print to JPEG file - optimal output sharpening

[PhotoMagical]

I have recently been making collages in the print module of Lightroom making print collections and saving to a final JPEG file (print to JPEG file)

My final goal is to print the collages, however, I want the flexibility to print them in various A sizes.

Until now I have just chosen A2 in the Page setup, as I thought it was better to downscale than upscale later on. Nevertheless, I've been wondering about output optimal sharpening🤔

will the process of saving to a A2 in a 300 ppi file resolution, and then later on printing to say a A4 result in non-optimal sharpening ?

also I am uncertain of the correlation of printning a bigger size to a smaller or vice versa and whether it results in over and under sharpening ?

hopefully, someone has some experience and  insight on this topic🤞

Best regards from Lars

[Marianne-Deiters]

The final, optimal sharpening is best done in original size and under visual control on the JPEG. Lightroom has sophisticated manual settings for it. Then you have to test the optimal settings by comparing the monitor and printout. After upscaling, there is definitely more loss of detail and sharpness than with reduced output.

[PhotoMagical]

Hey Marianne. Thank you for your answer but I don't feel that you were answering my specific question.

when I layout my collages , I need to choose a print size in the page set up and save as print collection. 
i've been figuring that the most flexible solution was to choose the biggest possible size, that I later might print. 
I then choose print to JPEG file and save out the collage to a single jpg file in 42 x59,41 cm with a file resolution of 360 PPI

but what happens if a later choose to import this single JPEG file into Lightroom and print in a smaller size say A4 ?

one additional question. What is the purpose of custom file dimensions ?  I only see this option in the print module for print collections.

The reason I ask this is, that I altered the numbers from A2 42,2x59,41 to A4 21x29,7 thinking that it would scale all the pictures in the collages appropriately to fit A4. However,  I now to my horror see that it was a mistake. it simply rearranged all the pictures and when returning to the original size of a A2 the pictures are still a mess.
Is there anyway to fix this ?

Best regards from Lars 🤗

[Todd Shaner]

The simpleset solution is to create the A2 layout JPEG file with the desired output sharpening. Then Use the LrC Export module to resize it to A4 dimensions and apply the same desired Output Sharpening in the Output module. That way you only need to use one layout and stll can control the target sharpening. I doubt you will see a difference in sharpening when compared to A4 layout Output Sharpening in the Print module.

What is the purpose of custom file dimensions ?

If outputting to JPEG you should be entering your page size in the Custom File Dimensions. More Help here:

https://helpx.adobe.com/lightroom-classic/user-guide.html

[Jao vdL]

You indeed absolutely want to do the output sharpening at the final printed size. The difference is very noticeable when you test (at least to me). Lightroom does the sharpening at the resolution you specify in the output and so the result is very different for a A2 print at 300 ppi as it is for a A4 print at 300 ppi. Even though the A4 file will be much smaller (about a factor of 4), the print from the A4 specific file where you had Lightroom scale and sharpen at the correct size will appear much sharper. 

Do note that the difference might not be observable to most people but I tested this double-blind with people asking what print they prefer and they generally prefer the prints that were sharpened at the correct size.

[Todd Shaner]

Just to clarify this is being done to prevent the need to create separate A2 and A4 collage layouts.

I totally agree with Jao. I tested my original suggestion of using the A2 print layout JPEG with resizing to A4 in the LrC Export module using the same sharpening setting. This results in an over-sharpened A4 image file!

What appears to work is to create a 2nd A2 print layout JPEG file with 'Print Sharpening' unchecked. Then resize it to A2 and select the same output sharpening in the LrC Export module. When I compared to an actual A4 size file created in the Print module the sharpening was virtually the same. Interestingly the  resized A2 file looks very slightly sharper than the Print module A4 sized output file. Not sure why.