Does Lightroom 'manufacture' the screen previews from RAW images? They are 3 to 6 mb and pretty good

[Ken Nielsen]

Long story short, I lost all of my RAW images on a recent download and needed to use a Lightroom plug-in to extract the screen previews of the images which were still present even though the original images were gone. Upon working with the now imported screen images back into Lightroom I was surprised at how good they were, giving plenty to work with and superb detail so even could be cropped and upscaled successfully and had I not known, would have been convinced I was still working with the original RAW images. These jpeg screen preview images were on average between 3.5 to 6 mp in size, much larger than I would have suspected. So I ask: Does Facebook create these screen previews and does it matter that they were RAW and shot on a 48mp camera? This has me taking a second look at why some shoot jpeg only... hmm...

[Jim Wilde · Accepted solution]

Can you clarify a couple of things:

1. You talk about "screen previews". Do you mean Lightroom Classic's "library previews" which are contained within the "catalog name Previews.lrdata" package alongside your catalog, and which can be extracted using an LrC plug-in to generate jpegs from them? In which case they can be workable quality, but that might largely depend upon the preview type, size and quality (set in the Catalog Settings). For instance, if there happened to be a 1:1 preview available it will be much larger in terms in the pixel resolution than a typical Standard preview (i.e. for a 48mp raw file, a 1:1 preview would contain the full 48mp). But the size of the Standard previews (which are likely to be the norm) will be smaller and governed by the aforementioned Catalog settings. But they would certainly be better than nothing, though not comparable to the original Raw file.

2. Your initial post title stated "Mb" (mega-bytes), but your post text stated "Mp" (mega-pixels). Which is correct?

3. Regarding your actual question, I have no idea how Facebook fits in here. Can you elaborate on that? The library previews I am talking about are created by LrC from the Raw file and then updated each time you edit to include whatever edits you make. 

[Ken Nielsen]

Ah... I have Facebook on the brain it seems and I meant to type in Lightroom, sorry and I don't think it's possible to edit that now. The megapixels applies to all as that is the file size, my error again. Also the 'library previews' would be the correct term. So, I see then that these previews can be regulated in catalog settings and go all the way up to 1:1. Is there a link to the manual for Lightroom online where it would cover this in detail. This would be a great help to me in discovering which settings I am working with right now and also for learning how to regulate these settings. Thank you for your very good reply and help Jim.