure control should not be used because the exposure meter
is calibrated for visible light only.
The optimum exposure values can best be determined through a series of
tests. The distance settings require some practice as well. As a rule of thumb for infinity focus under
IR conditions multiply the focal length by 300.
For example, for a 50 mm lens that is 15 meters.
These filters are available
in sizes E39, E46, E49, and E55.
Then there are a myriad of
special effects filters. These do apply
to equally to film as well as digital photography. The need or value of them can only be
assessed by the individual photographer.
It’s an eye of the beholder thing.
Finally, there is the issue
of lens protection. Many photographers
have UV filters permanently attached to their lenses as a means of protecting
them in case of a mishap. They certainly
offer a certain amount of protection and the argument that it is a lot less
expensive to replace a filter than a lens does make sense at face value.
During a visit with my father
to Leitz Wetzlar, when Leicas were still made there, we toured the entire
facility with Rudi Kraut as our guide.
He showed us everything from the camera production to the lens grinding
department to the lens assembly. At one
point he introduced us to a gentleman who was working in the lens design
department. During our conversation the
topic turned to filters. I simply asked
what, if any, advise he had regarding filters.
His face took on a rather stern look while he answered.
“If we had intended our
lenses to have flat pieces of glass in front, we would have designed them that
way.”
That caused me to research
the topic. After all, how bad can a flat
piece of glass in front of a lens be?
Flat is the keyword here.
Unfortunately, some filters are less flat than others. Ideally, a filter is made of high quality,
optical glass and ground from a blank, just like any lens element. The only difference is that the two surfaces
have no curvature. The same precision
and tolerances should be applied as with lenses. Only that will give the assurance that the
two glass surfaces are perfectly parallel to each other.
Unfortunately that is not
always the case. For one thing, there
are two distinctly different production methods. One is the grinding process. This is an expensive process that is only
applied by the top filter manufacturers.
Unfortunately, the majority of filters are made in a much cheaper
way. Here large, flat, narrowly rimmed
surfaces are filled with glass granules and then heated to melt the glass into
a large sheet. To make the actual
filters, these glass sheets are again heated to the point where they become
pliable and the filters are stamped in a process not unlike a cookie cutter, cheap
but not very precise. For one thing, the
two glass surfaces are not nearly as parallel as can be assured with the
grinding process. Secondly, the stamping
does add a considerable amount of physical distortion to the edges of the
filter which in turn does adversely affect lens performance.
Spectral transmission is
another, important issue. Many filters
need to be made with certain colorations to assure their proper effects. Here too we find considerable differences in
accuracy. High quality filters are
always dyed in the mass, meaning the glasses which the filters are made from
receive the correct coloration during the process of making the glass. Unfortunately this process too is subject to
considerable differences in accuracy.
A much less desirable
approach is to sandwich dyed gels between two pieces of clear glass to achieve
the proper coloration. Not only are
there differences in accuracy regarding the spectral accuracy of the gels, but
the problems of parallelism of the filter surfaces are doubled. This is actually an old, outdated approach
and hardly any filter manufacturer outside of Tiffen still uses this process.
Another criterion is the
thickness of a filter. Regardless how
perfectly flat a filter is made, it will add a certain amount of distortion to
any lens it is used on. The only
variance is the focal length of the lens, with wide angle lenses being more
affected by this than lenses of longer focal lengths.
The worst of all filters are
the ones made of acrylic rather than glass.
By nature these need to be a lot thicker to assure the desired
effects. In addition, even the best
acrylics are not nearly as clear as good, optical glass, thus adding to the
undesirable effects of these less expensive alternatives.
The problem lies in the fact
that when light hits the filter, it does not transmit straight through unless
the light hits the filter in a 90 degree angle.
There will always be a certain offset of the light path. The steeper the angle and the thicker the filter,
the more pronounced this is. The only
filters ever made to prevent this are curved filters. These are designed for certain focal lengths
where the curvature is such that the light path through the glass is always
reaching the filter in 90 degree angles.
These filters are prohibitively expensive.
Finally, there are the filter
mounts. Needless to say, we should stay
away from plastic ones. They simply
don’t offer enough precision to be worth any consideration. Most filter mounts are made of aluminum. However, most high quality lenses also use
aluminum for the lens barrels. Aluminum
against aluminum unfortunately has a huge amount of friction. This quite easily leads to filters being very
difficult to remove. The best filter
mounts are the ones made of brass. Brass
against aluminum has a very low coefficient of friction and therefore brass
mount filters are usually quite easy to remove.
This brings us back to UV
filters, permanently attached for protection.
Do we really want this, do we really need this? Based on the flat glass comment at Leitz
Wetzlar, I never use any filters unless absolutely necessary and I have done so
for years. None of my lenses have ever
been hurt because I take other safety precautions. The main one being that I always use a solid
lens shade. That gives any lens a
considerable amount of protection because the glass surface of the lens is
recessed by a certain amount. This
greatly eliminates the possibility of physical harm. Of course accidents can happen. I look at my insurance as a measure to
protect my lenses in those cases.
Of course when shooting under
condition where these measures are inadequate, a UV filter is definitely a good
idea. For instance when shooting under
extremely dusty conditions, or when wind whips up a lot of dust and fine sand,
we should not subject our lenses to such ill treatment. That is where a high quality UV filter is
definitely helpful. But personally, I
leave it at that.
Should we all use just Leica
filters? The simple answer is no. Leica is not a filter manufacturer. To my knowledge most of their filters are
made by Schneider through their B+W division.
B+W have proven to make some of the highest quality filters money can
buy. Equal in performance are the
Heliopan filters. Heliopan is owned by
Zeiss. Staying with those two
manufacturers will always give you the assurance of keeping the ill side
effects of filters to a minimum. The top
quality filters from Hoya could be added to that category as well.
Considering the overall
performance of Leica and other high quality lenses it just doesn’t seem right
to put flat pieces of glass in front of them except unless absolutely necessary. It especially doesn’t make any sense at all
to have the light pass through a cheaply made, low quality filter before it
even reaches the lens just to save a few bucks.
That approach has served me well over the years and will continue to do
so.
-------------------------------------------------------------------------------------------------------------------------------------------
Order: info@gmpphoto.com
Please make payment
via PayPal to GMP Photography
Click on ad to
enlarge
Order: info@gmpphoto.com
Please make payment
via PayPal to GMP Photography
Click on ad to
enlarge
Order: info@gmpphoto.com
Please make payment
via PayPal to GMP Photography
You mention that very high quality filters, like from B=W or Heliopan give a much greater assurance of being as perfectly plane as possible. Do you have any actual measurements?
ReplyDeleteYes, I do. These companies apply a deviation from being perfectly plane of no more than 1 second of arc. To put that in perspective; imagine a person standing on the Place de la Concorde in Paris with outstretched arms. Then draw a line from each hand to meet on the face of Big Ben in London, some 400 km distance. That constitutes 1 second of arc. With other words, the two glass surfaces cannot deviate from being perfectly parallel any more than that.
Delete