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12-21-2009, 10:39 AM   #1
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Pls Explain:3008x2000pixels = 2.5 meg files?

I'm trying to have a few 8x10s and 1 16x20 made with my K100D and I have the pixels set at 6Meg, quality at 3 stars (because my cpu photo viewer doesn't read RAW). When I am sending the files to the photo lab they are around 2.5 meg. Shouldn't they be 6 Meg or am I wrong thinking 6 million pixels is the same as 6 megabites of info?

Thanks in advance

B

12-21-2009, 11:04 AM   #2
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There's basically no reason to expect a calculation whose units are "pixels" to yield an answer whose units are "bytes", and in any case, the JPEG format also alters the equation.

A pixel in a JPEG file is actually three bytes (one byte each of red, blue, and green data), so an uncompressed JPEG (if such a thing existed, which is doesn't) from a 6MP camera would actually be 18MB, more or less. But JPEG files use compression to save space; that's how they work out to be so much smaller. The more compression, the smaller the file (and the lesser the quality). The way JPEG compression works, a savings of up to 10X or so is possible without making much of a noticable difference in quality.
12-21-2009, 11:04 AM   #3
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QuoteOriginally posted by bjstoys2000 Quote
I'm trying to have a few 8x10s and 1 16x20 made with my K100D and I have the pixels set at 6Meg, quality at 3 stars (because my cpu photo viewer doesn't read RAW). When I am sending the files to the photo lab they are around 2.5 meg. Shouldn't they be 6 Meg or am I wrong thinking 6 million pixels is the same as 6 megabites of info?
It would be >6Mb if the photo was in RAW (.PEF).

A JPG File is uses a lossy compression algorithm - and therefore the same photo (albeit with losses) is stored in much smaller file size.

Page 31 in the K100D manual shows the typical file sizes saved with different settings:

so a RAW file would be about 11-12Mb.
at 6Mp
Best JPG about 3.8Mb
Better JPG about 1.8Mb
Good JPG about 1.1Mb

You'll have noticed that the suggested average Best JPG size was about 3.8Mb - there's nothing wrong with your JPG -
Pentax estimates are probably on the conservative side - allowing for a photo with more/a lot of fine detail which tends to increase file sizes.
12-21-2009, 11:43 AM   #4
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Ok, that's all making sense now. Here's a follow-up question; Should I be using the RAW format instead of the three star (jpg) format when doing larger pictures? It's too late this time because the family pictures have already been taken as a jpg.
It seems to me that one of the dpreviews on my K100D said that there was little to no difference between the jpg. and RAW images. So just because the Raw images take up more "bytes" doesn't mean they are necessarily a much more detailed photo, right? ie. A 10meg Raw file at 6M resolution isn't 3x better than the 3meg jpg taken at the same 6M resolution.

12-21-2009, 12:33 PM   #5
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As the above posts have indicated, a jpeg uses compression to save file space. Depending on the subject detail, an image itself can vary in size (for instance, if you take a black photo with the lens cap on, the jpeg compression won't save each pixel's detail, but rather save the detail noting the black pixel, and for other pixels save detail stating to reference pixels around it - that is how I understand it - the information saved is kind of relative - if you have an area with similar color characteristics, those will be alloted the same value in order to compress information).
12-21-2009, 12:56 PM   #6
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The advantage of RAW is if you plan to do much PP - more than just small small adjustments to exposure or color. Then the additional info int he RAW file will pay off. Otherwise, it's just not going to matter. Shooting RAW doesn't magically give better pictures - it still has to be converted to the equivalent of JPEG in order to view it or print it. The only question is whether that conversion is done by the camera or by your computer. And depending on what settings you use and what software you have and what scene you are shooting and what your own personal tastes are, you might happen to prefer the camera's conversion or you might hapen to prefer the computer's, but wee're talking about tiny, msotly insignificant differences.

i've moved this, BTW to the Beginner's forum, because it seems more appropriate there.
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