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:<math>\frac{P Q_e t}{\sqrt{P Q_e t}} = \sqrt{P Q_e t}</math>.
Apart from the quantum efficiency it depends on the incident photon flux and the exposure time, which is equivalent to the [[Exposure (photography)|exposure]] and the sensor area; since the exposure is the integration time multiplied with the image plane [[illuminance]], and illuminance is the [[luminous flux]] per unit area. Thus for equal exposures, the signal to noise ratios of two different size sensors of equal quantum efficiency and pixel count will (for a given final image size) be in proportion to the square root of the sensor area (or the linear scale factor of the sensor). If the exposure is constrained by the need to achieve some required [[depth of field]] (with the same shutter speed) then the exposures will be in inverse relation to the sensor area, producing the interesting result that if depth of field is a constraint, image shot noise is not dependent on sensor area. For identical f-number lenses the signal to noise ratio increases as square root of the pixel area, or linearly with pixel pitch. As typical f-numbers for lenses for cell phones and DSLR are in the same range {{f/|1.5|2}} it is interesting to compare performance of cameras with small and big sensors. A good 2018 cell phone camera with a typical pixel size of 1.1 μm (Samsung A8) would have about 3 times worse SNR due to shot noise than a 3.7 μm pixel interchangeable lens camera (Panasonic G85) and 5 times worse than a 6 μm full frame camera (Sony A7 III). Taking into consideration the dynamic range makes the difference even more prominent. As such the trend of increasing the number of "megapixels" in cell phone cameras during last 10 years was caused rather by marketing strategy to sell "more megapixels" than by attempts to improve image quality.
===Read noise===
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Most consumer-level DSLRs, SLTs and mirrorless cameras use relatively large sensors, either somewhat under the size of a frame of [[Advanced Photo System|APS]]-C film, with a [[crop factor]] of 1.5–1.6; or 30% smaller than that, with a crop factor of 2.0 (this is the [[Four Thirds System]], adopted by [[OM System]] (formerly [[Olympus Corporation|Olympus]]) and [[Panasonic Corporation|Panasonic]]).
{{As of|2013|11}}, there was only one mirrorless model equipped with a very small sensor, more typical of compact cameras: the [[Pentax Q#Pentax Q7|Pentax Q7]], with a 1/1.7" sensor (4.55 crop factor). See section [[Image sensor format#Smaller sensors|§ Smaller sensors]]
Many different terms are used in marketing to describe DSLR/SLT/mirrorless sensor formats, including the following:
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|9.50||7.60||5.70||4:3||43.30||{{val|-4.32}}||4.55
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! scope="row" | 1/1.6" ([[F200EXR|Fujifilm
|10.07||8.08||6.01||4:3||48.56||{{val|-4.15}}||4.30
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|44.71||36.70||25.54||13:9||937.32||0.12||0.96
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! scope="row" | [[Red Digital Cinema Camera Company|RED]]
|46.31||40.96||21.60||17:9||884.74||0.03||0.93
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|54||45||30||3:2||1350||0.64||0.80
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! scope="row" | [[Pentax 645D]], Hasselblad X1D-50c, Hasselblad H6D-50c, CFV-50c,
{{cite web
|url=https://cdn.hasselblad.com/datasheets/x1d-II-50c/x1D-ii-50c-data-sheet.pdf
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|date=2019-06-01
|access-date=2022-04-09}}
</ref>
{{cite web
|url=https://fujifilm-x.com/global/products/cameras/gfx-50s/specifications/
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