Designed lens sharpness.

MMfoto · Nov 1, 2007

I've been looking a particular Pentax macro lens that is said to have extremely high sharpness. It's a A 50mm f/2.8. It's got me to thinking about lens designs. A lot of Macro lenses have very high resolution. These lenses are usually in the f/2.8 to f/4 range, but there have been some very good one at the f/2 and f/2.5 mark as well. So my question is this: Do lens designers have the ability to make traditional, non macro, lenses spectacularly sharp but do so because they would have unpleasently high sharpness, or perhaps because there would be too great a difference in sharpness between apertures, or is there something "magical" about macro designs that allow for such high resolution values.

Do lens designers limit gross resolution in order to achieve some sort of balanced lens design?

PS: I use the terms resolution and sharpness pretty loosely here, but I think my general point in pretty clear.

AgX · Nov 1, 2007

To my understanding macro lenses don’t yield better resolution and sharpness (it would be better to employ the MTF concept here) than lenses of the same focal length within the same class.
It is just that both types of lenses are calculated for different imaging scales resp. object distances. Concerning the issue of macro lenses yielding the same image quality at infinity as general purpose lenses I found that one renown lens company made contradictory statements on this issue…

Concerning an intented reduction in sharpness I refer to the design of lenses for digital capture. There at high resolution-frequencies which would be the same the very sensor would produce spurious resolution the lenses are designed to have a sharp fall-off in transmitted contrast. This is not an issue in analogue photography, and thus the lenses for this field can be calculated for maximum response at the highest possible frequencies. (Only limited by their price.)
And there are still those soft focus lenses which are calculated to yield in one way or another a high degree of spherical abberation.

DanielOB · Nov 1, 2007

Usually as lens opening go up one F-number (say from F2 to F2.8) the price go up around twice. So F2 is around twice expencive as F2.8 (similar focal length).
When F2.8 is twice more expensive than F2 something is missing. It is optical and mechanical design, glass kind, tolerances in manufacturing, mechanical design,... Lens price do not go by lens quality it go by lens production cost.

When you acouint what is above, (Zeiss Planar F2/50 mm ZF Macro) is around four times more expensive as (F1.4/50 mm Planar).

Standard lenses are corrected for indefinite, easy job. But who use it at indefinite? It is why quality is down, plus lower cost to develop, plus lower cost to manufactur, plus lower cost for materials (glass, to answer you question).

Macro we have from Leica and Zeiss are not real macro, but rather close-up lenses. They are also made from the best glasses and so very expensive. Something like Ford make different cars of different quality, for different pockets. However these macro (super lenses) are not very good for portrait, or we have to attach Zeiss Softar on it....

Daniel OB
www.Leica-R.com

Jim Jones · Nov 1, 2007

In the early 1970s I did a crude, but critical, resolution test of about 30 quality lenses for 35mm cameras and enlargers. Four were definitely sharper: Leitz 50mm f/2.8 Elmar, 45mm f/2.8 GN-Nikkor, Micro Nikkor 55mm f/3.5, and EL-Nikkor 50mm f/2.8. The first two were four element Tessar formulas, the latter two were six element. Faster lenses, like the Nikkor 50mm f/1.4 and f/2 and the Leitz Summicron f/2 were slightly less sharp wide open. All lenses designs are a compromise between sharpness, speed, macro capability, focusing distance, and cost. A few very fast lenses are optimized at widest apertures.

Mustafa Umut Sarac · Nov 2, 2007

You asked a critical question , problem is to know what designer can do and can not do ? How much art is there at lens design or how much it depends on more precise carving of glass and better more expensive glasses.
I am not a scientist but i learned something about 1942 zernike inventions and 1953 nobel prize. he had been found a easier way to balance many errors or corrections in a faster way. if it is not all mathematics , who says lets put some spherical aberration in to the design ? Which aberrations and how much of them put artificially in to the design ? I want to learn that.

Pinholemaster · Nov 2, 2007

Many macro lenses are designed to optimally render two dimensional objects (ie - paper, paintings, anything flat) well on 2-dimensional film, but not designed to record 3-D objects well in 2-D.

I'm only remembering what I've read from people who know the engineering of optics far better than me. Don't quote me as an expert.

What I've read is that a macro lens tends to flatten out the rendering of a 3-D object, as if it were 2-D.

Lenses are only tools. There is no one lens that can do it all. Also, I believe the notion that lenses should be selected for their 'sharpness' is over rated, when there are other features to be considered.

Ian Grant · Nov 2, 2007

Thats a very simplistic answer.

Macro lenses are designed and optimised for close up work typically 1:1 to 1:5

Compared to a similar focal length normal lens reversed a macro lens will give better depth of field and sharpness at the same apertures. They are not particularly designed for flat field work after all the medical Nikkors etc are used to photograph 3-D objects, and most are designed for all types of close-up applications. I've never heard of any nature/wildlife photographers complaining that their macro lenses caused flatter rendering of their images.

However Flat field process lenses designed for "Process Cameras" are different, but they are not designed for normal film camera use.

Ian

Dan Fromm · Nov 2, 2007

Pinholemaster said:
Many macro lenses are designed to optimally render two dimensional objects (ie - paper, paintings, anything flat) well on 2-dimensional film, but not designed to record 3-D objects well in 2-D.

I'm only remembering what I've read from people who know the engineering of optics far better than me. Don't quote me as an expert.

What I've read is that a macro lens tends to flatten out the rendering of a 3-D object, as if it were 2-D.

Total nonsense.

Nearly all lenses are designed to image a plane on a plane. You may be thinking of process lenses; they're intended to be used in applications where curvature of field can't be tolerated, so may have better field flatness than lenses not made for such exacting applications.

And macro lenses don't flatten. They are typically optimized, as Ian has already pointed out, for near distances. Prints made from negatives taken with a lens that's long for the format and viewed at normal viewing distance exhibit flattened perspective, but that has nothing to do with the conjugates the taking lens was optimized for.

Ronald Moravec · Nov 2, 2007

Slower lenses are easier to control aberations and macro lenses will focus on a flat plane if they are designed to do so. Field flatness is not a design optimizarion with faster lenses usually.

ehparis · Nov 2, 2007

There's much discussion of glass in this thread but I'm wondering what bearing, if any, the number of aperture blades has on the original question, i.e., sharpness (as opposed to bokeh etc.).

Dan Fromm · Nov 2, 2007

Ronald Moravec said:
Slower lenses are easier to control aberations and macro lenses will focus on a flat plane if they are designed to do so. Field flatness is not a design optimizarion with faster lenses usually.

Oh, come on, Ronald, curvature of field is almost always undesirable. The big exception is lenses designed to image a subject of known curvature -- usually a CRT -- on a plane.

The ideal lens is aberration free and so has no curvature of field. In practice, designers trade off aberrations against each other so may accept a little field curvature to get less of something else. But you can't tell me that they think curvature of field is ok.

Cheers,

Dan

DanielOB · Nov 3, 2007

Flatening?
There is something "like" flatening around macro lenses. It does nothing with 2 dim and 3 dimension picture.
Distance FILM-OBJECT AT CORNER of finder are larger than distance FILM-OBJECT AT CENTER of finder. So if one focus at center, corner is out of focus and will apper blur. Macro lenses are corrected to make both and center and corner "sharp" when we focus at center (or corner). It is caled "flat field", and again, does nothing with 3D od 2D.
Hex
Daniel OB

AgX · Nov 4, 2007

Daniel,

Yielding a flat image plane is the goal of the calculating of all photographic lenses since the beginning. Might they be `macro´ lenses or those calculated for much smaller image scales. (With the exception of very special lenses.)

However there are lenses such as `process´ or `lithographic´ ones, where the flatness of the image field is of even greater importance due the fact they are only intented to project flat object planes.

AgX · Nov 4, 2007

Something more on Field Curvature and Astigmatism:

http://www.vanwalree.com/optics/astigmatism.html

AgX · Nov 4, 2007

And as macro lenses are intended for large image scales, the dof will be rather small; thus there will be less tolerance to any kind of spherical image. And thus there is the necessity to even more reduce these aberrations.
To my understanding this will not gain a bettter image quality at infinity in comparison with lenses intended for smaller image scales.

Ed Sukach · Nov 4, 2007

AgX said:
Something more on Field Curvature and Astigmatism:

http://www.vanwalree.com/optics/astigmatism.html

This is a useful web site. Please note the emphasis on "oversimplification". Optical Design is, IMHO, one of the most complex of all sciences. It is NOT difficult, or unusual, in practice to find oneself out of the realm of "basics".

Every characteristic in the design of an optical system has its tolerance. "Process" lenses are designed to have an *exceptionally* "flat" field ... and more compromises with other characteristics; lessening maximum aperture, or even eliminating variable apertures altogether, are common. Performance at other focusing distances than those expected, may not merit close consideration.

It may come as bad news to some of us who are heavily anal, but lens Quality Control is not dedicated to the search for absolute perfection, but to limiting the acceptance of compromises, variations and imperfections involved - to produce a lens that is "good enough"!

Ole · Nov 4, 2007

The "basic" design criterion for process lenses is absolutely minimal distortion. Sharpness, flatness of field, contrast, astigmatism and everything else is secondary to that. Performance at anything but optimum distance is not even considered.

The fact that so many process lenses are quite usable for other photography is pure coincidence.

Designed lens sharpness.

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