resolution vs. physical size

[morrisstudio]

Hi all,
I really need to find out a (probably) elementary answer to a question. Is it better to have an 85" X 50" file at 72 ppi than an 8" X 10" file at 1200 ppi? I'm talking about specifically getting the most possible information from a scanned photograph. (no neg available).
Thanks if you can help.
~morrisstudio

[jonbalza]

Well there would be no difference in the amount of pixels, so you would have the same amount of information. One would just be smaller, and the other would be bigger, but the pixels would be more evident, but only because of the smaller size.

SO, unless you can actually print on an 85x50" piece of paper, the smaller one is "better".

edit: "detail" was changed to "amount of information"

[Shaggy]

JonBalza - the 8x10 would have more pixels. There isn't the same amount of information either. The pixels would be more evident on the 72ppi version @ 100% than on the 1200ppi @ 100%.

The smaller one is better only for the reason that it has more available information to work with - assuming that your scanner can actually, optically, scan at 1200ppi (which is somewhat doubtful with most home, flatbed scanners).

Morrisstudio - You need to know two things before you scan.
1) The capabilities of your scanner. If it says 1200dpi on the box, don't believe it... you'd be a victim of marketing hype. It would probably in the range of 400ppi.
2) Your intent! Is this going to be a printed image? Displayed on the web? Not sure? Print has the most demanding resolution requirements so your safest bet is to scan at as high of an optical resolution as you can.

400ppi would probably be safe. In Photoshop, play around with Image > Image Size. Uncheck the resample box and see what happens to the size of your image when you type in different resolutions. Then try typing in different sizes and watch what happens to your resolution. You will see that there is a relationship between the two.

[morrisstudio]

Jonbalza and Shaggy,

Thank you for your feedback. I am going to try to explain the purpose of this project so you understand my intentions:
It's a little complicated.
I reproduce historical wallpaper from (sometimes) old 100 year old photos. I was sent by the museum house a scan of a roomshot that was at 72 dpi but was 85 inches wide. It was quite blurry when I enlarged the crop of the desigh in the photo to the size of the final art work. After I groped through the very blurry reference to create the artwork I was then (many months later) sent a manipulated photo from the museum (done in PS) that showed a HUGE amount of more detail in the photos. I assume these were scanned from the original 8X10's and were then manipulated using levels, curves etc..
Of course I had originally asked for the original photo but was told they were not available. So my question is to determine whether the scan I was sent originally (a tiff at 72ppi that was 85 inches wide) has as much info in it as a scan that the lacki at the museum house made from the original photo print at 400-1200ppi (depending on the scanner quality of course) at 100% size.
No names will will be used in this inquisition! I just want to basically confirm that what Shaggy said is in fact correct. That there is in fact more info in a smaller physical size, higher res scan than a physically larger lower res scan.
If this is too much I understand cause i know there are many factors involved. But basically I think I agree with Shaggy on this one. But keep talking please and correct me if I' wrong!
Thanks a million. ~morrisstudio

[jonbalza]

Originally posted by Shaggy
JonBalza - the 8x10 would have more pixels. There isn't the same amount of information either. The pixels would be more evident on the 72ppi version @ 100% than on the 1200ppi @ 100%.

*DOH!* That's what I get for not thinking through the question all the way. I just assumed that it was the "standard" resolution/dpi/physical size question that you see lots of times. Thanks for the correction, Shaggy.

morrisstudio, Shaggy is absolutely correct. You DID get more information in the smaller (physcial) size scan, because there were more total pixels in the image to hold that detail.

...walks off, hanging head in shame...

[Greg Vander Houwen]

85 inches x 72px = 6120px long dim.
50 inches x 72px = 3600px short dim.

10 inches x 1200px = 12000px long dim.
8 inches x 1200px = 9600px short dim.

The second set has substantially more pixels and therefore more positions to define color and sharpness. If the second set was properly captured then it has more image resolution.

The real issue comes in as I look at the 6120 and divide it over 10 inches...= 612px per inch. 3600 by 8 inches = 450px per inch. You have a different set of proportions here (not 4x5) like the 8x10 set. And for your purposes I can't imagine any need for more than 450ppi. In fact 300ppi would seem like overkill.

Is this a spot color job by chance? What is the goal output tile size and resolution?

Greg

[morrisstudio]

First of I appreciate your help in this matter!
Greg -I was using those dimensions as a "general" example. But to answer your specific questions and to explain in more depth:
The file I was originally sent was 65" X 53" @72.
The scans that were sent after I worked for months on this project (that the museum house provided) are 8" X 10" @600.
Sometimes the old photos are not in the same dimensions that we are used to seeing now.
To answer your other question:
I use the photo (scan) and find the best area of information of the wallpaper. I then enlarge the area which is usully about an inch and a half square to 18 inches square. Thus you can see why the more info I can pack into the original scan the better.
The output is a (yes) spotcolor print and I work at a minimum of 300ppi. The output is not such a concern - your reply has been very helpful and answers my question.
For curiosity's sake I am attachng examples - even tho they are small res jpgs you can still see the "relative" difference in the quality of information.