Printing out a single photo on the poster printer? This is how to make sure it looks good.

We don't want pixelated printouts...

Today at the center, a student came in asking about printing a single photograph using the poster printer. This person wanted to know how big of a print they could make so that the poster looked good; so that the print wasn't too pixelated. We needed to figure out what the maximum size printout we could make was in order to have an acceptable amount of quality in the printout. Here's how you can do it:

There are two possible scenarios`. The first is if a person comes in and has the photo files with them. The second is if the person doesn't have the photo files with them, but they know which camera they took the photos with.

For either case, our definition of quality is going to be printing out the photo with a pixel density of 300ppi (pixels per inch). 300ppi is the standard value used for any high quality print work.

First case:

1. Find the file in the Windows file explorer (sorry Mac people).
2. Note the width and height (in pixels) of the photograph. This may be labeled under "dimensions" or you may have to open the properties menu for a specific file.



In Windows 7 you can find the photos width and height by clicking on the photo and looking at the bottom bar. In this example, the "windows7-desktop" photo has dimensions of 4000 x 2500, which is 4000 pixels wide by 2500 pixels tall.

3. Take the width and height in pixels and divide both numbers by your pixel density constant (which is almost always 300ppi). In the example above, you calculate 4000/300 and 2500/300, which are 13.33in and 8.33in, respectfully.
4. With that done, you now know how big of a photo you can print out while still having a high quality printout. For our example, we can print out a copy of that photo which is 13.33 inches wide by 8.33 inches tall.

Second case (The one you probably won't read, but should):

As a reminder, in this example we don't have the image files; we only know what camera the photos are going to be taken with. For this to work, we need two values:

1. How many megapixels (mp) does the camera have? This value should be found either by looking at the camera or by looking the value up. As two examples, the iPhone6 has an 8mp camera and the center's two Canon DSLR cameras are both 18mp.
2. What aspect ratio does the camera use to take photos? For more info about what this means, you can follow this link here. As a general rule of thumb, most smartphones or point/shoot cameras use a 4:3 aspect ratio; most DSLR cameras (like all of the DSLRs that we have at the center: the Canon DSLRs, the Pentax, and the Fujifilm) use a 3:2 aspect ratio.

You all ready for this? Warning: prerequisite of MATH121.

• To begin, let's review some math prefixes: mega means 10^6, so when a camera has 9 megapixels it also has 9,000,000 pixels or 9 * 10^6
• Also, the amount of megapixels that a photo has is equal to the width * height (both numbers in pixels). The example above was of a photo that was 4000 by 2500. When you multiply those two numbers, you get 10,000,000 pixels, which is 10 megapixels.
• We are going to assume that all cameras are set to their highest quality photo mode; this will ensure that we know how many megapixels the camera is using.
• For the following math, we need a number which I will refer to as the aspect constant. To find this number, take the first number of your aspect ratio and divide it by the second number. As an example, a 4:3 camera has an aspect constant of 1.33; a 3:2 camera has an aspect constant of 1.5.

1. Let's say a person comes in and is going to use the Canon T2i to take photos. How big of a print can we make for them using those photos?
2. Numbers: Canon T2i takes 18 megapixel photos (mp); T2i takes 3:2 aspect photos, so our aspect constant (ac) is 1.5.
3. We need to find out the width and height (in pixels) of a photo produced by this camera will be, from that we can find out the print size like we did above.
4. The height of a photo is given by the equation height = Sqrt[ mp*10^6 / ac ] where mp is the number of megapixels of the camera, and ac is the aspect constant. Sqrt is the square root of the number in brackets. From our example, height  = Sqrt[ 18*10^6 / 1.5 ]. Our photo height is equal to 3464 pixels.
5. The width of our photo is given by the equation width = ac * height, where ac is the aspect constant and the height is the value we previously got (in pixels). From our example, width = 1.5 * 3464. Our photo width is equal to 5196 pixels.
6. Take the width and height in pixels and divide both numbers by your pixel density constant (which is almost always 300ppi). In the example above, you calculate 5196/300 and 3464/300, which are 17.32in and 11.55in, respectfully.
7. With that done, you now know how big of a photo you can print out while still having a high quality printout. For our example, we can print out a copy of that photo which is 17.32 inches wide by 11.55 inches tall.

TL;DR If you have the file, divide the width and height by 300. If you don't have the file, you're on your own.

Have fun everyone! If you actually read through all of this: you're awesome, don't ever change!

 - Stephen