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
