If Nikon doesn't get their act together on DX...

Started Feb 7, 2013 | Discussions thread
jfriend00 Forum Pro • Posts: 11,326
Re: Who are "we"...

altair8800 wrote:

JimPearce wrote:

And what is it "we want to do" Dan?

We who disagree with your statement " A 70-200 f2.8 on DX is a 105-300 f4 equivalent" and want to disprove it. For exposure, it is a f2.8 lens in each case.

Here is a statement from another "equivalence expert":

Joseph James says:
"Consider two cameras, one with a sensor twice the size (four times the area) as the other. If we were to shoot the same scene with each camera from the same position, using the same lens, focal length, f-ratio, and shutter speed, crop the middle 25% of the photo from the camera with the larger sensor, then the resulting photo would be equivalent to the photo taken with the smaller sensor camera when displayed at the same size. If the sensors had the same efficiency and pixel density, then the crop from the larger sensor system would look exactly the same as the photo from the smaller sensor camera."

Do you agree with that statement? Why not?


f/2.8 is the same everywhere from a pure exposure point of view (recorded image brightness) regardless of sensor. But, it's not the same from a DOF or noise point of view when used with different size sensors. This is where the concept of equivalence comes from.

For example, a 105mm f/4 at 1/125 and ISO 200 on an full FX sensor will give you the same DOF and noise as a 70mm f/2.8 at 1/125 and ISO 200 on a DX sensor when you frame the same subject with the same framing for the whole sensor you are recording the image on.

If you want the same DOF on DX that you get on FX for the same whole sensor framing, you need a wider aperture on DX. If you want the same noise on DX as FX at the same shutter speed, you need a wider aperture on DX.

If you want the same framing on FX from the same subject distance as DX, then you need 1.5 more focal length pm FX.

There's no laws of physics being broken here - this is just how framing and noise work differently on different cropped sensors so people have come up with a notion a way of thinking about how you would get equivalent noise and framing between DX and FX. It's the same reason that an f/2.8 lens on a camera phone has tons of DOF, but the same aperture on a D800 has thin DOF.  If you wanted equivalent DOF on a camera phone, you'd have to change some parameters because f/2.8 doesn't work quite the same on a camera phone sensor as it does on a D800 when it comes to noise and DOF.

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