Noise and sensors – Canon A720 vs A650

[douglasM6]

A720 sensor size (diagonal) = 1/2.5”
A720 is 8 Mpixels

A650 sensor size (diagonal) = 1/1.7”
A650 is 12.1 Mpixels

My questions are 2:

a) The A650 has the greater sensor area per MPixel. Does this translate to better image quality?
b) Take the A650 for example, 12 Mpixels. More Mpixels for these small sensors = more noise. However, if I choose a lower resolution for a photo, say 1600 x 1200, will this give me less digital noise?

In general, how do people rate the A720 image quality vs the A650 image quality? I need to choose between the two on this basis.

Thanks for answering these questions (as you can detect I am a novice).

Doug

[bascom]

a) The A650 has the greater sensor area per MPixel. Does this translate to better image quality?
b) Take the A650 for example, 12 Mpixels. More Mpixels for these small sensors = more noise. However, if I choose a lower resolution for a photo, say 1600 x 1200, will this give me less digital noise?

In general, how do people rate the A720 image quality vs the A650 image quality? I need to choose between the two on this basis.


a) A greater sensor area per MP should give better quality, but to what degree? In this case I think the A650 advantage is so small that it is hard to detect in photos, espec 4x6. Look at the galleries in the A630/A640 and A710 reviews here. Do the photos look any better with either camera? They look similar to me.

b) From what I have read lowering the resolution will not reduce noise. However, in the Fuji forum there is a guy who bought the F50 and said lowering the res did reduce noise. http://dcresource.com/forums/showthread.php?t=34902&page=3

[flash110]

A720 sensor size (diagonal) = 1/2.5”
A720 is 8 Mpixels

A650 sensor size (diagonal) = 1/1.7”
A650 is 12.1 Mpixels

My questions are 2:

a) The A650 has the greater sensor area per MPixel. Does this translate to better image quality?
b
Doug

As far as my maths goes, i think in fact that the A720 has a slightly greater sensor area per mpixel:
1/2.5 / 8mp = 0.05
1/1.7 / 12.1mp = 0.048

[Sintares]

http://home.comcast.net/~plutarch/Sensel.html

Scroll down to "Point and shoot pixel area values"

And you are aware that a sensor quoted as (for example) 1/2.5in, doesn't actually mean its 1/2.5ins (0.4in) across ?

Its an archaic measurement from the olden days of 50's tv tubes..

http://www.dpreview.com/learn/?/Glossary/Camera_System/sensor_sizes_01.htm

[douglasM6]

yes I have learnt this recently - thanx.

dPreview do list the actual sensor sizes and the A650 does have slightly fewer
(16 %) Mpix per area.

I think that consenus is that it does not make a discernable difference.

Regards,

Doug

[Graystar]

Lower resolution images appear to have less noise because more photosites are averaged into each pixel. This has the effect of reducing noise.

However, if you take a high res image and reduce the size, the same thing will happen. Several pixel values are averaged to give one pixel. If the color of one of those pixels were off due to noise, then the noise will be blended away and the resulting pixel, while not perfect, won’t look so bad. Of course, the cost of this is reduced detail.

[bascom]

http://home.comcast.net/~plutarch/Sensel.html
Scroll down to "Point and shoot pixel area values"

Great link but I don't understand the math. How do they get 3.8 for the A650 pixel area? I know it's X/12.4mp = 3.8 but what is X?

Camera Sensor size Megapixels Pixel area (µm2)
Canon Powershot A650 1/1.7" 12.4MP 3.8
Canon Powershot A720IS 1/2.5" 8.3MP 2.6

And notice that the A650 has a larger pixel area than the A720, and thus less noise. But it's a small difference.

[Graystar]

http://home.comcast.net/~plutarch/Sensel.html

From the above link...
"Typically each sensel responds to either red, green, or blue, and a single "pixel" is comprised of four total: two greens, one red, and one blue. So if you have an 8 megapixel camera, it has 8 * 4 = 32 million sensels."

This is absolutely incorrect. Also, the calculations appear to have been performed with the effective MP of the camera, not the actual number of photosites. Finally, the photosites comprise about half the area of the sensor, with the rest dedicated to other electronics required to control each photosite.

I'm a bit dubious of the information on that page.