100% Crop vs Print Size

[jd101io]

Hi all.  I've been dipping my little toe into the waters of digital.  I borrowed an EOS 40D and put my trusty 85/1.8 on it, and have been trying things out handheld and on my tripod.  I now have some jpegs and some .CR2's loaded on my laptop and a trial copy of Bibble with which to open and view them.

One of the many things that I am very confused about is when people at various digital photography blogs and review sites put up a 100% crop as an illustration of some comparison or technical point, and then say "if printed at this size the print would be five feet across."  So, I tried figuring out the math for my own camera and laptop screen and come up with the following:  sensor 3888 X 2592 pixels; display res 1680 X 1050 pixels, diagonal size 15.4", 129 ppi.  Dividing each of 3888 and 2592 sensor pixels by 129 display ppi would give me a printed image of only 30" by 20", a far cry from "five feet across."

Am I missing something with an imperfect understanding of the terminology or the math, or are the bloggers/reviewers using displays with much less resolution than my laptop screen has?

Thanks much,

JD

[Slobodan Blagojevic]

Let's see... Sony A900 has 6048 pixels horizontally... my iMac24's screen has 96 ppi... 6048/96=63" or 5.25 feet... there you go.

[jd101io]

Thank you, Slobodan.  So my math is correct, and my confusion is due to having a display with a high number of pixels per inch.

-JD

[JeffKohn]

Thank you, Slobodan.  So my math is correct, and my confusion is due to having a display with a high number of pixels per inch.

-JD

That is correct. Laptops tend to have much higher PPI than desktop displays, due to being smaller and being viewed from shorter distances.

BTW the only correct way to figure this is by actually calculating the display's PPI as you did. The 'DPI' setting in the operating system is logical DPI and is only relevent to how text and some UI elements are displayed. It has no bearing on the actual number of display pixels per inch. So it is not true that all displays are 96ppi (or 72ppi as some really out-of-touch people might say). It varies from display to display. My 24" Eizo comes out to about 93ppi, while the 26" NEC with the same resolution will have a lower PPI, and some others might be higher.

[Jonathan Wienke]

All you need to do is divide the horizontal resolution of the monitor by the horizontal width of the display. A monitor 1920 pixels wide and 24 inches wide would have 80 pixels per inch.

[jd101io]

Now I know that when I am looking at "actual pixels" on my laptop I am seeing what a 30x20 print would look like?  If printed at 192 dpi?  Yes?  No?
And, it looks terrible, bearing in mind that I am looking at the "raw" RAW and I yet have no clue as to what to do with the RAW in Bibble to make it look better.

[JeffKohn]

All you need to do is divide the horizontal resolution of the monitor by the horizontal width of the display. A monitor 1920 pixels wide and 24 inches wide would have 80 pixels per inch.

True, just keep in mind that 24" Monitors have a diagonal of 24", not width. Have to get out the tape measure or the calculator.

[Jonathan Wienke]

True, just keep in mind that 24" Monitors have a diagonal of 24", not width. Have to get out the tape measure or the calculator.

Which is why I specified horizontal width instead of diagonal...

[Slobodan Blagojevic]

Now I know that when I am looking at "actual pixels" on my laptop I am seeing what a 30x20 print would look like?  If printed at 192 dpi?  Yes?  No?
And, it looks terrible, bearing in mind that I am looking at the "raw" RAW and I yet have no clue as to what to do with the RAW in Bibble to make it look better.

You are right, except for what I assume is a typo (i.e., you meant 129 dpi, not 192). As for looking "terrible", there are two additional steps to make it better: one is interpolation to at least !80 dpi, preferably to 240 dpi, and the other is proper sharpening after the interpolation.