“Canon EOS 20D”
“Canon EOS 300D / Digital Rebel”
I lent Nono my 20D for [what was soon to be considered] the last Ateneo – La Salle UAAP game.
Before I continue… let me tell you now that he’s the only person I can trust with my cam (without me physically present that is), so for those who want to borrow it – forget it!
Anyways, he noticed something about the “resolution.” The 20D stores its images @ 72dpi (Dots Per Inch), while the 300D stores its images at 180dpi.
What gives?
“Canon 1Ds”
“Canon 1D MarkII”
“Canon EOS 20D”
“Canon EOS 300D / Digital Rebel”
I had an inkling of a possible explanation from my experience with Photoshop before, but I never got to confirm it… until he pointed me to this reply.
The whole thing was better explained by the “pros” in our mailing list:
The dpi (or ppi) is actually secondary. It is the number of pixels that really matter. The 300D produces images made up of 6 million pixels, the 20D produces images of 8 million pixels. The dpi just refers to the arrangement of those pixels? How dense are they arranged. 72 dpi is the resolution for viewing images on the web, or on your computer. 300 dpi is the resolution recommended for printing. There is no loss when your camera dishes out the image as 72 dpi or 180 dpi as the total number of pixels remain the same. If you want to rearrange the density of the pixels, Go to photoshop, select IMAGE / IMAGE SIZE. A dialogue box pops up. Uncheck RESAMPLE IMAGE. In the box for RESOLUTION place the number of pixels you want your image to be arranged at (300 dpi, 72dpi, etc). Notice that the height and width of the image will change. The higher the value of the pixels per inch, the smaller the resulting image will be. The smaller the value of the pixels per inch, the larger the image size will be. No loss will occur because the same number of pixels that comprise the image remain. They are just arranged differently. Oh, click ok to exceute the command.
Fortunately, I understand it now (thanks to the post and some more fiddling with photoshop and a calculator), and I can offer a somewhat layman explanation – though extremely long and detailed.
It turns out that they [dpi and camera resolution] are totally unrelated as far as the digital image is concerned. Since when talking digital images, you’re talking about camera/sensor resolution. 8MP at any DPI, is still 8MP and does not change in quality since pixels are like “atoms” in digital imagery – you can’t make a pixel hold more detail than any other pixel, nor can you break it down to smaller units. Given the current pixels available to you (8MP, 6MP, whatever) it’s up to the sensor and processor to assign those values to the individual pixels… and that’s what affects sharpness, color, or detail.
So where does DPI/PPI come into the picture?
DPI/PPI (Dots Per Inch / Pixels Per Inch) are related to print resolution. Meaning that on a piece of paper, how many points are crammed/printed into an inch.
Yet DPI and PPI are further differentiated. DPI is printer specific, while PPI is the more appropriate term for the image’s print setting itself. However, it has been common practice to use DPI lightly and represent PPI most of the time. Image EXIF data [incorrectly] denotes the PPI as DPI.
PPI, DPI and all that jazz only will apply if you intend to print something. It is good practice to use the pixelcount in conjunction with the PPI you intend to use, to clarify things.
This is where PPI is important – assuming you have enough [mega]pixels to match your print resolution, the higher the PPI, the more “fine/detailed” your print will be. I had to mention megapixels matching print resolution because you can always try printing an 640×480px (1MP) image at 300ppi… you’ll get a really small (2×1.5in) picture. “Fine” probably, but hella small.
Simply put, any pixelcount can be printed a number of ways without resampling (changing the pixelcount, ergo retaining the original quality of the image). A 300×300px picture can be printed as a 1×1in picture using 300ppi… or larger using a smaller PPI setting – yet it still is the same 300×300px image with no modifications whatsoever.
Having difficulty imagining it? Think of it this way. If you had a black 1×1px image, and print at 1ppi (assuming it’s possible) – then you get a square inch of blackness (representing that single pixel). If you increase the PPI, it gets smaller.
If you print a 300×300px image at 300ppi, then you’d also get a square inch, but this time the whole 300×300px image will be printed on that square inch.
This is where pixels and PPI are relative to each other in digital photography printing. The higher the PPI translates to “finer” printing, because it can print more dots per square inch (duh!)
For printing (magazine, poster, etc.), the standard is 300dpi/ppi. So anyone trying to print pictures will most probably be trying to print at that “resolution” at the very least. You can use more (or less), but 300ppi is pretty much a safe standard since the eye, more often than not, cannot discern the individual printed dots/points beyond 300 per inch.
From that alone, assuming you know how you want to print your stuff, you can pretty much calculate how much [mega]pixels you need for your prints. Just multiply your intended print quality (PPI/DPI) to your intended print area.
Before I continue, let me paste these sizes (for future reference, yes even my future reference as I have lousy memory).
PRINT SIZE PHOTOSHOP FILE 3R = 3.5" x 5" = 4.45" x 6.25" 4R = 4" x 6" = 3" x 4.5" 5R = 5" x 7" = 4.5" x 6.25" 6R = 6" x 8" = 6" x 8" 8R = 8" x 10" = 8" x 10"
All in inches. The first column lists actual print sizes, next is the allowance in size you need in PS if you intend to have some printers print it (they sometimes crop your file). Though the second column may not be necessary for all people, I just included it in case it becomes applicable.
Now where were we? Ah yes. Say you wanted to print a 4R picture (4×6in). 4in @ 300ppi is about 1200px, and 6in @ 300ppi is about 1800px. But we’re not done yet! We need the area for the total amount of pixels needed. So 1200px multiplied by 1800px is 2,160,000 pixels or 2.16 megapixels. So the 2MP cameras are ok already to print 4x6es (at 300ppi), and those 4-5MP cameras are more than decent for everyday photography… and that’s assuming you absolutely must print at 300ppi.
Using the same math, approximately 7.92 megapixels are needed to print an 8×11in (letter sized) at 300ppi. So the 1D MarkII and 20D as of the moment, with their 8MP sensor count, are capable of such print resolutions. The 1DS can print up to 10×12in with its stellar 12MP count. And can be had for as low as 7.7k USD hahahaha.
But somewhere along the way, I’ve veered from my point. Point is that the 20D indeed takes pictures at 72ppi, and if you print at 72ppi, that would yeild a whopping 33×41in print! hahahaha. But you can always change the PPI to about anything… it’s all relative, and doesn’t change the fact that your pic was taken at 8 megapixels. But this is where DPI comes in.
If pixelcount weren’t an issue (meaning we had all the pixels in the world at our disposal) then we could simply print way beyond the standard… say 3200ppi? Yes we can, but can the printer support it? Same goes if you try to print a 300ppi image to a printer with a much lower DPI capability (say a dot matrix printer hehehe). At the end of the day, it’s always a balance between the image’s pixelcount, the image/software’s PPI setting, and the printer DPI capability when it comes to “what you need” for printing.
As far as original image quality is concerned however, these can be considered as irrelevant. The default of 72ppi is more of a nuisance, as you have to change the value everytime you intend to print at 300ppi (or any other print resolution). Then again, even the 300D requires you to change it (since it starts at 180ppi). Yet, in no way whatsoever, does it affect the quality of the picture in itself. That would be the sensor/processor’s job… and of course, the lens you’re using.
There’s a good article explaining this in case my explanation doesn’t cut it: http://www.tildefrugal.net/photo/dpi.php
To sum up, I have come to a realization – gradeschool math isn’t useless after all! hahahaha.