The Filters We Need With Digital...

Started Oct 5, 2008 | Discussions thread
DaSigmaGuy Forum Pro • Posts: 12,306
Re: Glass in front of the lens, negative SA and astigmatism...

Joseph S Wisniewski wrote:

DaSigmaGuy wrote:

Joseph S Wisniewski wrote:

DaSigmaGuy wrote:

Joseph S Wisniewski wrote:

The Protection Racket

Well, what about the “protection” filter: be it the traditional “UV”
or “haze” filter, or a so-called “digital” filter? The camera store
salesman told you that you need one. He may even have said you won’t
get a warranty if you don’t have one (a lie the smarmiest sales
people have been using for decades).

Funny you should bring up this subject Joe. I have just bought a
Tamron SP 300mm f2.8 LD IF and while searching for sites with more
info about it I came across a lens test with a remarkable
finding...As you probably know, Tamron used to offer an optional
112mm "normal" filter for the front of the lens. I dont know what a
"normal" filter is but apparently the tester has found his Tamron
300/2.8 to be considerably sharper with this filter on the lens than
without it!!!
He has even posted MTF graphs to show the difference, with and
without the 112mm normal filter.
If you have time, perhaps you would like to read his article to try
and work out whats going on. After reading it I now feel perhaps I
should try to get one of these filters or my lens wont be as sharp as
without one.

That sounds interesting. I'd love to see that article.

Here's the URL (replace "dot" with a "." and take out the spaces)

http://forum dot fourthirdsphoto.com/showthread.php?t=18918

Excellent. Thank you greatly.

Believe it or not, that article makes perfect sense, and I can
understand exactly why they are seeing what they are seeing.

There have been a couple of threads on the effects of the layers of
glass and crystal behind the lens. For fast lenses, a flat
transparent layer behind the lens increases spherical aberration and
astigmatism. (It moves the focal plane for "marginal rays" near the
outside of the lens's exit pupil farther back from the lens). The
lens would have to be designed to compensate for this to reach
maximum sharpness.

Microscope objectives are very fast, and the higher power ones have
"compensating collars" that you adjust to put in SA and astigmatism
compensation proportional to the thickness of the cover glass on the
slide.

I just did two crude "by hand" ray tracings, and the effect of a
layer of clear glass in front of the lens is exactly the opposite of
one behind the lens, it brings the marginal rays in earlier. And it's
not surprising that one would be able to measure the effect of this
on an Oly E-1, because that camera had the thickest pile of stuff in
front of the sensor of any camera of it's day: sensor cover glass,
two LiNbO3 crystal AA filters, clear IR blocking glass, and clear
glass for the dust shaker.

That must indeed be the reason why because I have made a few enquiries and at least two different Tamron SP 300/2.8 users have now confirmed that the 112m front filter makes no noticable difference whatsoever to the sharpness of the lens on their DSLR's...Therefore I wont bother buying one now.
Thanks Joe.

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