C/V 21/4 ltm focusing pat infinity?

[Pherdinand]

Hello all.

I just bought a 21/4 ltm voigtlander lens (my first c/v). I mounted it with a C/V 50-75 adapter to my M2 and when all seems to be put together fine, i can focus "past infinity". Meaning, i focus on very distant objects e.g. clouds aligning the rf patch and still, i can rotate the focus tab further than that so that the patch does not overlap anymore.
Close focus at 1m seems to be spot on (checked with ruler measuring from film position).

Is this normal??

I CAN use the 50-75 ltm to m adapter with it, right?

thanks

 

[Pherdinand]

It is.🙂The image plane will be infinitely thin, though, thus a small deviation from infinity focus will not be noticeable, being surrounded by blurry images in front and behind.
But since infinity focus lies somewhere in the continuous between pre- and past-infinity, it must be possible...

Edit: Thanks. So it is all fine.
A bit annoying though, since i never really want to focus PAST infinity 😉
and i will have to watch that i don't do it. Turning till it stops won't work on this lens.

 

[Pherdinand]

Well, infinity is where the lens is focused, when the image plane is exactly at the focal length!
Here. A definition to chew on. 🙂

It's actually a very difficult question. It involves h the Planck constant as well. I am sure it does.
Every difficult question does.

 

[tripod]

Don't worry, DOF will cover it.

 

[Pherdinand]

Actually, DOF won't cover focus mistake at infinity.
Theoretically: at infinity, any shift of the lens will mean infinite deviation from the correct focus distance, thus DOF does not cover it.
Practically: Focusing past infinity means the lens moves to a distance less than the focal length, with say, 0.1 mm deviation, therefore everything will be blurry.

We could say that DOF becomes negative at past infinity focus😀

 

[oscroft]

I CAN use the 50-75 ltm to m adapter with it, right?

Yes. All the different adapters do is bring up the appropriate viewfinder frame on cameras that have auto-frame selection. As there are no cameras with 21mm frames and auto-selection (there's only the Bessa-R4A that has a 21mm frame, and that has manual frame selection), it makes no difference with a 21mm lens.

 

[John Elder]

Try a leica adapter.

 

[Pherdinand]

u wanna say brand new voigtlanders' adapters do not work well on voigtlanders' screw mount lenses??

 

[rxmd]

Actually, DOF won't cover focus mistake at infinity.
Theoretically: at infinity, any shift of the lens will mean infinite deviation from the correct focus distance, thus DOF does not cover it.

In practice, the lens projects an image on film. At infinity, the lens projects all objects at infinity into a plane of focus that hopefully coincides with the film plane. What appears to be a point in this plane of focus is actually a little cone in front of the plane of focus, and behind it. So if you move the film plane forwards or backwards all you get is that points are projected into smaller or larger circles. It is irrelevant whether you move the film plane with respect to the lens or vice versa, and it is also irrelevant whether you move it forwards or backwards. The only difference when focusing past infinity is that as objects at infinity increasingly become out-of-focus, nothing else comes into focus instead. 🙂

DOF is a misleading idea here because it is measured in terms of distance, and there are indeed no distances past infinity. However this might lead to talk about circles of confusion again and here we're already dangerously close to involving the Planck constant 🙂

Philipp

 

[hou baloo]

And here I thought "circle of confusion" was the pet name for this website and its commenters!

 

[John Elder]

To Pherdinand: I believe the best adapters are made by Leica and further, am not alone in this opinion. I believe Ferider has posted to the same effect;he only uses leica adapters. Some CV adapters are not quite right. I believe Leica adapters had better quality control. This is not a new issue, google on photonet and you will see similar posts as mine.

 

[Pherdinand]

i see what you mean, John.
Thanks.