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

Started Apr 10, 2013 | Discussions thread
schmegg MOD
Senior MemberPosts: 4,978
Re: I'm still not understanding what he's said that's wrong
In reply to jayrandomer, Apr 14, 2013

jayrandomer wrote:

schmegg wrote:

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.

I took his statement to mean, given a minimum value of acutance (presumably what he deems as acceptable), the resolution at which the 5d reaches that value is higher in resolution than the 7d.  In other words, if I were to draw a line from at a constant 15%, I would intersect the 7d curve before the 5d curve.  One would say that, given the constraint of at least 15% MTF, the 5d achieves higher resolution (in this particular test).


In other words, if I took a test shot, made everything within 15% of middle grey grey, everything above white, and everything below black, and then tried to find the highest resolution lines, I would find more on the 5d image than the 7d image.  A contrived situation, for sure, but given the evidence from those MTF charts that statement seems correct.  Is that not the case?

That is correct. However, both cameras are still resolving detail at 60lp/mm, the only difference being acutance. By 70lp/mm the 5D is no longer resolving but the 7D is still resolving - therefore the graphs show the 7D out-resolves the 5D.

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).

I think everyone agrees the resolution of the 7d will be higher than the 5d, because it has more pixels.  That's not the statement I was confirming.  Perhaps not everyone does, in which case I'm sorry.

That's fine - no problem. I didn't take it that you were 'having a go' at me.

I'm pretty sure most people in this discussion, including yourself, already understand what I was attempting to explain. It was really meant for the benefit of people who are reading this thread and don't yet understand what MTF graphs are - as some of the claims made by one person in particular seem to be based upon a misunderstanding.

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.

Are you saying restricting acutance to a certain threshold to determine "maximum resolution" is not appropriate?  That somewhat subtler argument is not one I can disagree with.

Well, in terms of which of two sensors will resolve more, yes I am. If, for instance, you are interested in particular in detail at around the 70lp/mm frequency, then choosing a 5D would be futile, whereas a 7D will still give you some hope of recovering that detail as at least it will have been recorded, albeit at very low acutance. (remember, acutance is not fixed at capture time - the MTF graphs are for comparing sharpness delivered by different lenses on a given body, not for determining final image sharpness).

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