September 18, 2014, 03:52:51 PM

Author Topic: DxO & MTF Charts ... a little help please!  (Read 4118 times)

jrista

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Re: DxO ... a little help please!
« Reply #45 on: February 14, 2014, 12:18:44 PM »
Another point on MTF charts - you don't want to compare a super telephoto to a wide angle lens - as you can see the IQ looks insanely higher on the former, but it's not really that way.  You should compare similar focal lengths to each other if using MTF charts, i.e 24mm prime to a 24-70 lens at the wide end.

Oh, the telephoto focal lengths are indeed that good. Thats WHY I invested $12,000 in the 600mm f/4 L II! You have no idea how much better it is. And that conforms with the theory. At 600mm, the vast majority of incident light is collimated. There isn't much bending going on. Not nearly as much as at 70mm or 24mm. An MTF is an MTF. There are no special considerations for focal length. You can compare any MTF chart to any other. They are about as honest a review of lens quality at any given focal length as you can hope for.
jrista, this is something I had read from Canon (will have to look for it), and I guess I didn't phrase it correctly.  It's not to say that the MTF charts lie, just that wide angle lenses may look really soft in comparison to telephoto charts, but that doesn't mean that they are as soft as they appear, and the value of MTFs is better used to compare similar focal lengths to each other.

I see what you are saying. Indeed, a wider angle lens is compressing a very wide field of view into the same physical region of the sensor. That compression certainly mitigates the impact of higher angle light being bent into the lens. The MTF charts really don't lie, though, and they are a normalized plot, so they tell you the same thing about lenses regardless of focal length. The illusion would be when you think your ultra wide angle lens performs as well as a telephoto "for what it is"...simple fact of the matter is, quite often, they really don't! I have the 16-35 and the 600mm, two of Canon's most extreme lenses. The corner performance of the 16-35 is well and truly ATROCIOUS. Even its center performance isn't all that great on a camera like the 7D (with small pixels/high resolving power.) This is clearly indicated by the MTF:




At the wide end, sagittal performance drops to 0.3!! Meridional performance drops to almost zero!!! Even at the long end, resolution suffers considerably in the corners.

My 600mm lens, on the other hand, produces what I consider perfection from center to corner. Both lenses live up to their MTFs. Even though you don't necessarily "feel" as though the 16-35mm lens is all that bad, all it takes is a little bit of investigation of the corners to see the MTF doesn't lie. Corner performance on the 16-35 is about as bad as it gets. (Canon REALLY needs to update that lens.)

That makes me think of another great resource (even if it's not as current as the edition CPS members receive when they join) for learning about all of this stuff - Canon Lens Work.  You can download the 11 PDFs free from Canon's European CPS website:
http://www.canon-europe.com/Support/Documents/digital_slr_educational_tools/en/ef_lens_work_iii_en.asp

The 10th PDF, OPTICAL TERMINOLOGY & MTF CHARACTERISTICS, has tons of great information on optics and MTF charts.  It's not light reading, but it will tell you everything you wanted to know and probably a whole lot more!

Aye! I read all the EF Lens Work documents years ago. Excellent, if complicated, stuff.

One of the best explanations I've seen on how to read MTF charts is on page 14 of Sony's Alpha lens brochure which also contains great diagrams and explanations of all core lens concepts:
www.docs.sony.com/release/Alpha_Lens_Brochure_Fall_2012.pdf

I've never looked into Sony's documents. I'll have to check it out.
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Re: DxO ... a little help please!
« Reply #45 on: February 14, 2014, 12:18:44 PM »

candc

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Re: DxO & MTF Charts ... a little help please!
« Reply #46 on: February 14, 2014, 06:58:10 PM »
I read the Sony document. It is not a highly detailed technical paper but a summary of sorts. I found some useful info in the article. What I learned is that when interpreting an mtf chart the relative closeness of a dashed and solid line pair will generally indicate the quality of the defocused areas. That is to say if the dashed and dotted lines of the line pairs are close together or superimposed then that indicates the lens will have good bokeh. I didn't know that was something that could be measured but I guess it can. Maybe its not all telling but apparently a good indicator?

jrista

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Re: DxO & MTF Charts ... a little help please!
« Reply #47 on: February 14, 2014, 08:11:06 PM »
I read the Sony document. It is not a highly detailed technical paper but a summary of sorts. I found some useful info in the article. What I learned is that when interpreting an mtf chart the relative closeness of a dashed and solid line pair will generally indicate the quality of the defocused areas. That is to say if the dashed and dotted lines of the line pairs are close together or superimposed then that indicates the lens will have good bokeh. I didn't know that was something that could be measured but I guess it can. Maybe its not all telling but apparently a good indicator?

Aye, what Sony says is true. When the lines are on top of each other, you generally have very flat out of focus blur circles. They tend to be uniform from the center of the circle to the edge of the circle. When there is divergence, any number of alternate forms of blur circles can arise. Some even divergence and a relatively even falloff actually means the lens still has some spherical aberration, which usually results in good boke as well. When the meridional lines are all squirrely, then your boke will usually be pretty nasty.
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Current Gear: Canon 5D III | Canon 7D | Canon EF 600mm f/4 L IS II | EF 100-400mm f/4.5-5.6 L IS | EF 16-35mm f/2.8 L | EF 100mm f/2.8 Macro | 50mm f/1.4
New Gear List: SBIG STT-8300M | Canon EF 300mm f/2.8 L II

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Re: DxO & MTF Charts ... a little help please!
« Reply #47 on: February 14, 2014, 08:11:06 PM »