How does reducing pixle size work?
- Thread starter Robbo
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it uses a range of algorithms, different ones are good for different purposes. Photoshop gives you a choice.
Sure someone with several degrees in further maths could give you a better explanation...
Sure someone with several degrees in further maths could give you a better explanation...
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robbo,
you're combining quality and size and they are two different things. Size is how many pixels in dimensions of the picture. Quality is how many 'blank' pixels there are. Let's leave the size the same for now (large jpeg). When the camera takes a picture it uses all of the pixels in the sensor. When it converts to jpeg, it then throws away a number of pixels. How many and which ones depend on the processing and quality level. For instance, let's say you have a 3x3 grid of pixels. They all have roughly the same RGB values. On highest quality, the conversion will keep all 9 pixels. On a very high quality, the algorithm might eliminate the middle pixel from the file. When the opening program opens that file, it is missing the data for the middle pixel. So it makes a guess based on the pixels around it. If all of the pixels are similar, then it is an easy guess.
When you start using a lower quality, then maybe the algorithm deletes 4 of the 9 pixels. Again, the opening program will make some guesses about what data should be there.
Each time the opening program guesses, it may or may not guess correctly so it will make some slight differences between the new and original. When you save the new, you're saving the 'guesses' as the new data and the cycle continues. Not so much a problem for pictures with smooth gradients of color, but for sharp differences you'll see changes occur.
Just shoot RAW and use a non destructive editor and be done with it.
thanks
rick
you're combining quality and size and they are two different things. Size is how many pixels in dimensions of the picture. Quality is how many 'blank' pixels there are. Let's leave the size the same for now (large jpeg). When the camera takes a picture it uses all of the pixels in the sensor. When it converts to jpeg, it then throws away a number of pixels. How many and which ones depend on the processing and quality level. For instance, let's say you have a 3x3 grid of pixels. They all have roughly the same RGB values. On highest quality, the conversion will keep all 9 pixels. On a very high quality, the algorithm might eliminate the middle pixel from the file. When the opening program opens that file, it is missing the data for the middle pixel. So it makes a guess based on the pixels around it. If all of the pixels are similar, then it is an easy guess.
When you start using a lower quality, then maybe the algorithm deletes 4 of the 9 pixels. Again, the opening program will make some guesses about what data should be there.
Each time the opening program guesses, it may or may not guess correctly so it will make some slight differences between the new and original. When you save the new, you're saving the 'guesses' as the new data and the cycle continues. Not so much a problem for pictures with smooth gradients of color, but for sharp differences you'll see changes occur.
Just shoot RAW and use a non destructive editor and be done with it.
thanks
rick
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The simple answer to this is that at the time of capturing the camera sensor will capture all the pixels. Then depending on your setting it will resample the image data so going for a let's say medium size the camera will delete half the pixels hence the less mp. But one thing to remember is that when this happens the image size will change and not quality. So for example a 12.7 mp image is 4368 × 2912 pixels and when the image size is reduced to half (6.3 mp) the image size will be 2148 x 1456.
It is always suggested to shoot at the highest mp the camera allows because you can easily resample a photo in an image editing program and make it smaller later.
There is no difference in shooting RAW and jpeg when it comes to image size you can shoot medium quality RAW. The difference between RAW and jpeg is the information that is there for each pixel, in RAW all the details will be stored individually whereas in jpeg the information will be compressed in to the image.
It is always suggested to shoot at the highest mp the camera allows because you can easily resample a photo in an image editing program and make it smaller later.
There is no difference in shooting RAW and jpeg when it comes to image size you can shoot medium quality RAW. The difference between RAW and jpeg is the information that is there for each pixel, in RAW all the details will be stored individually whereas in jpeg the information will be compressed in to the image.
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Thanks for the answers, I shoot raw all the time.
My reason for asking is my gf has just bought a point and shoot, and the guy told one (for one of the features) that when shooting low light, it can look grainy with high iso, and this can depend on how many pixels are on the sensor, more pixels on the same size sensor, more grain. And that the camera has this feature when shooting low light, it will go from 12mp to 6mp as it combines 2pixels to create one, giving less noise??
Not sure on the theory he gave her is true for this kind of camera?
I was then wondering if the same would be applied to a dslr, but as you point out the camera uses all pixels then compresses it, kinda obvious now lol.
My reason for asking is my gf has just bought a point and shoot, and the guy told one (for one of the features) that when shooting low light, it can look grainy with high iso, and this can depend on how many pixels are on the sensor, more pixels on the same size sensor, more grain. And that the camera has this feature when shooting low light, it will go from 12mp to 6mp as it combines 2pixels to create one, giving less noise??
Not sure on the theory he gave her is true for this kind of camera?
I was then wondering if the same would be applied to a dslr, but as you point out the camera uses all pixels then compresses it, kinda obvious now lol.
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It is partially true. What it is doing is it is averaging the values of the two cells. This means that there is less apparent noise, but in reality, the accuracy of the recorded value is (likely to be in almost all cases) worse than if you hadn't used the feature.
In theory, when you use the noise reduction feature on your RAW file, it is doing a similar thing, but, only for those pixels which differ from all of their neighbours by an algorithm determined amount.
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OK, some haldff truths in this thread 
As to compression, JPEG compression works by taking a small (8x8) section of the image and working out what detail is in the image and then depending on the quality setting throws away the finest detail. The lower the quality setting, the more detail is thrown away.
Raw also uses a compression, but that is lossless (JPEG is lossy). Kind of like a zip file does for computers.
Yes. This is 100% accurate. The smaller the pixel size on the sensor, the worse noise performance it will have in low light. This is because the signal (light hitting the sensor) to noise (from the heat and electronics in the sensor) ratio gets smaller the smaller the pixels are. Lower signal to noise ratio means it's easier to see the noise -> more grain.
Yes, it will give less noise, but probably not much and may be less than the equivalent of halving the ISO speed (i.e. moving from 800->400 for example). Have a look in this thread: http://www.talkphotography.co.uk/forums/showthread.php?t=303432 for photos of the same thing on a 5Dmk2.
Yes, it does. See the thread above.
As to compression, JPEG compression works by taking a small (8x8) section of the image and working out what detail is in the image and then depending on the quality setting throws away the finest detail. The lower the quality setting, the more detail is thrown away.
Raw also uses a compression, but that is lossless (JPEG is lossy). Kind of like a zip file does for computers.