How to correct for back-focus

[David Solow]

Hi all,

I use a Linhof MT 3000 with a Cooke f/4.5 229mm lens. I have tested the focus four times from three different distances using the LensAlign testing setup. In all cases, the camera/lens combination back-focuses about 2". I can compensate, of course, by focusing 2" in front of the plane I want in focus. Is there anything else I can do?

Thanks,
David

[Greg Miller]

I'm assuming your are focusing on the ground glass, then developing the sheet, and seeing the actual point of focus on the film(?).

If so, then your ground glass is not positioned properly.

[taulen]

http://www.vintagelargeformat.com/20...ft-focus-lens/

Maybe this can help ?

[Frank Petronio]

If you don't want to send the camera and lens to a technician like Marflex or Precision Camera Works then you would likely add a very thin shim to move the ground glass further away from the lens.

[Bob Salomon]

It sounds like your gg is not lying in the film plane. Has someone installed a non-factory gg? Marflex is also in NC in Havelock. Currently he is under the weather but you should have him set the shims to factory specifications if they have been set improperly.

Also, is the grain side of your gg facing towards the lens? That is where it belongs, resting on the gg shims. Do you have a Fresnel and, if so, is it on top of the gg closest to your eye? That is also where a Fresnel belongs on a Master Technika.

[David Solow]

Greg,
Thanks for asking for clarification. You are correct in your assumption. I did focus on the ground glass using a Rodenstock 6x Aspheric Loupe. I developed the sheet and saw the actual point of focus on the film. I did this numerous times from three different distances. Each time, all of my results were the same.

Taulen,
Thanks for the link. I am not sure how this will work with my lens. I didn't see any halos, but I will look into this in more depth.

Frank,
Thanks for the suggestions. I will send my camera and lens to Marflex, but it will be a while. Right now, I can't afford to be without them. I have a lot of work to do with them over the next two months.

Thanks again everyone,
David

[jp]

It's a portrait lens. The OP has figured out the way Kodak suggests focusing their portrait lens. There is a different shape to the in-focus area in terms of where and how it goes in and out of focus.

If you stop it down after focusing, you also throw off the focus. Due to the designed aberration, axial rays focus in a different distance than the rays going through the center of the lens. This is the physics that provides the diffusion, mixing the soft and sharp. When you stop it down, you go from mostly axial rays to mostly central rays, altering the focus. Thus, focus at the aperture to be used.

[David Solow]

Jp498,

Thanks for your response.

I never stop down after focusing. I always focus at the aperture I'm shooting, which 90% of the time is wide open at F/4.5. For now, I have shot only in the studio and use a powerful tungsten fresnel in order to focus, which has no problem in providing a well lit image on the ground glass, even at f/22, which is the smallest aperture I've used.

thanks again,
David

[David Solow]

Bob,

Thanks for your response. I have left a message with Martin concerning my camera and lens. I understand he will not be available for while, which is fine. I'll continue to compensate until he returns to work. I won't be able to be without the camera or lens anyway until sometime in February. I'll send him both at that time.

As far as the gg is concerned. It is original to the camera. I bought the MT3000 a few months ago brand new from B&H. I have treated the camera with the utmost care and have not added anything to the camera. The lens is brand new as well. I am using the lens board you recommended, which was also bought new at B&H.

Thanks again,
David

[Frank Petronio]

A quick way to check whether it is the lens focusing technique or the ground glass positioning would be to shoot with a "normal" lens wide open using either your testing stuff or the simple newspaper on the table technique....

If it were the lens I would expect Clive or someone from Cooke to mention it requires a certain technique to focus properly.