Can light reflected by mirror back into lens cause edge blurriness?greenspun.com : LUSENET : Large format photography : One Thread
I recently tried to photograph a person half-figure laying on top of a LARGE (4ft x 5 ft) mirror using a 4 x 5 camera, 210 Sironar S lens, and one 2040 Dynalite strobe head (undiffused, with reflector) as light source about 4 feet from subject, at a 45-degree angle in relation to camera, pointed down onto the person and mirror and experienced what I can only term a MAJOR ACUTANCE BREAKDOWN throughout the picture-- in other words, a major loss in edge sharpness or "ghosting" along the entire length of the frame surrounding the mirror, as well as along the edges of various objects that I had placed next to and around the mirror, which made the whole picture look as if there had been either camera or enlarger-shake during the shooting or printing, both of which did not occur.
Does anyone know if it is possible for light reflected/refracted from a mirror (or any other highly reflective surface for that matter) back into (and around, possibly) a camera lens, to cause such a loss of sharpness of various objects in various, discrete areas throughout a picture?
The whole photograph (8x10 print enlarged 2x from 4x5 BW neg) in fact looked REALLY REALLY BAD: mushy, "aqueous", maddeningly unsharp in various places throughout the frame; some parts were normal looking and sharp while other parts were fuzzy; there were fuzzy areas within sharp areas and sharp pockets next to fuzzy edges... Even the subject's face looked fuzzy--as if she had moved slightly during the exposure, which she hadn't...
I have racked my brain, wondering if such effects could have been caused by any other factor (including the lens, the camera, the film, the holder, the film processing, etc., etc. etc), but seem to have eliminated all variables, and yet I find it a little hard to believe that reflected light could cause such a degradation of sharpness.
Is this really possible?
Has anyone else ever encountered this in your picture-taking?
If so, is there any way to still take this type of a picture without incurring such a loss in sharpness? Dulling the mirror? Dulling the light?
-- nick rowan (email@example.com), July 12, 2001
I have done a little reflective shooting and have never had that happen, it sounds like defective film to me. I have a friend who has a friend in Canada that that happened to shooting landscapes, I will check and see if he remembers what the problem was, and the solution to it. Pat
-- pat krentz (firstname.lastname@example.org), July 12, 2001.
Answer: Yes, using a mirror this way can cause blurry egdes, and it's happened to me.
Your camera can see something your eyes and brain have been "trained" to ignore. Because common mirrors are reflectively coated on the rear surface, there are two reflected images -- one from the uncoated front surface, and one from the coated rear surface. The faint image from the front surface becomes more vivid as the angles of incidence and reflection depart from perpendicular, or as light increases. Stand in front of a lavatory mirror, for example, and look at the reflection of the rear of the faucet or something else far below eye level. There are two images if you look for them, but most of us have trained our brains to ignore the fainter one.
The camera lacks a brain, and because the two reflected images are at different effective distances, one of them may be out of focus at the film plane. This enhances the effect, as does the bright, undiffused EF. If the flash hits the mirror, and is then reflected onto the subject, you effectively have multiple sharp light sources creating multiple shadows in multiple directions. Plus, you may have projected reflections of other bright and/or shiny objects nearby.
I seem to be a lot better at describing than solving, but the reflections from the front mirror surface can be minimized by polarization, and the judicious use of barndoors and scrims can solve most of the problems created by the light-source reflections.
-- Lyle Aldridge (email@example.com), July 12, 2001.
Is this double image only in the mirror reflections by any chance? I can't believe there's anyone that doesn't know that you get a double reflection from a rear silvered mirror!
-- Pete Andrews (firstname.lastname@example.org), July 13, 2001.
Thank you very much for your extensive reply. It really helped me better understand what may have happened. Despite the arrogant and condescending calumny of Pete Andrews's response, I was NOT aware that one mirror could wreak such acutance havoc across an entire picture field. To be sure, I am still having trouble coming to grips with the notion that a whole picture could become bathed in a kind of maddeningly subtle blurriness--across whole AREAS (such as a person's face) and NOT JUST ALONG EDGES, especially edges NOT directly facing or adjacent to the mirror-- and in places within the picture where I wouldn't have expected the mirror to have been able to bounce light TO. I mean the subject in my photograph was laying with her back, afterall, on top of the mirror and her head facing up to the camera, so, for example I don't quite know how the center of her face (her nose, for instance) could receive reflection from this perspective and subsequently become blurred, but maybe the refracted light rays somehow reached there... And as I just mentioned, all of this didn't just amount to a breakdown of sharpness along edges, but across areas too... Maybe the fact that these "areas" acted like different "geometric planes" within the picture with respect to the light source and mirror, that they in turn ended up reflecting or receiving reflected light in the way that the edges did too...
Lyle, do you think that if I put a polarizing filter over the lens that all of this VERY VERY SUBTLE GHOSTING and DOUBLENESS of LIGHT would be eliminated COMPLETELY? I absolutely need for these pictures to be razor sharp, as they will need to stand enlargement by a large factor, from a 4 x 5 negative to mural size.
Also, is it possible to purchase SINGLE-coated mirrors in very large sizes (4 ft x 5 ft), and if so, would that then cut down or completely eliminate edge/area blurriness? Or would the reflection from just one layer of mirror coating still wreak sharpness havoc?
-- Nick Rowan (email@example.com), July 13, 2001.