Those Magic Numbers
Presented by Glenn Springer, TIF, RHCC

Did you ever go to print a picture and it looks like Garbage?
This is probably why...

I’m back in teaching mode today! What prompted this column was a question that was raised on a Photoshop and Lightroom forum I’m involved with, and the unbelievably convoluted and flat out wrong answers that some very knowledgeable people gave. Even the pro’s get confused. The most frequently asked questions seem to revolve around “pixels” and “megapixels” and “dots-per-inch” and “pixels-per-inch”. And the mistake that people most often make is to send too small an image, so it prints fuzzy (I’ve used a highly technical term here!). OK, so what do these magic numbers mean and why should you care?


  

This is very difficult to illustrate on-screen because the screen resolution is already low. The left side of this picture is what it would look like if you tried to print from a file that has been sized for the screen, not for printing. The right side is what a full-resolution version would look like.

Digital photographs are made up of an array of little dots of light, called “pixels”. If you have an image that is 4200 pixels wide and 3000 pixels high, that’s 12,600,000 pixels, or 12.6 megapixels (Mp for short) in techno-nerd speak. Remember these numbers, there’s a test at the end.

Now how many pixels do we need to show a full-sized picture on a normal computer screen? Even a really good high definition screen might be set at 1920x1080 pixels, that’s only 2 Mp! A normal picture on the web, say on FaceBook, is usually around 800x600 pixels, or only one-half of a megapixel. But you need more if you want to print the picture.

  

Again, the version on the left is screen size and the on on the right is print-size. It's a mock-up because I can't show you on-screen.

 

 

Almost all photoediting programs have a way to resize photos. That's the good news.

The bad news is, they can't do anything if they didn't have the information to begin with. For instance if an image is only 600 pixels wide and you want to enlarge it to 2400 pixels, all the program does is spread the 600 dots over a bigger area, then try to guess what to put in between.

Some of the programs have fancy algorithms that work better than others (you get what you pay for!) but I'm here to tell you, you can't quadruple the size of a picture without introducing all kinds of bad artifacts and stuff.

 

 

 

Most photo printers spray ink on paper in a pattern that has about 300 dots (of ink) per inch. A newspaper, a little less. So let’s do some arithmetic: if you want a picture to print 6” wide, at 300 dots/inch, how many dots do you need? Let me get out my old slide rule, do some tensor analysis using quantum theory in 10-dimensional Reimann space… I get 6 x 300 = 1800 dots. So I have to make my picture AT LEAST that wide or else some of the dots won’t have matching pixels, and the printing program has to guess what to do. Of course it won’t be quite right, so the colours will be off, and the picture will be fuzzy (that technical term again).


You can get pretty good results, even on an inkjet printer (they call the process giclée so they can charge more for it. That’s how most digital pictures are printed), at 240dpi. So you can get a nice 4x6” picture printed from a 1440x960 pixel image. That’s only 1.4 Mp! To get a good 8x10, you need about 5 Mp.

 

 

So here’s the test I promised: if you want to send someone a picture so they can print it, how many megapixels does it have to be?

Answer: ask them how big they want to print it. Multiply the size in inches by 300, that's how many pixels it should be.

Example: if they want to print an 8x10, the picture should be AT LEAST 2400x3000 pixels.

Note: if you just take a small picture (say 800x600) and enlarge it, it is NOT going to look very good. Start with the original, larger image.

 

OK, did your eyes glaze over about 3 paragraphs ago? Let me summarize it for you. To post a picture on FaceBook, it doesn’t need to be bigger than 800x600, or ½Mp. If you want to email a picture of your kids to your mom so she can look at it on her iPad (she doesn’t have one? Shame on you. Oh well, Christmas is coming…), send her something slightly bigger, around 1000 pixels on the long side. Less than 1Mp. If she’s going to want to print an 8x10, make it about 4 or 5Mp. You don’t need huge files, especially for the web.

The smaller it is, the easier it is to send it. How do you set the size? The problem is, I can’t see from here what program you’re using on your computer. That part I have to leave to you. If you’re using Lightroom (that’s what I use), you tell it to export a jpeg and what size you want. By the way, jpegs have different “qualities”. Choose somewhere around ‘8’ or 80%.

A normal DSLR has about 10 or 12 Mp. A high end one might be somewhere in the 24 Mp range. Did you know that the latest generation of iPhone and Android cameras will give you an astounding 41 Mp? (there’s a quality issue here we’re not going into today). This is ‘way overkill, unless you’re trying to print a picture the size of a bus.

"Glenn's Angels!" A shot from a recent workshop.

 

 

 

If you’re thoroughly confused, come do a mini-workshop (go to www.photography.to and click on ‘workshops’). The 3-hour ones are probably enough. Or join the new Highlands Camera Club, this is sure to be one of the first topics we discuss. Go to www.highlandscameraclub.ca for a look-see!

PS: my workshop schedule is wide open for the fall/winter. I do small groups, so you choose the date! An inexpensive way to learn how to make better pictures! Check out the workshops here.


Links to FACzen Photography:
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