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false_Aesthetic
10-Oct-2007, 04:08
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

Robert A. Zeichner
10-Oct-2007, 06:06
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.

Geert
10-Oct-2007, 07:12
T,

Is the stability of the front standard OK? You could be moving the lens when mounting the filter.

G

Brian Ellis
10-Oct-2007, 07:47
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).

PViapiano
10-Oct-2007, 08:11
Hey Brian...

Tell me more about this John Sexton method!

RichardRitter
10-Oct-2007, 08:45
Why the filter?
It makes for long exposures and the chance of camera or tripod movements.

Brian Ellis
10-Oct-2007, 09:23
Hey Brian...

Tell me more about this John Sexton method!

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.

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
10-Oct-2007, 09:39
Why the filter?
It makes for long exposures and the chance of camera or tripod movements.

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.

Alan Davenport
10-Oct-2007, 10:53
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.

Asher Kelman
10-Oct-2007, 10:54
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.
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

Brian Ellis
10-Oct-2007, 11:21
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.

You're right, it was my assumption that it was graduated but he didn't say that so maybe it's not.

Robert A. Zeichner
10-Oct-2007, 12:57
I think it's a pretty good assumption that with a 3.0 ND filter (10 stops) that the OP is trying to use the lens wide open or close to it. If not, I'd be very curious to know what he is using as a light source or how fast an emulsion he is using that could require such attenuation of incoming light. As far as the question regarding depth of focus posed by Asher, the point I was trying to make is that with shorter lenses, the angle of intersecting rays is far greater than with long lenses where these rays are closer to parallel with one another. What this means is that any error in the coincidence of the gg to the film plane could result in the image being decidedly out of focus with a short focal length, where with a longer lens, that might not be as much of a problem. I wasn't really making any reference to penetration of the emulsion.

Colin Robertson
10-Oct-2007, 14:12
False is trying to make loooong exposures of the rising sun, I believe.

Asher Kelman
10-Oct-2007, 14:25
As far as the question regarding depth of focus posed by Asher, the point I was trying to make is that with shorter lenses, the angle of intersecting rays is far greater than with long lenses where these rays are closer to parallel with one another. What this means is that any error in the coincidence of the gg to the film plane could result in the image being decidedly out of focus with a short focal length, where with a longer lens, that might not be as much of a problem. I wasn't really making any reference to penetration of the emulsion.
Thanks Robert,

I see your point. Then we could also bring in ant curvature of the film. That might account, in part, for this issue of decreased focus at open apertures?

Asher

false_Aesthetic
12-Oct-2007, 12:20
Why the filter?
It makes for long exposures and the chance of camera or tripod movements.


80 min exposures during the day @ f32

John Schneider
12-Oct-2007, 12:47
Using a filter in front of the lens will not shift the focus, as opposed to using it behind the lens. I confirmed this by a conversation with a physicist, with a Rodenstock engineer, and my own critical tests using a thick (~0.5") aerial camera filter. If any filter would have caused a focus shift, that one would have done so. As suggested above, there must be another culprit...

false_Aesthetic
12-Oct-2007, 18:42
Using a filter in front of the lens will not shift the focus, as opposed to using it behind the lens. I confirmed this by a conversation with a physicist, with a Rodenstock engineer, and my own critical tests using a thick (~0.5") aerial camera filter. If any filter would have caused a focus shift, that one would have done so. As suggested above, there must be another culprit...

Thanks!

Now to figure out where I've gone wrong.

Cheers,
T

Colin Robertson
13-Oct-2007, 02:50
False-
are you shooting outdoors? Is the camera exposed to the wind? What is it standing on- if on concrete or asphalt, are you near a source of vibration, like traffic? Could the locks on your tripod allow the head to 'slump' during a long exposure? Is the camera exposed to sunlight during these long exposures- perhaps something is heating andmoving through thermal expansion?
Can you post a sample of an image to help distinguish between pure 'out of focus' and some kind of movement?

Scott Kathe
16-Oct-2007, 09:32
Since the film is just a little bit smaller than the space in the film holder maybe the film is 'falling' in your film holder over the course of the exposure. Do you smack the film holder to get the film to rest against the bottom before you take your shot?

Scott

Kirk Keyes
16-Oct-2007, 09:48
If it is film movement during exposure, search the web for instructions on how to make a vacuum film holder - people doing astrophotography with large format use these. The basic idea is to drill through the back of a film holder to make air channels from the front of the holder into the back, seal up the back of the holder, and then connect a small pump to the holder - something like an aquarium pump. It makes the holder single sided by doing that.

Or switch to glass plates!

Kirk Keyes
16-Oct-2007, 09:50
By the way, I've found that you can photograph the disk of the sun with a ND 5.0 filter. You can use the "sunny 16" rule to determine shutter speed.