Equivalent Sharpness?

Started Sep 21, 2013 | Discussions thread
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Jack Hogan Veteran Member • Posts: 6,410
Equivalent Sharpness?

This 'Equivalence ' stuff is a hoot. Just when you think you have it figured out, out comes the next bit... Take for instance sharpness.

In an equivalent image (same FOV, same relative AA, same sensor technology, same number of pixels, same viewing size and distance, same DOF, same SNR) it turns out that to have 'equivalent' sharpness the lens on a crop sensor needs to be 1.5x as sharp as the lens on FF - the mFT lens needs to be almost twice as sharp as the one on FF. Well, are they?

Here is your archetypal lens, a 50mm f/1.8. This type of lens has been in use on 35mm film for over 100 years. It has been designed and re-designed so that it is optimized for typical usage, with peak sharpness between f/2.8 and f/11 - because that f/number range was most useful to photographers shooting with it during that time span. This is what its MTF50 performance looks like in a recent incarnation, courtesy ofLenstip.com

Lens Mounted on a D3X, with about a 6um pixel pitch

It follows that if a photographer were investigating a switch of formats, say to mFT, they would want a prime that would behave much like it - assuming that photography subjects have not changed much over the last century. If a century of experience is worth anything, chances are that the photographer would want performance equivalent to this (the red line represents an mFT lens that would perform equivalently to the 50mm f/1.8 on FF)

Equivalent Lenses producing Equivalent image sharpness on mFT and FF

Such a 25mm lens, in an equivalent situation, would result in similar sharpness in images produced by mFT as the 50mm lens above would on FF. The problem is that it doesn't seem to exist. Here is for instance a Leica DG Summilux 25mm f/1.4 aspherical plotted against the ideal 50mm f/1.8 equivalent

The 25mm Leica on mFT appears to come up short in terms of equivalent sharpness compared to the 50mm on FF

Not even close. The thing is I looked and I looked - at every focal length - and every time the mFT came up short on an apples-to-apples comparison*, even ignoring the f/number shift and simply concentrating on peak performance. Am I doing something wrong? Is FF it for the sharpness conscious amateur photographer?


*DON'T get me started on the absolute lack of standardization in sharpness measurements that make it very hard to compare apples to apples.

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