High megapixel crop mode

Started 1 month ago | Discussions thread
FingerPainter Senior Member • Posts: 7,610
Re: High megapixel crop mode
6

Doug Larvey wrote:

Hi Fingerpointer.

The shallow depth of field yes.

The noisy photo. Nope

Doug,

The noisiness of a photo depends primarily on the amount of light captured in it. If you are cropping to any significant extent, you will have thrown away most of the captured light. What remains will be noisier than the uncropped photo.

If you crop so the width and height of the crop are only 1/4 the width and height of the full image, the crop will only have about 1/4 the SNR of the uncropped image.

How noisy an image looks depends on its Signal to Noise Ratio (SNR). The higher the SNR, the less noisy the image appears. The amount of noise in light (shot noise) is equal to the square root of the signal (the amount of light). This means that the SNR is also equal to the square root of the amount of light. Capture more light and the SNR goes up. The image will look less noisy.

You are aware that cameras with smaller sensors produce noisier images, at the same exposure, aren't you? That's because the same exposure means the same amount of light per square mm of the sensor, and a larger sensor has more square mm so it captures more light.

Perhaps this analogy will help you understand.

Imagine a choir in which a number of singers equal to the square root of the total number of members in the choir sing off key. All the other choir member sing on key.

If the choir only has four member, 2 singers will be off-key. You will hear that for sure. If the choir has 16 members, four of the sixteen singers will be off key. There's still a pretty good chance you will notice them. Now imagine if you got 324 members of the Mormon Tabernacle Choir together and 18 of them sang off key. You almost certainly would not notice. They be drowned out by the other 306 voices. Even though more choristers are singing the wrong notes in the 324-voice choir than in the four--voice choir, the much greater number of voices singing the correct notes makes it harder to detect the variation.

That's how noisiness works in a digital image. When you crop an image down to a small part of the original, it is like picking a four-voice choir out of the mass choir - you are going to hear the wrong notes (see the noise) much more readily.

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