Return of Interesting article on DxO about 5D III and D800...

Started Apr 10, 2013 | Discussions thread
schmegg
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Re: Return of Interesting article on DxO about 5D III and D800...
In reply to jayrandomer, Apr 14, 2013

jayrandomer wrote:

schmegg wrote:

qianp2k wrote:

Steen Bay wrote:

qianp2k wrote:

Steen Bay wrote:

qianp2k wrote:

Steen Bay wrote:/

We can also see that the 7D actually has a higher resolution. The 5D's MTF curve stops at app. 61 lp/mm because it runs out of vertical pixels (app. 61 lp/mm represents the 5D's Nyquist frequency), while the 7D's MTF curve continues to app. 72 lp/mm (on 24 x36mm). The contrast is rather low, but the 7D's resolution is nevertheless higher.

I'd suggest everyone reads these articles to understand these concepts first and understand what that means in real world photos.

http://www.normankoren.com/Tutorials/MTF.html

http://www.imatest.com/docs/sharpness/

From above two DXOMark MTF tests, we can see 7D only extends at MTF 15% or below that virtually invisible by eyes in real world photos even at base ISO when 7D photos buried by higher noises/grains. So that 7D higher resolution (18mp vs 12.8mp from 5D) is only on paper or reflected in larger CR2 file size but unless you have an eagle eye, you just cannot see it

A bit of sharpening makes it easier to see. It's clearly visible if comparing the resolution test chart shots in DPR's 5D and 7D reviews.

a) that depends from shooting distance.  A meaningful test should be done from at least 50x focus-length distance; b) depends on which lens. 7D needs a really good prime such as 100L (85/1.8 is also very nice lens) under perfect neutral gray and grainless/noiseless light condition at base ISO 100; c) doesn't reflect in real-world photos in outdoor harsh light or indoor poor light.

Not 100% sure, but think that DPR uses EF 85/1.8 on Canon FF and EF 50/1.4 on Canon APS-C. Agree that the 5D has better sharpness/contrast/acutance, but the 7D's higher MP count will most often also give it a higher 'resolution' (at least at low ISOs).

As shown in the two snapshots of old MTF charts between 5D and 7D, even with two excellent prime lenses, 100L and 85/1.8, 7D still only outresolves 5D around MTF 15% and below ....

Fail.

It doesn't show that at all. You have a fundamental misunderstanding of what these graphs represent it seems.

It simply shows that the 5D has higher acutance down to around MTF 15% (where it runs out of resolving ability all together and the 7D keeps going) - not that it resolves more! And this is from a raw image that has not been adjusted in post processing at all - it has only a passing bearing on the obtainable final image detail and the test is simply designed to compare lenses on a given camera.

What's unacceptable about comparing two curves at fixed ordinate value?  That is certainly a standard way of using graphical information.  It may not be the way the data were generated, which is typically by measuring (or calculating) the acutance at a given resolution, but the ability to compare curves at fixed ordinate values is one of the reasons scientists and engineers employ graphs in the first place.

In other words, if one asks the question, "what is the maximum resolution which has a minimum MTF of 15%?" the value assigned for the 5d would indeed be larger than the value assigned for the 7d.  What he says is correct given his caveat of 15% MTF.  If he had said, "the 5D has higher resolution than the 5D" without such a caveat he would have failed, but we already know that the absolute resolution of the 7D is higher than the 5D simply from the number of pixels.

Nothing at all wrong with graphing data and comparing it.

When he said .. "7D still only outresolves 5D around MTF 15% and below", I take that to ipso-facto imply that he believes the graphs show that the 5D out-resolves the 7D above MTF15.

So, what is wrong though is that those graphs don't show what he claims. What they show is that the 5D, at almost all levels of detail it is capable of resolving, will have higher acutance than a 7D at the same level of detail. They do not, however, show that the 5D out-resolves the 7D - as he claimed.

From those graphs it can be seen that the 5D runs out of resolving capability at just above 60 lp/mm, whereas the 7D continues to record detail up to and beyond 70 lp/mm. The acutance is lower, but the detail is still recorded on the 7D whereas it isn't on the 5D. So, in fact, the graphs show the opposite to his claims - that the 7D out-resolves the 5D, albeit with lower acutance as lens quality drops. (It'd be interesting to see the 300/2.8L II, for instance, as even the 100 macro is very close in terms of acutance).

However, acutance is not fixed at capture time like resolution is. It can be, and usually is, modified in post processing. And this is not considered at all in the MTF data because it is concerned with showing the relative sharpness of different lenses on a given camera - and changing acuity in post processing is of no relevance for the purpose of comparing lens sharpness. So not only do the graphs not tell the whole story, but they also show the opposite of what he claimed they show.

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