user6420: I'm trying to figure out what is more important - the screen resolution or the source resolution?
OK, so a Full HD 1080p file might be able to be played on a small HTC One mini screen, but due the the screen only being HD (1280 x 720) is there any scaling back? Or is there no scaling back and it plays the normal file but can only actually see the 1280 x 720 resolution? (I guess it also depends on the playback application...?)
On a screen that size, can anyone actually tell the difference between a 1280 x 720 HD and a 1920 x 1080 Full HD movie?
Further to this, what's the difference between a screen resolution of 1280 x 720 on a 4.5" screen than to a 50" screen? The pixels are bigger on a 50" screen so it looks 'worse' (Or pixels are smaller on a 4.5" so it looks 'better')?

user3592: If you're only delivering to the HTC One mini, then yes, screen resolution overrides source resolution, as you put it.  Your assumptions are correct about 1080p video either having to be scaled or cropped to fit the display, and you're right, whether the source media is scaled or cropped does depend on the playback application.  
As a general rule, you should try to match your final output to the format upon which it will be viewed.  In your case, because the HTC One mini is only capable of displaying 1280x720 (roughly 900,000 pixels), there's no need to send it 1920*1080 (about 2 million pixels).  Doing so is certainly a waste of bandwidth, and arguably a waste of CPU cycles and battery life.
But to be clear, this advice only applies when you know the video will never be displayed on any other device.  If your final output is 1280x720, then anything with a better display will have to scale the image UP, making it blurry or blocky.  By limiting your resolution to the HTC One, you'll notice sub-optimal performance on HD-TVs, for instance, or perhaps next year's model phone.

On a screen that size, can anyone actually tell the difference between
  a 1280 x 720 HD and a 1920 x 7080 Full HD movie?

Probably not.  It depends on the quality of the phone's onboard scaling vs the quality of whatever application you'd use to scale from 1920x1080 to 1280x720.

Further to this, what's the difference between a screen resolution of
  1280 x 720 on a 4.5" screen than to a 50" screen? The pixels are
  bigger on a 50" screen so looks 'worse' (Or pixels are smaller on a
  4.5" so it looks 'better')?

Yes. If you have the same number of pixels on a bigger screen, the pixels themselves will just have to be bigger.  But you have to remember that a 50" screen isn't meant to be viewed at arm's length like your phone.

user2836: They both matter.  What really matters is the combination of the two.  Scaling always produces a loss of quality unless you are using an even multiple of the resolution.  This is because interpolation must occur during a scaling operation that isn't an even multiple or divisor of the original resolution.  This is due to the fact that some pixels must contribute to two different new pixels.
That said, it is even more complicated when deciding what to shoot with for your camera if it is coming from a camera.  The actual imaging sensor has a native resolution itself and will internally apply scaling to fit to whatever size you need.  In some cases, like DSLRs, all formats actually end up getting scaling applied since the actual sensor is much higher quality, but that also means the down-scaling can be more accurate if implemented properly.
If you know that you will only ever use output in a given resolution, start with that resolution as early as possible to make use of any source side optimizations that may exist rather than going to an unnecessary intermediate format that will only be scaled again (resulting in another interpolation).
If you are targeting multiple output formats, then use whatever the highest quality format you can is (even if it exceeds the maximum format you intend to output) since the more information you have, the more accurately you can downscale later.
The exception to this would be if the majority of your usage will be on the highest quality format that you will be distributing, then go ahead and use that quality for source since you will gain a little quality at that level at a cost of a little quality on the lower end videos.
As for the side question about screen size, yes, it can make a difference, but angle of view is what matters.  A 50 foot screen two miles away is still going to have less visible resolution than a similar pixel count screen that is 4 inches across and 3 inches in front of your face.  I touch on this in a little more detail in this answer.

6420: <p>I'm trying to figure out what is more important - the screen resolution or the source resolution?</p>

<p>OK, so a Full HD 1080p file might be able to be played on a small <a href="http://www.gsmarena.com/htc_one_mini_2-6345.php" rel="nofollow">HTC One mini</a> screen, but due the the screen only being HD (1280 x 720) is there any scaling back? Or is there no scaling back and it plays the normal file but can only actually see the 1280 x 720 resolution? (I guess it also depends on the playback application...?)</p>

<p>On a screen that size, can anyone actually tell the difference between a 1280 x 720 HD and a 1920 x 1080 Full HD movie?</p>

<p>Further to this, what's the difference between a screen resolution of 1280 x 720 on a 4.5" screen than to a 50" screen? The pixels are bigger on a 50" screen so it looks 'worse' (Or pixels are smaller on a 4.5" so it looks 'better')?</p>
 3592: <p>If you're <strong>only</strong> delivering to the HTC One mini, then yes, screen resolution overrides source resolution, as you put it.  Your assumptions are correct about 1080p video either having to be scaled or cropped to fit the display, and you're right, whether the source media is scaled or cropped does depend on the playback application.  </p>

<p>As a general rule, you should try to match your final output to the format upon which it will be viewed.  In your case, because the HTC One mini is only capable of displaying 1280x720 (roughly 900,000 pixels), there's no need to send it 1920*1080 (about 2 million pixels).  Doing so is certainly a waste of bandwidth, and arguably a waste of CPU cycles and battery life.</p>

<p>But to be clear, this advice only applies when you know the video will never be displayed on any other device.  If your final output is 1280x720, then anything with a better display will have to scale the image UP, making it blurry or blocky.  By limiting your resolution to the HTC One, you'll notice sub-optimal performance on HD-TVs, for instance, or perhaps next year's model phone.</p>

<blockquote>
  <p>On a screen that size, can anyone actually tell the difference between
  a 1280 x 720 HD and a 1920 x 7080 Full HD movie?</p>
</blockquote>

<p>Probably not.  It depends on the quality of the phone's onboard scaling vs the quality of whatever application you'd use to scale from 1920x1080 to 1280x720.</p>

<blockquote>
  <p>Further to this, what's the difference between a screen resolution of
  1280 x 720 on a 4.5" screen than to a 50" screen? The pixels are
  bigger on a 50" screen so looks 'worse' (Or pixels are smaller on a
  4.5" so it looks 'better')?</p>
</blockquote>

<p>Yes. If you have the same number of pixels on a bigger screen, the pixels themselves will just have to be bigger.  But you have to remember that a 50" screen isn't meant to be viewed at arm's length like your phone.</p>
 2836: <p>They both matter.  What really matters is the combination of the two.  Scaling always produces a loss of quality unless you are using an even multiple of the resolution.  This is because interpolation must occur during a scaling operation that isn't an even multiple or divisor of the original resolution.  This is due to the fact that some pixels must contribute to two different new pixels.</p>

<p>That said, it is even more complicated when deciding what to shoot with for your camera if it is coming from a camera.  The actual imaging sensor has a native resolution itself and will internally apply scaling to fit to whatever size you need.  In some cases, like DSLRs, all formats actually end up getting scaling applied since the actual sensor is much higher quality, but that also means the down-scaling can be more accurate if implemented properly.</p>

<p>If you know that you will only ever use output in a given resolution, start with that resolution as early as possible to make use of any source side optimizations that may exist rather than going to an unnecessary intermediate format that will only be scaled again (resulting in another interpolation).</p>

<p>If you are targeting multiple output formats, then use whatever the highest quality format you can is (even if it exceeds the maximum format you intend to output) since the more information you have, the more accurately you can downscale later.</p>

<p>The exception to this would be if the majority of your usage will be on the highest quality format that you will be distributing, then go ahead and use that quality for source since you will gain a little quality at that level at a cost of a little quality on the lower end videos.</p>

<p>As for the side question about screen size, yes, it can make a difference, but angle of view is what matters.  A 50 foot screen two miles away is still going to have less visible resolution than a similar pixel count screen that is 4 inches across and 3 inches in front of your face.  I touch on this in a little more detail in <a href="https://video.stackexchange.com/questions/8603/dslr-raw-video-1080p-or-720p/8607#8607">this answer</a>.</p>
