Trying to get my arms around this concept. A medium raw file for the Canon 7D is 8MB and 3,456 X 2,304. An SRaw file on a 50D is 12.6MB and 3,267 x 2,178. So the larger file size has smaller dimensions?
Also that same 50D raw file when exported using LR set to 80 will produce a 2.1MB jpeg file at the native resolution of 3,267 x 2,178. When the slider is bumped up to 95 the file size jumps 6.3MB but the resolution dimensions stay the same.
I guess my question is how does the file size get bigger if the dimensions don't change? Is it cramming more pixels (making them smaller) in to the same space? If so does this improve the quality for printing?
Also when it comes to making large prints (13x19 and up) I assume that I not only want a large file size (7MB+) but also big dimensions (5,000 x 3,000).
MCams wrote:
I guess my question is how does the file size get bigger if the dimensions don't change? Is it cramming more pixels (making them smaller) in to the same space? If so does this improve the quality for printing?
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when your talking about Jpeg you are talking about levels of compression. IE how much data is thrown away that cant be recovered.
I would say always keep the RAW (or original Jpeg) and work from that.
As a test try someting with alot of detail in it and save as a jpeg at a medium compression then open that and do the same again and then compare the 2 file sizes. the 2nd will be a fair bit smaller and you may notice a worse image (jaggies and such). do the same again and a few more times and view the results.
All of the above files will have the same pixel dimesions.
If you are going to work on and save an image to maybe work on again a Tiff file is a much better idea (or PSD)
What you may be missing is that RAW files are compressed, just like jpegs. But the RAW compression is "lossless" while the jpeg compression is "lossey". But in both cases, the amount of compression and resulting file size is highly dependent on image content.
An image with lots of fine detail will compress less than an image with lots of broad flat areas. For example, an image with a lot of blank blue sky will compress a lot.
The key number for prints is PPI (pixels per inch), often confused with DPI (dots per inch). So often confused that they have come to mean the same. DPI has lost it's original meaning.
You'll hear different arguments, but mine claims you need a minimum of 150 PPI for a good print. More is OK, less is bringing on trouble. So for a 13" by 19" print you need an image with a minimum of 1,950 pixels by 2,850 pixels.
