Again, another box you PROBABLY don’t want to check very often, despite how great it seems at first. The tool tip that pops up when you hover your mouse over it reads “If preview files are already generated… selecting this option will use those preview files and make rendering faster.” Basically, if you have rendered clips in your timeline (i.e. you’ve turned red or yellow lines green with the Render command), then Premiere will use those rendered clips to speed up the export. Sounds great, right? Well, the tool tip only tells you the positive aspect and not the negative!
This process will usually result in a double generation loss in the quality of your video. Let’s talk about a very common scenario. Perhaps you shot your video on a Blackmagic Pocket Cinema Camera in ProRes 422. Then, you added some effects that caused poor playback in Premiere, so you decide to render the clip (red line above the clip goes green). Then when you export, you export to H.264 for the end client and you check “Use Previews.” Danger, Will Robinson, you just produced a lower quality output because Premiere rendered part of your timeline TWICE: once from ProRes 422 to your sequence’s Preview Format (probably MPEG I-Frame, the most common default), and then again from MPEG to H.264.
This setting is usually only good to select in two scenarios - rough cuts (when quality isn’t necessarily key) and IF you carefully set your Preview Format settings in Sequence Settings and are exporting to the same codec as that Preview Format.
This setting is useful if you are exporting to a different frame rate than your sequence is set to. Let’s say your sequence is set to 23.976 fps but you need to fork your video over to a TV network that wants it in 29.97. Sure, it’s easy enough to override the frame rate settings under the Video tab above, but this frame rate conversion will result in the duplication of occasional frames which, in some shots, such as slow panning shots, could produce a noticeably jerkiness to your video. That’s where the three options found within this setting come in.
Frame Sampling, the default setting, basically means Premiere isn’t going to do anything special, it’s just going to duplicate (or subtract) frames as needed to change frame rates. Frame Blending is more or less what it sounds like. It tries to smooth out any jerkiness caused by duplicated or missing frames by blending adjacent frames together. This is rather quick and effective with only a modest slowdown to the export process. Lastly there is the big daddy, Optical Flow. With this selected, Premiere will actually generate whole new frames by analyzing the movement of pixels on two frames. In theory, this is the highest quality option, but is often hard to tell the difference from Frame Blending and will dramatically slow down your export. Keep in mind, this is a different setting from the Time Interpolation setting found in the Speed/Duration window in the app.
When All Else Fails…
If you are left more confused than ever after this article, or are skeptical about any of the recommendations, here is the most important thing to remember - test, test, test. If you are unsure whether or not to use one setting or another, export a short portion of your sequence using the Source Range controls with settings batch A and then do it again with settings batch B and compare.
When do you use Maximum Bit Depth? Have any unique exceptions to the common scenarios above to share? Talk about it in the comments below!