Focusing issue at higher aperture

[nicolas]

Hi everyone,

I have looked for infos on the forum (and I'm pretty sure I has been discussed here before but couldn't find it. Sorry if I double-thread)

I keep having the same focusing issue while shooting portraits at large aperture(f 5,6, 8 and 11) on a 4x5 field camera (toyo field) and a 135mm lens.

The focusing point seems to be often a foot in front of the place where I did focus on the ground glass. ( I focus normally at f5.6 on the ground glass and then stop down to the right F stop).

My ground glass is in right position and not reversed, I lock my focusing knobs correctly and everything, and I do take my time to focus correctly, using a loupe. And I have doubts it happens also with some sharp pictures I have, I just can't really notice it because of the smaller aperture (f32 and f64) and greater the depht of field.

I talked to some serious LF landscape photographers who had the same problem doing portraits at larger aperture like me, and didn't know also why and how it happened.

As a newcomer in LF photography, I seek for your help!

Thanks!

[Jac@stafford.net]

What brand and type of lens, please.

It is not unusual to find lenses with focus-shift, but in my modest experience focus moves behind the original point-of-focus when stopped down.

[swmcl]

Portraiture with a foot (300mm) change in focal point on a 135mm lens ?! It can't be focus shift in a lens coz I assume the composition is taking place at or near wide open and then the shot is only a couple of f-stops from there. There is more of a change in focus shift from say f5.6 to f32 I believe.

I know with my camera that I must work on something different in the locking mechanism of the focus knobs because when I have everything just right and then go to lock the focus knobs, the locking action shifts the focus point. I'm not sure about a change of a foot though.

To get a foot of change on a head and shoulders portrait with a 135mm lens would be a bellows movement of what? maybe a half inch at a guess ... maybe more. It seems a bit crazy.

[Vaughn]

Align your standards well, lay a ruler out flat in front of the camera, one inch mark close to the lens, 12 inch mark away from the camera. Focus at 6" and use different f stops. Using the same film holders you normally use, check negatives for the amount the focus has drifted away from the 6" mark. Using a yard stick and using approximately the same distance you take your portraits would be good. This should help you see what is going on.

[ic-racer]

Shim your ground glass back 1.5mm (a way from the lens) if you are off a foot (30cm) at about 2 meters with a 135mm lens.

[koraks]

Almost sounds like you're loading the film in the slot where the dark slide is in instead of the slot where the film is supposed to go in.

[Randy]

Weird!

Focus on a brightly lit object at wide open, then lock focus and check the ground glass again for sharp focus, then close aperture down and check focus again. If object stays in focus through all that, sounds to me like the ground glass may not be at the correct distance (same distance as the film when film holder is inserted). Did the Toyo have a fresnal in front of the ground glass, but has since been removed? That may cause the ground glass to be in the incorrect position.

[mdarnton]

I'd add to Randy's suggestions that you should also load a holder, pull the slide, put it back in, pull the holder, and see if the focus has changed. You could be moving something when inserting holders.

[nicolas]

First of all, thanks to everyone for the hints and advices!! Much appreciated! I will try a few things you told me and I will come back to you in the coming days.

The lens is a Schneider Symmar-S 135mm f/5.6 MC. The ground glass still have its fresnel and have been checked by a pro. I suppose I do something wrong while manipulating the focusing knob, but it could be a spacing problem (which would be still surprising because it is the original ground glass and the body was almost mint when bought.)

And I assume I'm loading my films correctly... :)

[RandyB]

Do you wear eyeglasses? If so, you may need to update your prescription. As I've gotten older and my eyesight changes it definately causes problems with my getting correct focus. Most times when I get new glasses the problem goes away. You might also check your focusing loupe, it could be the poblem.