Zeiss 25mm/f2.8 not sharp in the corners when focused on short distance

G

Guest

I bought a second hand 25mm/2.8 and noticed that when focused on short distance (eg. 30cm) the center is sharp but the corners are not sharp. When I turn the focusing ring to 25 cm without moving the camera (then, of course, the center is not sharp anymore) the corners become sharp !!!

Is this a peculiarity of the 25mm lens, or did I buy a broken lens???
 
G

Guest

I guess you are focussing on a flat surface as the subject. Only macro lenses are "flat field" lenses, capable of keeping a flat surface all sharp while close up. When close up, the depth of field is very shallow, and a non-macro lens will not have a flat plane of focus. For non-macro lenses the plane of focus is more like spherical. The focus distance is actually more like the "radius" from the lens. If you stop down to a small enough aperture, you should see the sharp area get larger as the increasing depth of field starts including more of the flat surface.
 
G

Guest

Craig,

Yes, I am focusing on a flat surface. Your explanation sounds logical.

There is only one thing I don't get: If I understand your explanation well, I would expect that when the center is sharp when the focusing ring is set at 30 cm, the corners will be sharp when the focusing ring is set at > 30 cm (e.g. 35 of 40cm) because the distance from the corner of the subject to the middle of the lens is more than the distance from the center of the subject to the middle of the lens.

With my lens it is opposite!

Or did I misunderstand your explanation? (see the schematic drawings below)


WHAT I EXPECT AFTER YOUR EXPLANATION:

(Flat surface) oooooooooooooooooooooooooooo
- - -
- -
- -
(Plane in focus)
- - -
- -
- -
(plane of lens)



WITH MY LENS IT IS OPPOSITE:
Plane in focus)
- -
- -
- - -
(Flat subject) 0000000000000000000000000000


- - -
- -
- -
(plane of lens)
 
G

Guest

:-( Sorry for the crap above. The spaces in my drawing were deleted by the software of the internetsite... So, that won't be of much help.
 
G

Guest

i had this lens about 6 years ago (i bought it as a new one); I made the experience that it wasn t
sharp at all focussing at a distance about less than 3 metres with aperture 2.8
 
G

Guest

Hi Craig,

I checked my 28mm 2.8, at the lens's closest
focus (.25m)
And yes, I have to set a greater distance to focus the edge. It means that the field in
focus is a convex surface from the camera's
viewpoint. Not concave. It is counter intuitive.
But that's what the optics is.
Of course it could be different at other focus setting. I can imagine that as the focus setting is increased, a point can be reached where the field is flat and then concave beyond that.

Good to know these lens characteristics.

Kim Teo
 
G

Guest

Kim,

Hmmm... than the field in focus of your 28mm is a concave from the camera's viewpoint???

If your center is sharp with the focussing ring at .25m, and the corners are sharp with the focussing ring at > .25m, it means that when the focussing ring is at .25m the point in focus at the edge lays closer to the camera then the point in focus at the center --> concave? For me that is rather intuitive. Or do I miss something?

With my 25mm it is opposite: I have to set a smaller distance at the focussing ring to focus the edge!

Sander
 
G

Guest

Martin,

So, you are saying it is a bad lens? (a wide angle lens that can't be focused well under the 3 meters sounds as a bad lens to me). How is that with the 28mm lens?

Sander
 
G

Guest

No, it s not a bad lens at all; from aperture 4 on it was great at all distances;
but it was a little bit disapointing that such an expensive lens is unsharp at low distances at f 2.8
 
G

Guest

Someone told me that the lens is most likely out of collimation. Is that possible? I doubt it a little, because all corners behave the same way with resprect to being sharp or unsharp with a specified focusing distance.
 
G

Guest

Hello Kim,

I still have a question about your message of November 9th. You say you checked your 28mm 2.8 lens at the lens's closest focus and had to set a greater distance to focus the edge.

My question: how much cm or inch did you have to change the distance to get the edges sharp?

Thanks in advance for answering my question!

Sander
 
G

Guest

Hi Sander,

Yes I got it the other way around.
I have to focus nearer to get the edges in focus.
To put it another way, when focusing on a card,
I have bend the edges of the card away from the
camera to get everything in focus. So the field
of focus is convex. How much? Maybe half an
inch at the edges of a 7 inch card. You can
try it with your 25mm lens.

It is quite significant. I found this post
while investigating a couple of plant close-ups that
I had taken recently with 2.8 aperture. The edges
were not only out of focus, but is fuzzy like
the effect of astigmatism, instead of a smooth
blur, which would look better.

Like you, I started wondering if it was a bad
lens. Also I noticed the out of focused background did not look pleasing. So I started
examining out of focus views for the 28mm,35mm,45mm,50mm and 85mm lenses for both in-front-of and behind the point of focus.

My conclusion is that the 45,50,85mm have what
I regard as pleasing
bokeh - lines goes smoothly in and out-of focus.
For the 28mm and 35mm, it is different. A bright
out of focus line grows thicker but still form
a bright/dark edge instead smoothly going from
bright to dark. The result is not a particularly
pleasing bokeh. My guess is the retro-focus
design of the Distagons have something to do
with it. I also notice a picture in a book
by Freeman Patterson. There was deliberately
out of focus picture of a neon sign, with
clear edges. Don't know what lens was used.

BTW, the 35mm also has convex focus field
at the shortest distance, but not as sever.
So I learned 2 things in this investigation.
1. Convex field for the 28,35 and your 25mm,
important at close distances and wide aperture.
2. Different bokeh, between 28/35 and 45/50/85.

Are our lens defective? I don't think so.
Just use them according to their characteristics.

Kim
 
G

Guest

Thanks to you all... I also got the advice from someone to sent an email to zeiss. I got an excellent answer which told me that the curved field of a wide angle is much more appearent then that of tele lenses. My 25mm is pefectly normal. The CZ 21mm lens has a floating element, which is much better corrected for curved field.

Sander
 
G

Guest

Sander,

I don't think that it's a siple case just to discusse it by e-mail whit Zeiss. First of all you have to test your lens with different bodies (maybe you allready have done it). If the result is the same with each bodies than you have to sell the lens. My oppinion is that a CZ lens cannot be unsharp. Within my CZ line I also have a Sigma 2,8/24 MM, I have never taken unsharp pictures with it. I'm a fan of CZ lenses mostly of CZ made in West Germany but I can reccomend the Sigma 2,8/24 MM for everyone.
( I can't believe that Sigma can be better than CZ!)

Zoltán
 
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