Previews - Let's Talk Preview Files.

[GregJ]

Lightroom Version Number

Latest Version of Classic via Adobe Cloud

Operating System

1. Windows 11

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/139148982@N02/albums

 

[Y.K]

LrC uses the standard previews and the 1:1 previews only in the Library module. You do not have to update them manually - they get updated automatically whenever you do any edits on the photo(s). (While in Develop module, LrC uses other - sort of - previews, stored on the Adobe Camera Raw Cache.)

LrC cannot be used without these Library previews: they are jpg representations of how your raw (or other files) “look” after their latest edits -if any. They are stored with your catalog so that LrC works fast and that you can at least view your photos even if the original files are not available.

You can elect not to automatically create them on import and you can delete the previews folder. But LrC will automatically start creating them on the fly whenever you view your photos in the Library module, either on the grid or in the Loupe View. If you are in a collection or a folder and while browsing the photos, you will experience a small delay as LrC creates the previews of the next batch of photos in this current view.

So what you save, during import, on disk space and process time, will later make LrC run slightly slower whenever you wait for the current source’s previews to be generated on the fly.

You can set the size of Standard Previews to less than your working screen’s dimensions - to save some disk space. When LrC uses these previews will have to recompute them automatically to fill your screen - which again will take some milliseconds. 1:1 previews are generated automatically (if not already present) whenever you zoom to 100% or more. Their size is dependent on the dimensions of the original files - you cannot set it. It goes without saying that LrC can automatically (on the fly) generate the Standard and 1:1 previews only if the original files are present: if the disk with the originals is not connected you cannot view the photos if you haven’t already created the previews.


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[GregJ]

Interesting. Thanks.
What do you mean by LrC? Cloud or Classic? Cloud I assume. I never touch Cloud. I'm not ready to be conversing with the cloud with my big raw files. Not yet. Maybe in 3 to 5 years. But I doubt it even then. I want my raw files on a fast internal SSD.
LR Classic for sure uses the 1:1 previews in the Library module. That is what they are for. And with the latest update, the literature on that update claims it now uses previews in the develop panel, whereas before they said that every image generated the preview when you opened the image for editing in the develop panel. I could be wrong but that is what the update said.

Yes, I know you don't have to generate previews on import and don't have to later either, and of course when you open a file in LR it generates a preview if one does not already exist. And it atomically generates a 1:1 preview if you zoom.

But you said something I did not know about preview files. Let's say I delete all my preview files - delete the previews lrcat folder. So no previews exist. Then I open a file in LR and zoom to 1:1. LR of course immediately generates a 1:1 preview because one did not exist already on the disk to grab. But then I thought it would just disappear when you moved on from that image and not save it to the preview folder (or keep it in the cache until you exit LR). When LR generates a preview when you open a file and that preview didn't exist before, it will now save that preview in the preview folder? I don't think so but I will check. That's very interesting. So, if I have no preview files (didn't create them on import), and I then edit all the files, LR creates a preview for each file you open and then stores them in the preview folder so you will end up with them anyway? Does anyone know?
By the way - my preview folder is huge. Lightroom Catalog\Lightroom Catalog-2-2-v10-v11 Previews.lrdata on my disk id 500GB! Half a TB. That's a lot of 1:1 preview files. I'm thinking of deleting that folder. Let's say I delete it. Every time I open a file in LR and it creates a preview, will that file then store in that folder so it starts to buiuld again? Or does it disappear because you didn't tell LR to build preview.
Also ... If I highlight a file and tell it to build a preview, LR asks if you want to build preview for all or just the selected file. What if I said "all"? Would it build previews for every file linked to the catalog or just that folder? If it's the whole catalog I would be building 100,000 previews if none already existed! Who knows the definitive answer on that?

 

[Hal P Anderson]

Let's say I delete it. Every time I open a file in LR and it creates a preview, will that file then store in that folder so it starts to buiuld again?

Yes. Once LRC (Lightroom Classic) builds a preview, it will keep it. You never have to tell LRC to save a preview.

 

[Y.K]

LrC is Lightroom Classic.
Lr is Lightroom (cloud based)

Lightroom CC should not be used ever anymore because it used to refer to both the predecessor of Classic and then the Cloud version.

When LrC opens it creates a fresh previews folder (if you deleted the previous one) where it puts any standard and 1:1 preview it generates on the fly. You can use the Library menu to generate (or discard) previews for certain selected files. If you select a file and LrC asks you whether to create previews for the selected or all files, “all” refers to your current grid view: the folder or the collection you are in; if you are in “All photographs” then, of course “all” is everything.

I would suggest that you run the “find all missing photos” command from the Library menu: if some are found, your current previews will help you view which files are misplaced (deleted, moved, renamed) But if none are found you can delete the old massive Previews folder and generate the standard previews from scratch. You can choose,in catalog settings, a smaller standard previews size for all your older photos. Don’t create any 1:1 previews and set LrC to automatically discard them after a week or a month anyway. You can choose to generate 1:1 previews only for the photos you are currently working: a certain old batch and any new imports.


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[GregJ]

LrC is Lightroom Classic.
Lr is Lightroom (cloud based)

Lightroom CC should not be used ever anymore because it used to refer to both the predecessor of Classic and then the Cloud version.

When LrC opens it creates a fresh previews folder (if you deleted the previous one) where it puts any standard and 1:1 preview it generates on the fly. You can use the Library menu to generate (or discard) previews for certain selected files. If you select a file and LrC asks you whether to create previews for the selected or all files, “all” refers to your current grid view: the folder or the collection you are in; if you are in “All photographs” then, of course “all” is everything.

I would suggest that you run the “find all missing photos” command from the Library menu: if some are found, your current previews will help you view which files are misplaced (deleted, moved, renamed) But if none are found you can delete the old massive Previews folder and generate the standard previews from scratch. You can choose,in catalog settings, a smaller standard previews size for all your older photos. Don’t create any 1:1 previews and set LrC to automatically discard them after a week or a month anyway. You can choose to generate 1:1 previews only for the photos you are currently working: a certain old batch and any new imports.


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Thanks. Great advice on the find missing photos. I haven't done that in a while.
I don't agree with you on doing the lower res previews vs 1:1. I always generate 1:1 on import when I'm about to do a lot of heavy lifting and I import a bunch of new shots (like you advisaed). I'm not anywhere near short of disk space anyway. I might delete the preview folder though.
Let's face it, we all have images we haven't looked in ages and if we ever do want to bring them up in LR we can generate them then. Computers are getting far more powerful and storage far faster, better and cheaper.
Thanks for the advice on what "all" means. It would just be the folder I'm in on that edit.
Good post. Thanks.

 

[GregJ]

Yes. Once LRC (Lightroom Classic) builds a preview, it will keep it. You never have to tell LRC to save a preview.

Thanks Hal. I get it now. If there is no preview already made and LR generates one when you view a file, it will store it in the preview folder.

But, I experimented today and when there was a preview already generated in the preview folder and I did a few additional edits and then exited, LR did not save that updated preview that included the new edits into that folder. I think it just kept the old one and I had to tell it to update the preview in order for it to save over the one that was there. I could be wrong.

 

[Y.K]

Then use Standard previews corresponding to the dimensions of your computer screen. Standard previews are separate from 1:1 previews.


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[Denis de Gannes]

Interesting. Thanks.
What do you mean by LrC? Cloud or Classic? Cloud I assume. I never touch Cloud. I'm not ready to be conversing with the cloud with my big raw files. Not yet. Maybe in 3 to 5 years. But I doubt it even then. I want my raw files on a fast internal SSD.
LR Classic for sure uses the 1:1 previews in the Library module. That is what they are for. And with the latest update, the literature on that update claims it now uses previews in the develop panel, whereas before they said that every image generated the preview when you opened the image for editing in the develop panel. I could be wrong but that is what the update said.

Yes, I know you don't have to generate previews on import and don't have to later either, and of course when you open a file in LR it generates a preview if one does not already exist. And it atomically generates a 1:1 preview if you zoom.

But you said something I did not know about preview files. Let's say I delete all my preview files - delete the previews lrcat folder. So no previews exist. Then I open a file in LR and zoom to 1:1. LR of course immediately generates a 1:1 preview because one did not exist already on the disk to grab. But t

Quote “ What do you mean by LrC? Cloud or Classic? Cloud I assume.”
LrC indicates Lightroom Classic.
Lr indicates Lightroom Cloud based desktop and Lightroom Mobile apps

 

[clee01l]

You can delete the Previews folder and LrC will create a new on on startup. It will only contain the previews in the sizes that LrC needs to display in the module requested.
In my Import dialog I have the import create only "minimal" previews. With your really fast machine, the delay that you might see when there is not a preview size available and LrC needs to create one, is IMO imperceptible. After all you are only creative one set of previews for one image and not a whole camera card full of images. My Nikon NEFs are 48mp and I have a 4K monitor generating lots of Pixels.

 

[GregJ]

Cletus, that is well said and convinces me that I'm about to delete that folder. It's no big deal. I have plenty of storage space. My Operating System, Programs and LR Folders / Catalog are sitting on a 2TB M.2 PCIe 4 SSD that is not even 40% full. I am really more interested in how the previews behave and function. I know exactly how they update now, although I'm still not sure in one case:
My first post here talked about how I edited a file and then updated the preview (one already existed). It reported that it updated the preview. Then I did it again immediately. LR reported that there was already an updated preview. Then I edited the file and updated the preview. It reported that the preview was updated. That means LR did not do it automatically and then save the new preview over the old one. Unless it does it when you exit LR.

I'm also curious about how the preview behaves in the develop module. On the latest LR update, I read that previews are now used in the develop module. I think I read a couple of years ago that LR does not use pre-generated previews in the develop module but generates a new preview at 1:1 for every file you open in the develop module. I would like to know the ground truth about that. Why? I'm not sure, except that I'm curious.

 

[Johan Elzenga]

I'm also curious about how the preview behaves in the develop module. On the latest LR update, I read that previews are now used in the develop module. I think I read a couple of years ago that LR does not use pre-generated previews in the develop module but generates a new preview at 1:1 for every file you open in the develop module. I would like to know the ground truth about that. Why? I'm not sure, except that I'm curious.

I am not sure what you read, but nothing has changed. Lightroom uses different previews in the develop module than in the other modules. It has always done that.

I am really more interested in how the previews behave and function.

Lightroom is a non-destructive editor. That means that the original files are not changed. Edits are stored in the catalog, not applied to the images. Of course you’ll want to see what your edits look like however (they are applied to any exported image, but you obviously want to see the edits too when you look at your images in Lightroom), and that is why Lightroom uses previews. Whenever and where-ever you look at an image in Lightroom, you look at a preview.

 

[GregJ]

OK John - you aren't reading my posts. I've been using LR for 13 years. You are telling me that it is a non-destructive editor. That's like lecturing me that the world is round. With DNGs, you can write edits to a container in the DNG file because there is no sidecar. But yes, of course it is non-destructive. But I'm talking about the behavior of the preview file and when and how it overwrites the one that can be created at import (after further edits). I'm not talking about the DNG file. (I say DNG in this case because Leica is causing me to have to use DNG files because that is their format, much to my displeasure).

 

[Johan Elzenga]

I’m not talking about DNG either. I’m trying to explain to you why Lightroom uses previews and why they are updated when you edit an image.

 

[Califdan]

I think you are over thinnking this and spending way too much time stressing over it. Keep it simple.

My choice:

1) Generate "Minimal" previews on import. (why waste time generating larger previews on images that may never need them?)

2) Don't worry about it after that as Lightroom Classic builds bigger ones as needed

3) From time to time (every year or so) delete the previews file. (this may not be necessary anymore as 11.4 includes 'automatic prviews purging'

Done

 

[Jim Wilde]

I think you are over thinnking this and spending way too much time stressing over it. Keep it simple.

My choice:

1) Generate "Minimal" previews on import. (why waste time generating larger previews on images that may never need them?)

2) Don't worry about it after that as Lightroom Classic builds bigger ones as needed

3) From time to time (every year or so) delete the previews file. (this may not be necessary anymore as 11.4 includes 'automatic prviews purging'

Done

I'm sort of similar, except that I use the Embedded & Sidecar preview option on import. That way I can cull in the Library module at 100% without having to spend any time building previews (which of course get tossed as soon as I start editing). Once done editing, the standard preview will be built automatically when I access the images in Library. I never have to specifically build 1:1 previews, though they will obviously be created should I zoom to 100% in Library (but discarded after 30 days).

 

[GregJ]

I think you are over thinnking this and spending way too much time stressing over it. Keep it simple.

My choice:

1) Generate "Minimal" previews on import. (why waste time generating larger previews on images that may never need them?)

2) Don't worry about it after that as Lightroom Classic builds bigger ones as needed

3) From time to time (every year or so) delete the previews file. (this may not be necessary anymore as 11.4 includes 'automatic prviews purging'

Done

I think you are over thinnking this and spending way too much time stressing over it. Keep it simple.

My choice:

1) Generate "Minimal" previews on import. (why waste time generating larger previews on images that may never need them?)

2) Don't worry about it after that as Lightroom Classic builds bigger ones as needed

3) From time to time (every year or so) delete the previews file. (this may not be necessary anymore as 11.4 includes 'automatic prviews purging'

Done

I disagree and I'm not sure you are understanding what a 1:1 preview really does, and why we (I) write them on import. In the past 5 years, I have shot tens of thousands of high-res 100-200 MB Medium Format RAF files with Fuji GFX cameras and every GF lens in existence. On every single one of those shots, I generated a 1:1 preview on import. Why? Because on every shot I have ever taken and edited in LR in the past 5 years, the first think I do in the library folder on a 4K 32 inch IPS pro monitor, is zoom onto 1:1 on each shot to check res ate the point of focus. If for any reason the shot is not bone-numbing sharp, I delete it. By creating the 1:1 previews, I am doing something ahead of time that is going to happen anyway.
I have asked two questions that no one knows the answer to, and I count 5 items on this thread of answers that I know are wrong. That's OK. We are having a discussion here.
You probably don't need 1:1 previews. I absolutely do on that first big editing drill. I might not need to keep half a TB of them though. I have 48GB of 1:1 preview files on mu OS disk.

 

[GregJ]

I'm sort of similar, except that I use the Embedded & Sidecar preview option on import. That way I can cull in the Library module at 100% without having to spend any time building previews (which of course get tossed as soon as I start editing). Once done editing, the standard preview will be built automatically when I access the images in Library. I never have to specifically build 1:1 previews, though they will obviously be created should I zoom to 100% in Library (but discarded after 30 days).

Jim, I'm rereading your post here. This may be what I'm after on having a complete understanding of the 1:1 preview behavior on that first big edit after a shoot and when LR creates new previews on its own and how it stores them, and whether or not it overwrites the old preview on its own, or discards it. I wish I could talk to you on the phone. The verbiage is a bit hard to follow on topics like this. And whoever said I need to not stress about this, I'm not stressing, just trying to nail down a basic LR nuance for my own reasons. I think your post here is the key and the best of this thread. I need to try to understand it better.
I don't discard after any days - I keep them forever at 1:1. I of course know that any zoom causes a 1:1 preview to be built right then anyway. But I'm still wondering if on every edit, and new preview saves to the preview folder overwriting the older version that does not contain the new edits. I don't think it does. And I want to know that answer for other reasons.
I also think LR literature via help does a poor job of articulating this.
I'm also curious about how the develop module uses already built preview files. I read in several places that LR does not use pre-built preview files but builds one from scratch on every file you open in the develop module. But on a recent LR update, I read that LR changed that behavior and does use prebuilt preview files in the develop module, which is even more reason to build them on import.
But there are a lot of people on here that are saying things about preview files that are not 100% correct.
And no one has yet answered the 2 original questions. That's OK.

 

[Johan Elzenga]

But on a recent LR update, I read that LR changed that behavior and does use prebuilt preview files in the develop module, which is even more reason to build them on import.

Can you find a link to that? I’m asking because I am 99.99% certain that this is not true. The previews that Lightroom uses for all modules except the develop module are AdobeRGB. The develop module uses a special version of ProPhotoRGB however, so that is also the color space of the develop module previews. https://helpx.adobe.com/lightroom-classic/help/color-management.html

 

[GregJ]

I disagree and I'm not sure you are understanding what a 1:1 preview really does, and why we (I) write them on import. In the past 5 years, I have shot tens of thousands of high-res 100-200 MB Medium Format RAF files with Fuji GFX cameras and every GF lens in existence. On every single one of those shots, I generated a 1:1 preview on import. Why? Because on every shot I have ever taken and edited in LR in the past 5 years, the first think I do in the library folder on a 4K 32 inch IPS pro monitor, is zoom onto 1:1 on each shot to check res ate the point of focus. If for any reason the shot is not bone-numbing sharp, I delete it. By creating the 1:1 previews, I am doing something ahead of time that is going to happen anyway.
I have asked two questions that no one knows the answer to, and I count 5 items on this thread of answers that I know are wrong. That's OK. We are having a discussion here.
You probably don't need 1:1 previews. I absolutely do on that first big editing drill. I might not need to keep half a TB of them though. I have 48GB of 1:1 preview files on mu OS disk.

Jim, I'm rereading your post here. This may be what I'm after on having a complete understanding of the 1:1 preview behavior on that first big edit after a shoot and when LR creates new previews on its own and how it stores them, and whether or not it overwrites the old preview on its own, or discards it. I wish I could talk to you on the phone. The verbiage is a bit hard to follow on topics like this. And whoever said I need to not stress about this, I'm not stressing, just trying to nail down a basic LR nuance for my own reasons. I think your post here is the key and the best of this thread. I need to try to understand it better.
I don't discard after any days - I keep them forever at 1:1. I of course know that any zoom causes a 1:1 preview to be built right then anyway. But I'm still wondering if on every edit, and new preview saves to the preview folder overwriting the older version that does not contain the new edits. I don't think it does. And I want to know that answer for other reasons.
I also think LR literature via help does a poor job of articulating this.
I'm also curious about how the develop module uses already built preview files. I read in several places that LR does not use pre-built preview files but builds one from scratch on every file you open in the develop module. But on a recent LR update, I read that LR changed that behavior and does use prebuilt preview files in the develop module, which is even more reason to build them on import.
But there are a lot of people on here that are saying things about preview files that are not 100% correct.
And no one has yet answered the 2 original questions. That's OK.

Can you find a link to that? I’m asking because I am 99.99% certain that this is not true. The previews that Lightroom uses for all modules except the develop module are AdobeRGB. The develop module uses a special version of ProPhotoRGB however, so that is also the color space of the develop module previews. https://helpx.adobe.com/lightroom-classic/help/color-management.html

This is one of the main reasons I wrote this post. I don't know. I read it digging around trying to educate myself on everything there is to know (generally) about preview behavior. But for years I have thought that the existing preview files in your preview folder are not used in the develop module, so it got my attention when I read that LR Dev Module uses them now to speed up stuff. Hey - it's the internet. It could have been a bogus posting or someone articulating a wrong point. But I think it was on the Adobe site..... And I still want to know if LR saves them after every edit and writes over the old preview file in the preview folder. That is key. I want to know for sure. I don't think it does. If you want that to happen you have to tell it to after edits.
So, I could delete my entire 480 GB preview folder and regenerate 1:1 previews for all 200,000 files and let my computer go to town on that job. That would make every preview saved as the latest edit. Will I do that? No. I'm speaking theoretically. Would it help speed ops? Maybe.
That's why I wrote this post and no one has answered it yet because no one knows.

 

[Jim Wilde]

And no one has yet answered the 2 original questions. That's OK.

There seem to be a lot more than two questions in this thread, so would you like to restate them and I'll do my best to answer them (as everybody else is trying to do!).

Library previews, basically the standard and 1:1 previews, are not used in Develop. When you open an image into the Develop module, LrC initially shows you either the ACR cache entry (which is built automatically when the first raw conversion is done), or the smart preview if you have chosen to build one (the smart preview would take preference if it exists). That is how LrC can get you up and running with the histogram initialised and the develop sliders enabled, whilst at the same time in background the raw file is being read and converted to a "scene-referred" cached preview which will replace the initial temporary ACR cache entry or smart preview. When you subsequently switch from the image in Develop, that cached preview will be discarded.

Once you start editing the image, it follows that the existing library preview is obsolete and is therefore discarded (but not immediately in case the user wants to revert back to the initial opening state and exit Develop without making any adjustments). But assuming the user continues to develop the image, a NEW library preview file is created (i.e. the old preview is NOT overwritten), but initially it will contain only the smaller thumbnail previews which would be used in the filmstrip and navigator (in case you are not aware, a preview "file" contains a collection of differently-sized jpegs which are needed for various display purposes in the Library and other modules). When you finish editing the image and move to a new image or exit the Develop module, the new "standard" preview will then be created automatically and silently when the image next becomes visible on-screen in the Library (i.e. on the filmstrip, in the grid, or opened in the loupe view). If the old preview contained a 1:1 preview then be aware that it is NOT automatically re-created until the use either specifically chooses to build it, or zooms into 1:1 view.

Regarding what you read about Develop using prebuilt previews, I suspect the author has confused you. One of the performance changes that were made in an earlier version of LrClassic was the automatic pre-building of the special Develop previews for the next xx number of images in the filmstrip when opening an image into Develop (so that when you move on to the next image the full "scene-referred" preview is instantly available). But these "previews" are written into the system cache (i.e. they will not exist on disk anywhere), and cannot be specifically user-generated. How many can be pre-built will be determined by the system capabilities at that time.

 

[Y.K]

But I'm still wondering if on every edit, and new preview saves to the preview folder overwriting the older version that does not contain the new edits. I

When you edit a photo, LrC automatically marks the older Standard preview as "invalid". It will automatically replace (overwrite) the old version with a fresh one. Whether it will do it a) just after you change to a different photo in the develop module or b) when you next view this particular photo on the Library module (grid or loupe) is imaterial - it will certainly happen. I don't think LrC will update the other 1:1 preview corresponding to the file in this scenario: this will certainly be created on the fly (replacing the older 1:1 preview) when you next zoom in the photo in question in the Library module. But it's possible the 1:1 preview gets updated along with the standard preview.

The experimental way to check all this is to create checksums of all the files in the previews folder and then check how many of them have changed (and when) after you edit the photo in the develop module.

I'm also curious about how the develop module uses already built preview files.

It doesn't ! The Develop module doesn't use at all the standard or the 1:1 previews we are talking about. It caches image data read from the original files in the Adobe Raw Camera Cache. These are "sort of" previews but not like the jpg ones used by the Library module. They make the Develop module run faster.

 

[GregJ]

Jim and YK, that is what I was wanting to read and 95% of that is exactly what I have thought to be true for ages. But I read something on the Adobe site that threw me off.
Jim's very well written and clear explanation should be pasted into the Adobe help site when you search for preview. It is way better than what is there now. They should add in your comments as well. Very good. Thanks.
I experimented yesterday and it seemed that after edits the 1:1 preview file was not changed in the preview folder. I had to tell it to update them. Then it did right then. But I bet I needed to exit LR for it to happen, so I didn't see it. I bet it happens on exiting the program. It doesn't start overwriting older preview versions until it knows you are done editing. I'm guessing though. They key is that it does happen and every possible edit is included in a new stored preview file. Even if you edit on a raw file in the dev module every day for a year.
Thanks so much. This is clear now and

 

[GregJ]

I've been thinking.... You guys didn't know all this until after this thread started. That's cool ... but you either did some fast research or the Queen told you the answer. Hey - we all learned something.

 

[Jim Wilde]

Greg, let it go, you've got your answers so leave it at that. Tossing out cheap insults to folks who are giving up their own valuable time to try to answer questions here is distinctly uncool.