Focus shift when stopping down a dialyte?

[matti]

Didn't find this one by searching. I find my Ernman dialyte hard to focus correctly on my Tachihara. I like the funky aberrations I get but sometimes I wonder if it wouldn't be a bit better if at least something was in focus
I don't recheck focus after stopping down a bit. To do that, I would need a better dark cloth.

But dialytes don't have focus shift while stopping down, or do they?

/matti

 

[Ole]

Dialytes shouldn't have "funky aberrations"?

Focus shift is a subjective term. I seem to be less plagued with it than many others using the same lenses I use; possibly because I know the difference between "edge focus" and "center focus": it looks different on the GG - and if you get it right, there is no focus shift with any lens!

 

[matti]

Hm, maybe I am just adding to the confusion now but:

I attached an image from the lens. I was trying to focus on the small tree growing on the overturned tree root but tried to have the ground out of focus. But the focus is too high up. Maybe this image just looks odd, but it is the only one I have at hand with this little Ernemann Doppel-Anastigmat.

Maybe aberrations is the wrong word. Let's just call it out of focus areas.

But I am pretty sure the little tree was in focus while I focused. Then I stopped down maybe two stops.

I have one other image that doesn't have the focus plane slanted where it is easier to see what I mean. But I don't have it in the computer.

There is no focus shift with any lens?

/matti

 

[rknewcomb]

"because I know the difference between "edge focus" and "center focus":

OK, I'll bite, what's the difference between edge focus and center focus, unless it's just focus in the center or on the edge of the groundglass?

Robert N.

 

[Ole]

...
There is no focus shift with any lens?

/matti

Not when I use them, there isn't!
But I acknowledge that most others report focus shift with just about any lens, so I assume it must be something I do differently. Like focussing by a different definition of "sharp".

 

[Ole]

Robert, the image looks a little different depending on whether you focus the image formed by the central parts of the lens or the outer parts of the lens. The outer parts give the brightest part of the image, but that disappears when you stop down. So by learning which focus is which you can focus by the image formed by the central parts of the lens, thus eliminating focus shift.

 

[matti]

Well Ole, i uploaded a picture. Do you find it sharp?

/matti

Not when I use them, there isn't!
But I acknowledge that most others report focus shift with just about any lens, so I assume it must be something I do differently. Like focussing by a different definition of "sharp".

 

[Ole]

It's sharp in an almost horizontal plane, which makes it impossible to see if you missed the focus due to focus shift or something else.

 

[matti]

And I thought this was an easy issue if the dialyte had a focus shift or not...
Ole, are you about looking at the light rays formed by the center part of the lens? Isn't this exactly what you do by stopping down. But you do it by just looking at the light rays that have passed the center part of the lens? I would believe these light rays are all over the ground glass. So you mean, there can be no focus shift because the aperture just eliminates the outer light rays of the lens. Makes sense but if it is so, why do we have the expression "focus shift"?

/matti

 

[Ole]

With the lens wide open, the image is formed by rays coming from the entire lens.

With any lens with significant spherical aberration, the brightest part of the image is formed by light focussed by the outer part of the lens - which are focussed at a different place than the light that has passed through the center of the lens.

So if you focus so that the brightest image is sharp, you can experience focus shift when stopping down after focussing; because that part of the lens you used for focussing no longer contributes to the image. One way around this problem is to check focus after stopping down to shooting aperture - which can be difficult at f:32 or smaller on a bright day. Another way is to learn to see the differences in "focus" when the "center image" is sharp or when the "edge image" is sharp.

It's difficult to explain, but it really works!

And BTW: Good dialytes should have very little spherical aberration, and the Ernemann one should be quite good.