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DxOMark scores lens sharpness with Perceptual Megapixel

Photo.net Industry News , Dec 20, 2012; 12:27 p.m.

When you want to tell someone your expensive new glass is really good — how can you express that? What numbers quantify or clarify speed, sharpness and fidelity?
Now DxO Labs says its “Perceptual Megapixel” is a “a measurement correlated to human vision,” and provides photographers with a means of instantly understanding the performance of a lens by quantifying its impact on the perceived resolution of the camera with which it is coupled. Perceptual Mpix is also based on the scientific findings of the French space agency CNES “pertaining to the optimization of digital acquisition, notably in the field of satellite imagery.”
“This new unit of measure weights MTF measurements [Modulation Transfer Function] with the sensitivity to detail of the human visual system,” the company says, “thus providing a score for the sharpness perceived by our eyes. We think this new measurement will be more understandable and practical for photographers who use our website.”
The Perceptual Megapixel measurement allows for making numerous comparisons among cameras, DxO adds, “and for the first time quantifies the impact of lens sharpness on camera resolution.”
How much impact? An average of 45 percent of all resolution is lost, DxO claims. “The perceived resolution for a camera can be significantly different from the native resolution depending on the lenses with which it is coupled.”
For example, DxO notes, when coupled with a Canon EOS 5D Mark II camera with a 21MP sensor, a Sigma 35mm F1.4 DG HSM lens has a Perceptual Mpix score of 17.2, whereas a lens such as the Carl Zeiss Distagon T* 35mm f/1.4 ZE has a Perceptual Mpix score of 15.2.
The example is based on data from DxOMark’s database of test results for more than 2,700 camera and lens combinations.
This metric can also tell you whether it’s better to buy a new camera, or a new lens, DxO adds.
Perceptual Mpix is on DxOMark, DxO Labs’ site for measuring the image quality of cameras and lenses. Much more information is here.
Source: http://pmanewsline.com/2012/12/18/dxomark-scores-lens-sharpness-with-perceptual-megapixel/

Responses

JDM von Weinberg , Dec 20, 2012; 04:07 p.m.

Here's another excerpt from the DxOMark page linked to. It helped me to figure out what the graph above was claiming to show:

For example the Sigma 35mm f1.4 DG HSM A when mounted on a Canon EOS 5D Mark II scores 17.2 P-MPix, on the Canon EOS 5D it scores a near perfect 12. Samyang produce a lens with the same specification: the 35mm f1.4 AS UMC. It is around 30% cheaper so it is worth looking at. On the Canon EOS 5D it scores pretty well at just under 10 P-MPix a difference which many photographers would be prepared to live with. When you mount the Samyang lens on the newer Canon EOS 5D Mark II however, the score is 11.5 P-MPix, a loss of 45% of the camera’s resolution. Effectively the results from the Samyang will be only marginally better from the Canon EOS 5D Mark II than from the Canon EOS 5D.

I leave the validity of the metric to those better qualified to judge, but there is not universal acclaim for DxO in other assessments of cameras and lenses.

Wouter Willemse , Dec 21, 2012; 03:25 a.m.

there is not universal acclaim for DxO in other assessments of cameras and lenses

Just what I was thinking when I saw this. It seems just another confusing figure. It shouldn't come as any surprise some lenses might not render the full resolution some sensors are capable of. And some seem to be able (on the current available high resolution cameras). I didn't really need a measurement for that. On top of that, DxO (willingly or unwillingly) gives a wrong impression that resolution is all that counts.
Let them count pixels. I'm not going to stop shooting lenses wide open because I loose a megapixel here and there. It's about photos, in the end.

Zack Zoll , Dec 23, 2012; 05:58 p.m.

Great, that's another thing I'll have customers whinge at me about. "I just bought this lens for $200, and DxO says that whenever I use it, I only get to use half my megapixels. $200 is a lot of money, so why can't you sell me a lens that lets me use ALL my megapixels?"

Thanks, DxO. I hate you.

Pete S. , Dec 25, 2012; 12:07 a.m.

I think it's good because it makes it easier to compare different combinations of camera bodies and lenses to make informed decisions. Resolution doesn't make a good picture in itself but sometime resolution is important.

I actually proposed the same idea as Dxomark's Perceptual Megapixel here on photo.net a year and a half ago.

How many megapixels do you really have?
http://photo.net/wedding-photography-forum/00YTUy

PS. It's just too bad that dxomark removed the mtf graphs for us that actually knows how to read them.

James P. Jones , Dec 30, 2012; 10:36 a.m.

At what aperture and how many lenses were tested? If lens rentals measurements tell us anything it is that there is significant sample variance.

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