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Archive 2012 · Canon EF 24-70mm f/4L IS MTF Chart comparison
  
 
alundeb
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p.4 #1 · p.4 #1 · Canon EF 24-70mm f/4L IS MTF Chart comparison


Steve Spencer wrote:
Corner performance is probably best understood as from about 18mm to 21.6mm from the centre on the x-axis.

jorkata wrote:
A FF sensor is 36mm wide, so anything beyond 18mm on the MTF chart is actually outside of the frame (and hence irrelevant).
I'd consider 15-18mm on the MTF charts as extreme corner, whereas 12-15mm can be considered 'edge'.



Back to the school bench with rectangles, circles, diagonals, how was that again?



Nov 13, 2012 at 05:36 PM
Ralph Conway
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p.4 #2 · p.4 #2 · Canon EF 24-70mm f/4L IS MTF Chart comparison


jorkata wrote:
A FF sensor is 36mm wide, so anything beyond 18mm on the MTF chart is actually outside of the frame (and hence irrelevant).
I'd consider 15-18mm on the MTF charts as extreme corner, whereas 12-15mm can be considered 'edge'.


Is this so? I do not know about MTF charts but remember a bit to maths. Steve is right about the corners. In a 36x24mm frame, the corners are about 21-22mm from the center (square root of 24x24 + 36x36, result divided by 2).



Nov 13, 2012 at 06:50 PM
Steve Spencer
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p.4 #3 · p.4 #3 · Canon EF 24-70mm f/4L IS MTF Chart comparison


Yup, in landscape position from the centre it is 18mm to the side edges, 12mm to the top and bottom edge, and 21.6mm to the corners (a squared (18 squared or 324) + b squared (12 squared or 144) = c squared (or 468). The square root of 468 is 21.6. That is the distance to the corners.


Nov 13, 2012 at 07:18 PM
Ralph Conway
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p.4 #4 · p.4 #4 · Canon EF 24-70mm f/4L IS MTF Chart comparison


I still remembered it! I am wonderfull!



Nov 13, 2012 at 07:58 PM
jorkata
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p.4 #5 · p.4 #5 · Canon EF 24-70mm f/4L IS MTF Chart comparison


alundeb wrote:
Back to the school bench with rectangles, circles, diagonals, how was that again?


Heh. I guess my habit of ignoring everything on the MTFs beyond 18mm backfired on me today .


Edited on Nov 13, 2012 at 08:25 PM · View previous versions



Nov 13, 2012 at 08:06 PM
alundeb
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p.4 #6 · p.4 #6 · Canon EF 24-70mm f/4L IS MTF Chart comparison


jorkata wrote:
A FF sensor is 36mm wide, so anything beyond 18mm on the MTF chart is actually outside of the frame (and hence irrelevant).

alundeb wrote:
Back to the school bench with rectangles, circles, diagonals, how was that again?

jorkata wrote:
Heh. I guess my habit of ignoring everything on the MTFs beyond 18mm backfired on me today .




According to Paul van Walree, whose optical knowledge I respect, all Canon MTF charts should be ignored. So these charts may backfire on us all some day.



Nov 13, 2012 at 08:25 PM
 

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skibum5
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p.4 #7 · p.4 #7 · Canon EF 24-70mm f/4L IS MTF Chart comparison


S Dilworth wrote:
Steve, looking at the corners alone is insufficient for a complete picture, but people are widely concerned with corner performance at the wide-angle setting, presumably because itís often poor in this type of zoom lens.


although I wonder if reading it at 20mm or just a hint past isn't a bit more realistic than peaking the farthest extremes of the extreme corners, nobody cares too much if the last 20x20 block of pixels in the most extreme corner is a little blurry. 20mm seems like a reasonable spot to me, 18mm far edges might not be quite enough, but full extreme just doesn't make any sense to me and if two lenses both do the same at 20mm but one suddenly dives the last mm does it really matter much?



I hear fewer complaints about the long end, since it's usually better, and the corners are often out of focus at longer focal lengths (in portraits, etc.).



Edited on Nov 13, 2012 at 10:37 PM · View previous versions



Nov 13, 2012 at 10:34 PM
skibum5
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p.4 #8 · p.4 #8 · Canon EF 24-70mm f/4L IS MTF Chart comparison


alundeb wrote:


According to Paul van Walree, whose optical knowledge I respect, all Canon MTF charts should be ignored. So these charts may backfire on us all some day.


And yet using them to compare Canon lenses relative to one another they often do seem to do a relatively decent job at matching to relative reality, if not 100% of time and maybe to either more or lesser degree of difference implied.




Nov 13, 2012 at 10:35 PM
SeverianTL
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p.4 #9 · p.4 #9 · Canon EF 24-70mm f/4L IS MTF Chart comparison


Fred Miranda wrote:
Below is a MTF Chart comparison between Canon's 4 offerings in the 24-70 range.
In this order: Canon 24-70mm f/2.8L, Canon 24-70mm f/2.8L II, Canon 24-70mm f/4L IS and Canon 24-105 f/4L IS.
Any theoretical conclusions can be drawn from these charts?


Canon publishes their MTF data in practically the most confusing and eye-straining manner you can think of. It's very hard to read their stamp-sized images. So I decided to tease apart some of their charts and extract the data from the curves in a form that allows them to be enlarged and/or combined arbitrarily without pixellation. In essence, I converted them from raster images into vector graphs then enlarged them.

Examining some of the results below you may note some of the curves have tiny bumps or squiggles not seen in the originals; those are minor artifacts of the conversion procedure that I haven't bothered to correct by hand. Also some of the curves are so tightly jammed together at the top of the chart that it was very hard to distinguish one curve from another. You can check the charts from Fred's original post together against with my versions so that you can verify how good or bad of a job I've done.


So here they are. Take 'em for whatever you think they're worth.














































Nov 21, 2012 at 06:22 AM
SeverianTL
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p.4 #10 · p.4 #10 · Canon EF 24-70mm f/4L IS MTF Chart comparison


The unrealistically high MTF values for the thin blue lines (30 cy/mm @ f/8) in Canon's charts suggests that they are calculated (not measured) without any regard to the effects of diffraction on actual lens performance; in other words, the graphs look to be the geometrical MTFs based on the optical designs of the lenses. Fortunately when the spatial frequencies are low (e.g., 10 and 30 cy/mm) and aberrations aren't severe, the actual physical MTF can be approximated well by simply multiplying the geometrical MTF by the diffraction-limited MTF. I've done that in my adjusted versions of the graphs below. I've rescaled the individual curves that make up the charts by the maximum possible MTF value allowed by diffraction for the aperture (f/2.8, f/4, or f/8) and spatial frequency (10 or 30 cycles/mm) that the curve represents. Not only does this rescaling make the charts more physically realistic, it also has the side effect of making them much more readable!

Of course the diffractive scaling depends on three variables: aperture, spatial frequency, and wavelength of light. The question then arises as to what the proper wavelength should be. This choice is important as you can see in the plot below which shows the effect of diffraction as a function of spatial frequency for the extreme ends of the visible spectrum. The first idea is to scale for the wavelength that our eyes are most sensitive to: green light @ 555 nm. Another simple (seemingly simplistic) idea is scale for the average wavelength in the spectrum(which turn out to be not too different: 570 nm). But the most natural choice is an unweighted average of scale factors over the visible spectrum, which corresponds to the effect of diffraction in white light with equal intensities at every visible wavelength(CIE E). In practice, the difference between these three different measures is miniscule for the spatial frequencies that concern us (10 and 30 cy/mm).

These two plots show the MTFs of a perfect (i.e., diffraction-limited) lens under various conditions. The two vertical lines mark 10 and 30 cy/mm. Just as a point of reference, the Nyquist frequency of the 5D3 sensor is 80 cy/mm and that of the 7D is a bit more than 116 cy/mm (for you Nikonians: D800 = 102 cy/mm).













Here are the MTF charts of the two new 24-70 lenses adjusted for the effects of diffraction.
























Nov 21, 2012 at 06:31 AM
SeverianTL
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p.4 #11 · p.4 #11 · Canon EF 24-70mm f/4L IS MTF Chart comparison


S Dilworth wrote:
...

In any case, the Canon MTF charts have too much information to absorb at once, so we have to concentrate on one aspect or another of performance to take in anything at all.

Back to the corner numbers, then. Iíd expect two people to read the charts slightly differently, perhaps by a few per cent in some cases, but some of your numbers are very different from mine.
...
According to EF Lens Work, the curve in question for the above data (tangential (or meridional) MTF at f/8 and 30 lines/mm) is the dashed, blue, thin line. For that line, for the
...Show more


Yeah, the Canon charts have way too much information crammed into way too small a space. You need a magnifying glass and a lot of patience to tease out all the relevant info. This is especially true of the newer lenses where you have a lot of lines squeezed in at the very top of the graph. But having converted the graphs from Canon's postage stamp-sized raster images into a mathematical representation I can freely combine elements of several MTF charts into single image which makes comparison of lenses (or rather of their idealized MTFs) a snap.

Below are comparisons of the 24-70/2.8 I & II, the 24-70/4 IS, and the 24-105/4 IS at 24mm and 70mm (except not the 24-105 at 70mm of course) both wide open and at f/8. The wide open data has been separated from the f/8 to minimize clutter, but a fair degree of clutter is inevitable when so much data is being presented. The graphs have been enlarged to make it easier for tired eyes.

In every chart the 24-70/2.8 (version 1) is represented by black lines, the 24-70/2.8 II by blue lines, the 24-70/4 IS by red lines, and the 24-105/4 IS by green lines. As in the original Canon charts: thick lines represent contrast (10 cy/mm), thin lines represent micro-contrast/sharpness (30 cy/mm), solid lines are sagittal (radial), and dashed lines are meridonal (tangential).

All data adjusted for diffraction.
























Nov 21, 2012 at 06:41 AM
skibum5
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p.4 #12 · p.4 #12 · Canon EF 24-70mm f/4L IS MTF Chart comparison


As I said all lenses are NOT equal at f/8, look at those 24mm differences and that is between all Ls.



Nov 21, 2012 at 08:42 AM
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