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Previews - Let's Talk Preview Files.

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I think I have read everything I can find about Previews and I've just about decided I don't need them, despite every LR article that suggests how much they speed up work in the Library and now in even the Develop module. But my computer is fast and generates a 1:1 preview almost instantly. Besides, Previews confuse me.

I'm a high-res shooter (50-100 MP always), and for years I have just automatically created 1:1 previews on import. But I've never understood exactly when they are used or how they behave. I know more now because I've been experimenting today.

First of all, generating previews is a CPU-intensive task. For example, I watched my desktop generate 2,400 1:1 previews from a 50 MP Leica Q2 a couple of weeks ago when I imported 2,400 DNG files into LR. My computer is about as fast as you can build right now (PC desktop-wise) and I watched that i9 11th gen CPU crank to 100% load and max MHz for quite a while. My rig is water-cooled and I watched the temps climb and the fans were all spinning at max (8 fans in that baby). The GPU (which is a 3080 Ti) was hardly touched. I don't think it engaged, even though I think Adobe says they utilize the GPU for generating previews now. (Note: I do notice that the GPU is extensively engaged on exporting jpegs.)

Anyway, I had 1:1 previews built on import and then I started fine-tuning the editing on many of the images - probably about 1000 of them. I had already edited the whole batch on the road on a laptop so the edits were already 95% complete on the import when the previews were built.

I did a little tinkering today. I selected one photo and did some minor edits. Then I went to Library - Previews - Build 1:1 Preview. LR asks if you want to build previews for all or just the selected photo. I clicked on just selected photo. The 1:1 preview built in about 1 second. Remember, I already had a preview built on import. Then LR reported "Preview was built."

So then I did it again immediately after no further edits. I just selected the same image and did it again. LR said "The Preview was already up to date."

So that told me that once a preview is built and you do any edit later, if you want it to include the latest edits you have to tell it to build the preview again. So I think the preview just speeds up the process of loading for LR to do its editing work. But what if you didn't update the preview after extensive further edits? When you are in the library module, I assume the preview will load that you created at some earlier point and then LR will apply any newer edits that were added since the preview was built. It won't update the preview file on your disk, but it will load what you have and update the edits on the screen via the catalog. Right?

I experimented with this for a while. Any slight edit and you can build a new preview after that. If there is no edit, LR will tell you the preview is already built.

My preview folder is huge because I have 1:1 preview file for every one of my bazillion raw files. Therefore I have a bazillion 1:1 preview files because I tell LR to never delete them. But do I really need them? I don't think so because what difference does it make? I open an image in the Library or Develop module and when there is no preview it takes an instant to build the preview. I guess having them is helpful and LR sure wants you to do that in all of their literature. They say you should generate 1:1 if you have the space. I want 1:1 because I have a big 4K monitor and high-res files and plenty of disk space. But do I really need them? And should we update the preview files after further editing? I guess after editing a shoot you could update all the 1:1 previews before walking away from the desk, because the previews you have were generated at import.

Should I update them? Should I even have them at all? Comments? What do the gurus say about Previews? Did I describe the behavior correctly?

Thanks,

Greg Johnson, San Antonio, Texas
https://www.flickr.com/photos/[email protected]/albums
 
Last edited:
Joined
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"Cheap insults" are strong words Jim. Insults of any kind were not in any way intended, especially to guys who are volunteers who have helped me in the past.

This thread was just a running conversation among enthusiasts. Not every post here by the Gurus has to be spot-on with the first response or demonstrate total knowledge on every algorithm deep in the bowels of LR. None of us are Adobe programmers and sometimes ideas and explanations evolve on a thread on any forum.

I was just curious about the behavior of the preview files and system because of some weird experiences I have had recently, and the Adobe literature on the Help system on this topic is not great. In fact, you and YK (on those last two posts) provided a very impressive explanation of the exact behavior of the preview system and files that I think should be adopted by Adobe in their help system and literature. You nailed it with precise language and made it clear exactly what goes in the preview file arena of LR.

But you are right. I got my answer (as we all did) and I really appreciate it.
 

ChrisKet

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I read this thread with interest, because I have been evaluating just how critical it is for me to have that Previews.lrdata backed up to my "disaster recovery" files. I back up my Mac using Time Machine, so it is backed up there; I also have a cloud backup using Amazon photos, but didn't want to pay for extra storage there if it was not necessary. After reading through this, I have decided that I will not worry about uploading that big file to my cloud backup, because if I happen to "lose" it in some disaster, it will be rebuilt for me.

So, thank you all for your insights!
 
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I read this thread with interest, because I have been evaluating just how critical it is for me to have that Previews.lrdata backed up to my "disaster recovery" files. I back up my Mac using Time Machine, so it is backed up there; I also have a cloud backup using Amazon photos, but didn't want to pay for extra storage there if it was not necessary. After reading through this, I have decided that I will not worry about uploading that big file to my cloud backup, because if I happen to "lose" it in some disaster, it will be rebuilt for me.

So, thank you all for your insights!
Yes, Lightroom Classic is able to regenerate Previews in the Previews folder as needed. Some times the Previews folder get enormous and may previews might never get accessed again. So you can safely delete the Previews folder and Lightroom will regenerate a new folder and the previews it needs on the fly.
More properly, the "XX.lrdata" is a folder "package". Normal folders do not have an extension (.lrdata) but there is nothing in the filesystem rules to say that the person (.) is not a valid folder name character. You can ope the Previews folder using context menu and choose "Show Package ContentsP.
 
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I didn't reread the whole thread above, but I think I said a couple of times that I was close to just deleting my entire preview folder because it is huge (almost half a TB). That is because I have generated 1:1 previews (which I suppose are really full-size jpegs) of big GFX files - every raw I have imported LR for many years.
So what happens if I delete them all? Well, we have them there for a reason. The reason is so in the library module we move along much faster as we scroll through viewing a folder full of images (which I do a lot). If there is no preview, LR has to build one anyway. It builds one when you click on the file and if you zoom in then it builds a big 1:1 preview file.
If you had no preview files already generated and you started scrolling through a bunch if images and zoom to full res 1:1, then you are generating the preview files anyway, but it will slower that way. Generating previews is a hard-working CPU-intensive process and not as snappy as if they are already there, no matter how fast your computer is.
Anyway, I decided to just keep my huge previews folder and it is on the boot 2 TB M.2 PCIe 4 SSD. If I start filling that up, I will just delete the preview folder.
We already covered the fact that LR will generate a new preview and overwrite the old one when you edit a file in the develop module. The preview will reflect your latest edit.
So, I think generating a 1:1 preview is no different than exporting a full-size jpeg from LR because a preview file is a jpeg file. Am I right? If so, what quality is the preview jpeg created at? 100%? 90? 80?
When I export a full-size jpeg from LR, I do it at 85%. I am absolutely convinced that it is impossible to discern the difference of a full-size jpeg at 100% 90% or 85%. I am a pixel-peeper and obsessed about resolution and image fidelity. I view all my big Medium Format GFX 100 MP files on a 4k IPS Pro 32-inch monitor and I cannot tell the difference between a full size jpeg exported at 85% or 100% quality when I view at full resolution 1:1.
There has been much discussion about this on the medium format camera boards and most medium format photographers I have talked to agree with that viewing assessment. So, most pros export from LR at full-size 85 or 90% quality. Going from 100% quality to 85% quality on LR jpeg exports can be a huge difference in size.
Remember, exporting jpegs from LR uses both CPU and GPU (as of the most recent update). Generating preview files uses only the CPU. We talked about that on another thread. But I think generating a preview is no different than generating a jpeg for export. I would think Adobe would use the GPU for both processes. But right now, Adobe only uses it on exporting jpegs - not generating previews.
Are preview files jpegs? They have to be. If so, then why can't you see them as jpeg files in the preview folder? You just see proprietary gibberish in that folder, like in most backup programs when you see just indecipherable gibberish inside the folders, not the files that you named (which is why I love GoodSync- you see your exact files and folder structures on all disks you sync to - identical to your main data disk if you view it in explorer or any file app). I'm sure Adobe has a good reason for that. Still, I think the preview folder is really just a folder full of jpegs.
 
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So, I think generating a 1:1 preview is no different than exporting a full-size jpeg from LR because a preview file is a jpeg file. Am I right? If so, what quality is the preview jpeg created at? 100%? 90? 80?
It depends on the quality selected in Catalog Settings > File Handling > Preview Quality. I don't know the exact numbers offhand. And yes they are essentially JPEGs.

I would think Adobe would use the GPU for both processes. But right now, Adobe only uses it on exporting jpegs - not generating previews.
They likely will in future, but rewriting export for GPU was a major undertaking, and it will be for previews too.
 
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