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Exporting JPGs from Lightroom Smart Previews VS Originals

From time to time I accidentally hit the export button in Lightroom when my original files are not connected. When this happens, I cancel the process, connect my external drive, and re-export. But every time I hit that cancel “x” on the export window I noticed the export SEEMED to be going a lot faster than usual. This would make sense given that Lightroom need only reprocess a very small DNG file into a JPG versus a 25+ MB raw file. The reason I’ve always canceled the export in the past is because I figured the quality would be better if the develop adjustments could first be applied to the original and then exported.

So I went about searching “Lightroom original versus smart preview export quality” and couldn’t find any answers on the first page of Google, and I couldn’t be bothered digging any deeper than that. So naturally, I decided to invest three hours into running and writing up my own experiment.

Hypotheses:

JPGs up to 1285 px on the long end exported from Lightroom Smart Previews will:
a. export 25%-50% faster than from the raw files
b. be identical in quality for all web based purposes

Methodology

Test Subject: 100 5D Mark IV CR2 Files from the BCCK Cocktail Night Networking Event

I always prefer real world testing so I’ll be pulling the first 100 files I delivered for the last event I covered. Events are typically always shot under less than ideal conditions. This one is particularly dark so if quality issues arise I think they should be especially prevalent on these images shot at ISO 3200 and up.

I’ll be using Lightroom to export a set of “Web Ready” images for my clients which is preset to 2048 pixels on the long edge at 80% quality. I chose these dimensions based on Facebook’s maximum HD image upload size. I chose 80% quality because I haven’t been able to notice any quality difference and the file size is substantially smaller. I’ve also heard Facebook will apply more aggressive compression when images are larger in file size.

It should also be noted that Lightroom Smart Previews seem to have a maximum long edge of 2560. The files out of my 5D Mark IV get resized to 2560 x 1706. From what I’ve read the previews would be smaller if your original file resolution were smaller. Logically, exporting JPGs that are larger than the smart preview would be detrimental to image quality and I believe it’s not even possible. I’ll check right now… okay, I tried exporting a smart preview at 5000px and the result was a 2560px JPG regardless of if “Don’t Enlarge” was checked or not. (I wonder why Lightroom doesn’t warn you about this in the export dialogue.)

The original files were hosted on an external SSD connected via USB 3 on a Mid 2012 MacBook Pro. The JPGs were directed to export into my dropbox folder on a 5200 RPM internal spinner in said MacBook Pro.

Results

Speed Results
100 5D Mark IV CR2 Files Exported to 2048px JPGs  = 15:04.60
100 Smart Previews Exported to 2048px JPGs = 1:49.77

Analysis
WOW! So there is a MASSIVE speed difference!!! (904 seconds / 109 seconds = 8.29) The export is over 8x faster!!! That’s A LOT of saved time which means a lot of saved processing and battery power too if you’re not plugged in. All this saved speed isn’t worth much though if the image quality is poor. Lets check that out next.

Quality
100 5D Mark IV CR2 Files Exported to 2048px JPGs  = 65.8 MB
100 Smart Previews Exported to 2048px JPGs = 63.2 MB

Before examining the images I decided to do a quick comparison of folder sizes. Interestingly the folder containing the JPGs that were exported when Lightroom had access to the original files is larger. This along with the longer processing time is a clear indication the Lightroom is doing something different when connected to the originals.

Folder size differential between 100 JPGs exported from original file vs smart preview.

Individual image size differential between JPG exported from original file vs smart preview.

Now let’s see if that 0.04% size difference has any visible impact.

Below we will look at some side by side comparisons. The JPG exported from the RAW files is always on the left. The JPG exported from the Smart Preview is always on the right. I have also cropped at 100% so we can see the detail. (Keep in mind this is a RAW converted to DNG screen-shot as a PNG, converted to JPG agin,  compressed for the blog and then in some cases recompressed by wordpress again because was too lazy to resize the screen shots to native blog size)

A sponsor shot from the event under good light at shallow DOF. See any difference?

Same shot as above at 100% crop.

I chose this image because there was noticeable noise in the shadows. It doesn’t get noticeably worse when exporting from the smart preview.

Same image as above at 100% crop.

Again I was looking for poorer quality in the darks/shadows.

Same image as above at 100% crop.

Here I wanted to see if skin tones rendered alright. Fill flash was used.

I thought we should bring in a well lit studio style portrait to see if quality loss might be more noticeable on something clean. Again I see no difference.

Same images as above at 100% crop.

Analysis
There seems to be no noticeable quality loss to the images both at original and 100% crop.

Verdict

If you need JPGs that are smaller than 2560 for strictly digital usage then there in no reason not to export those suckers at 8x the speed from Lightroom’s Smart Previews. Not only will you save a ton of time, you will put less stress on your processor and reap all the benefits that come with that like less power consumption and heat dissipation. If you’re originals exist on a separate drive it’s as easy as not plugging it in when you export. If your catalogue, previews and images exist on the same drive you can simply modify the name of the file containing the images and lighroom will loose the link to the file. I simply add an “X” to the end in my finder/explorer window, ie. “03-Selctets” becomes “03-SelectsX”. Once I’m done editing/exporting I remove the “X” from the file name and all is linked up again.

Feedback

If you see flaws or oversights in this experiment please point them out in the comments below. Hopefully this info will help save all of us heaps of time. Thankfully I can now makeup in export time the few hours I spent putting this post together 🙂 If it helped you please share it with your photo friends!

 

 

 

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