DxOMark Introduces the “Perceptual Megapixel”

A couple of weeks ago the website DxOMark, operated by DxO Labs, introduced the “perceptual megapixel”, which is in effect a measurement of the sharpness of individual lenses.

As you are probably already aware, there are already measurements intended to provide you with a sense of the relative sharpness of a given lens. In most cases that information is presented as a number of line pairs per millimeter, for example. A lens with greater sharpness is a lens that is able to resolve a greater number of pairs of black and white lines in a test chart. Pretty simple. Sort of.

The concept of counting line pairs is pretty simple. That’s especially true when you consider that in order to determine how many line pairs a lens can resolve, you actually need a target image that consists of various line pair options. But that’s where things get a little tricky. There’s a certain amount of subjectivity involved in deciding at what point a particular line pair density has gone beyond the point of being a line pair rather than a gray blob.

Well, subjectivity often has an answer, and in this case the answer was the modulation transfer function (MTF). MTF values can be plotted on a graph, so that you have nice curves visually presenting very complicated information about the behavior of a particular camera and lens combination. The trouble is, as much as this quantitative approach to lens evaluation can be very helpful in evaluating lenses, it isn’t all that helpful in terms of being able to actually interpret the data. Sure, I can see a series of curves on a graph, but I don’t necessarily know how to evaluate a single chart for a single camera and lens combination. And things can get even trickier when comparing different lenses, possibly even with those lenses mounted on different camera bodies.

This is exactly the challenge addressed by the perceptual megapixel, presented as “Perceptual MPix” by DxO Labs. Instead of a graph that you may not know how to interpret, or a line pair value that can be tricky to compare across a wide range of equipment, you simply have a single number to evaluate.

DxO Labs presents a sample using the Canon EOS 5D Mark II digital SLR, which has a sensor with 21.1 megapixels. That information certainly provides a sense of what the camera is capable of, at least in a general way. But how do you evaluate different lenses on that camera body? In this example, DxO Labs presents the Sigma 35mm f/1.4 DG HSM lens compared with the Carl Zeiss Distagon T 35mm f/1.4 ZE lens. The Sigma lens achieve a Perceptual MPix value of 17.2, while the Zeiss lens achieves a score of 15.2. In other words, these lenses are effectively reducing the resolution of the camera by virtue of reducing sharpness to some degree.

I think the Perceptual MPix values provide a simple and potentially very effective method of evaluating different lenses, and even different lenses with different camera bodies. The question is, with this new score for camera and lens combinations be widely adopted? And will photographers actually pay attention to this method of evaluation when planning an equipment purchase?

What do you think? Are you likely to use the Perceptual MPix rating for specific camera and lens combinations to help you make a purchase decision? Feel free to share your thoughts in the comments below.

And you can get more information about the Percetual MPix through the DxOMark website here: http://www.dxomark.com/

About Tim Grey

Tim Grey is in the business of making photographers smarter and happier. He is the author of more than a dozen books on digital imaging for photographers, has written hundreds of magazine articles, and publishes the Ask Tim Grey e-mail newsletter as well as the Pixology digital magazine. He also speaks at a variety of events and leads photography workshops around the world.
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2 Responses to DxOMark Introduces the “Perceptual Megapixel”

  1. Tom Judd says:

    MTF stands for modulation transfer function, not ‘modular’

    • Tim Grey says:

      Tom,

      I’m going to blame spell-check for that one, though I’m not sure that’s plausible. I may have simply typed it wrong. Thanks for the catch, and I’ve now fixed it in the post…

      Tim

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