In trying to decide whether to go all-out for the 32GB iPhone 4 or save a hundred bucks and just get the 16GB, one of the considerations is photo and video size. My whole music library won't fit on any iPhone (or any iPod for that matter), so that's not going to be the deciding factor. But will I regret it if I have a small iPhone and decide to take some video? We don't yet know exactly how big the iPhone's 720p HD video will get, but fortunately, someone has done the calculations and the answer seems to be that you'll want all the space you can get if you're planning on shooting video. Feel free to skip the next few paragraphs if you want to avoid the math.
Justin Horn at WhenWillApple? did a similar estimate last year when trying to decide whether to get a 16GB or 32GB iPhone 3GS. To determine file size for still images, he just does a simple scale up from the size of the files generated by the 3GS's camera: since the 3GS generates an average file size of 1.3 MB at 3.2 megapixels, Justin figures that the pictures taken on the 5-megapixel camera of the iPhone 4 will be about 2.7 MB each. Not that big a deal… even with a bunch of music and apps on your iPhone, you can probably take a couple hundred pictures before noticing it.
In order to figure out video size, Justin goes to Apple's best frenemy Adobe. They have a guide up on their website that gives simple formulas for calculating the amount of space that video will take up. First, you determine the size of a single frame using the formula:
Frame size K = ([Pixel Width x Pixel Height x Bit Depth] / 8) / 1024Figuring that the 720p HD camera will image at a bit depth of 24 colors, Justin calculated ( [960 x 720 x 24] / 8 ) / 1024 for 2025 KB per frame. Then, he multiplied that by 30 for the number of frames per second and got the figure of 60.75 MB for each second of uncompressed video. Justin had estimated a 60:1 compression ratio for the H.264 video on last year's 3GS but found that to be a bit high in practice. Assuming a 40:1 compression ratio made the math come out right, so he's making the same assumption for the iPhone 4's video compression, he ended up with 1.518 MB per second of compressed video. So if you want to store as much as 250 minutes of video on your iPhone 4, say 50 five-minute clips, he calculates it will take 21.73 GB (1518.75 KB/sec compressed * 250 min * 60 s / min * 1 MB / 1024 KB * 1GB / 1024 MB)
Bottom line: unless you want to take four hours of HD video on one go on your phone, or carry that much around in your pocket, you'll probably be fine with the 16GB model. However, if you're taking a bunch of videos, and carry a lot of music, sideload a bunch of apps and have a whole library of books, 32GB might be a reasonable decision. At least, that's what I just talked myself into...