Does changing aperture move the focus point

[Dan Fromm]

The lens is a Goerz 12" dagor it's barrel mounted and I have the sinar copal shutter.

It's too dark to see. As I close the aperture it seems to get sharper. I'm focusing on some wood grain and it gets sharper and then it gets too dark to see.

You're confusing two effects.

Dagors are known to shift focus on stopping down. This should cause the image on the GG to lose sharpness.

Most, not all, lenses' gain coverage and, for a while, sharpness on stopping down. This because stopping down reduces some aberrations, off-axis ones in particular, until diffraction, which increases on stopping down, overwhelms the gains from reducing off-axis aberrations. And yes, stopping down increases depth of field.

As an aside, I've always wondered whether all Dagors and dagor-type lenses shift focus on stopping down. This because (a) Goerz Dagors' were manufactured for many years and were recomputed from time to time and (b) my Boyer Beryls, which are Dagor clones, don't shift focus on stopping down.

If you have a 4x5 camera, use it to take two test shots with your 12" Dagor, one wide open and the other at your intended shooting aperture. Set the camera up at an angle to something like yardstick (if you're going to be shooting close) or a brick wall or picket fence (if you're going to be shooting at some distance), mark the subject so you'll know where you focused, and go.

[BrianShaw]

It might be worth exploring the question using a depth of field calculator and the parameters of the shot you are planning. With the exception of Imagon, I would think that under “normal conditions” dof accounts for any minuscule focus shift that may occur.

[afotandolaciudad]

Another point: focus shift while stop down happens with convertible lenses when using converted. So seems to be related with alterations in the iris position or configuration. This barely occur with smaller formats.

[tgtaylor]

Well I have the Nikkor 135mm AF DC f2 lens and the Nikon instruction manual says to focus after setting the DC (defocus image control) ring or the subject will be out of focus. The purpose of the DC ring is to control the focus of the foreground or background. So I would focus at the shooting aperture.

Thomas

[BrianShaw]

Well I have the Nikkor 135mm AF DC f2 lens and the Nikon instruction manual says to focus after setting the DC (defocus image control) ring or the subject will be out of focus. The purpose of the DC ring is to control the focus of the foreground or background. So I would focus at the shooting aperture.

Thomas

It’s not likely for that lens to be used on a large format camera!

Even in its intended application... is “unique”.

[lassethomas]

In any case, why would LF lenses change focus points and DSLRs don't? Every shot one takes with a DSLR stops down when you press the shutter. I've never heard of this problem. I've got enough problems learning LF. Let's not create more problems for me.

Focus shift is well known for some (D)SLR lenses too. For example, the Zeiss Contax 45mm f/2 for c/y mount has a ridiculous amount of focus shift.

[Drew Bedo]

Maybe testing the lens described in the OP would help. However, shooting LF film today can be costly and takes time. A slightly inclined ruler or yard stick/meter stick under strong flood lights would allow a visual check of focus shift as it is stopped down.

[Alan Klein]

If you can't handle this, you had better give up now and stick with your digi's

What are you saying? Do I have to refocus with my Nikkor, Fujinon and Schneider lenses after I stop down on my 4x5? I haven't read that anywhere.

[Alan Klein]

Maybe testing the lens described in the OP would help. However, shooting LF film today can be costly and takes time. A slightly inclined ruler or yard stick/meter stick under strong flood lights would allow a visual check of focus shift as it is stopped down.

Have you ever done this, Drew? Was there focus shift?

[Jim Noel]

I have never had the problem with any of my Dagors, in fact I have never had to stop down when using any lens other than a few from the 18th century. I do recheck focus after stopping down and if there is any focus shift, it is too small to detect on the ground glass.