Topic: Funny focussing

[choppert]

Hello world,

Simple and probably stoopid question.

When photographing close-up still lives (lifes?) on my Bronica S2 the subject looks in focus.

When developing the subject is not in sharp focus!

Can SLR focussing be out?  I've heard of back and front focussing problems with RF cameras.  Can there be issues with SLRs too?

Thanks,



Chops

[gregor]

typically with an SLR what you see is what you get. are you using a tripod?  An photo example of your problem would be good to see - including lens, CU diopter used (if not a close up lens), & camera settings.

Unless you're confident that you have a steady hand, macro photography can fall out of focus very easily - just pressing the shutter release can make enough hand shake to change the focus.

I rarely use a tripod and seem to have the luck of a steady hand.

On my Bronica I often gaffer tape a smaller CU diopter over the PS80 lens as it gives a more pronounced DOF which also gets me in just a bit closer...

[moominsean]

i was misled...

[Sorry, image deleted during forum software upgrade. Please re-upload if so inclined.]

[Pete_R]

Can SLR focussing be out? 

Yes it can, but I don't quite see how it could happen only when photographing close-ups.

SLR focussing errors would be caused by by a difference in distance between lens and film and lens and focussing screen. Maybe due to mirror position being wrong or focussing screen position being wrong. But the problem would always be there.

Maybe you do have a problem with the camera but it only shows up when depth of focus (as apposed to depth of field) is small. Now I just need to work out if depth of focus reduces in proprtion to depth of field - logic says not, in which case I'm talking b******s.

Thinks...

But I guess a small depth of field would show up focussing errors and you may have focussing errors due to a fault of the camera so depth of focus doesn't come into it.

I think you have a problem.

[moominsean]

it may just come down to camera shakes, as said above. when i do macro with my great wall, it's difficult to keep it steady when the mirror slams down...

that and depending on the camera, lenses don't always present themsleves the same on film as they do in the viewfinder. i find that the DOF is much stronger on film compared to what i see through the viewfinder of my slr and tlr cameras.

[Francois]

Mirror shake is a definite problem. Mirror lock-up is always recommended for close-up shots. I also doubt the camera would be at fault... you would probably notice some blur on shots taken with a wide aperture. But if you want to make sure, you could do like the repairman do
Favorite Classics Repair Articles
and try a collimation technique to see if the camera focus is accurate.

[Pete_R]

The more I think about it the more I agree with Francois that you would probably notice a problem, if there was one, at other times, not just when doing close-ups.

[Photo_Utopia]

My 2 pennies worth is that it could be a problem with the pressure plate or some film flatness issue, this would be magnified with closer focus distances where focus is critical.
Mark

[Pete_R]

My 2 pennies worth is that it could be a problem with the pressure plate or some film flatness issue, this would be magnified with closer focus distances

I don't think that's true. Let me have another go at this one because I think I confused myself at the first attempt.

If there is a problem with the camera, it will be a difference in distance between the lens to film and the lens to screen. This might be due to the screen being out of alignment, the film not being flat or the mirror out of position. The question then is, will the error in distances show up more if the lens is focussed close-up?

I think the answer is no. Whether the error is noticed will depend on whether the depth of focus (like depth of field but at the film plane) is sufficiently large to accomodate the error. If it is, then no difference would be seen. If it's not, then the image on the film will look out of focus.

So, will the depth of focus be effected by the lens being focussed close-up? The depth of focus will change similarly to depth of field when the aperture is changed. It will also change if the distance between lens and film changes similarly to how depth of field changes if the subject to lens distance changes. But, the distance from lens to film may not change if the lens is focussed close-up. Take for instance, if a close-up supplementary lens is used. The lens may be focussed on infinity though the subject plane is brought much closer by the supplementary lens. In this case, the depth of focus would be the same as if the lens was being used to photograph a subject at infinity. Also, if the lens itself is moved to enable the closer focussing, it would move AWAY from the film which would actually INCREASE the depth of focus so making the problem less obvious.

So, my conclusion is, if there is an error with the camera, it will be no worse when doing close-ups than at any other time. It would, though, be worse, or more obvious, when using larger apertures.

I thank you....

Chops, what setup were you using? Any other clues you can give us?

[choppert]

Chops, what setup were you using? Any other clues you can give us?

A statue of Ganesh.
Dining table tripod 
1/60 s and f2.8 focussed to the lens' closest and the camera moved until the statue appeared to be in focus.

Happened outside though with small aperture, fast shutter and the twig that was meant to be in focus wasn't but the leaves a little closer were!

Don't think it's motion blur folks - even with the rather agricultural mirror slap.

[Pete_R]

Hi Chops,

I think you need to do some sort of check of the focus accuracy at infinity - then read this

http://web.archive.org/web/20000930155240/www.smu.edu/~rmonagha/bronfocusfix.html

and this (scroll down to the Adventures in Tuning Focus and read the bit about the mirror)

http://web.archive.org/web/20001021110233/www.smu.edu/~rmonagha/broncrisp.html

Doesn't explain why the error when close focussed but there does seem to be general focus problems with these cameras.

[db]

I suspect a camera problem ( I haven't read all Peter's Broni links I admit). I've had focus problems with a blad (focus screen not sitting properly in place) and with a Nikon lens- (lens helicoid took a knock and back focused everything- evident at range as well as c/up)

So it might be worth burning a roll of film on a focus test where you design a test range which includes a measuring tape and some targets at various distances that you carefully focus on. Use a tripod. Take notes. Focus on, say 6 inches, shoot, note what you did for that frame and process to see if the tape in the image is actually sharp there, or is it sharper on 7 inches?  Do it for each of your lenses to figure if it's a problem with the body or just one lens.

Not meaning to make light of your problem, but a mate of mine had a hell of a time trying to nail down a focus problem. He did all the tests, spent a fortune on camera mechanics ...no luck!  But the issues all went away a few months later when he got glasses