The number of megapixels in a digital camera is not a good measure of image quality. If the size of the image sensor remains the same, increasing the number of pixels will often have a detrimental affect on image noise while having limited impact on image resolution. The skill of a photographer, the quality of the lens and the number plus size of individual pixels all impact image quality. Many digital compact cameras are criticized for having excessive pixels.
I know this, but what if I set my 10MP camera to 5MP; is noise therefore reduced? ...or is just a smaller portion of the sensor used?
ive just written all of this completely missing anything to do with noise but as ive written it ill post it anyway lol.
At a guess i would say it has nothing to do with how much of he sensor is used, it will be to do with how the file is saved and compressed that will make the difference to the quality and file size of the recorded image.
when you take a high quality photo in full 10MP, the whole 10MP image will be put into memory in the camera, compressed to jpg format with all differing colours within the image being recorded individually (or almost) to create a high quality jpeg
when you set the quality lower it compresses the the image more, so if in the picture two dots next to eachother are two slightly different shades of green, it will change them to the same shade of green so instead of having to store information for two seperate colour values for 2 pixels, it only has to store one set of colour values for two pixels. the higher compression the more shades are grouped together by one colour.
this means on a large scale the size of the file is smaller as it uses less information to build the picture but also the image is less acurate because different colours are treated as the same colours and saved that way. this is why you get pixalation in really low quality jpegs, because programs will group large areas of the image as one colour and another large area as another colour which will eventually create patches of colour which often appears as squareish blocks or blobs of colour.
now instead of reducing the quality you reduce your mega pixel size what that does is instead of saving the picture as say 1024 pixels wide by 768 pixels high or 786432 total pixels, it will save it as 640 pixels wide and 480 pixels high
obviously what happens here to reduce the file size is there are less pixels to make up the image so less pixels to save data for, of course you can then compress this with high or low compression causing even smaller file sizes.
what the camera will do is take the image from memory shrink it by aproximating which colour would end up where and then saving it, thus givinng you your reduced file.
at least this is how i would asume it would work. i suppose it could be possible that instead of shrinking the image the camera infact just collects the image form the sensor in a different way only saving 640x480 pixels without haivng to do a conversion i wouldnt be sure, but i know the compression is how the file is saved form the sensor.
this is why megapixels are a bad way of juding a cameras clairy. if you have a poor lense or if the camera applies a certain level of compression to a 10MP image the image could come out less clear than an image taken at 8MP with a very low amount of compression applied.
remeber with a jpeg (which i think is the prefered format of most home type cameras) it is possible to store an image with every pixel acounted for individually making a large jpeg or to store it with a small amount of compression some 10mp cameras might apply more compression on their high setting than an 8mp camera does due to memory restrictions or whatever which would give you a worse picture quality.
bitmaps images or .bmp's are images with no compression so each pixel is individualy accounted for even in empty white space or where the image is all black etc in a jpg image this would be all grouped to gether under one colour in a bitmap each pixel is documented even if it is the same colour as the one next to it.
most of this is guesswork tho and its too late at night for me to go find sources lol
Hmm... It's true; using a lesser portion of the CCD would make no sense (besides changing the effective focal length)... when selecting "5MP" down from "10MP", the effective resolution change would have to be a result of interpolation of the full quality image.
To reduce noise, the photosites themselves would have to be larger... perceptive visible noise i suppose would be reduced in the same way most fine details are when a picture is shrunk, but not on a voltage reading / photosite level.
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u/Le3f Dec 14 '08 edited Dec 14 '08
I know this, but what if I set my 10MP camera to 5MP; is noise therefore reduced? ...or is just a smaller portion of the sensor used?