DXO MARK sharpness scores.

[volks]

Since I have never heard about or read about this, I'm hoping someone can shed some light on this. Just out of curiosity I compared a couple of Nikon FX lenses at DXO Mark web site and found the following.

70-200 F4 on a D610 gets a sharpness mark of 20P mpix
When mounted on a D7100 the sharpness mark drops to 13P mpix

Similarly the 24-120 F4 on a D610 gets sharpness mark of 13P mpix,
and drops to 9P mpix when mounted on a D7100.

Am I missing something ?

[Franglais]

Here is the link to how they calculate the result:

Metric Scores - DxOMark

The mpix rating is an indication of how much detail is recorded on the image.

How much detail you actually get on your images depends on how much detail your sensor can record and how much detail your lens can resolve BUT it is less than either. There is a formula to calculate it but I've forgotten it.

Lets say you have a 24MPix FX sensor and a lens that is able to resolve 24Mpix of detail on a FX size sensor (when looked at through a microscope). You won't get 24Mpix of detail on your image you'll only get 13 Mpix (or whatever).

If you put the the same lens on a DX body with a 24Mpix sensor then you might expect to get only 6.5Mpix of detail (DX sensor is half the size of a FX one so for a given lens you expect half as much detail). But because the resolving power of the 24Mpix DX sensor is higher than the 24Mpix FX one you actually get more detail (lets say 9Mpix). Plus the edges are going to be better than on FX because you're only using the center of the image.

Addendum: I've found the calculation:

1/(system resolution) = 1/(lens resolution) + 1/(sensor resolution)

[volks]

Thanks again Charles. What you explained makes sense and I understand that part.
I guess ( as a novice ) what I found confusing is the term "sharpness".
The way I interpreted that data was that when those lenses are mounted on a DX camera that the photos are less sharp, like softer or not as clear. But that is obviously not the case, right ?

[Franglais]

Thanks again Charles. What you explained makes sense and I understand that part.
I guess ( as a novice ) what I found confusing is the term "sharpness".
The way I interpreted that data was that when those lenses are mounted on a DX camera that the photos are less sharp, like softer or not as clear. But that is obviously not the case, right ?

For the lens its the opposite. Put a FX lens on a 24Mpix DX camera and the lens will actually resolve more detail than on a 24Mpix FX camera. As the sensor on the DX camera is only half the size of the FX one, overall you are actually recording less image data. And the DXOMark score tries to indicate how much useful data you are getting out of a given combination.

You mentioned "softness" and "clarity". This raises a point about the sensor itself. Sometimes an image from a FX camera has a sort of effortless clarity that I've never seen a DX camera achieve. Photo-John thinks its something to do with light diffraction in the smaller pixels on the DX sensor. I need more experience with the D7100..

[volks]

As mentioned earlier, looking forward to your D7100 comments when returning from your trip.

[Franglais]

I've just written a review of the D7100. There isn't an entry yet for the 18-140.

I used the D7100 + 18-140 intensively in Stratford-upon-Avon and in Budapest. I tried lots of subjects - walking the dog in the street at night, old buildings in overcast conditions in black and white, birds, old building interiors, parties.

The combination exceeded my expectations. I was always delighted with the results. There was only one disappointment when the 18-140 showed some light falloff at the edges of the image. Haven't figured that one out yet.

When I got home and I put the images in with the others shot on the D200 and D300 - hummm - the D7100 ones don't really stand out as being better. Whereas some of my D800 images have something extra. They look clearer and nicer.

[volks]

Thanks for the update on the 18-140 lens Charles. Good to hear that it exceeded expectations. Will get one if I ever manage to sell my 16-85, but no takers so far.