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View Full Version : drop resolution in point and shoot to reduce fringing?




pna
Aug 19, 2008, 11:59 AM
Hi all,

My basic question is: I know that the the newer point and shoot cameras that have 8 or more megapixels shoved into a tiny sensor are more prone to noise and purple fringing, more so than, say the 5 megapixel cameras that were top of the line not too long ago. If I drop the resolution to shoot at 5 megapixels, will that effectively give me the same picture (with less purple fringing) as the older cameras? Will it just use more pixels on the CCD to sample the image, and use them all in generating the 5 MP image?

The back story is this:
My Canon powershot A610 just stopped working with the dreaded E16 error. I loved that camera for lots of things, most notably the flip-out screen, and the 5.1 megapixels were actually plenty for taking some fantastic landscape shots when I didn't have my dlsr, and they looked great even blown up.

Now that it's done, Canon is offering me the 'canon loyalty program' option to 'upgrade' to a SD1000, or an SX100 (10X optical zoom) at a significant discount. Both of these have greater resolution on the same (or smaller) size sensor, and I'm a little concerned about the fringing. I think I'd be totally happy using them and dropping the resolution of the images, if that would take care of the issue.

Thanks for any advice you all have.



KidneyPi
Aug 19, 2008, 12:44 PM
Can you point me to an example of this fringing? The only thing I can think of that would be like a colored fringe would be chromatic aberration. That is where the different wavelengths of light are in focus at different points. This is caused by a poorly designed or damaged lens and has nothing to do with the sensor.

Backlighting of the subject could cause something similar too, though I would imagine the light would have to be gelled or the white balance would have to be horribly wrong.

In either case, having not seen any example images, I would have to say this is an example of the photographer using faulty equipment or not knowing how to properly use what he has. The only way I can imagine anything like what you describe being caused by the sensor having smaller pixels is if somehow the pixels were small enough that a single photon could completely cover one. Since photons are what we call very, very tiny, there is no way one could make off with a whole leg, er, completely cover a whole pixel.

Westside guy
Aug 19, 2008, 12:52 PM
Dropping the resolution won't accomplish what you want, since the basic photosite size stays the same. I'm not sure what pixel size has to do with "purple fringing" (maybe it does; I just haven't heard that), but it does of course affect low-light noise performance and diffraction.

I don't suppose Canon would just repair the A610? :D

pna
Aug 19, 2008, 01:11 PM
Can you point me to an example of this fringing? The only thing I can think of that would be like a colored fringe would be chromatic aberration. That is where the different wavelengths of light are in focus at different points. This is caused by a poorly designed or damaged lens and has nothing to do with the sensor.

Backlighting of the subject could cause something similar too, though I would imagine the light would have to be gelled or the white balance would have to be horribly wrong.


I haven't seen the effect in any of my previous shots, but had heard it was a potential issue as they crammed more and more pixels into the same size sensor. I interpreted it as a noise issue -- that once you increase the pixel density to a certain amount, that noise at the transition between items of high contrast results in purple fringing around the transition.

There are some examples towards the bottom of the page on the dpreview sx100 review here:

http://www.dpreview.com/reviews/canonsx100is/page5.asp

It's not an effect I'm overly worried about, the real question is if you can avoid it completely just by dropping the resolution. I had thought that, by dropping the ultimate resolution, it would use more pixels any given pixel in the end image, thereby increasing the signal/noise ratio.

Canon can fix the A610, but for the same cost (or more) as one of the newer SX100 or SD1000's...

ChrisA
Aug 19, 2008, 02:12 PM
I had thought that, by dropping the ultimate resolution, it would use more pixels any given pixel in the end image, thereby increasing the signal/noise ratio.

Some astronomical cameras can be operated in a reduced resolution mode where the pixels are combined ('binned" is the term) into larger pixels right on the CCD chip. The pattern used to shift charge on the CCD is changed and the charge is combined before it is digitized. But this technique only works on a monochrome CCD camera. All consumer cameras use a color mosaic filter such that adjacent pixels have different color filters over them. Because of this resolution reduction must be performed by means of digital image processing, you can't actually change the size of the pixels on the sensor like you could with a monochrome camera.

So if you are going to reduce the resolution you may as well do it after the fact in Photoshop where you have more control over exactly how the image is down sampled.

KidneyPi
Aug 20, 2008, 12:10 AM
There are some examples towards the bottom of the page on the dpreview sx100 review here:

http://www.dpreview.com/reviews/canonsx100is/page5.asp

It's not an effect I'm overly worried about, the real question is if you can avoid it completely just by dropping the resolution. I had thought that, by dropping the ultimate resolution, it would use more pixels any given pixel in the end image, thereby increasing the signal/noise ratio.

The page you linked gives a much better explanation of the problem. They say it is caused by large differences in contrast. I'd go a bit farther. I'd say it is that, combined with the fact that digital cameras in general do terrible with highlights. Put a highlight right next to a dark area and the camera just has to guess what is there. The Canon sx100 apparently does a poor job of guessing. Some are better than others at this. If you are worried about it, I'd check dpreview.com for this problem with any camera you are interested in or just avoid shooting into the sun. It has nothing to do with the number of pixels being used.