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Old 01-22-2013, 09:06 PM   #1
Skibble
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Question Maximum Image Size (in KB) to Use for Kindle Formats?

For those who don't need a background story to go with the question, I'll get straight to it: What is the maximum file size (in KB, not dimensions) that can be used for images in Mobi files? What about AZW3 files?

For those who are willing to read a background story -- who knows, it might provide some useful context -- here goes.

I owned a Sony reader for years, but am now converting my library to use on a Kindle Paperwhite. Some of my old ePubs have images in them that are ~250 KB. They always showed up on my Sony reader at full quality -- or at least what appeared to be full quality to my middle-aged eyes. However, so far my experiments in converting these ePubs to Mobi with Calibre have resulted in a noticeably lower level of image quality. Artifacts, blurriness, loss of detail -- not exactly what I like to see when referencing a historical map or painted portrait, even if it is in black and white.

I've done my best to research the issue, and have learned that images with "large" file sizes are compressed during conversion to Mobi. However, I cannot figure out what constitutes a "large" file size. What is the "magic number" of KB over which the images will get compressed, and under which they are left alone? I found a reference to 127 KB being the limit, but I also found a reference to 250 KB being the limit.

I would be most grateful to anyone who can teach me. My goal is to downgrade the "large"' images on my own, to maximize their quality while still appeasing the Kindle. Obviously, knowing my upper limit would be terribly helpful.

Last edited by Skibble; 01-22-2013 at 09:16 PM.
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Old 01-22-2013, 09:37 PM   #2
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In Calibre, try changing the device output type to tablet. By selecting tablet, the images will not be changed. They will stay as is. I have no idea if they will work or not, but it won't hurt to try.
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Old 01-31-2013, 01:39 PM   #3
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According to Amazon Kindle Publishing Guidelines it depends on book format:
Quote:
3.5.2 Image Guideline #2: KindleGen Performs Automatic Image Conversions
The limitation on the image size depends upon the book’s format:
- Reflowable books (fiction and non-fiction): 127 KB maximum
- Fixed-layout books (other than comics): 256 KB maximum
- Comic books: 800 KB maximum
But then they also recommend saving in 300dpi at a minimum size of 600x800.... who knows??

I haven't seen anything different for .azw3 ???
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Old 01-31-2013, 02:07 PM   #4
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Quote:
Originally Posted by Turtle91 View Post
According to Amazon Kindle Publishing Guidelines it depends on book format:


But then they also recommend saving in 300dpi at a minimum size of 600x800.... who knows??

I haven't seen anything different for .azw3 ???
300 dpi is meaningless for display; it's a print setting. If you print it, 600x800 at 300 dpi is 2" x 3.something".

You can test this in an photo editing program. if the file is 600x800, changing from 300 dpi to 220 dpi or 72 dpi or anything else doesn't change the file size at all.
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Old 01-31-2013, 02:12 PM   #5
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Thanks for that info, Turtle, and also your suggestion, JSWolf.

I've done a bunch of experimenting, and have gotten the best results by converting the ePubs to AZW3. My primary test file was an ePub with a cover image that was 322 KB. The image doesn't appear to have been downgraded at all in conversion to AZW3. I stress the word appear, as I cannot confirm what is (or isn't) happening behind the scenes during conversion. The results speak for themselves, though. I cannot discern a difference between the cover in the AZW3 file and the original image, except for it being displayed in black-and-white. This was not the case when I converted the ePub to Mobi; the Mobi image was very obviously downgraded.

I'm going to proceed with converting everything to AZW3 without tweaking the original images in the ePub files. I'm relieved that I won't have to bother with that latter step -- I have a lot of books with illustrations!

Last edited by Skibble; 02-05-2013 at 03:31 PM.
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Old 01-31-2013, 02:43 PM   #6
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Quote:
Originally Posted by derangedhermit View Post
300 dpi is meaningless for display; it's a print setting. If you print it, 600x800 at 300 dpi is 2" x 3.something".

You can test this in an photo editing program. if the file is 600x800, changing from 300 dpi to 220 dpi or 72 dpi or anything else doesn't change the file size at all.
Wow! My day is complete - I have learned something new!!

I was always under the impression that the same image dimensions at a higher number of dots per inch would equate to a larger file size. But then I was thinking image dimensions as "6 inches x 8 inches" not 600x800 ...

Thanks!
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