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Vignetting on some C-mounts as you stop down
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PostPosted: Wed Jun 20, 2012 3:08 pm    Post subject: Vignetting on some C-mounts as you stop down Reply with quote

Apologies in advance if this is a stupid question, folks.

I have a couple of lovely C-mounts as you probably know which almost cover the APS-sensor wide open with only a touch of vignetting but vignette more as I stop them down.

I have heard people adapting the Schneider 50/0.95 to Leica M8 which is a larger sensor, which got me thinking that perhaps it is the mount opening (which is of course very small in the c-mounts) that gets in the way and whether a change of mount will remove that problem.

I am currently using them on a NEX-5n but if I ever start selling pics I will upgrade to a Leica M14 or whatever it will be in 12 years!

Any insights will be greatly appreciated!!!


PostPosted: Wed Jun 20, 2012 3:56 pm    Post subject: Reply with quote

General rule is that longer c mounts covers a greater area of the sensor Very Happy A 50mm may cover both ff and aps-c, but a 25mm will have heavy vignetting on ff and less on aps-c...


PostPosted: Wed Jun 20, 2012 8:19 pm    Post subject: Re: Vignetting on some C-mounts as you stop down Reply with quote

DanielT74 wrote:
I have a couple of lovely C-mounts as you probably know which almost cover the APS-sensor wide open with only a touch of vignetting but vignette more as I stop them down.


Interesting. Stopping down reduces, doesn't increase, mechanical vignetting.


PostPosted: Thu Jun 21, 2012 6:29 am    Post subject: Re: Vignetting on some C-mounts as you stop down Reply with quote

danfromm wrote:

Interesting. Stopping down reduces, doesn't increase, mechanical vignetting.


It depends - it reduces the vignetting till some point ( let say 'f8' ) and after that it starts to increase again.


PostPosted: Thu Jun 21, 2012 7:56 am    Post subject: Re: Vignetting on some C-mounts as you stop down Reply with quote

DanielT74 wrote:

I have a couple of lovely C-mounts as you probably know which almost cover the APS-sensor wide open with only a touch of vignetting but vignette more as I stop them down.


The vignetting is always there
the only difference is that by stopping down it becomes more visible.


PostPosted: Thu Jun 21, 2012 10:28 am    Post subject: Re: Vignetting on some C-mounts as you stop down Reply with quote

Orio wrote:
DanielT74 wrote:

I have a couple of lovely C-mounts as you probably know which almost cover the APS-sensor wide open with only a touch of vignetting but vignette more as I stop them down.


The vignetting is always there
the only difference is that by stopping down it becomes more visible.


This is a very enigmatic statement. I am not familiar with invisible, or less visible vignetting.

The fact with these two lenses is that wide open, vignetting is very slight, on the Schneider just the very extreme bits of the corners are black and a touch of gentle vignetting leading to them. On the Switar gentle vignetting but no black. As you stop down vignetting progressively increases on both lenses.


PostPosted: Thu Jun 21, 2012 2:50 pm    Post subject: Re: Vignetting on some C-mounts as you stop down Reply with quote

DanielT74 wrote:

This is a very enigmatic statement. I am not familiar with invisible, or less visible vignetting.


Take a very fast lens, mount it on SLR, set it wide open, and put a finger in front of the lens, almost in contact.
Than look through the veiwfinder: can you see the finger? No. At the most you can see a blurring, and some fall of light.
Now stop the lens down to f/8 or f/11 and look again: now you can see the finger, at least a discernible shadow of it.

There are two origins of vignetting:

1- Optical origin: the glass causes a light fall-off at the edge of the image. This type of vignetting improves stopping down,
because the outer part of glass is progressively excluded from the contribution to the image.

2- Mechanical origin: a mechanical factor, like an object that obstructs the view, or an image circle that is not large enough to cover the frame (your case).
This type of vignetting gets worse stopping down, because the circle of confusion is reduced and the edges of the mechanical problem
(in your case, the edges of the image circle) become more visible (while wide open they are blurred).


PostPosted: Thu Jun 21, 2012 11:20 pm    Post subject: Reply with quote

Wollensak velostigmat 25/1,9 has opposite thing. Vignette reduced if stopped down


PostPosted: Sat Jun 23, 2012 5:50 am    Post subject: Re: Vignetting on some C-mounts as you stop down Reply with quote

Orio wrote:
DanielT74 wrote:

This is a very enigmatic statement. I am not familiar with invisible, or less visible vignetting.


Take a very fast lens, mount it on SLR, set it wide open, and put a finger in front of the lens, almost in contact.
Than look through the veiwfinder: can you see the finger? No. At the most you can see a blurring, and some fall of light.
Now stop the lens down to f/8 or f/11 and look again: now you can see the finger, at least a discernible shadow of it.

There are two origins of vignetting:

1- Optical origin: the glass causes a light fall-off at the edge of the image. This type of vignetting improves stopping down,
because the outer part of glass is progressively excluded from the contribution to the image.

2- Mechanical origin: a mechanical factor, like an object that obstructs the view, or an image circle that is not large enough to cover the frame (your case).
This type of vignetting gets worse stopping down, because the circle of confusion is reduced and the edges of the mechanical problem
(in your case, the edges of the image circle) become more visible (while wide open they are blurred).


This makes more sense, Orio, but I tested the Schneider again last night and it is reasonable in the corners wide open and sharp by f1.4, whereas by f8 those corners are black.

So I am still a bit confused...


PostPosted: Sun Sep 02, 2012 10:14 am    Post subject: Re: Vignetting on some C-mounts as you stop down Reply with quote

Hi!

DanielT74 wrote:
I have a couple of lovely C-mounts as you probably know which almost cover the APS-sensor wide open with only a touch of vignetting but vignette more as I stop them down.


Unlike others, I don't have an explanation for the phenomenon, but I see it the same as you in some of my C-mount lenses. In fact, I just uploaded a series of photos to this website that show it quite clearly. See below for more information:

http://forum.mflenses.com/viewtopic.php?t=52439&sid=d2adbf8839d346d11bc6cb53e78cacc1

Is this similar to what you see?

Cheers!