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MF Digital, myths or facts? - A larger format is less demanding of the lens

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Article Index
MF Digital, myths or facts?
Collecting more photons
Comparing the top of the line MF back with the top of the line DSLR
Is Capture One better on IQ180 files?
A larger format is less demanding of the lens
The Really Right Stuff
OLP (AA) filtering
The 16 bit issue, truth or myth?
Do MFD have better DR?
MFDBs have better color, truth or myth?
Flexibility
Summary and conclusions
Are large pixels better?
Read also
Experts's comments
Thanks!
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A larger format is less demanding of the lens

A lens transfers an image from the subject to the sensor. The image will be deformed in different ways. Small details will be smeared out. If we think of a small point of light it will be spread over a larger area. Imaging is about light, darkness is lack of light, so that is always light that diffuses into darkness. This happens also be a law of nature.
For small details this diffusion will lead the loss of sharpness. The contours will be fuzzy, instead of being sharp. There is a measurable quantity for this diffusion, it is called Modulation Transfer Function (MTF). MTF says how much of the contrast between two nearby objects will be lost. MTF is essentially 100% for large patterns and goes to zero for small patterns. MTF is a very pactical measure, lens designers produce lots of MTF data, for different light, different focus and so on. It probably takes dozens of MTF graphs to describe a single lens.
The example below was made with two DSLRs having different sensor size, but using the same lens. Both cameras were 24MP one was APS-C and the other full frame. MTF shows how a lens can transfer contrast. The curves shown here illustrate how much contrast the lens/sensor combiantion can transfer at different frequencies.

What we can se in the images is that the sensor on the left can transfer more contrast at each cycles/mm. For instance at 40 lp/mm it would transfer about 58% on the APS-C camera while on the full frame it would transfer around 50%. But, because the sensor is smaller, the relevant comparison is to compare 40 lp/mm on FF with 60 lp/mm on the cropped frame. So here we see that the cropped sensor would transfer about 36% contrast at 60 lp/mm to compare 50% contrast at 40 lp/mm on the full frame. So, using the same lens the full frame camera has a significant advantage in sharpness.


Now, the upper dotted line shows the diffraction limit. An optically perfect lens would have MTF like diffraction limit. on APS-C such a lens would reach around 75% MTF. It is thus theoretically possible to compensate for the format size by building a better lens, in practice that would be very difficult.


The images below demonstrate this difference:

With no sharpening (full frame to left and crop frame to the right):


Or properly sharpened (full frame to the left, crop frame to the right)



Sharpening has a major inpact on digital images. I have not looked into this, at least for now.


Real world MTF comparison based on test images from Imaging Resource:


The image samples above were made using different small format Sony cameras, using essentially the same lens. Below is an MTF based comparison  of the Pentax 645D and the Nikon D800E. This NTF curves are based on test image shot at Imaging Resource. In this case the Pentax 645D was tested with it's standard lens, while the Sigma 70/2.8 Macro was used on the D800E.


Carefully comparing the MTF curves below I would say that the Pentax 645D does have a very small advantage, but the difference is more like an Imprial Quibble (that is small enough not to be significant).





Last Updated on Thursday, 13 December 2012 11:56  

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