Why is there a variance in pixels in my photos?

[brock7105]

This may be a really stupid question but i cant figure it out for myself...

I have a 400D with a 700-200 f4 L attached ,and I have it set on file size large/fine, which I believe is its highest level of recording other than shooting in RAW.

My problem is some photos I take come out at 8-9 megapixels while others taken at the same location, time etc are only 2-3 megapixels... I dont think I doing anything different.

Is my only option to shoot in RAW and will this give me a constant file size?

Any advice would be muchly appreciated

 

[Toothie]

IIRC file size will depend on how many pixles there are of the same colour, the more colours the bigger the file size.

 

[bigbigblue]

The varying file size is due to the JPEG compression applied to the image by the camera's firmware. The compression algorithm will be able to compress to much smaller sizes images with large areas of constant colour (such as a uniformally blue sky) than one which has lots of detail.

The changing file size is NOT an issue, just a result of the content of the image and the compression algorithm. The smaller images are just as good "quality" as the larger ones, it is just the compression algorith has been more successful at compressing it.

If you convert the images to a lossless file type (such as uncompressed TIFF) you will notice the image files are much closer in size.

Shooting RAW has many advantages in post processing (such as more exposure lattitude and the ability to adjust white balance after the fact) and I shoot RAW all the time for that reason alone, but if you are happy with the results you achieve then stop worrying about the file size

Hope that helped.

 

[arclight]

I think that some colours use more pixels than others, if my memory is correct.

 

[brock7105]

Thanks for your replies, that explains a lot. Ive had a look at some of my pics again and it is the ones that have a lot of uniform colour ( i shoot quad bike motocross so there is alot of blue skies ) that seem to have the smalller file sizes.

 

[Richard King]

imagine photographing a perfect white background.. how many KB would it take to explain how to reproduce that - not many - you could literally say: "take the colour of the first pixel, and repeat it for the next 6000"
on the other hand - shoot a intricate street sceen in full colour - that would take some more describing. JPEG compression, tries to do scenario 1 as much as possible

 

[Garry79]

imagine photographing a perfect white background.. how many KB would it take to explain how to reproduce that - not many - you could literally say: "take the colour of the first pixel, and repeat it for the next 6000"
on the other hand - shoot a intricate street sceen in full colour - that would take some more describing. JPEG compression, tries to do scenario 1 as much as possible

Correct, and RAW would hold each pixel of the 6000 seperately.

I think the OP is confusing pixels with file size. As long as your setting remains the same, each image will have the same number of pixels but as Richard King rightly says the file size will vary depending on the scene.

 

[oldgit]

I think that some colours use more pixels than others, if my memory is correct.

Sorry that's wrong. Each pixel has the same number of bits associated with it at the time of taking. Its what happens afterwards in the compression that causes the difference.

imagine photographing a perfect white background.. how many KB would it take to explain how to reproduce that - not many - you could literally say: "take the colour of the first pixel, and repeat it for the next 6000"
on the other hand - shoot a intricate street sceen in full colour - that would take some more describing. JPEG compression, tries to do scenario 1 as much as possible

That's almost right. Except it's not how JPEG works.

You'll find that modern RAW files do vary in size (a few of the really old/original cameras used to be Xpixels * Ypixels * Nbits per Pixel). They use "lossless" compression so that as RK says taking a shot of a pure white background or a Macaw in a flower garden will result in different file sizes, but they will not have lost any information.

JPEG is not lossless. It uses clever algorithms to chop up the image into lots of little bits and then says that "this" and "that" bit are "near enough the same" so it will only store one of them (and forget the other and its differences). You can see this for yourself if you crank the accuracy slider all the way down when saving jpegs. They get kinda square blotchy blobs all over them.

 

[Richard King]

lol - that was my realy simple answer. far too early for complex maths

 

[arclight]

Sorry that's wrong. Each pixel has the same number of bits associated with it at the time of taking. Its what happens afterwards in the compression that causes the difference.
QUOTE]

Cheers - my memory was not correct then.