Megapixels and print size

Let’s continue on our discussion of image sharpness and resolution.  So far we’ve limited our discussion to sharpness as limited by the number of pixels and seen how that determines required pixels per inch when you are displaying your wonderful image on a computer screen.  Today let’s consider the issue of displaying your images on paper – that is printing them.

The situation here is very similar to that of displaying on a computer screen.  The limitation here is how fine can the human eye resolve?  This number is something like 200 pixels per inch.  For good measure I like to crank that up  to 300 pixels per inch.  Ok then, this means that if you’re going to print an 8” X 12” print  you need 2400 pixel by 3600 pixels; so 8.64 M pixels.  Boy, that was easy!

Let’s consider the maximum number of pixels required for different size prints.  The first column gives you the number of megapixels,  MPz, requirement assuming 200 pixels per inch while the second column gives you the required MPz assuming 300 pixels per inch.

6” X 4”

.96 MPz

2.2 MPz

8” X 10”

3.2 MPz

7.2 MPz

8” X 12”

3.8 MPz

8.6 MPz

11”X14”

6.2 Mpz

13.9MPz

12”X18”

8.6 MPz

19.4MPz

 

The other way to look at this is that for a Canon T2i with its 5186 pixel by 3457 pixel (or 18 MPz) sensor array, if we assume 200 pixels per inch as a requirement, then the biggest picture you should print is 5186/200 x 3457/200 , then the biggest picture you should print is 5186/200 x 3457/200 or 26” x17”.  For a 300 pixel per inch requirement this becomes 17” X 12”.  So now you can understand while I won’t print from my Canon T2i any bigger than 18” X 12”.

This entry was posted in Technical Aspects of Photography.