downsizing a big image

I've photomerged an image considerably larger than necessary (18000 x 12000 dpi)
Don't want to crop anything...want to end up with a 20 x 30 inch print.
Would I increase sharpness by merely downsizing to 9000 x 6000 pixels?

Am I being clear with my question?

In Photoshop's Image Size dialog there are resampling options. Try all three of the Bicubic options - Bicubic, Bicubic Smoother and Bicubic Sharper. Compare and see which you prefer.

Truly, though, it doesn't matter worth a damn. It all comes down to how you sharpen for print after resizing.

chauncey wrote in post #18328944
I've photomerged an image considerably larger than necessary (18000 x 12000 dpi)
Don't want to crop anything...want to end up with a 20 x 30 inch print.
Would I increase sharpness by merely downsizing to 9000 x 6000 pixels?

Am I being clear with my question?

No, you aren't being clear. "18000 x 12000 dpi" doesn't make sense. Do you mean 18000 x 12000 px?

For a 20"H x 30"W image at 300 pixels per inch (which is what many commercial printers want as a minimum) you provide a image file which is 6000H x 9000W pixels, and you can simply use Resize in many programs to shrink 12000 x 18000 pixels by 50%

Wilt wrote in post #18329001
For a 20"H x 30"W image at 300 pixels per inch (which is what many commercial printers want as a minimum) you provide a image file which is 6000H x 9000W pixels, and you can simply use Resize in many programs to shrink 12000 x 18000 pixels by 50%

Why would commercial printers want 300 ppi? Isn't that what a customer would want?

If the requirement is 300 ppi, then a 20 megapixel camera will only produce files for 12x18 inch prints and smaller.

Don't ask me why, apart from the fact that if they failed to specify that, the naive, uninformed consumer asks for a 20" x 30" print from a 2000 x 3000 pixel image, and then complains to the printer that "It looks like schit" and asks for a (free) reprint in the next breath.

I can tell you that occurred to me on my very first order of a large print from a well known commercial printer resulted in an email response to my order that I had not provided them a large enough image for the print size ordered...that happened about 14-15 years ago, with a photomerged double-photo image from a 4MB Canon G2!

I must have a fundamental misunderstanding event...it would seem that if you compressed a 300 dpi 60 x 40 print down to 300 dpi 30 x 20 print,
it would inherently be considerably sharper.

chauncey wrote in post #18329273
I must have a fundamental misunderstanding event...it would seem that if you compressed a 300 dpi 60 x 40 print down to 300 dpi 30 x 20 print,
it would inherently be considerably sharper.

No, it depends on what algorithm you use to resample. See Damo77's reply above. He mentions three bicubic algorithms in Photoshop. If you use IrfanView to downsize, there are five more algorithms available.

Is this valid...http://kronometric.org …%20sampling%20m​ethods.htm

The dpi value has an effect for OFFSET PRESS lithographic printing of photos only. Printed at 300dpi, the photo would have reasonable good quality like on a National Geographic page, but printed at 75dpi it would look pretty pathetic with daily newspaper quality

But If you took a 6000 x 4000 image (dpi value irrelevant) and printed it on a large format inkjet printer to 20" x 30" the resulting print has 200 PIXELS per inch, and the 200 ppi would indeed appear 'sharper' print, while if you printed the same image to 40" x 60" that print has 100 PIXELS per inch,

But BOTH PRINTS printed on the inkjet have been printed at the INHERENT DPI of the the printer (and ignoring the DPI value in the EXIF)...depending upon which Epson model printer, it might have 4800dpi or 9600dpi (dots of INK per inch).

Canon calls it dpi but it is really ppi. In PS resolution it is ppi which is for prints. Like Wilt said dpi is for offset printing and I can back that as I was in print media for over 30 years.

This video spends most of the time on upsizing but there is info on downsizing. While I have attended classes and read numerous posts about using bicubic sharper the presenter recommends just bicubic when downsizing. This is also the first video I came across that said ppi has no effect on screen or web files.

It may be helpful.

https://www.youtube.co​m …4mV3NsLmXw&feat​ure=relmfu

Here is one on sharpening if you are interested. I have applied key features from both for my resizing and sharpening action but I have never worked with file as big as yours.

https://www.youtube.co​m …ure=player_embe​dded#at=20

I always sharpen as my last step after any uprez or downrez... As has been stated many times, there are many different types of algorithms written to take an image and resize it, and depending on the original detail of the image, choosing an inappropriate method may make the image less sharp. Again, I mitigate that a bit by always running a small amount of USM just to make sure, after I downsize.

TeamSpeed wrote in post #18330084
I always sharpen as my last step after any uprez or downrez... As has been stated many times, there are many different types of algorithms written to take an image and resize it, and depending on the original detail of the image, choosing an inappropriate method may make the image less sharp. Again, I mitigate that a bit by always running a small amount of USM just to make sure, after I downsize.

That was what the presenter said. Applying bicubic sharper can tend to over sharpen and you can make up for it sharpening at the final size which is the correct thing to do.

digital paradise wrote in post #18330074
Canon calls it dpi but it is really ppi. In PS resolution it is ppi which is for prints. Like Wilt said ppi is for offset printing and I can back that as I was in print media for over 30 years.

Even those of us who understand 'dpi' vs 'ppi' can mis-speak! In the above statement, it really ought to state,
"Like Wilt said DPI (dots of ink per inch) is for offset printing and I can back that as I was in print media for over 30 years."

I even corrected my own post 10 only 5 minutes ago, to fix another 'mis-speak'!

And Canon is not the only software supplier to 'mis-speak' within the software user interface. Photoshop was also known to mis-use the term 'DPI' in its UI when it really meant 'Pixels per inch' (and not 'dots per inch')

Noted and corrected Still on Thailand time and completely messed up. Can't keep my eyes open 7 PM and still getting up at 4 AM