There isa good reason why DxO is looking at the four criteria
- Signal Noise Ratio
- Dynamic Range
- Tonal Range and
- Color Sensitivity
If you imagine these criteria being tetrahedron, the pivot point for what you want to do is somewhere between these four edges and implicitely what want to doand how you want to do it. The tetrahedron get reshaped depending on which equipment (camera) you use and which ISO you need to shoot with, where the most significant influence is created by ISO because it basically makes the tetrahedron smaller.
Although the values all four values decline with growing ISO values, because that's basically the physics of optical sensors. There are considerable differences to consider.
The second thing that they did for good reason is distinguishing measured vakue depending on the media you use for displaying the resulting photos. In many cases the differences between cameras / sensors are much bigger for active display, i.e. projection by menas of screens or beamers, than they are for passive display, i.e. print. I have seen pictures looking great as a large format print but were pretty much unusable for large scale display with a high res beamer.
IMHO for images that are downsampled and converted to sRGB for internet publications more or less any shot that is properly taken is good enough if looking at the modern DLSR generations with the exception of publishing hi res images or even native res images for download.
Because I wanted to get a good decision whether to replace my D750 and D7200 by one D850 or not and whether or not to keep my D4s for low light or not, I did some research along this line. Due to the fact that "you can tweak data by means of software, but you can't cheat physics", I prefer to avoid or minimze noise in the pictures as early as possible and this is at the time you take the image. Yes, I know and I love things like DxO Deep Prime, but I still prefer to use it only for stretching the limits if needed in cases where I can't reduce the noise to an acceptable level upfront, rather make it a default.
Keeping in mind that one of the key criteria from physical perspective is the size of a sensor receiving light and transforming it into an electrical signal - and fporm there into data - we need to look at the size of an individual pixel, because it is the one causing the noise.
I was comparing in two directions, one being the capability of the D850 replacing my D750 and D7200 and the second being the question of whether I can get any better regarding IQ with really high ISO. Leaving the fist part aside in this context here, I looked at my D4s, the D5 and the D850 (I would have taken the D6 but they don't have that in the database yet). Here are the charts:
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From the points where the camera lines cross the dotted red line you can see that the D5 is about 1 stop better compared to the D850 and the D4s another 0.7 to 1 stop better than the D5.
Regarding the dynamic range D4s and D5 pretty much on par from ISP 1600 onwards while the D850 is behind for high ISO, sometimes up to 1.5 stops.
Similar observation is made for the Tonal Range as well as the Color sensitivity, exceapt that thecurves are not that mixed up towards low IOS values.
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What does that mean keeping in mind the pixel size thing ?
- If you stay more or less within one generation of a sensor technology you can compensate part of the disadvantages of smaller pixels by means of more advanced in-camera software, but the smaller pixel can't substantially ovetake it s bigger counterpart.
In terms of sensor performance the D5 can catch up to the D4s in three of four discipline despite having higher resolution and thus smaller pixels and in the SNR discipline it stays behind its older counterpart, whose pixels are about 1/3 bigger.
Of course there are other criteria like AF speed, precision and accuracy in low light and there the D5 certainly wins, but from the sensor perspective there would be no point for upgrading.
- The D850 is for sure one of the best - if not the best - DSLR that Nikon has built so far, but as physics tells us, it has to pay a price for its monster resolution and the currency in which it is payed is pixel size. In three disciplines it is one stop behind the big single digit machines and it terms of noise it is 1.7 stops compared to the D4s.
That said, don't get me wrong here: I loved the D850 from the moment I took the first test shots after taking it out of the parcel for its IQ and versatility !
The conclusion was
- to keep the D4s for the situations where light gets really critical,
- to get the D850 for everything else and replace my D750 and D7200,
- keeping the chance to have to high end bodies usable in parallel if required including having the additional reach I used to have with my D7200, because the D850 provides already in DX mode about 20% more resolution than the D4S and only about 6% less compared to the D5
Again and always it all depends on you, what you do and how you do it. In terms of sensor capabilities it comes down to getting the pivot point of your work inside the tetrahedon defined by your camera and only if you find you can't get it inside you need to think about changing somthing.