APS-C IMAGE QUALITY VS fULL fRAME AT TESTING

Started Feb 24, 2013 | Discussions thread
A bit more help.

joger wrote:

stepping in late (busy these days at work)

I am not so sure that any of these above mentioned lenses exist in real world for consumers. They might exist for technical applications or in microscopes or in appliances we normally don't use in photography.

Well, per the PZ test of the 60 / 2.8 macro on a 50D:

and the 100 / 2.8L IS on a 50D:

the 60 / 2.8 macro wins across the aperture range.  In other words, there's an example of a lens designed for a smaller format outperforming a lens designed for a larger format.

Now, when comparing between formats, let's go back to what I said:

Now, let's see what happens when we compare across formats, for example, the 100 / 2.8L IS macro on a 50D (1.6x, 15 MP) to a 5D2 (FF, 21 MP), and ignore differences in perspective and/or framing (the reason being so we can eliminate lens sharpness as a variable when testing my claims about the effect of the enlargement ratio and pixel count).

For example, at f/2.8 in the center, PZ records 1400 lw/ph for the 50D, and 2050 lw/ph for the 5D2.  Based on the difference in pixel count, we would expect the 5D2 to resolve 1400 x sqrt (21/15) = 1657 lw/ph.  Based on the enlargement factor, we would expect the 5D2 to resolve 1400 x 1.6 = 2240 lw/ph.  Note that the tested value of 2050 lw/ph lies neatly between these two values.

The same is true for DPR's test.  The 100 / 2.8L IS macro resolves 2333 lw/ph on the 50D, and 3391 lw/ph on the 5D2.  Based on the difference in pixel count, we would expect the 5D2 to resolve 2333 x sqrt (21/15) = 2673 lw/ph.  Based on the enlargement factor, we would expect the 5D2 to resolve 2333 x 1.6 = 3733 lw/ph.  Once again, note that the tested value of 3391 lw/ph lies neatly between these two values.

In fact, the tested value lies 24% above the expected difference for pixel density and 8% below the expected difference for enlargement factor with the PZ test, which is in remarkable agreement with the 27% / 10% difference of the DPR test.

In other words:

http://www.josephjamesphotography.com/equivalence/index.htm#lensvssensor

In short, the resolution as a function of sensor size and pixel count can be summed up as follows:

• For a given lens sharpness, the difference in resolution (lw/ph) between two systems will be between the ratio of the sensor heights (enlargement ratio) and the ratio of picture height in pixels (linear pixel density).  The sharper the lens, the closer the balance will be to the ratio of linear pixel density, the less sharp the lens, the closer the balance will be to the enlargement ratio.

However, the lenses specifically designed for smaller formats are often sharper than the lenses designed for larger formats.  Whether or not this is enough to overcome the difference in sensor size depends on the specific lens-sensor combination, and, of course, whether or not we are comparing at the same AOV and DOF.

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