Lens "sweet spots"...or aperture for peak MTF

Canon Bob

Canon Bob

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It's a topical question at the moment so I thought I'd publish the list below to give some idea where to find the ultimate "sharpness" for certain lenses...Canon only folks, I'm afraid.

The peak MTF is a reasonable measure of the maximum resolving power that a lens can have. Some lenses (by design) have a fairly flat line whilst other peak somewhere in the aperture range. Zooms often have different "sweet spots" (aperturewise) at different focal lengths....see the chart below.

"My lens is sharp from wide open"....heard it frequently? Well, if it is, then your copy is probably faulty. Physics pretty much decrees that reducing the size of the aperture will improve the MTF until diffraction or other limitations start to play a part in the equation. If it doesn't get "sharper" then something's not right.

The apertures quoted in the chart are peak values and it may well be that one stop lower has almost the same MTF.

Bob
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Thanks for taking the time Sue Bob :thumbs:
 
More than half of my lenses aren't on the chart :p

24-70L, 300L f4, 100L IS are :thumbs:

24L TSE MkII, 90 TSE, 65 MPe, 200L f2.8 MkII aren't :thumbsdown:
 
Sorry to be picky Bob, what what is meant by peak MTF? Where? What exactly? And peak sharpness varies with format, as diffraction cuts in earlier with smaller sensors.

Those numbers look to me like they probably refer to full frame (guess) which, on average, is usually about f/8. On crop format, the same lenses peak around a stop earlier at f/5.6.

Good example here, nifty-fifty 1.8 from dpreview. The lens sharpness widget shows how the recorded peaks vary between APS-C and full frame, and also across the image http://www.dpreview.com/lensreviews/canon_50_1p8_ii_c16/page4.asp
 
HoppyUK said:
Sorry to be picky Bob, what what is meant by peak MTF? Where? What exactly? And peak sharpness varies with format, as diffraction cuts in earlier with smaller sensors.

Those numbers look to me like they probably refer to full frame (guess) which, on average, is usually about f/8. On crop format, the same lenses peak around a stop earlier at f/5.6.

Good example here, nifty-fifty 1.8 from dpreview. The lens sharpness widget shows how the recorded peaks vary between APS-C and full frame, and also across the image http://www.dpreview.com/lensreviews/canon_50_1p8_ii_c16/page4.asp
Click to expand...

50/1.8 looks sharpest at f/4.5 on both 1ds3 and 450d as illustrated on that site.

I think the sensor density might affect the sharpness limiting aperture, rather than the sensor size. Diffraction is to blame for this. For example equal areas of equal density APSC and FF sensors (1Ds III and 30D are similar in that respect) should show the same sharpness with the same lens and same settings.

all about MTF: http://www.normankoren.com/Tutorials/MTF.html
 
daugirdas said:
50/1.8 looks sharpest at f/4.5 on both 1ds3 and 450d as illustrated on that site.
Click to expand...

If you look at overall sharpness across the frame, I would say that lens is best at f/5.6 on crop format, and at f/8 on full frame. Either way, there's about one stop between them (as there should be).

I think the sensor density might affect the sharpness limiting aperture, rather than the sensor size. Diffraction is to blame for this. For example equal areas of equal density APSC and FF sensors (1Ds III and 30D are similar in that respect) should show the same sharpness with the same lens and same settings.

all about MTF: http://www.normankoren.com/Tutorials/MTF.html
Click to expand...

It does get complicated, there is diffraction (mainly), nyquist frequency and pixel density all playing their part as you say, and all interelated. But it's the different degree of enlargement that is making the difference between crop format and full frame. And the difference is, in theory (and all things being equal, which they rarely are!) the crop factor, ie f/number x 1.6 = 1.28 stops.

That looks to be born out in practice pretty much, if you look at a few lens tests on DPReview.
 
HoppyUK said:
Sorry to be picky Bob, what what is meant by peak MTF? Where? What exactly? And peak sharpness varies with format, as diffraction cuts in earlier with smaller sensors.

Those numbers look to me like they probably refer to full frame (guess) which, on average, is usually about f/8. On crop format, the same lenses peak around a stop earlier at f/5.6.

Good example here, nifty-fifty 1.8 from dpreview. The lens sharpness widget shows how the recorded peaks vary between APS-C and full frame, and also across the image http://www.dpreview.com/lensreviews/canon_50_1p8_ii_c16/page4.asp
Click to expand...

Richard.....picky is good :thumbs:

Yes, all readings are for the lens and not the entire imaging system. This requires a full frame sensor and hence no EF-S lenses.
Peak MTF aperture is the setting that will resolve the highest spatial frequency of an extended image circle (24mm diameter) on the sensor....this will exclude the corners.
For zoom lens users it's possibly useful to see that focal length has a bearing on the outcome and different zoom lenses react in different ways...ie, some need stopping down more at the long end, some in the middle and some at the wide end.

Clearly my attempt proved a little too simplistic but I didn't want to frighten the average snapper away with an overly technical appraisal of the lenses' performance.

Bob
 
Canon Bob said:
Richard.....picky is good :thumbs:

Yes, all readings are for the lens and not the entire imaging system. This requires a full frame sensor and hence no EF-S lenses.
Peak MTF aperture is the setting that will resolve the highest spatial frequency of an extended image circle (24mm diameter) on the sensor....this will exclude the corners.
For zoom lens users it's possibly useful to see that focal length has a bearing on the outcome and different zoom lenses react in different ways...ie, some need stopping down more at the long end, some in the middle and some at the wide end.

Clearly my attempt proved a little too simplistic but I didn't want to frighten the average snapper away with an overly technical appraisal of the lenses' performance.

Bob
Click to expand...

Haha! I'm not sure picky is always good Bob. It often only serves to confuse and the basic point of your post is well made.

Some less considerate posters have gone on to frighten people with talk of nyquist and stuff, and while I could go on about it - simply because I'm interestested, not because it changes anything - I'll stop now :D
 
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