What technically happens when you shoot at a lower MP rating?

[crash331]

I am looking at getting a camera that is 10MP. I really don't need 10MP, in fact I think it would produce files that are too large. If I switch the mode to 6 or 8MP in camera, what happens? Does it just turn off the edges of the sensor, or does it do a software crop? Both seems it would upset the FOV.

Is image quality reduce (aside from the obvious reduction on MP)?

[Jon]

It merges actual pixels into "virtual pixels". You'll still get exactly the same picture, but it'll be as if you "downsized" or "resized" it in post-processing. So, taking an extreme case (for ease in math), a 10 MP camera shooting at 2.5 MP resolution will combine 4 pixels from the sensor for every output pixel.

[alan_potter]

This presumably improves noise at high ISOs, yes?

regards,
/alan

[watchtherocks]

No it does not, unfortunately. Well, in SLRs at least. The sRAW modes don't offer increased noise performance.

[Jon]

Originally Posted by alan_potter

This presumably improves noise at high ISOs, yes?

regards,
/alan

At least theoretically it will.

Originally Posted by watchtherocks

No it does not, unfortunately. Well, in SLRs at least. The sRAW modes don't offer increased noise performance.

sRAW, at least in the initial implementations, probably doesn't work the same way. Every JPEG pixel value is derived from the values of multiple adjacent sensor sites. Except in the case of Small JPEG, the JPEG image dimensions don't reflect a straight n:1 mapping of sensor sites to pixels, and even in that case deriving the colour for a pixel is best served by sampling the "real" adjacent sensor sites rather than by choosing every nth sensor site as a contributor, or by computing the entire image at native size and resampling it. An sRAW image (excluding the sRAW1 of the 5D II and 50D) can be simply picked up by collecting 1 in 4 sensor sites for each colour. As sRAW1 on the newer cameras contains 1/2.25 the pixels of a full RAW image, you'd need to compute intermediate values for "pseudo-pixels" to minimize problems with blocking and "jaggies" due to pixel misalignment from sampling 4 of every 9 sensor sites. Given that need, it'll be interesting to compare noise levels of sRAW 1 and sRAW 2 on these with those of sRAW in the earlier cameras.