JPEG: lower resolution or higher compression?

[SRH]

In an ideal world, I would save all my shots in RAW. However, on long trips I have limited file space available. To reduce file size I save shots as JPEGs, and I have a choice of selecting the resolution (pixel dimension) or selecting the quality (compression ratio), or some combination of the two parameters. What are the advantages of using either method to reduce file size? Specifically, should I maximize the pixels, or minimize the compression ratio, or select some intermediate combination?

[McQ]

Very interesting JPEG storage dilemma. In short: always opt for the higher resolution file with greater compression as opposed to the lower resolution file with less compression. After all, downsizing an image is just another form of image compression-- it's just not as perceptually efficient as JPEG image compression. In other words, you get the most "bang for the byte" using more JPEG compression instead of downsizing.

Just to be sure, I performed a test on the photo below, which contains regions of high texture, low texture, bright tones and dark tones. This was a good way to examine regions all in the same image where the JPEG compression algorithm is typically the most and the least apparent.

Note that the above photo is just a compressed version of the original (uncompressed) image which was used. This uncompressed photo was then either (i) downsized 50% with JPEG compression set to maximum (100% in photoshop) or (ii) left at the size shown above but with a JPEG (greater) compression which created a file the same size as in (i). In both cases, the compressed version was 88 Kbytes. I then upsized the downsized image from (i) in order to do a side by side comparison, as shown below for an example crop:

left: lower resolution file with less JPEG compression from (i)
right: higher resolution file with more JPEG compression from (ii)

I think it's clear that retaining the full resolution image is a *much* better option, even if that means having to use a higher compression in order to have a small enough file size.

[tmyuen]

Thanks McQ for such a convincing comparison. Just to ensure I understand it correctly, I'd like to take two of the options available on my camera (a Canon XSi/450D) as an example:

Option 1) Large/Normal : 4272 x 2848, approx. 2.2 MB (Higher resolution, more compression)
Option 2) Medium/Fine : 3088 x 2056, approx. 2.5 MB (Lower resolution, less compression)

Based on what you are suggesting, Option 1 should give a better JPEG quality even the file size is smaller. Am I correct?

[McQ]

When the file sizes are different the choice becomes a little more difficult. In this case they are close enough that I would still contend that option 1 would produce an image that would make a sharper and more detailed looking large print.

The only caveat I would give (that I have yet to observe) is that there might be situations with very low contrast dark texture that would be better preserved by using option 2. However, there's a reason JPEG does not prioritize that type of texture-- it's generally less perceptually important.