Another resolution question

First, there's a difference between ppi and dpi. Pixels per inch is ppi. A pixel is the smallest "thingy" in the CCD. The CCD is the electronic "film" in your camera. Your CCD has 7,077,888 total pixels (approximately 7.1 million pixels or 7 MP), which are arranged as 3072 pixels in one direction X 2304 pixels in the other direction.

If your entire photo were to be printed on 8" X 6" paper, the ppi would be 3072/8 or 384 ppi (or 2304/6, which also equals 384 ppi). Unfortunately, there is no such thing as 8X6 paper so you would probably print on something like 7" X 5" paper. HOWEVER, the available paper does not have the same "aspect ratio" as your CCD so you'd need to cut off some pixels to make it fit. You would print 3072 pixels X 2194 pixels and throw some away! Have I confused you yet?

Allow me to interupt myself. This problem has existed since the days of Matthew Brady but, up until now, the photographer was never involved in the details.

Despite what you think you saw, iPhoto does not change the ppi value. Well, not exactly. If you look at the pixel length X width, you'll see that that did not change. However, it is now assumed that you will print on a much larger piece of paper than originally assumed. Actually, the change was to 72 ppi first (to match your monitor) and then the paper/monitor size was calculated.

When you're viewing photos on your screen, you're viewing pixels and your screen displays 72 pixels per inch. Period. Now, when you wish to print, your computer sends dots to the printer and dots per inch (dpi) becomes significant. In one hellacious oversimplification, figure that you can never have more dots than pixels because the dots are created from the pixels. Most good quality prints are printed at 300 dpi.

If all of the above has confused you thoroughly, think of a child's "connect the dots" puzzle. The more dots per inch on the puzzle, the better the graphic after the child finishes. Again, dots are on paper while pixels are on monitors. You can control printer dpi but your computer controls ppi.

Now, let's talk about compression. That is nothing more than a technique to keep the size of the computer file manageable. (It's automatically decompressed when you used it.) However, there ain't no free lunch. Orange juice is compressed (condensed) for shipping. It's smaller than it originally was but it's not usable in the compressed condition. When you receive it, you add water to decompress it. You and I both know that it's not as good as the original but is it good enough for your purposes?

The finest compression is the least amount of compression and it keeps the image closer to the original than less fine compression.

jpg files are compressed and they're perhaps 1/4 the size of uncompressed tiff files. You can relate jpg files to condensed orange juice and tiff files to non-condensed juice.

When you're viewing photos on your screen, you're viewing pixels and your screen displays 72 pixels per inch. Period.

Actually this is one of those old things that is no longer true and simply will not die. The screen displays how ever many pixels per inch the monitor resolution is set for. Rarely does it happen to equal 72 DPI.
As an example, my eMac is set for the max resolution of 1280x960 pixels. My eMac's viewable area is roughly 9.25 inches tall by 12.75 wide. So that puts my screen display at a bit over 100 pixels per inch.
If you are still running some old computer from the 1970s with a fixed screen resolution, then it might be true. But for modern computers this statement is simply rarely true and never a rule.
Patrick

Hi callandra,

Resolution is the actual pixel size of your file, yours is at 3072 x 2304, resolution often is a reference to size as per the virtual world. dpi on a print is different than dpi on a monitor. To see the info on your image, open it in Preview and Get info, it may display more details...

Good Article -> DPI is a function more or less by print size or in general terms 'finished, printed or displayed' size. Your image file will NEVER contain any more infomation than it does. You CANNOT up the dpi or lower the dpi. For example, your file at 3072 x 2304 at 180 dpi is a real world size of @ 12.5" x 18". If you were to make the same file above a print 20" x 30" your dpi would be @ 100 dpi. The reverse will happen if you print your file at 4" x 6" your dpi will be @ 500 dpi. As you can see if you keep the orginal resolution, dpi becomes a function of the finished product. For printing purposes, I would never change the resolution of any image.

The best way to grasp dpi is comparing it to grain in film photography. The larger the print the more grain you see, hence some will say detail is lost, the smaller the print the grain disappears, some will say you see better detail. But it is still the same negative, same information. The finished size determines grain in film and dpi in digital images.

Your resolution of 3072 x 2304 is high enough to print an acceptable 24 x 36 photo, just think what it would look like at 8 x 10" or 4 x 6"... I would keep shooting at your current settings. Good Article --> ISO

Rick

iMac G5 iSight 20" - 30G iPOD in Slimming Black - Mac OS X (10.4.7) - HP Pav 15" WS and Toshiba Sat 17" WS LP's - Canon 20D & A620

Your camera dpi cannot be set...

This is why 'Pros' use digital cameras with 12 megapixels or more. If you read my post above, there is a 'native' size to all digital images, 7.1 megapixels at 180 dpi translates into a 12 x 18" print. In order to get this size print at 300 dpi your camera would need to be @ 1+ megapixels... Your 7.1 megapixel camera will make great prints up to 12" x 18" with no apparent lost of quality. With the resolution of your camera shot at 180 dpi, if you were to order an 8x10 print the resolution would be @ 300 dpi...

Rick

iMac G5 iSight 20" - 30G iPOD in Slimming Black - Mac OS X (10.4.7) - HP Pav 15" WS and Toshiba Sat 17" WS LP's - Canon 20D & A620

Many cameras do allow adjustments in pixel quantity but, obviously, the adjustment can never be more than the maximum that the CCD can produce. Adjustments are DOWN from that figure.

You're confusing dpi with ppi and then you throw in total pixel count. Be that as it may, 12 MP is peanuts when your goal is a very large print. Would you believe 39 MP? (The price is somewhere around $30,000.)
http://www.hasselbladusa.com/