The ongoing resolution question!


What do the say in the different forums on the Internet?    Some quotations...


red bullet gif image Jim Kroger (jimkk@removethisumich.edu) asked 2002-12-30 :
How many megapixels do I need to be able to enlarge to 8 x 10? Double that? Jim
red bullet gif image jdunn (jdunn@plano.net) answered 2002-12-30 :
If you print at 300 dpi (which is a good resolution to use) then simple math shows

8 in x 300 dpi = 2400 pixels
10 inches x 300 dpi = 3000 pixels
2400 pixels x 3000 pixels = 7.2 megapixels

16 inches x 300 dpi = 4800 pixesl
20 inches x 300 dpi = 6000 pixels
4800 x 6000 pixels = 28.8 megapixels

for more info go to www.scantips.com

red bullet gif image Mark Roberts (mark@robertstech.com) answered 2002-12-30 :
Depends on how good you want the print to be and, to some extent, where the file comes from; you can get away with lower output resolution - pixels per inch - with a file from a digital camera than you can with a file from scanned film (largely because there's no grain/grain aliasing with an image from a digital camera).

Absolute minimum output resolution for prints is 200 pixels per inch (ppi). I personally won't go lower than 250 ppi and try to stay at 300 ppi or better whenever possible. There's insignificant improvement with resolutions over 360 ppi in my experience.

For an 8 x 10:10 inches at 250 ppi gives you 2500 pixels along the long axis and 8 inches at 250 ppi gives you 2000 ppi along the short axis. 2500 x 2000 is 5,000,000 or 5 megapixels. If you lower your standards to 200 ppi you might get away with 3.2 megapixels. If you raise your standards to 300 ppi you'll need 7.2 megapixels.

For a 16 x 20 multiply the results above by 4.

-- Mark Roberts
Photography and writing www.robertstech.com


red bullet gif image Alan Justice (me@privacy.net) asked 2003-01-01 :
How do megapixels relate to file size (megabytes)?

300 dpi is typical, but is it overkill? Can one see the difference between 300 and 150 dpi without a loupe?

-- - Alan Justice


red bullet gif image Raymond Rangel (ray.rangel@cox.net) answered 2003-01-25 :
..... the answer is:
MB = MP x (CD / 8)

Wherein:

MB = Megabytes of the uncompressed image
MP = Megapixels
CD = Color Depth in bits
8 = Byte size in bits and is a constant

Thus:
My P&S digital has 3.1MP @ 48 bits of color information so it takes 6 bytes for each pixel which yields an uncompressed (in this case a TIFF format file) or about 12MB. By the same token, a camera capable of only 24 bit color depth would yield a file half as large i.e.6MB.

Now, my film scanner has a resolution of 1800DPI, this equates to a full frame 35mm being about 2586x1708 pixels, or 4.5 (roughly) MP. At 48 bits of color depth this will yield a file about 26MB. At this rate, I can get one 24 exposure roll on one CD (640MB / 26MB = 24) if I scan the film at 48 bit color of two rolls at 24 bit scanning.

All of this affects some decisions concerning the digital (or digitized) photo. Storage, obviously, is a factor. High resolution scanners like those capable of 3600DPI or more at 48 bits of color will produce huge files. For instance, a 3600 DPI scanner has twice the vertical and horizontal pixels as my 1800DPI scanner. This results in four times the number of MP, i.e., 18MP. Hence, at 48 bits of color depth one would get a file that was about 108MB yielding 5 or 6 frames per CD.

Of course Photoshop 7 can only handle 24 bits of color, so images specifically destined for alteration by this program could be half a large. Still, 50MB is a pretty good sized image. If images are destined for a computer screen or digital projection, True Color depth is 32 bits...of course the DPI for CRTs is much lower (72 or 96) which will result in comparatively small files.

But, back to Photoshop and a printable 8x10. A printer capable of 300DPI will produce the best results if the file is at the same resolution. Thus an 8x10 at 300DPI = (8x300) x (10x300), or a requirement for 7.2 million dots, i.e. pixels. Now since one might clean up the image with Photoshop, the color depth will be 24 bits. This yields a file that is about 21.6MB (29 8x10s @ 300DPI @ 24 bit color per CD). As can be readily seen, the translation of MP to MB is pretty straight forward. It is a important thing to know when deciding such things as numbers and size of memory cards that might be required for a digital camera, hard drive space, computer processing speed, and long term storage requirements.

red bullet gif image Robert E. Williams (mytbob@cox.net) comented above 2003-01-25 :
Your Formula should also contain a term for jpeg or similar compression
So: MB = MP x (CD/8) x C* , where C* may vary from about 0.25 (superfine) to 0.025 (normal).
I suspect most people would print an 8 X 10 from a superfine jpeg. I have not seen any difference in print quality when printing from tiff or superfine jpeg.
Bob Williams

red bullet gif image DigitalCameraBasics answered 2003-01-26 :
And for those who do not feel the need to take night classes to earn a degree in math (as in to the original poster)....

Common English...

"Technically", you need something close to a 6.5 megapixel camera in order to get a "35mm quality" 8 x 10 print, but this is being very specific. A good 4 megapixel camera can produce a good print (lab print or from a very good inkjet on good photo paper) that when viewed from a normal wall-hung viewing distance, will look just as good to many as any film print.

There are again, variables, such as how good the quality of the lens is on the camera in question, the size of the CCD imager being used, lighting conditions when the photo is taken, etc., but that is the very very basic answer to your 8x10 question, at least... no math required.


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