Confused about DPI and image size and depth

Report · Jul 03, 2018

I have been scanning my college days (circa 1972-1980) 35mm color slides into the computer. In the (Epson V550) software, I set the depth to 24bit and the DPI to 4800, in case I want to blow any printed up to poster size, and I set the image dimensions to 1.28 x .86 (which is the actual measure of the photo portion of the slide.

After editing and saving an image in Camera Raw, I open the images in my preferred viewer (Lyn for Mac), and it lists the DPI of the PE edited image as 300. The image resolution remains the same (4060 x 6113 pixels). But when I view the numbers inside of PE, it lists the resolution of my edited and 'saved as...' file at 4800 DPI and the size is very close to the original: 1.274 x .846. Also, I see in Finder the file has greatly increased in size (3.9 to 6.6 meg).

Clearly confused here! And then there is the depth. I don't fully understand it, but from my audio experience, I thought depth dealt with quality, so that 24bit had more information and thus larger file size, but here, dropping from 24 to 16 added over 2 meg to the file size.

Most important to me is the DPI, and I have no idea why it would read 4800 inside of the PE image>adjust size but only 300 in a photo viewer. I don't know why PE does not support 24 bit. And I don't understand why reducing the DPI and bit would result in a significant increase in file size.

Report · Jul 04, 2018

Some thoughts:

DPI = dots per inch. This is determined by your printer.
PPI = pixels/inch. This is the resolution of the file, and can be altered by re-sampling.
The expectation that one can enlarge the tiny bit of information inside the 35mm. slide to poster size is unrealistic.

What are you really trying to do? Where is the problem?

4 Have you tried scanning these old slides with a dedicated scanner, e.g. Wolverine 35mm Film to Digital

Converter?

Report · Jul 04, 2018

Hi again! When I was searching for scanners for this project, I did not know of the Wolverine or anything specifically dedicated to such purposes. I did not know to not limit my search to flatbed scanners.

I'm doing two things, One is to be able to view my slides on a large TV. While I currently have a 65" TV, in the not too distant future, I expect to have a much larger one for a dedicated home entertainment center. Second, I may want to print a few of my best into a size larger than 8x10, with the size perhaps being limited by the image quality.

I'm entirely self taught, and what I read said to make the scan size the same as the original, and the more DPI the better (which is what Epson labels it...there is no PPI setting I can find). I chose 4800 as a compromise. It takes 1:30 to scan one slide in at this resolution.

Report · Jul 04, 2018

I have digitized about 200 35mm.slides using the Wolverine converter with excellent results. There are other machines for this purpose as well - check on Amazon. One inserts a camera card into the machine, and uploads to the computer via this medium. The resolution of the digitized file is 72px/in. Generally speaking, it is desirable to have resolution in 240-300 px/in range for printing, but I have printed at lesser value without undue pixellation.

As for display on your TV, I have no experience with this, but you can try it with one of your files.

There are also commercial labs that digitize, e.g. https://dpsdave.com/slide-scanning-prices/

Report · Jul 04, 2018

Well, I'm about a third of the way through my color slides (plus too many B&W negatives to count), so I'm stuck with what I've got. They look great on the 65" TV. In fact, even on my 27" iMac I can't always tell if the focus is right on. It has to pass the big screen test before inclusion in the final collection.

I think I'll just ignore the issue then. Maybe I'll email Adobe with the question of why the same image reads 4800 DPI in PE but 300 in my image viewer.

Report · Jul 04, 2018

cglenn7777 wrote
Maybe I'll email Adobe with the question of why the same image reads 4800 DPI in PE but 300 in my image viewer.

I'm not aware of an email address for Adobe Support.

There is a chat:

Contact Customer Care

Report · Jul 04, 2018

Yes, I found that out. I don't have much faith in chat people when it comes to this sort of question, so I'll call tomorrow. It's easier to get escalated on the phone than in chat.