Oly, perhaps a new player in this market segment

Started Aug 18, 2012 | Discussions thread
crames
Regular MemberPosts: 181
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Re: Oly, perhaps a new player in this market segment
In reply to bobn2, Aug 23, 2012

bobn2 wrote:

michaeljones wrote:

"If I crop in post and enlarge to the same size, then I get a noisier image than using the full frame. If I had set my shutter speed to the minimum that gave me an acceptably noisy photo, I will now find my output image is more noisy than acceptable, and will be wishing I had used a slower shutter speed or wider aperture when making the capture."

Bob, you wrote the above before.

Yes, I wrote that because it is correct, according to optical physics.

Bob, when I crop and enlarge, the image usually looks smoother, rather than noisier. Where is the increased noise coming from when you enlarge? If not done badly, I would expect that enlarging/zooming/interpolating would neither increase nor decrease noise.

Do you honestly think that cropping your image in post changes the number of photons that were incident on any given pixel at the time of capture?

No, of course it does not, but anyone who understands just a little bit about the process of capture of an image would realise that the 'number of photons incident on any given pixel' is not what determines the SNR of the image as presented enlarged to the same size (see what I actually wrote). What determines the relative SNR is the number of photons captured in like portions of the image, and a single pixel in the cropped image represents a greater proportion of the (same size) final image than does a single pixel of the image made from the full sensor. Imagine for instance we make an image from a 36MP FX sensor and one from the 15MP DX crop, at the same exposure and enlarged to the same size. Say that the exposure used represents 10000 photons per pixel. In the 15MP image each 15 millionth sample represents 10000 photons, and therefore the SNR measured over a 15MP bandwidth is 100. In the image from the full sensor each 15 millionth sample represents 24000 photons, so the SNR over the same bandwidth is 156.

What's confusing is that your example is not a demonstration of cropping and enlarging. It appears to be a down-sampling of the 36MP image to the size of the 15MP crop. Otherwise, how do you get from 10k to 24k photons? Of course the down-sampled image will have higher SNR (less noise) compared to the untouched 15MP crop, due to the combining of pixels.

While down-sampling can reduce noise, it doesn't follow that up-sampling increases noise.

Can you present a scenario that illustrates how noise increases in an enlarged crop?

Cliff

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