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

Started Feb 7, 2013 | Discussions thread
altair8800
Senior MemberPosts: 1,202
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Re: Who are "we"...
In reply to jfriend00, Feb 9, 2013

jfriend00 wrote:

altair8800 wrote:

If you want the same noise on DX as FX at the same shutter speed, you need a wider aperture on DX.

You have to take pixel density into account. In the situation where the FF has fatter pixels, this is true. If the FF has 28Mp for same pixel density as D300 and same level of efficiency, then same noise with same lens.

OK, we're mostly in agreement, but this part of your supposition isn't right. Noise in an overall image is related to total photons collected not to pixel density. And an FX sensor collects way more photons than a DX sensor (because it's larger) and thus delivers lower noise.

For example, a D800 has about the same pixel density as a D7000, but much lower noise at the same exposure when the whole sensor is used in both cases.

You may be talking about per-pixel noise (where fatter pixels would appear to be better), but per-pixel noise is not an important characteristic in the visual perception of noise in a viewed image. If that was the case, then the D800 would have much worse noise than the D4 (because it has much smaller pixels than the D4 and worse per-pixel noise than the D4) but, the D800 does not produce images with worse visual noise in them. Further, fatter pixels make more visible noise. In fact, the impact of the D800 per pixel noise is reduced because it has such tiny pixels so the noise artifacts are smaller and more random and thus less visible in a printed or screen viewing of an image.

Nikon has proven with the D800 that they can make smaller and smaller pixels while still making overall images with less visible noise. Fat pixels aren't necessarily better than tiny pixels. It's the overall sensor size that really makes the bigger difference. Collect twice as much light (with a larger sensor) for the same image and you get half as much noise. It's really that simple.

Lets look at this again:

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."

He says  "would look exactly the same" which would include noise.

If we do his test with a D800, we would take a shot in DX mode, then another in FX mode without changing exposure or position. If we print both images at the same DPI, the FF image would have wider FOV but both would have the same DOF. If we crop the FF image to DX and print at same DPI, it will be identical to DX print including noise. The same pixels are being used.

If we want the DX image to have same FOV as full FF image, we must move back. We now have a wider DOF, not because of sensor size, but because of increased distance. To prove this, take FF shot at the same distance.

So, a f2.8 lens has same exposure and DOF when used with FF or DX at same distance and shutter speed. When you try to match the different formats, the different distance to the subject creates the DOF problem.

For the types of photography I do, shallow DOF is not wanted. I am just starting in macro, but it is common to stop down to tiny openings to increase DOF. For birding with long lenses, DOF is often too shallow as I do not want the bird's legs to be in focus, but not his eyes.

Dan

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