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Looking for new photo gear? DxOMark's Perceptual Megapixel can help you!

Monday December 17 2012

Lens Insight

The optical science community has long used Modulation Transfer Function or MTF, to measure and describe the sharpness of a lens. They will continue to do so as it is a great way to characterize the physical performance of a lens, however it’s both rather complex to understand and difficult to communicate how well a lens performs from the perspective of the human visual system. At the end of the day photographs are primarily produced for humans to look at: so DxOMark is introducing the Perceptual MPix measure, a more intuitive way to describe lens and camera perceived sharpness.

Most of our readers were not looking at MTF graphs

Can you define MTF? Do you know if an MTF20% of 50lp/mm is better or worse than an MTF 50% of 30 lp/mm? And when reviewing an MTF chart, can you distinguish which curve is best? The short answer is probably no.

DxOMark P-MPix

Modulation Transfer Function or MTF, describes how well a lens and a camera reproduce a scene’s details and contrast in a final image. An integral part of DxOMark’s image evaluation toolkit, MTF is understood by optics experts and by some of the ‘geekiest’ photo enthusiasts, but for the vast majority of people, MTF is an abstract measurement represented by opaque numeric scores, and swooping lines on a graph.

MTF provides a lot of information about optical performances, but can be overwhelming to manipulate for scoring and comparison. Therefore, DxOMark is introducing a new sharpness measurement unit called Perceptual MPix. This unit will help users to compare the perceived resolution of lens and camera combinations.

Perceptual MPix: a much simpler tool to score and compare lenses

P-Mpix is the unit of a sharpness measurement. The number of P-Mpix of a camera/lens combination is equal to the pixel count of a sensor that would give the same sharpness if tested with a perfect theoretical optics, as the camera/lens combination under test.

For example, if a camera with a sensor of 24Mpix when used with a given lens has a P-Mpix of 18MPix, it means that somewhere in the optical system 6Mpix are lost, in the sense that as an observer you will not perceive the additional sharpness that these 6Mpix should have added to the photos if everything was perfect.

In other words it indicates the ability of the lens and other optical components of a camera to utilize, from a visual perspective, the number of pixels of the camera sensor.  P-MPix expresses the result using a figure that can easily be compared to the camera sensor’s MPix figure to show the quality of the lens.

This measurement bypasses the problems inherent to MTF:

  • Describes resolution with a single number,
  • Correlates with the way the human vision perceives resolution
  • Uses a unit that is well-known to photographers — the megapixel.

The result is a more easily understandable measurement for users that makes comparisons between camera and lens combinations very simple: the higher the Perceptual MPix score, the better the perceived resolution.

Photographers of all types can relate to megapixels, as it is a figure that camera manufacturers provide to describe the resolution of their cameras’ sensors.

45% of megapixels are lost due to lens or sensor defects

The Perceptual MPix measure provides photographers with a value that is more strongly associated with the true resolution of their camera sensor when coupled with a lens, or vice versa. For example, a photographer who shoots with a 20-megapixel sensor might produce images that are realistically only 15 megapixels in resolution. A number of factors can cause this loss in megapixels and resolution, including such lens defects as optical aberrations, light diffraction, or an ineffective anti-aliasing filter. The difference in number between a sensor’s megapixels and Perceptual MPix quantifies this loss.

Comparison of perceived resolution between combinations of lenses and Canon EOS 5D mark II.

The example above is based on data from DxOMark’s database of test reults for more than 2,500 camera and lens combinations. These tests reveal that roughly 45% of the resolution is lost due to lens or sensor defects.

A valuable tool for photographers looking for sharp images

Perceptual MPix quantifies the impact of lens sharpnes on camera resolution. This can be seen clearly when you look at the scores for two, superficially similar lenses when used on cameras with different resolution.

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.

What this is actually showing is the inability of the cheaper lens to resolve detail well at the scale of the pixels in the 5D Mark II, while the Sigma lens seems to be limited only by the resolution of the sensor when used on the 5D and gives much more perceived detail when used with the 5D Mark II.

To further emphasize this, if you mount the Samyang 35mm lens on the Canon EOS 7D which is an APS-C camera and in which the pixels are about half the size of those in the older 5D, the P-MPix score drops to 8.9. This is less than half of the EOS 7D’s actual sensor resolution, a camera with 50% more pixels but a Perceived MPix score lower than the 5D and that, using only the centre of the lens.

Comparison of perceived resolution between combinations of lenses and Canon EOS 5D mark II, 5D, and 7D bodies. The left bars show the Canon sensors’ number of megapixels; the right bars show Perceptual MPix values.

Perceptual MPix can be particularly valuable for DxOMark users looking to purchase a new camera.

Looking at combinations of cameras and lenses rather than just the lens resolution makes it clear that there are times when there will not be a significant advantage in sourcing the finest lenses. The Sigma 35mm f1.4 DG HSM A is an excellent lens but used on the Canon EOS 5D performs only slightly better than the cheaper Samyang 35mm f1.4 AS UMC, but when the camera sensor no longer the limiting factor in the combination you see the quality benefit only with the better lens.

Perceptual MPix thus lets photographers take manufacturers’ announcements about resolution with a pinch of salt, and answers an essential question when changing equipment: Is it better to buy a new camera or a new lens? 

A great metric backed up by science and the industry

DxOMark’s new Perceptual MPix measurements are based on acutance and human contrast sensitivity function (CSF) published in recently-released image quality standards from the International Standards Organization (ISO) and the International Imaging Industry Association (I3A). A member of the working groups involved in image quality, DxO Labs has been working diligently with giants in the digital imaging industry such as AMD, Nokia, Kodak, Nvidia, Fujifilm, HP, RIM, Intel, Microsoft, Google, and others.

DxOMark has also relied on the very recent scientific research of CNES (the French space agency) with respect to the optimization of digital acquisition systems, notably those for satellite imagery.

 What are the key lessons for the photographic community?

The Perceptual MPix measure confirms certain rules of thumb such as “a 12 MPix full-format camera is sharper than an 18 MPix APS,” and can verify if the kits offered by manufacturers will properly support a hardware update, etc. Many exciting issues are now quantifiable, and DxOMark will soon offer detailed analyzes based on these scores.

Combined with DxOMark’s long-standing position as the industry leader in image quality evaluation, this new Perceptual MPix measure with its single numeric score, its strong correlation with the human perception of image quality, and its usefulness for easily comparing combinations of digital cameras and lenses, will give consumers, journalists, and experts in the field a better understanding of image quality, and more specifically, of lens and camera resolution.