It is impossible with the current state of technology to make a lens
that has all the various aberrations from perfect focus at all visible
wavelengths, linear distortion, etc. etc. well corrected at all focusing
distances. Or if possible, amazingly expensive. That's why most 'normal'
lenses don't focus very close. It's also why reasonably priced macro
lenses tend to be rather slow, making the design, materials and
production problems less. Most single focal length lenses are focused by
simply varying their distance from the film plane. With its series of
fast F2 lenses, Oly added internal movement that changes the
relationship of internal elements, allowing better overall optical
performance at close focus than in conventional designs without
compromising distant subject performance.
This is why a true Macro lens is preferable to a 'normal' lens with
extension tubes for close work where a flat plane of focus is needed.
For many kinds of macro shots of things where the part of the subject
that is within the DOF is in the central part of the image extension
tubes or even close-up lenses can work quite well. For flat things that
need to be sharp edge to edge, they will generally be disappointing. For
that, you need a lens designed for flat field at close distances, a true
macro lens.
There was a thread back around June of last year about close up
performance of the 21/3.5, with some sample pics of maps. I believe that
the conclusion was that the 21/3.5 is sharper at center than edges in
all cases, but edges still very good at a distance. Close up, it appears
that field curvature is the major problem. Of course, field curvature
can work for or against you, depending on the direction of the curvature
and the shape of the subject!
Mark Lloyd's 6/21/02 post summarized it's close-up performance pretty well for
me:
"The 21/3.5 I have also has very bad edge resolution at f3.5 close up. I
doesn't have that problem at infinity and at f8 it follows Gary's test closely in
that it's tack sharp.
I have gotten very good closeup results with this lens, however, since the
blurriness is not noticeable to about 500ff center and anything in the center
is sharp so it highlights things in the center while a flat field macro keep
everything sharp. Also stopping the lens down to f11-16 improves edge
performance dramatically to the point of being virtually unnoticable at 4x6.
The 21/3.5 is a great lens I've found out, it's tiny very sharp at infinity and
makes very good pictures closeup if you are willing to work around it's
limitations in that area."
As to differences between the f3.5 and the f2, C.H. had the following
observations:
Okay everybody, here below is my quick observation on the viewfinder with my
OM4 (the OM1/2's finder is a little poorer in edge resolution) and Zuiko lenses
wide open.
21/3.5 at closest focusing distance of 0.2m, mag. around 1:6, it gave very poor edge
resolution, could be similar to the Map result Jochen posted. At 1:15 it is still not
good but improved a lot, I bet it would be fine at 1:40 as Gary's result was not bad at
F5.6 ("B" grade).
At the same time, I have also tried 21/2, 24/2, 28/2, 35/2 and 35/2.8. At
closest focusing distance, the best performer being the 21/2 and the second are
24/2 and 28/2 the third is 35/2.8 and at last is the 35/2 (which does not lag
much behind). Even the 35/2 was poorest among them, it is still much much
better than the 21/3.5. The 21/2 has outstanding edge resolution but with
extremely high barrel distortion.
It is a bit disappointed for the poor performance of 21/3.5 (as it is very
expensive) but I think there is very little chance that you need a flat field wide
angle for close up work as in the real life most objects are 3-D. I hope there will
be some other quality that the 21/3.5 is better than the 21/2. (Actually, I always
think the 21/2 was not so hot on the color dept.)"
In return for digging up all this info, if you find another 21/2 that cheap, I want it! :-)
Moose
iwert wrote:
Hello,
Reading through documentation on the wide angles I often see the words
"close focus aberrations". I have a zuiko 21f3.5 and zuiko 21f2 (close to
mint @ 351$!), of which the latter has a special mechanism correcting for
"close focus aberrations". Which difference should I see when clos focussing
with these two lenses?
Anybody who can enligthen me on this?
Iwert.
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