I have been wondering if the following is true and the implication for mirrorless cameras. DSLR camera sensors are only exposed to light when the picture is taken, so the sensor presumably produces less noise than if it was always exposed to the light. I find support for this in the way Canon warns about Live View extended usage. Is this true?

If this is true, then how do mirrorless camera deal with the extra noise from the sensor?

DSLR camera sensors are only exposed to light when the picture is taken, so the sensor presumably produces less noise than if it was always exposed to the light. I find support for this in the way Canon warns about Live View extended usage.

It's not the exposure to light that induces that Live View warning, it is the heat generated by the sensor being continuously energized. This heat is generated whether there is light falling on the sensor or not.

In the case of a camera being used in Live View, it's also exacerbated by the heat being produced by the camera's processor (almost always located closely behind the sensor inside most DSLRs and MILCs) that is providing 15-30 fps to the LCD screen on the back of the camera, the heat being produced by the always-on LCD screen, and the heat being produced by the battery providing energy to the sensor, the processor, and the screen.

This was particularly a problem a decade ago when DSLRs with Live View first started appearing. For instance, when some production companies first started using the Canon 5D Mark II for shooting television shows (most famously House), they had to swap out camera bodies after about 10 minutes of use due to the heat affecting the image quality.

As sensors have improved, it has been less of an issue. But a warmer sensor will always produce more read noise than the same sensor at a cooler temperature. That's why astrophotographers spend (tens of) thousands of dollars on actively cooled imaging sensors.

Canon tends to be very conservative with what they promise their customers about the capabilities of their products. In actual usage they often exceed those published performance parameters. Based on years of observations of the way Canon does business and the way they present their products to customers, my guess is that Canon still includes the warning lest any customer get upset that their camera is noisier when used with the sensor energized in Live View for several hours and then accuse Canon of not warning/informing them of the fact that image quality will be reduced as the sensor is heated.

There have been other camera companies that rush products to the marketplace without a vigorous testing program first. Some of them have had serious heat-related issues in the past due to small, compact cameras that couldn't passively shed heat as fast as they were generating heat.

  • House like in House. M.D.? – Alexander von Wernherr Aug 9 at 13:54
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    @AlexandervonWernherr Yes. Well, sort of. I just looked it up. The entire season finale in 2010 was filmed with Canon 5D Mark II cameras (rather than the 7D). – Michael Clark Aug 9 at 13:58
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    Typo in penultimate paragraph: s/fro/for/ (not enough rep here for such a trivial edit). – Toby Speight Aug 9 at 19:46

To my knowledge and experience sensors don't produce more or less noise by being exposed continuously allowing the use of live view.
The exposure time for actual pictures is still exactly the same, and so is the amount of noise generated.

I have used a Canon DSLR, a Fuji Mirrorless and currently a Sony SLT.
The last two continuously use live view.
Comparing shots between those I see no noise difference not explained by newer and better technologies.

Quoting from CANON EOS 80D knowledge base:

Image Quality
-When you shoot at high ISO speeds, noise (such as dots of light and banding) may become noticeable.
-Shooting in high temperatures may cause noise and irregular colors in the image.
-If Live View shooting is used continuously for a prolonged period, the camera's internal temperature may rise, and image quality may deteriorate. Always exit Live View shooting when you are not shooting.
-If you shoot a long exposure while the camera's internal temperature is high, image quality may deteriorate. Exit Live View shooting and wait a few minutes before shooting again.
(Emphasis mine)

As you see the issue is with rising internal temperatures because of the continuous processing required.
Camera's designed for continuous live view are likely less sensitive to this issue.

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    In the Canon warning they specifically mention that image quality will deteriorate. – John Smith Aug 9 at 13:16
  • So it does, see my edit – HTDutchy Aug 9 at 13:23
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    Yes, visual noise can increase with temperature at the time of image capture, which is why, for example, precise measurements of visual noise are performed in temperature-controlled rooms. However, this is a different issue than the idea that sensors in mirrorless cameras are collecting noise between image capture moments. – Alexandra Aug 9 at 13:45
  • On the subject of temperature, see photo.stackexchange.com/questions/79669/… – HTDutchy Aug 9 at 13:49

The sensor noise you are asking about is temperature induced and not the type induced by the signal amplification (using higher ISOs).

Since both type of cameras are designed differently and technology has advanced quite a lot, both sensors are designed to have non o extremely low noise with when working within a wide range of temperatures.

In a photographic camera with a mirror sensor (SLR) is only exposed to light when while the electromechanical shutter plane opens and close at max. resolution to take a shoot, so the dissipation of the sensor is not generally so optimized.

Mirrorless cameras have the sensor exposed to light during all the time you have the camera on, and without a lens cap. But it is not taking pictures to do it so it uses a much lower resolution than when you take a shoot and the sensor is designed with more optimized dissipation at those resolutions to maintain its temperature well below it maximum operating temperature. This is the mode used while recording 1080p or 4K video.

But when you take a shoot, the mirrorless will enter in full resolution mode (stressing the sensor like the ones in DSLR) so the heat noise from each system, ceteris paribus, will be the same or even better in when using a premium mirrorless to shoot relatively very long exposure/s.

The exposure will be made, depending on the quality of the camera and the exposure speed, by clearing the sensor just before an electromechanical shutter plane opens and closes or if using extremely high exposure speeds will just reset and read the sensor extremely fast.

Lower quality cameras will just use the latter method because they don’t have electromechanical shutter planes.

As a matter of fact, today most full frame sensors (on any brand) are developed and manufactured by just one company (Sony), who is considered by the industry to bethe best and most advanced developer and maker of FF sensors in the world.

Sony’s highest quality cameras are all mirrorless. So don’t be afraid, you are in good company.

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