help....uneven focusing problem

[andy_shand1]

could anyone please shed some light on why sometimes my images seem

to be out of focus on one side (say the left) and pin sharp on the

middle to right . Using a sinarx view camera and only using

horizontal and vertical shift but also stopping down to f22 and

focusing carefully on the ground glass everything ought to be in

focus....or at least if i screwed up the focusing then equally out of

focus. But to be sharp on one side and not the other?? My only

thoughts are that having two filters (a grad and a cc gel on a lee)on

a wide angle lens(sa 72 xl) at dusk with some bright

lights/highlights may be causing problems. Also being next to a road

might cause some vibrations. But would this only affect just one

side....maybe my rig or my lens might be at fault..but to be stopped

down to f22 and still out??..it doesnt happen all the time...only

occasionally. Anyone have a similar experience??

[tim_curry]

Check the basics. It sounds like you have the front or rear standard rotated slightly out of the "0" position. If there were a vibration from the road, it would be apparent in a "soft" image across the film, not in one small area of the shot. Another problem could be an inproperly loaded film holder into the back of the camera. If it isn't seated properly, there wouldn't necessarily be a light leak at dusk or later in the evening.

 

In any event, take two shots and are both out of focus the same and in the same area? Only at night or at dusk sounds like a focusing issue.

[leonard_evens]

Misalignment of the standards is one possible explanation, but if you still see it easily at f/22, it might be something else. If you are using relatively large vertical and horizontal shifts, then field curvature might be a factor. The "plane" of exact focus is not actually a plane but is instead a portion of a the surface of a sphere with a very large radius. When the frame is centered, the departure from planarity as you move from the center of the image is quite small, but if your frame is shifted so it is no longer centered, you no longer have a symmetrical situation and you might observe field curvature on one side only.

[brian_kennedy]

FWIW, I had a similar problem a couple of weeks ago because my rear swing movement wasn't locked down. I use rear swing so rarely, I hadn't checked to see if it was locked. Anyway, when I inserted the film holder behind the GG, I swung the rear slightly out of the zero position without realizing it.

[andy_shand1]

now come to think of it i was giving some pretty heavy shifts both sideways and vertically on my last two out of focus images(architecture).....but it was only showing focusing issues from left to right not top to bottom....but i think it may be something to do with heavy lateral shifts...and long exposures at night...but why?.....maybe the 72saxl is wide but poor to focus at its extremes....but its supposed to be a good lens.....any more ideas would be welcomed...thanks guys!

[andy_shand1]

im pretty sure that all the swings were zeroed and locked....and my film holder wont open unless its fully seated

[joe_hunt1]

Also the 72 XL is pretty close to film at infinity and alignment would be quite critical. Would you be able to check the GG to the sides at dusk?

 

Could it be a film flatness problem - film not seated right in holder at one end?

[david_gagnon]

Going out on a limb here, Andy, but you were shooting near dusk. Were the camera and film holders at relatively the same temperature? Film can change from flat to bowed with a temperature change that involves humidity. You didn't say what film you were using, so we don't know how thick it is. The thicker the film is, the less this would affect film flatness (IMHO). Also, are you experiencing this in a vertical or horizontal format, or both?

 

Thanks.

 

DG

[andy_shand1]

im using provia and yes its dusk but the temp hardly changes here....slow to change and between 25-32 oc. Im begining to think it may be something to do with extreme wide angle shift with two filters in front of the lens during dusk with high contrast lighting all around. What might be happening is mirroring between the two filters increases as you move away from the centre of the lens...so with a shift the side that is in the middle has no ghosting while the other does. On close examination its acctually ghosting ..not just blur but its only really apparent in the high contrast areas...otherwise it just looks out of focus.This might be the problem and it makes sense to me. I may have to experiment further...i will try shooting off a couple of tests with one control shot. Very usefull to hear all your thoughts...thanks...and anymore ideas would be great ...this problem has been driving me nuts.

[john_burns1]

I had a similar problem with my 4x5 tachihara. The problem was that the camera back (toward the lens) where the film holder sits was not planar. To detect this and correct it I used a depth gauge. The correction was using shims of tape to get the depth to the film plane the same at all four corners. Hope this is helpful.

[andy_shand1]

interesting....im acctually having a sinar rep/expert drop by to check out my rig just in case that might be the problem....however did that problem show up even at f22?

[doremus_scudder1]

Andy,

 

This sounds to me like you have simply exceeded the coverage of your lens. I don't know the specs for the 72mmxl, but many lenses have a smaller usable image circle than the actual one, i.e. the outermost edges of the image circle are optically inferior. If your "out of focus" areas are consistently on the side you shifted to and the shifts were extreme, then I would suspect this problem.

 

Regards

[andy_shand1]

this is true to a certain degree but i have found with the 72 that it usually softens slightly and then falls off to black...this problem is really much worse than that.