rehess Loc: South Bend, Indiana, USA

When too many thoughts are in a Original Post, people seem to get lost. Just one request here - how large in MP does an image have to be to "look right" from a decent viewing distance, to not look pixilated?

burkphoto Loc: High Point, NC

Well, 1080P HDTV looks good from its diagonal distance away from the screen (i.e.; 5' or more from a 60" screen) and it's only about 2.1 MP!

4K TV looks good at up to half the distance that 1080P HDTV looks good (i.e.; 2.5' or more from a 60" screen), and it's only about 8.3 MP.

An 8x10 at 240 PPI (extinction resolution for the average human eye when viewed at 13") requires around 4.6 MP.

At the lab where I worked, we wanted 300 PPI for a 5x7, 250 PPI for an 8x10, 180 PPI for a 16x20. I know this is counter-intuitive, but it is true for average scenes that are viewed as WHOLE images (you're far enough back to see the whole image). We printed 16x20 and larger sizes up to 40x60 on an Epson inkjet at 1440x2880 dpi, so we liked images to be 180 PPI.

If you view a print from the same RELATIVE distance that you would view a smaller print, it can have the same MP count. (i.e.; a 16x20 viewed at 26" and an 8x10 viewed at 13" will look the same, if printed from the same 250 PPI (5MP, 2000x2500 pixels) file. But most folks prefer more than 125 PPI resolution in a 16x20, on the off-chance that someone will get really close to it. That's why we preferred 180 PPI as the minimum when we made 16x20s. Photos of large groups of people (>100) would require 250 PPI or even 300 PPI, because viewers pixel peep them (view very closely) to see individual faces. The same would be true for a landscape scene that will be scrutinized thoroughly at close distances. That's where the 100 MP Hasselblad backs come in handy... (I'm only slightly serious about that!)

Remember that visual acuity usually decreases with distance, so smaller prints usually require MORE detail than larger ones. This is why very large prints can be made from very small sensors and still look good. Few people pixel peep a 40x30. If you have sufficient resolution for a photo quality 15x10 at 250 PPI, it will make a nice 40x30, so long as the subject matter does not require close scrutiny. A print that big is so large, most folks will view it at its "normal" or "standard" viewing distance of 50 to 75 inches. That would be like viewing the 15x10 at 25".

In all my examples here, I'm assuming that "PPI" refers to spreading at least that many ORIGINAL, FROM THE CAMERA pixels over each inch of the print. If you are going to enlarge an image, use a program that is good for that, and get the pixel count right for the lab. Photoshop will do it, but ON1 Resize is better for that purpose.

I hope that helps.

rehess Loc: South Bend, Indiana, USA

Thank you. I didn't want to influence answers too much by my question, but I'll add some detail now. Back in 2003, when I was first thinking of going digital, I sent some slides to a professional to be scanned. He returned 3000x2000 scans {I think of everything in terms of pixels}. I set up my projector in our family room next to the computer and compared scans to slides; every detail I could find on a slide was also on the corresponding scan, so I decided that I would go digital when cameras that provided at least 3000 pixels horizontally were available. At that time, recognizing that I don't always have the most expensive lenses, my wording was "that matches the quality I've been willing to pay for". More recently, I've been using a Pentax K-30, which allows me to use again the Pentax-A 50mm f/1.7 kit lens l'd used to take most of the slides in my original test, and the resulting images are sharper than the slides had been, so my projection system {dumped several moves ago} is still "suspect", but the lens was not a serious issue.

Today, when I scan slides, I typically crop / downscale down to around 3000 x 2000, but my cameras are producing much larger images. As I look at the scans before I process them, I don't see anything being lost, but I was wondering whether there is something else that should be bothering me.

burkphoto Loc: High Point, NC

As a rule of course, when I photocopy my slides and negatives with a macro lens on my GH4, I save the maximum size image I can capture in raw. Storage is cheap.

I agree, 3000x2000 is sufficient to make a photo quality 12x8 inch print at 250 PPI, and that is good enough to enlarge or reduce to any reasonable size, so long as the content will be viewed at a distance equal to or greater than the diagonal dimension of the print. W^2 + H^2 = D^2 so the square root of (W^2 + H^2) is that diagonal.

Here is an example of an image copied from a 35mm Kodachrome 64 slide, and another of an image copied from an HP5 negative:

Rusty Gears and Chain, 1983

(Download)

Lynda, Charleston, 1986

(Download)

Gene51 Loc: Yonkers, NY, now in LSD (LowerSlowerDelaware)

This has the answer to your question, which is essentially boils down to - it depends - on viewing distance (print size is implied here), eyesight, subject material, medium used for the print:

http://www.photokaboom.com/photography/learn/printing/resolution/1_which_resolution_print_size_viewing_distance.htm

Uuglypher Loc: South Dakota (East River)

An excellent link!
Jeff Schewe, in “The Digital Print” (pg 129) cites a similar but slightly abbreviated chart of viewing distance and recommended DPI compiled by the late Bruce Fraser that agrees closely with the chart in the photokaboom link.

Dave

burkphoto Loc: High Point, NC

Yes... it depends.

However, if our lab had relied on these guidelines, we most likely would have lost some customers. I don’t know any well-reputed pro lab that wants fewer than 240 PPI for an 8x10.

At one point, for a brief time, we had a Kodak Professional Digital Multiprinter. It was a huge, expensive disappointment. If we ran it when a train was passing on the tracks 100 yards behind our building, well, we had to reprint! It printed 203 PPI 8x10s. They were soft, and too many customers could see the pixels when they got close to the prints in good light. We dumped it and bought Noritsu mini-labs.

The use of viewing distances farther away than 1.5 times the diagonal of the print is unrealistic, except for very large prints. Most pro labs figured that out in the late 1990s. We had to get prints past our clients and customers. Photographers are consistent pixel peepers.

Our standard became 1X the diagonal for a print viewing distance. 250 PPI was our standard for 8x10s. It scaled up and down very well from 11x14 to wallets. It was acceptable for 16x20 head-and-shoulders portraits, but not for 20x16 groups or 20x8 panorama groups. We needed at least 180 PPI for those.