Macro Lens Sharpness

[ga6742]

From my research it seems to be commonly accepted that macro lenses are among the sharpest. I own the sigma 70mm macro and I can attest to that. It is as sharp or sharper, across the aperture range, as my 70-200mm L canon. What is there about macro lens construction that makes them inherently sharp? I look at corner to corner sharpness on all these fast primes and they seem to all be soft in the corners until f4+. My macro is flat corner to corner at f2.8 and above. If there is a construction distinction, why aren't all primes this way?

[Dave Humphries]

Hi George,

I suspect the simple answer is that they are prime lenses - far less compromise in design compared to a zoom lens.

If you had a prime (non macro) lens that cost as much as a macro lens does (i.e. thus excluding the cheaper "nifty fifty" range), it will probably be just as sharp.

Cheers,

[Jim B.]

Hi George,

Macro lenses also have a floating element in their design. I believe it's to help lessen focus shift.

[Black Pearl]

Fast primes are not usually used (wide open) for their corner to corner sharpness, they are generally use for isolating a subject from the background so the corner performance is irrelevant.

Also - when under normal circumstances is corner to corner sharpness the main reason for choosing a lens? I would have thought the focal length/lengths it offers, the maximum aperture it can achieve, the subjects it is likely to be used for, the size/weight and the price all come before the extreme corner performance. Yes it is of technical interest in lens tests and so worth comparing but in real world use - not flat field copy work where a macro lens will excel - it doesn't actually matter.

[Glenn NK]

Hi George,

Macro lenses also have a floating element in their design. I believe it's to help lessen focus shift.

Jim:

Thanks for the information - I didn't know that. So I googled floating lenses and came up with:

http://www.photozone.de/focusing-systems

I found it useful and interesting.

Glenn

[chauncey]

I have a whole host of Canon "L" glass but 99% of my work involves two lenses mounted on my 1DsIII...180 macro or a 300mm f/2.8. They, according to, Canon's MTF characteristics...http://software.canon-europe.com/fil...Book_10_EN.pdf
are two of their best. I do wish my skills were on a par with them.

[ajohnw]

Macro lenses are also usually, or should be, optimised for low distortion and shorter working distances. The latter is similar to the use of 50mm lenses and others reversed on bellows. The rear end is optimised for short distances and often offers a significant advantage as magnifications get higher and the bellow length increases.

The term macro on long zooms is a bit of a misnomer. What it really means is hey we have found a way of making it focus closer.. I reckon that's often detrimental to how well it works elsewhere.

Combinations of the arrangements shown in the .de link. Even on old lenses such as this one. Others may alter the distance to the sensor as well or even more lenses.




The rates the lenses move at can be different too. Can't find one of a technical lens. Interesting read here. The Cooke triplet started a lot of lenses off.

http://en.wikipedia.org/wiki/History...ic_lens_design

John
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[DanK]

I think the answer is that macro lenses are designed to have a flatter plane (really, 'flatter surface') of focus, but it beats me how it is done.