Sensor sizes and f stops

Started Jan 25, 2014 | Discussions thread
Christof21 Veteran Member • Posts: 3,637
Re: No real change in noise (SNR) in final image

TTMartin wrote:

Christof21 wrote:

darklamp wrote:

The cameras have exactly the same algorithms to determine the aperture so that the amount of noise is about the same,

No. This will not work.

The camera will, using the same shutter speed and ISO and using the same metering system, will use the same f-number.

If you do not do this then one shot will be darker than the other and to equalize the shots you will have to brighten it. This is essentially equivalent to raising ISO and thus increasing noise relative to signal.

So you gain nothing by using a different aperture to equalize noise ( technically SNR ) instead of using the correct aperture.

Assuming you make both sensors the same way ( !! ) and they have the same number of pixel but different sizes, then the larger sensor should have less noise.

It will not depend on the number of pixels. You do not need to make this assumption. Noise does not depend on the pixel density.

Because of the way noise is generated in small sensors I would expect the difference to be less than e.g. doubling the size might lead you to hope. However it's very hard to make generalizations in these cases.

I would suggest that the difference between older sensors and newer ones ( small sensors ) is also less dramatic than people like to think. Large sensors have improved a lot, but there are reasons for that which do not translate so well into small sensors.

We can answer precisely to this by looking at the sensor efficiency. Dxomarks ratings is an excellent reference. Smaller sensors tend to be slightly more efficient but, in general, sensors efficiency does not vary with sensor size.

But, a larger sensor has more light reaching it. This can be illustrated with a full frame camera and and APS-C camera and the same full frame lens. A full frame Canon 6D camera has 864 square millimeters area that the light from the lens hits. An APS-C camera like the 700D has 332 square millimeters area that the light from the same lens hits. So even if both sensors have the same efficiency, the full frame camera will have over twice the performance, because it is collecting over twice the light.

Interesting . Take the same full frame lens but use a speed booster adapter with the aps-c  (more than 1 stop gain) and the advantage vanishes !!!

Just the proof of equivalence.

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