Difference in file size: LR and ACR

[Barnack7]

Why are raw files opened through LR twice the size of a file opened through ACR, when no adjustments were made in either, and saved as a TIFF? Raw files opened via LR are also larger than when opened via ACR and saved as a PSD. Why is this? Australian photographer Peter Coulson says LR creates a DNG when a raw file is opened, but ACR does not. Is he correct?

[JohanElzenga]

No, he is not, but you have to explain in more detail what you are doing. Lightroom does not 'open' raw files. It adds adjustments non-destructively. If you get a TIFF or a PSD, them you must have made a second step. Like sending the file to Photoshop or exporting as TIFF or PSD. 

[Barnack7]

Thank you for the quick reply! After highlighting the raw file in LR, I did
a Command-E to open it in LR, after no changes were made in the develop
module. When I saved it as a TIF, I selected No Compression, and made no
adjustments in PS either

In the Mac Finder, I dragged the raw file on the PS icon and hit Open in
ACR with no adjustments being made, and again Saved with no adjustments in
PS, as above.

Sincerely,

Barney

[Conrad C · Accepted solution]

I’m assuming when you say “opened,” you specifically mean “opened into Photoshop” and then saved as a TIFF?

The file size of a document opened from Lightroom Classic into Photoshop depends on the settings in the External Editing preferences.

The file size of a document opened from Camera Raw into Photoshop depends on the settings in the Workflow preferences.

If those settings are the same in both applications, the resulting file sizes should be about the same.

If one is twice the file size of the other, the first place to look is the Bit Depth. If one application is set to 16 bits per channel and the other is set to 8bpc, the file size will be about 2x. The second place is to see whether either application has any resizing (specifically resampling) applied.

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@Barnack7 wrote:

Peter Coulson says LR creates a DNG when a raw file is opened, but ACR does not. Is he correct?

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I have never heard of anything like that happening. Various types of files can be created in the course or viewing or exporting a file, such as a JPEG-based preview or thumbnail, but anything DNG-related, such as a DNG file or a DNG-based Smart Preview, would be created only on demand, not by viewing/opening an image. Do you have a link so we can see how he was describing it?

[Barnack7]

You are exactly correct! It was 8 bits/channel when the raw file was run
through ACR, and 16 bits/channel when run through LR.

I will search for the link to Peter Coulson's video. He claims he can see a
difference in files run through ACR (they look OK) versus those run through
LR. That's what made me run this test. I cannot see any difference.

Thank you again!