user16139: I have some 720p 30FPS H.264 video that I want to downsample for online backup. I want to get to somewhere around 10% to 20% of the existing size, which will evidently require a large sacrifice in quality. I will use H.265 as the destination format.
Now I have two main knobs I can turn to achieve this: I can lower the resolution, or I can lower the quality factor for the encoder. Of course, I can also combine both approaches.
How should I approach this to preserve as much quality as possible? It's pretty clear that turning the quality way down while keeping the resolution at 720 results in some pretty terrible video, but I'm not sure that I should draw the conclusion that I should mostly use resolution decreases to achieve the size reduction.

user14504: In addition to suggestions above, you could also do a pulldown from 30fps to 24 fps. 
This would likely not be noticeable to the average viewer; as 24p is the standard for film and narrative tv production. 
You thus decrease the amount of discrete frames by a factor of 1/5 (20%). 

user18314: Decreasing the size a little won't help much, decreasing it a bit more induces a fair bit of aliasing (unless it smooths it also) butchering the quality without much gained.
Cutting the size 50% will reduce errors and double the jaggyness and be easily enough to save a lot of space, question is: Do you want 360P Video? You really wouldn't want any smaller.
So while that is one way to save space for Archiving I would recommend using 'Q' at 40 to test one file at 720P (output) and see if you can tolerate the quality.
H.265 has superior output quality as well as an approx. 50% reduction. Decreasing the resolution in half means an approx. 25% reduction alone, along with approx. 50% from H.265 your going to be under 15% WITH the same quality.
So decreasing the resolution from 720P will be either too drastic or induce greater errors in the output (it will begin to look like mush). Best to try using 'Q' alone as see where between 30 and 40 you can stand.
Here's an example from one particular Video YMMV.

Note that going from 24 to 40 gets you down to 10% (in that one example).

16139: <p>I have some 720p 30FPS H.264 video that I want to downsample for online backup. I want to get to somewhere around 10% to 20% of the existing size, which will evidently require a large sacrifice in quality. I will use H.265 as the destination format.</p>

<p>Now I have two main knobs I can turn to achieve this: I can lower the resolution, or I can lower the quality factor for the encoder. Of course, I can also combine both approaches.</p>

<p>How should I approach this to preserve as much quality as possible? It's pretty clear that turning the quality way down while keeping the resolution at 720 results in some pretty terrible video, but I'm not sure that I should draw the conclusion that I should mostly use resolution decreases to achieve the size reduction.</p>
 14504: <p>In addition to suggestions above, you could also do a pulldown from 30fps to 24 fps. </p>

<p>This would likely not be noticeable to the average viewer; as 24p is the standard for film and narrative tv production. </p>

<p>You thus decrease the amount of discrete frames by a factor of 1/5 (20%). </p>
 18314: <p>Decreasing the size a little won't help much, decreasing it a bit more induces a fair bit of aliasing (unless it smooths it also) butchering the quality without much gained.</p>

<p>Cutting the size 50% will reduce errors and double the jaggyness and be easily enough to save a lot of space, question is: Do you want 360P Video? You really wouldn't want any smaller.</p>

<p>So while that is one way to save space for Archiving I would recommend using 'Q' at 40 to test one file at 720P (output) and see if you can tolerate the quality.</p>

<p>H.265 has superior output quality as well as an approx. 50% reduction. Decreasing the resolution in half means an approx. 25% reduction alone, along with approx. 50% from H.265 your going to be under 15% WITH the same quality.</p>

<p>So decreasing the resolution from 720P will be either too drastic or induce greater errors in the output (it will begin to look like mush). Best to try using 'Q' alone as see where between 30 and 40 you can stand.</p>

<p>Here's an example from one particular Video YMMV.</p>

<p><img src="https://i.stack.imgur.com/JbtCt.png" alt="Compare H.264 vs. H.265 File Size"></p>

<p>Note that going from 24 to 40 gets you down to 10% (in that one example).</p>
