Re: Warren's comment that, "I don't care for it [ polarizing filter ] for day-to-day train shooting, mainly because you have to be careful how you set it up, and you never know exactly how it's going to affect the exposure" prompted this explanation from me.
It is a popular misconception that polarizing filters change the film exposure as you rotate them, something that even the filter makers allude to on their instruction sheets when they show the exposure change as "1.5 ~ 3.0 f/stops", or "2.0 ~ 4.0 f/stops" or whatever (I never saw two of these figures alike in the old days of the 1960s). This is only partially true and is very misleading. What happens is that a camera's TTL meter reads the amount of bright light off a reflection and tells the viewer to close-down the exposure, thus resulting in a too-dark photo with a not-so-bright reflection--the TTL meter was fooled. By adding a pola filter to the front of the lens, at certain viewing angles the pola filter will remove bright reflections from a surface, thus allowing the viewer to see the true color of the surface that heretofore was hidden by the reflection. The camera's TTL meter now reads this darker surface without the glare, the meter needle drops because less light is now striking the meter, and the meter tells the viewer to open-up the exposure. Again, the TTL meter is fooled and the result is an over-exposed photo. This difference between the meter reading with and without the reflection is what the instruction papers are referring to, and is the confusing part of polarizing filters, when there should not be any confusion at all. You all know that the TTL meter can get fooled by overly-bright or overly-dark subjects, resulting in wacky exposures. The writers of these instruction sheets probably were public relations staff, not professional photographers, and therefore did not realize that just because the TTL meter says "This is the exposure", the exposure really was something else. They wrote the instruction papers based on how they saw the TTL meter react to the bright reflection and to the dark subject, just as they would react to the meter if no pola filter had been applied to the lens. Other than the change in the reflection, a polarizing filter admits exactly the same amount of light through it no matter how far it has been rotated--that is, 1-1/3 stops of light. Period. Don't believe me? Place one over a hand-held exposure meter and take readings as you rotate the filter--no change in exposure! Put the filter on your lens and stand facing any wall, diesel or building at a 90-degree angle to that vertical surface, and take meter readings as you rotate the filter--no change in exposure! Don't believe me? Run some tests for yourself. If you see a scene with a glare or bad reflection, take a meter reading off of the subject area (but not directly off the glare) and shoot a slide. Then hold the pola filter up to your eye and rotate it to see what affect it has on the reflection. If the glare disappears or is reduced in its intensity to your liking, put the filter onto the lens, rotate it to that position (you can see this right through the lens), open the exposure by 1-1/3 f/stops and make your exposure. After your film has been processed you will see that both slides have exactly the same exposure, only some areas have had the reflections removed. You might want to give a little more exposure if the non-reflective subject (e.g. Norfolk Southern diesel) appears too dark, or give a little less exposure if the non-refective subject (e.g. Soo Line diesel) appears too light, just as you would have done had there been no reflection and no pola filter used. CSXT "rolling gray card" diesels need no exposure adjustment. Do not get fooled like your TTL meter does, and do not be afraid to use the polarizing filter--it is your friend, not the enemy! Once you get the hang of it, you will ask yourself, "How did I ever live without this filter?" To remove all surface reflections for special set-ups in the photo studio, I have used large 14-inch pola filters over the lights in addition to a pola filter on the lens. All three filters were turned to remove the glare from the flat surface of the object being photographed, and the exposure was determined by reading the light falling onto a hand-held incident meter at the subject plane...and then opening up 1-1/3 f/stops. Back in the late 1980s (sorry, I do not know that date off-hand) Mainline Modeler magazine ran an article entitled "The Universal Filter" that I wrote about this very topic of polarizing filters and their proper use. I had to laugh, though, as a few months later Railfan Magazine ran its own knock-off article on pola filters in its Camera Bag section, even though Jim had told me that he never used pola filters. It was obvious (to me, anyway) that my article was the impetus for theirs since they suddenly decided to "dust -off" the old, unused polarizing filter in the bottom of their camera bag, and covered the same situations that I had. The kicker was that they even wrapped up their article by zinging me in the last paragraph by disagreeing with my claim that a polarizing filter was a "universal filter" (their quotes). I guess that imitation is the greatest form of flattery. Anyway, look up these two articles for more info about polarizing filters, and then go out and try them. They can be life-savers and scene-makers. John B. Corns _____________________________________________________________________ You don't need to buy Internet access to use free Internet e-mail. Get completely free e-mail from Juno at http://www.juno.com Or call Juno at (800) 654-JUNO [654-5866] -> SPORRS: Serious Photographers of Railroad Related Subjects -> Web Site: http://www.anet-stl.com/acphotog/sporrs -> Message © SPORRS® 1998 - All Rights Reserved
