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80mm lens shootout - for coin photography
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ChrisR
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Joined: 14 Mar 2009
Posts: 7689
Location: Near London, UK

PostPosted: Sun Feb 18, 2018 10:00 am    Post subject: Reply with quote

karlmera wrote:
rjlittlefield wrote:
Feff=10.8 has a lot more diffraction blur relative to the MFT sensor than Feff=12.8 does to the FF sensor. As a result, when you make same size prints, the MFT sensor gives a more blurred image to go along with the greater DOF.



But if we want the same sharpness, we should take the relative aperture, p.e.

eff. 8 for MFT and eff. 16 for FF? And the dof is the same, isn't it?


DOF, not sharpness, but yes, eg

on 18mm sensor, M=1, marked f/4 becomes eff/8 DOF= 0.19mm
on 36mm sensor, M=2, marked f/5.33 becomes eff/16 DOF= 0.19mm

I like seeing the numbers Wink.
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ray_parkhurst



Joined: 20 Nov 2010
Posts: 1261
Location: Santa Clara, CA, USA

PostPosted: Sun Feb 18, 2018 10:19 am    Post subject: Reply with quote

rjlittlefield wrote:

Again, the requirement for same diffraction, same DOF, is that Feff must scale in proportion to sensor size

...

--Rik


I guess in simple terms..."use smaller apertures for larger sensors".

But please don't blame me for saying that diffraction was the reason for preferring smaller sensors! My reasoning was purely related to the magnification range required for a range of coin sizes and whether that range crosses over the 1:1 line, plus whether the range encompasses magnifications which are optimum for a wider range of lenses (eg enlarging lenses). The diffraction discussion was ancillary.


Last edited by ray_parkhurst on Sun Feb 18, 2018 10:26 am; edited 1 time in total
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karlmera



Joined: 05 Feb 2012
Posts: 12

PostPosted: Sun Feb 18, 2018 10:24 am    Post subject: Reply with quote

ChrisR wrote:
karlmera wrote:
rjlittlefield wrote:
Feff=10.8 has a lot more diffraction blur relative to the MFT sensor than Feff=12.8 does to the FF sensor. As a result, when you make same size prints, the MFT sensor gives a more blurred image to go along with the greater DOF.



But if we want the same sharpness, we should take the relative aperture, p.e.

eff. 8 for MFT and eff. 16 for FF? And the dof is the same, isn't it?


DOF, not sharpness, but yes, eg

.


And the sharpness is with FF better?
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rjlittlefield
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Joined: 01 Aug 2006
Posts: 18868
Location: Richland, Washington State, USA

PostPosted: Sun Feb 18, 2018 11:43 am    Post subject: Reply with quote

karlmera wrote:
rjlittlefield wrote:
Feff=10.8 has a lot more diffraction blur relative to the MFT sensor than Feff=12.8 does to the FF sensor. As a result, when you make same size prints, the MFT sensor gives a more blurred image to go along with the greater DOF.



But if we want the same sharpness, we should take the relative aperture, p.e.

eff. 8 for MFT and eff. 16 for FF? And the dof is the same, isn't it?

Yes.

For clarity, let me take this again from the top.

I'm concerned with DOF and sharpness in the final images, and for comparison those final images have to be the same size. Because the MFT sensor is half the size of the FF sensor, the MFT image has to be magnified twice as much to get the same final image. Then to get the same blur in the final image, after twice the magnification, the diffraction blur (Airy disk) on the MFT sensor has to be half the size it is on the FF sensor. The size of the Airy disk is proportional to the effective f-number, so that's why effective f/8 on MFT gives the same diffraction blur in the final image as effective f/16 on FF.

I realize that I'm repeating myself, but when the DOF is the same, so is the impact of diffraction on sharpness, regardless of sensor size.

There are other reasons to prefer large or small sensors, having to do with convenience, available lenses, and achievable noise levels. But those are completely independent of the DOF/diffraction tradeoff which is the same for all size sensors.

ray_parkhurst wrote:
please don't blame me for saying that diffraction was the reason for preferring smaller sensors! ... The diffraction discussion was ancillary.

Agreed. The diffraction discussion started with my reaction to austrokiwi1's misleading statement that the smaller sensor gets more DOF and requires less stacking. It does not, except as a side effect of somehow encouraging him to set his lens aperture to do that.

--Rik
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