I use 1024 x whatever keeps the aspect ratio. It's probably somehwat less than the retina display can handle, but it gives me file sizes of around 1.25GB for a ful-length movie, which suits me.
I'd expect 1440 * x would give file sizes about souble that - so 2.5-3 GB. Experiment with somehting like Handbrake which has some presets. The Apple TV presets work well for iPad
It depends on the viewing distance. 30 cm (12") like watching videos in an aeroplane / in the bed? Definitely 1080p (or even 2k - see below). Much higher like doing the same in, say, a gym? SD will suffice.
You may also want to read the article at http://www.iphonelife.com/blog/87/new-ipad-3-whats-real-difference-between-720p- and-1080p-two-major-high-resolution-video-form . The one at http://www.iphonelife.com/blog/87/new-ipad-3-2k-video-playback-definitely will also be of interest if you have video material exceeding 1080p in resolution (then, you might want to convert it to 2k format instead of 1080p).
"If I convert a movie to that resolution it would be 20G. A 1920x1080 movie already takes up 7G, which is too much."
You may want to stick with 1080p, unless your source media is of higher resolution (not very probable as 2k/4k consumer videos are still very rare). Then, 7G is the standard size for a 1.5...2-hour-long movie, assuming 10 Mbps bitrate, which is perfectly OK with 1080p H.264 movies. In no way should it be 20G, unless it's a 4-hour-long movie.
"I only have the 16G version so space is an issue."
Can you also stream (are you at home / do you have the Seagate GoFlex Satellite or something similar)? Then, use 1080p and a player that has excellent streaming support. (GoodPlayer is the best, as long as you don't want to play back MP4's over uPnP.)
"I think maybe an upper limit of 2G per movie is reasonable...?"
Definitely not with 1080p (too bad video quality because of the 3-4 Mbps bitrate). 720P would look OK with H.264.
If I want to put my own movies on the ipad (from dvds that I own, I have all the conversion tools I need), what is the recommended resolution?
DVDs are SD, which is 720x480 NTSC or 720x576 PAL.
Converting to a higher resolution is pointless, it will just blob out the pixels.
You may want to remove the anamorphic conversion (wide screen movies stretch the horizonal size by 1.4 or close to it).
If you do this it will make sure that the iPad shows the video correctly.
To do so would be 1024X480 for NTSC, or 1024 x 576 for PAL.
The iPad will stretch it out to look full screen, and it will look sharper played out at the origonal reslution and resized on the fly, then it would doing an up-res in a domestic video converter.
Out of interest, the final video size is more determined my the compression ratio / datarate then the pixel size.
You could make a 4000x4000 pixel video in less the 1gb, just lower the compression; it would look rubbsih though.
Go for about 1500 to 3000 mb/s for SD, and higher for HD content in MP4 or Quicktime H.264.
I find most domestic or plain video converters do a bad conversion, and often tend to make files way to big for the quality of the source material. You can lower the data rate quite well in H.264 or MP4 without much noticable quality loss. Saving space for more videos in less space.
....the difference between 1080p and 720p is quite visible.
Sure, if the source material is HD.
There is difference from SD footage blown up on an iPad screen by the built in decoder versus converting the footage to 1080 before sending it to the iPad.
Why upscale SD footage to HD on re-encode?
What can you possibly gain from making the pixels twice the size, taking twice the space?
The iPad can do this live when you play the video.
I think converting DVD videos to 1080 is a waste of space and time.
Yup, I've assumed the OP wanted to watch 1080p source content on his iPad 3 - it was only later that I realized he actually wants to upsize his video to 1080p.
All in all, nicros: never ever think of "upsizing" a DVD to HD or Full HD resolution. Convert it as is - but, if I were you, I'd try to get the Blue Ray version of the same movie instead, partiuclarly if the viewing distance is no more than 30cm/12". It'll pay off, full HD playback is so much better than SD on the iPad 3. (Again, when watched this close but not from more far away!)
"Yup, I've assumed the OP wanted to watch 1080p source content on his iPad 3 - it was only later that I realized he actually wants to upsize his video to 1080p"
Actually I didn't say anything about upsizing my video to 1080p... sorry if that was implied! More to the opposite. Just to clarify and avoid confusion, lets remove the 'DVD' part of the question, as I also have bluray, downloaded 720p content, etc.
If I have DVD source, I'm not going to upscale, I'm going to watch it at it's higest native resolution (as people have mentioned). I get that, and shouldn't have said anything about DVD- I just spaced. What I'm really interested in is what to downscale to. If its HD source, say 1020p or 1920x1080, it's too large on disk after conversion to m4v, and there is no observable benefit over, say, 1440x900. Or even maybe 1280x720.
What I was trying to get to (which is admittedly subjective) is given normal viewing environment (airplane, lap, etc.) what is a reasonable resolution to downconvert to? If my eye can't discern any detail improvement going from 1280x720 to 1440x900, then I want 1280x720.
I have been using handbrake with 1280x720 and it seems pretty clear... but was wondering if others had evaluated this stuff before me...
Again, all depends on the viewing distance. Do you do it from around 12" (30 cm)? Then, 1080p is the way to go, particularly if you put up with only being able to synch one movie to your iPad (a usual, 1.5-2-hour-long 1080p movie encoded at around 9-10 Mbps takes about 7 Gbytes) at a time OR using an external, Wi-Fi-capable media store like that of GoFlex Satellite Hard Disk (particularly if you install the http://www.hackseagatesatellite.com/wordpress/welcome/ hack on it.)
From more far away? 720p or even SD will suffice.
There's a VERY nice article explaining all this and linking to a lot of other comparisons (including comparing Apple's own 720p and 1080p encoding quality) at http://www.iphonelife.com/blog/87/new-ipad-3-whats-real-difference-between-720p- and-1080p-two-major-high-resolution-video-form , I really recommend it.
The resolution of DVD video is usually 720*480 (NTSC standard) or 720*576 (PAL standard), ripping your DVD to a higher resolution won't improve the output quality, so "Keep Original" is highly recommend when you convert DVD to iPad supported videos.