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Re: [OM] 80/4.0 Macro resolving power?

Subject: Re: [OM] 80/4.0 Macro resolving power?
From: "Carlos J. Santisteban" <zuiko21@xxxxxxxxx>
Date: Tue, 20 Oct 2009 22:32:19 +0200
Hi Dawid, Edward and all,

From: Dawid Loubser <dawidl@xxxxxxxxxxx>
>I need massive resolving power at around 1:2,

That's interesting, because I'm planning to "scan" slides with my new GF1
and the 80/4!

>even at the expense of contrast

Again, quite interesting for slides... IME, the 80/4 (mine is the Auto
version) has very high contrast, though.

>The 90/2.0 Macro is *superb*, but does begin to get out of its depth by
1:2.
>At 1:5-1:10 though, I have never seen a sharper lens.

I have borrowed a 90/2, as soon as I get some time I'll give it a good try.

>I am hoping the 80/4.0 is a fairly cheap route to achieving superb
resolving
>power (better than the 50/20 or 90/2.0) at around 1:2, and still wish
>somebody could dig me out some figures, MTF graph, or something...

The point of the 80/4 is that it's a totally symmetrical design so, if the
_whole_ system is symmetrical (the placement of both the object and its
image, i.e., 1:1 scale), most aberrations will be cancelled -- astigmatism
being the main exception, but keeping the space between the two halves as
short as possible, that shouldn't be an issue.

According to Oly, the 80/4 is (obviously) optimized for 1:1, but should give
great performance in the 2:1 to 1:2 range -- thus the 1:2 we're interested
in is at its limits, I don't know if contrast will suffer before resolution,
or vice versa.

>I still want to know whether it's basically diffraction-limited at around
>f/4.0 or f/5.6, or not. On the one hand, well-made normal lenses (like
>the 50/1.4 [>1.1m s/n]) can be diffraction-limited at f/5.6,

I'm not sure that such lenses are _really_ diffraction-limited. According to
Rayleigh criterion, diffraction limits the resolution (in linepairs/mm) of
any optical system to about 1500/F (the f/number), thus f/5.6 means nearly
300 lp/mm "aerial" resolution. This will of course diminish on film (or
sensor), but still seems a very high figure.

>on the other hand, we have the 20mm Macro lens (both versions) which
>is also apparently diffraction-limited around wide open,

This is a different matter, since in extreme macro the F-number is no longer
valid in calculations... the focal length increases dramatically, thus
multiplying the _actual_ F-number, leading to much lower (and manageable)
resolutions.

>A Zuiko Macro lens e.g. the 80/4.0 was made to photograph an area of around
4x4cm,

Not any macro, really...

>and to project it to a capture area the same size.

That's the exclusive feature of the 80/4. Other macros are expected to
project a smaller (50's, 90, 135) or larger (20 & 38) image.

>Also, since the 80/4.0 is completely symmetrical, the direction in
>which it's mounted should make no difference, save perhaps for lens
coatings, etc.

So I think.

From: "Sawyer, Edward" <Ed.Sawyer@xxxxxxx>
>If the Oly 80/4 was really a world-beater it would be in more demand, I'd
think.
>Their high-demand macros are the 90/2, 20/2, 38 (maybe)...
>the 80 and 135 certainly have demand but not like those others.

I believe this is because the 80/4 is a very specific tool -- it doesn't
even focus to infinity! But for the ocassional macro-shooter, a "standard"
macro (maybe with help of an extension tube) could do 1:1 --not quite up to
the same performance, but still acceptable for the non-expert.

The 135 can focus to inifinity, but it's useless without the 65-116 tube or
bellows. The same would apply to the 20 & 38, but their magnification range
is way out of the reach of "standard" macro lenses, no matter the
accessories used, so there's no real competition for them.

>(relavent tests posted on http://www.edsawyer.com/lenstests/ )

Thanks for the link!

> Surely, the lens designs are totally different
>Lens diagrams: Rodenstock : http://tinyurl.com/y998lyv
>Oly: <
http://www.mir.com.my/rb/photography/hardwares/classics/olympusom1n2/shared/zuiko/images/80mmf4opticNW.gif
>
>They are almost identical in appearance of the optical formula...
>so likely more alike than different in many ways.

However, it _looks_ like the Rodenstock has comparatively less spece between
groups, thus _theoretically_ less affected by astigmatism. But in real
world, probably won't be an issue, unless you're looking at the very corners
-- astigmatism effect increases with the third power of the angle from the
axis.

>From the image above it doesn't look totally symmetrical.

The only things to take into account about optical design are the surfaces'
curvatures and the distance between vertex -- plus the type of glass, but
that won't show on a scheme. The elements' diameters and shape of the rims
could be different but won't alter the design or performance.

Cheers,
-- 
Carlos J. Santisteban Salinas
IES Turaniana (Roquetas de Mar, Almeria)
<http://cjss.sytes.net/>
-- 
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