smaller sensor pixel size = more noise?

Started 2 months ago | Discussions thread
Michael Fryd
Michael Fryd Forum Pro • Posts: 15,935
Re: smaller sensor pixel size = more noise?
4

JohnS7 wrote:

Can we formulate a rule of thumb regarding relationship between sensor pixel size to level of noise?

with regular photo with all else equals bigger pixel size on sensor will deliver more light and less noise correct?

So phone vs APS-C is always going to lose with low light.

P.S. I am not touching on any smart solutions to conquer this problem the software way because they really don't work at night and if we talking about taking photos with exposures more than 1 second.

Smaller pixel sizes have more noise per pixel, but each pixel contributes less to the final image. The end result is that the visual noise in the final image is relatively independent of pixel size.

The largest factor in low light image noise is usually the shot noise. This is the noise inherent in the quantum nature of light. Shot noise goes down as you increase total light captured.

At the same angle of view, same shutter speed, same subject and same aperture diameter you get the same total light captured (and same depth of field).

If "by all else being equal" you mean same shutter, aperture diameter etc, pixel size and sensor size don't make much of a difference in image noise.

In practice, large aperture diameters are generally not available for smaller sensors. Therefore a shot taken on a smaller sensor may have a smaller aperture diameter, greater depth of field, and more image noise.

A common mistake is to compare cameras at the same angler of view, but different aperture diameters.

A full frame with a 50mm lens at f/4 has a 46° angle of view and a 12.5mm aperture diameter ("f/4" means the aperture diameter is the actual focal length divided by 4). A 2X crop body with a 25mm lens at f/2 also has a 46° angle of view and a 12.5mm aperture diameter.

The full frame with a 50mm f/4 yield the same results as a 2X crop body with a 25mm f/2.  That's the same depth of field, and same image noise, and same motion blur (assuming same subject and same shutter speed).

Now if you compare at the same f/stop, you will find that you have different depth of fields, and the image with the shallower depth of field has more total light, and less image noise.

Remember, the "f/stop" is not a measure of something.  It's the relationship between the focal length and the aperture diameter.  If you change the sensor size, you need to change the focal length to maintain the same angle of view.  When you change the focal length, but not the aperture size,  it's natural for the ratio between the two (the "f/stop") to change.

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The bottom line is that if you maintain the same depth of field, smaller sensors don't lose out.

Smaller sensors lose out in situations where you have to use a smaller aperture and accept greater depth of field.

In situations where you need to stop down the larger sensor for depth of field reasons, they offer no noise advantage over a smaller sensor (assuming you can match that aperture on the smaller sensor).

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