Capture One Sessions and How They Work

May 21, 2019  •  Leave a Comment


Capture One offers two systems to manage images, Catalogs and Sessions. This document focuses on Sessions for the simple reason of the simplicity of the design. The basic difference is the storage of the adjustments per image are stored in the Catalog database and depends on the database to create a directory structure. Sessions on the other hand allows you to create the directory structure that makes sense to you and not the black box. (Sorry but I have several old catalogues form years ago that will not work. The RAW images are there but digging them out is painful.)

In my past life I worked for a large software company designing software and doing analysis of other software and how they work. I’m not a programmer but a good “hacker”! So I put on my Hacker hat on and took a closer look at Sessions.

Sessions are relative simple with little mystery about them. Basically a Session is a Folder with a controlling file called the Session (name) .COSessiondb extension. The db at the end would indicate that it is a database and is some sense it could be consider it to be but I would rather call it a controller and tracker file. So what does it track? Well we will get into that as we go. The main goal to to get Image files into the Session. These are again directories called Session Favorites in the interface. There are two ways to Session Favorite, Importing within Capture One Import took or by using the Finder or Explorer to create and populate a Folder. If you do the later you need to tell the Session what and where this folder is located by using the + next to the Session Favorites and navigating to the Folder that contains the image files. The puts the   Folder name in the CO interface and lets the COSessiondb know about it such as it’s location and so on. As soon as one adjustment is made to an image a Sub Folder is created in the Session Favorite folder called CaptureOne. Under the CaptureOne folder there are two folders called Cache and Settings120. The settings for each image are stored in the Settings120 folder with the image name .cos files. More on this later but for now just know that the settings for each image are stored in the same folder structure as the Session Favorite folder name so it creates a well organized package that can be added to moved and so on. No Mystery, No Black Box!

Note: In the above the sub folder called settings120 the 120 is the version of Capture One engine. If you migrated from say version 11 the you will see a folder called settings110. So if you want or need to go back a version on the engine you still have the old settings. Nice!

More Details!:

Time for more details about Session Library, Session Folders, Session Albums and Session Favorites.

Session Folders:

Session Folders are just that, Folders that are created when you create a Session called Capture, Selects, Output, and Trash.  The names are hardcoded in the interface but each one can be changed if you so desire to what ever you want. For if you want to rename the Trash folder to Junk you can but in the interface it will still be called Trash. Here is how you do this:

  • In the CO Library interface select the Trash Folder
  • Right Click and you will see New and a sub dialog with Trash Folder
  • Create a new folder called Junk
  • It will ask you if you want to save the existing Trash folder in the Session Favorites. You can say yes or no in either case you can deal with this later if you say yes!
  • Now when you put images in the Trash in CO they will go in the Junk folder. You can put this anyplace in your directory structure you want but I suggest keeping it under the Session folder or a sub folder. 

You can do the same for the other three folder if you so desire. I have tested this but I just accept the defaults.

Now if you do this and forget where you put the folder or something you can Right Click the Trash Folder in CO, in the sub dialog you will see three options Show in Library, Show in Finder and Show Details. I use Show Details. Right Click on a detail and you will get a Hide button.

The renaming and locations are stored in the COSessiondb file from what I can tell. (Thats where my hacking experience comes in!)

Session Albums:

There are two albums supplied by default called Five Stare and All Images. There two types of Albums, Smart Albums and just Albums. Smart Albums are nothing more that a collections of Filters like Ratings, Color Tags and Keywords. They get any image that fits the criteria of the filter that exists in the folders in the Session Favorites. Albums are what ever you want to drag and drop into that Album. 

By the way Session Albums can re added and renamed easier than the Session Folder. Just follow the interface.

The criteria for the Albums are stored in the COSessiondb file,

Session Favorites:

Session Favorites are simply a Folder with Image files and a CaptureOne folder with sub folders to contain information relative to the Image files. So think of this as a CaptureOne container. 

Now that said you can do most everything from Finder to populate a folder with Image files. Remember from above that I said the COSessiondb files keeps track of Session Favorites or I can say Session folders so we need to tell this file what is going on. This is done from within Capture One Session Favorites.  Lest start with and example:

Lets say I just did a shoot of a Saw Mill and I want to download the image files from the camera or memory card to a folder called Saw Mill. You can do this will Explorer of Finder with a simple Copy. You can move the Saw Mill folder where you want on the hard drive. Now we need to tell Capture One about this folder. Open Capture One and either navigate to or create a new session. To tell the CoSession.db about this folder you need to go to the Session Favorites  tab and click on the + and navigate to the Saw Mill folder. This makes the entry with the COSessiondb file and puts the folder name in the Session Favorites list. Simple! No other way to do this that I know of. Same way if you want to remove the folder you just use the - in button.

So we have made the relationship and we are ready to work. As soon as you make one adjustment on one of the files in Saw Mill, CO creates a sub folder in the Saw Mill folder called CaptureOne. In CaptureOne folder there are two sub folders called Cache and Settings120 (the 120 is the version number of Capture One. Handy to know!) 

In the Settings120 directory there are two file types that are created; a .cos file that contains all the settings for the Image file and if you create a mask there will be a .comask file. Both have the full name of the image file.  Now you can rename these files through finder but you may have problems with Albums if you do this with Finder so it is best to remame the image file through CO. This will take care of everything for you including Albums.

In the Cache folder there are two folders called Proxies and Thumbnails. Not sure of the total functions of these files and I just don’t think it is important to totally explore these for this discussion. But since they are in a Cache finder I know these are files used to speed up the operation of CO. They can be deleted and CO will regenerate these files if they are missing but  it is best to just leave them alone and let CO do what it needs to do. Just be aware of these files.

Same way with the cos files. There is nothing you can do within the files other than to know where the settings for each image are stored. If you delete a .cos or .comask file then you will loose you settings for that image file. Best not to touch these file!

Do’s and Don’ts of Sessions!

So now that you know what a Session Favorite is and how it is organized there are some things you can do and somethings you should avoid. In General its best to let Capture One do as much as possible but you can override as if needed. Why would you need to override? Well you may want to split a Session that would require the use of Finder to move folders for example. What ever reason you can recover. In this section I will try and give some examples of what to do or not do and the easiest way to get there.

Can a Session be renamed?

Yes but but be careful! This can not be done in CO that I have found but can be done in Finder. Remember a Session is a folder with a  tracking file, the .cosessiondb file. If you rename the Session folder rename the .cosessiondb file with the same name. Best to use Copy and Paste to make sure there isn’t a typo. I have gotten away without renaming but ran into problems down the road. You will have to rename using the Finder or Explorer and make sure you are not in the Session during the name change.

Can a Session be split?

Yes but not in CO.  What you can do is in Finder just move the Session Favorite folder form one Session to the new Session as you want. Now you have to make the .cosessiondb happy so where is what you will need to do.

    • In the session folder that you have moved the folder from you will need to open that session and remove it from the Session Favorites with the -. This will tell the cosessiondb that they folder has been removed. Now depending on where it it moved to there may or may not be a warning icon next to the favorite in the interface.
    • In the new Session you will need to use the + in the Session Favorites to associate it with the cosessiondb file. Done!

Can a Session Favorite belong to two different Sessions?

Yes they can. One thing I should point out is a Session Favorite folder doesn’t know who it belongs to, only the Session cosessiondb knows who belongs to the Session. So it can belong to two or more Sessions. Simply use the Session Favorites + to grab any Favorites folder anywhere.

Now why would I want to do this? Lets say you want to grab several images from a different Session you can grab that Session Favorite folder and select the Images you want and Drag and Drop! When you are done just remove the Session Favorite with the - tool.

Can I backup or restore a single Session Favorite?

Yes. I have done this with a three year project where the full Session is bigger than I have room on my hard drive. Generally I will work directly on the USB hard drive on that Session it there are little changes but if there is major work then I just pull a partial session on the Mac and back up when done. If you have done anything with Albums make sure you update the .COSessiondb on your backup.

The other technique I use is to make a Temporary Session and just bring in one or a few Session Favorite Folders and backup just those folders.  (For reference I use a backup software called Backup Guru. The no frills version is simple and doesn’t do any compression or packing in some strange format. Keep it simple!) 

What do I do if something goes wrong?

For some reason I have gotten blank images in the Browser. I haven’t been able to pin down the cause but it is generally when I look at a Smart Album in a session I have moved or removed a Session Favorite. Generally what happens is there is a duplicate image one with an image and one blank. Select the blanks and put them in the Trash.

If for some reason a Session Favorite is blank then next to the Session Favorite there is a warning icon. If that is the case just remove the Session Favorite with the - tool and get re assign it with the + tool.

You can try changing the search settings in the Session Favorite form Absolute to Relative to Session in the Session Favorite Show Info but I have not had much luck doing this. Just Right Click the Session Favorite in question and select Show Info. The only thing you can change here is the search method form Absolute to Relative to session. Now doing a little testing on the Mac it doesn’t behave like I expect. I blame this on the fact that Mac uses a Unix operating system with a Mac front end. As I recall from years ago Unix search is different than Windows and was more robust so this may search differently on the Mac. (I currently don’t have a Windows computer to test this so just an educated guess!)


To me Sessions are the more user friendly than Catalogs. Less black box and more versatile. The only thing missing is being able to search for an image outside of a Session. My question with a Catalog is what happens when you run out of disk space? Start a new Catalog? Same with Sessions!

The bottom line in this discussion is to let Capture One do as much lifting as possible, they do a great job for the most part. But if you need to do more I hope you will have a better understand of how Sessions work. My belief is if you understand how something works you can understand how and why to do things.

Disclaimer: All the testing was done on a Mac Book Pro and not on Windows so there may be some differences.  I have been using Capture One since about version 3 and started with a Windows machine. In 2015 I switched to Mac, a move I have not regretted!  




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