Heya,
I'm shooting with a rather dark ND filter on the front of my lens. It seems that every photo I take is a bit out of focus. I'm wondering if maybe a shift in focus is happening because I focus w/out the ND filter on the lens.
Camera: Toyo 45A
Lens: Fujinon 125 5.6
Filter: B+W 3.0 ND
Thanks
T
I seriously doubt that the filter, if mounted on the front of the lens is causing a shift that would result in focus problems. What might be the cause is accuracy of your ground glass alignment or the performance of the lens at the taking aperture. What I would suggest is making a couple of test negatives where you use the same aperture setting, but different exposure times, one with and one without the filter. Aim at a 2-dimensional target with fine detail, like some newsprint. If the condition of focus is the same on each, that will rule out the filter. With shorter focal length lenses, depth of focus is shallower because of the angle at which rays intersect the film plane. If your gg/film plane coincidence is slightly off, that could result in out of focus negatives.
T,
Is the stability of the front standard OK? You could be moving the lens when mounting the filter.
G
Focus shift is a change in the plane of focus that most commonly occurs as a lens is stopped way down. If by chance you're always stopping way down when you use the filter then the problem could be foucs shift caused by that rather than by the filter as such. Focus shift can also result from spherical or chromatic aberrations. So if the filter is causing either of these effects then it could be creating focus shift.
Are you photographing in color or b&w? If color only then you should be seeing color fringing around the edges if the filter is producing chromatic aberrations. If b&w only then the result of chromatic aberrations is a soft photograph, which is what you're getting.
The obvious first step here, if you haven't already taken it, is to compare photographs made with and without the filter. If they're all soft then try some of the things Robert suggested. If only the ones with the filter are soft then the filter is the source of the problem but I don't really know what to do about it short of trying a different ND filter, possibly one with only one or two stops of added density instead of three. Or you could eliminate the filter entirely and deal with the contrast range of the scene in another way (e.g. by holding a dark slide over the top part of the lens for a portion of the exposure, which is a method John Sexton teaches in his workshops).
Brian Ellis
Before you criticize someone, walk a mile in their shoes. That way when you do criticize them you'll be
a mile away and you'll have their shoes.
Hey Brian...
Tell me more about this John Sexton method!
Why the filter?
It makes for long exposures and the chance of camera or tripod movements.
It's pretty simple. The assumption is that the contrast range between the sky and the foreground is too great to capture both well on the film. So after composing and focusing you hold the darkslide in front of the lens at about the place where the horizon meets the foreground (you could use almost anything, the darkslide is just convenient). You note the position of the darkslide by looking at writing, numbers, or something else on the edge or rim of the lens that will allow you to place it back in that same position while the exposure is actually being made. You then proceed as usual to make the exposure but if you have say a five second exposure for the foreground you hold the darkslide in position (based on the writing, numbers, etc.) for maybe two seconds of the five total, moving it around very slightly. It doesn't work for every subject but it works pretty well if the horizon line is reasonably distinct and relatively uncluttered.Originally Posted by PViapiano
I hope I've explained it accurately, it's been quite a while since I took the workshop and I don't actually use it myself, graduated ND filters work o.k. for me.
Brian Ellis
Before you criticize someone, walk a mile in their shoes. That way when you do criticize them you'll be
a mile away and you'll have their shoes.
Are you asking what he's trying to accomplish or why he's using a filter to do it?
If you're asking what he's trying to accomplish, it sounds like he's using a graduated neutral density filter to better balance the exposure between something very bright in the scene and something darker. The typical situation is a sky that's much brighter than the foreground so that if the exposure was based on the sky the foreground would be underexposed and if the exposure was based on the foreground the sky would be overexposed and if the exposure was somewhere in between neither would be correctly exposed.
If you're asking why he's using a filter to accomplish that, presumably because he prefers it to other ways of accomplishing roughly the same thing such as by reducing his development times or doing something else (flashing the film maybe?) that might achieve a similar result.
Brian Ellis
Before you criticize someone, walk a mile in their shoes. That way when you do criticize them you'll be
a mile away and you'll have their shoes.
The OP never said it was a grad ND; that may not be the correct assumption...
It does sound like there's a focus shift, either due to the filter (less likely if the filter is in front of the lens, more likely if it's on the rear element) or most likely due to the lens' focus shifting when he stops down.
Hi Robert,
Is this really what's happening? I can understand that the distance light penetrating the film plane travels in the emulsion is greater with shorter lenses and wider image circles. I'm not sure that would be measurable unless the emulsion is pretty thick.
This kind of "travelled path" issue seems to be problem with the thin IR filter in fromt of the CCD of the M8 Leica digital camera with very short lenses and causes a color shift.
Here you are talking about a change in focal depth. Chromatic aberations I can understand but are you saying that the DOF gets less as one goes from the center to the periphery of a large lense image circle?
Asher
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