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

[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???

 

[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.

 

[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)

 

[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.

 

[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

 

[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

 

[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

 

[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

 

[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

 

[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.