Hi,
I'm wondering if there's any down-side to putting a filter on behind the lens instead of in front of it.
I'm not so interested in whether or not it'd be a PITA. I'm more interested in the uhm optical quality of the image.
Specifically, I'm wondering if putting a filter behind the lens will cut down on some of the flare I'm getting when shooting with the sun so low and in the frame. I realize I'm not going to get rid of all the flare, but I'd like to minimize it.
Thanks
T
I read somewhere but, of course, can't remember where, that there is a focus shift if filters are used on the rear element. I don't quite understand why that would be, nor do I particularly believe it (unless someone can show some proof that is better than opinion/speculation).
Calumet once, and maybe still does, sell a gel filter holder that mounts to the back of the lensboard. It had an odd name - something like "Xenophon". I use gel filters when I use filters on LF but they go in front of the lens, in a filter drawer that is part of my lens hood.
A lens hood, by the way, is perhaps a better way of reducing flare... unless the sun is setting and you are shooting to the West!
Filters have an optical thickness that is different from the physical thickness (if that makes sense).
So if you focus without a filter and then put a filter on the back of the lens, focus will shift a little bit. That's why lenses made for rear-mounted filters (like fisheyes) come with a neutral glass filter for use when no filter is needed - the filter is a part of the optical system!
You can get away with a thin gel filter, and the change is less with longer focal lengths.
Ahhhh.
that there is a focus shift if filters are used on the rear element.
There are two effects of inserting a plane & parallel piece of glass behind a lens that focuses an image at a finite distance (the usual photographic situation)
The first effect is a simple focus shift, the amount of which can be found in all basic textbooks in optics, the displacement of the image is equal to t.(1-1/n) when "t" is the tickness of the glass an "n" its refractive index. Textbooks usually take n=1.5 which yields a focus shift equal to one-third of the glass thickness. Gelatin filters being thin will induce a very small focus shift. Note that when the glass is inserted in front of the lens, the image is shifted as well, but shifted from infinity to infinity plus or minus something small ;-) so the image, on film (or silicon
So this focus shift with a filter behind the lens could be problematic for a rangefinder camera, but for a view camera where you focus, I should say : you re-focus all the time on the ground glass, no problem. For reflex SLR camera, provided that you can re-focus, what you see on the ground glass (GG) is what you get on film as well ! ( provided that the filter is located just behind the lens in front of the reflex mirror so that both optical paths in the reflex camera are identical between viewing/GG and film plane.)
So in practice the only drawback I see of putting a filter behind the lens is that you have you take the lens off to attach/detach the filter.
The second effect is more subtle and is visible only in certain situations like microscope lenses, could be visible for an extreme wide-angle view camera lens.
Even if you correct for the focus shift, the filter introduces a certain amount of spherical aberration if it is inserted between the lens and the image, again in a situation where the image focuses somewhere near the focal plane.
I have no idea if this subtle effect is noticeable in real life for photographic lenses.
If the filter is placed in front of the lens and the object located at infinity, as strange as it may sound, if the piece of glass is really flat and parallel, no spherical aberration is introduced even if incident rays fall at a high angle ! the only image degradation is : flare in any case and of course image quality degradation if the glass is not of good optical quality.
The additional spherical aberratiion induced by a piece of flat glass inserted in a converging/diverging beam is taken into account for high-power microscope lenses used for biology where you focus through a standardized 0.17 mm piece of covering glas.
But for photographic use, using a filter behind the lens it is worth the experiment ! I'm quite sure that it could reduce some flare.
Ahhhhh!
The focus shift you get from doing that is WELL WITHIN the margin of focus error anyway - so, practically speaking there is no difference. But it would be good practice to refocus with the filter ON - in general. Otherwise, to answer your question- YES, it would yield a better result (again - at least in theory) than being exposed to the source of illumination - where you could pick up flare, etc... If it's buried inside a really good lens hood setup - it would be just about the same result, front or back.
Oh, oh, slow it down! It's surely not true for the short focal lengths (wide angle lenses) if by the "focus error" you mean the depth of focus...Originally Posted by JW Dewdney
By 'margin of error of focus' - I mean (the distance you actually focus on vs. the distance that you INTENDED to focus on) x (the location of the film vs. the manufacturer's ideal intention) x (whatever other mitigating factors come into play - i.e, wind, vibration, focus shift on stop-down, etc etc..). It MAY be more noticeable on a wide angle (i.e. -result in a larger 'circle of confusion')
Along with the mentioned focus shift of 1/3rd the thickness of the filter there is also a very good possibility of image degradation from any marks or scuffs on the filter. In addition, filters that rotate; polarizers, grads, etc. are impossible to use conveniently behind the lens.
The only time a filter should be placed in or behind the lens is when that filter is designed to be part of the optical system. An example would be the corrector plates that screw in to the back of the Apo Sironar Digital HR lenses when they are used for film.
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