Standard Preview Size/Preview Quality

Question

This may be a silly question, but in Library mode, under Edit>Catalogue Settings>File Handling, you have options under Preview Cache for 'Standard Preview Size' (1024/1440/1680/2048/2880 pixels) and Preview Quality (High/Medium/Low)... but what do these settings actually do; I've tried changing them & not noticed and difference??

Answer 1

Les_Cornwell wrote:
Thanks again Rob
You bet .
Les_Cornwell wrote:
I've tried all the different size options, namely 1024 through to 2880 & low/Medium/high and none make any obvious difference at all.
So are previews only created as required when you view a picture in full screen mode or does LR create a preview for all your files?
Every image you look at in Library module comes from the (library) previews, there are up to 8 possible jpegs:
* a tiny thumbnail in root-pixels.db
* up to 7 jpegs ranging from small to 1:1 in the "preview pyramid" (each smaller is half the dimensions of it's bigger sibling).
Try this with a 10 photo test catalog:
If you have a big monitor and set standard preview size to 1024, then (with Lr closed) delete all previews, then restart Lr and wait for all the "..." indicators to be extinguished (indicating standard previews have been built), then step from photo to photo in loupe view with all panels collapsed (loupe view "real-estate" maximized), you should see "loading" indicator, since it needs a bigger preview than you've got built. What it will do then is build 1:1 previews and all the smaller ones along with it, which is suboptimal from a performance point of view. If you try and zoom in to 1:1 after the "loading", there will be no additional loading, since 1:1 previews were already built.
Then, repeat the test with preview size at max - no loading indicators, right? (when stepping in loupe view after standard previews have finished being built, I mean). Except now if you try to zoom in there will be "loading", since 1:1 preview were not required to display the loupe view, they will need to be built for the zoomed (1:1) view.
The only difference between big enough and too big will be an ever-so-slightly greater lag when stepping in the loupe view and no 1:1 preview exists (when preview is too big I mean), since it's loading a bigger standard preview than is actually needed. Reminder: if preview is not big enough, there will be an ever-so-slightly bigger lag when stepping in loupe view too (e.g. vs. just big enough), since it's using the 1:1 preview instead of standard (which wasn't big enough). So, tester beware... (somewhat counter-intuitively, in some cases, it will be faster loading a preview when settings are, in general, too big, because it can get away with loading the next size down, which is an even better fit, e.g. if image is cropped just so - all of these little nuances make it especially tricky to test & evaluate, so consider doing initial tests using uniform-size uncropped images, to reduce the number of variables - it's confusing enough as it is ;-}).
Note: as previously mentioned, there is considerable complexity (and bugs) in the preview system, and I may not have described it perfectly, so it wouldn't surprise me if your results were not exactly like that, but I just went and retested on my system, and what happened is exactly as I described above (win7/64), as I read it anyway...
Regarding quality, you should see difference in some photos not others, but ONLY if it didn't resort to the 1:1 preview which may be higher quality than the standard and is independent of the standard quality setting. (I think somebody may have stated that you'd need to zoom in to see differences in standard preview quality settings, but that is wrong - the only way to see differences in standard preview quality settings is if you are in fact viewing standard previews, which you aren't when zoomed in to 1:1, and anyway it can be ellusive - see paragraphs above...).
PS - If you want to compare jpeg quality of standard previews, one way is to export them using PreviewExporter. Again, it's tricky, since you need to assure you aren't exporting a scaled down version of the 1:1 instead of a true standard preview. After exporting you can compare outside Lightroom, so you don't have the "preview of a preview" issue going... I use Beyond Compare by Scooter Software for doing objective comparison of like-sized jpegs, but you can compare subjectively using any ol' viewer, e.g. as built into OS.
Too much?
UPDATE:
Les_Cornwell wrote:
does LR create a preview for all your files?
No - they are created on an as-needed basis (thus the reason we hear many complaints about how stale or non-existent previews should be built in the background, to minimize "loading" in library module, e.g. after making dev changes to a large bunch), but note: standard previews may be considered "needed" when thumbnail is in view in grid or filmstrip (but not considered needed if thumbnail is off-screen, even if existing in filmstrip and/or grid).
R
Message was UPDATED by: Rob Cole