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Author Topic: Strange Results  (Read 252 times)

Offline acaball1

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Strange Results
« on: March 08, 2017, 04:23:21 am »
This a comment for this page on the website
I'm curious. The D500 is a 24 MPix camera. However, here it's only rated at 14 PMPix.
A similar result is shown for the D7100 which is also a 24MPix. The D750 on the other hand rates at 21 PMPix. All with this exact same lens. Why this difference?
It's true that they have a 1.5 times smaller sensor. However, these cameras have very small pixels about 4um 1.5 times smaller than the D750 (6um). Which gives them a 1.5 greater resolution. This can be verified by measuring MTF. In addition, these smaller sensors look at a smaller image circle. Most lenses perform better towards the center. So this should give these cameras a little extra advantage on sharpness. Can anyone explain why DXO rates these cameras much lower in sharpness? Maybe someone for DXOMark?

Offline Renarik

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Re: Strange Results
« Reply #1 on: April 05, 2017, 04:48:19 am »
I agree it does seem strange, it took me quite a bit of reading before I understood why this was the case. Not many authors or manufacturers talk about this.

With a smaller sensor, the lens has to focus all that detail onto a much smaller space. That means that the lens has to be all that much sharper to take advantage of all those megapixels. I don't believe I've ever seen a lens that was so sharp that it would provide you with full sensor resolution on an APS-C sensor.

Take for example the new king of sharpness, the Sigma 85mm F1.4 DG HSM A. On a Nikon D750, it provides the full 24Mpix of resolution (36Mpix on a D810), while on a D7100, only 20Mpix, and on a D500, only 19Mpix. That's really impressive performance on APS-C for any lens, but you can see by that example, that it's not up to providing a full 24Mpix on an APS-C sensor.

In general, the larger the sensor, the more forgiving it is, because there's a larger surface area to focus on.

The interesting corollary to this is that often, full frame lenses are built to lesser tolerances (or were, before full frame sensors started reaching resolutions that provide the same level of challenge as an APS-C sensor does) than APS-C lenses. For example, my Nikon 28-300mm f/3.5-5.6 ED VR lens gave very poor performance on my Nikon D7000, but it's quite a bit improved (though still not super sharp) on my D750.

Hope that helped!


Offline acaball1

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Re: Strange Results
« Reply #2 on: April 11, 2017, 08:49:10 pm »
I'm glad you became interested!
I'm beginning to believe that the present DxoMark scoring method automatically penalizes smaller formats assuming that they will be enlarged by a greater amount because they are smaller in size. That is to say mm wise. That happens when you use line pairs/picture height or similar score. However in reality a picture is enlarged based on its pixel size. In other words any 24 megapixel picture when zoomed to 1:1 or 100% will be the same size in your monitor regardless of sensor format. Full or APS-C. They are both 6000x4000 pixels.
This has been recognized now specially by wildlife photographers that are using high resolution APS-C cameras such as the D500 and even D7100. This contradicts the indication by the DxoMark that the lens performs much better in the full frame camera.
It would be nice if Dxo would comment on this issue and clarify any misunderstandings.