Understanding Resolution in Photography

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Understanding Resolution in Photography

Understanding Resolution in Photography

This tutorial is the second in a multi-part series covering Aspect Ratios, Resolution, and Cropping vs. Resizing.

Resolution is one of those non-intuitive things that all digital photographers have to figure out eventually.  Why?  Because resolution has a direct effect on the quality of your printed photos.

Resolution is the number of pixels a digital image contains. This number is measured in megapixels.  If you buy a 17 megapixel camera, this means that the highest quality image that camera can produce will have 17 million pixels.  Think about a 4×6 with 17 million pixels – those pixels are going to be so tiny that you can’t see them, and your photo is going to look natural and realistic.

Say, however that the same 4×6 only has 100 pixels.  Divide that image up into 100 boxes, and fill in each box with a color.  Your image will look like a bunch of squares and will bear little resemblance to your subject.  This is what we call a pixelated image.

When we refer to printed images, we discuss resolution in terms of dots per inch (or DPI). DPI refers to the number of color dots your printer puts into every inch of your image.

When we are talking about images on the internet, computer screens, tvs, etc, we discuss resolution in terms of pixels per inch (or PPI).

There are a couple of important questions for us as digital photographers.

First, how big can I make my image before it becomes pixel-y looking? In other words, does my digital file have enough pixels that can stretch across a large image without becoming visible to the eye?  The maximum print size of your image is limited by the number of pixels your camera put into it.  (Now, there are ways to add new pixels in Photoshop so that you can enlarge your image even further, but that’s a discussion for someone else to lead!)

Let’s return to the example of the 4×6.  Say that this image is 2400 pixels wide.  2400 divided by 6 inches = 400 pixels per inch.  That is more than enough to produce a quality print.

However, say we wanted to enlarge that 4×6 to a 40 x 60.  Now we have to divide 2400 pixels by 60 inches, giving us 40 pixels per inch.  That is not going to be a pretty print.

As far as the optimal DPI for a good print, it depends on the printer.  Photo labs or your home printer should have recommendations for you.  When I print, I aim for a minimum resolution of 240 DPI.

The 2nd question for digital photographers is, “What is the optimal size for displaying my pictures on the internet or emailing them?” The maximum PPI that that monitors, TVs, and other screens can display is 72 PPI.   If your image has a PPI larger than 72, those extra pixels are essentially wasted space.  This is an issue because they are going to slow down your upload and download times on the internet, and take up valuable hard drive space.

Technically, an image that is imported from a camera doesn’t have a DPI/or PPI setting.   But our importing software often assigns one for us, and sometimes cameras program a resolution number into the EXIF data of an image.  To get a good print, you might need to change this resolution of your SOOC picture, or you might not.

To view the current resolution/PPI of your image, type control+alt+i (command+opt+i on Mac) in either Photoshop or Photoshop Elements.  That is option+command+i on Macs.

Note that the resolution is only 72 PPI, but the width is 24 inches.  If I were to print it right now, as a small print, say 4×6 we would be fine.  If I tried printing it as a 24×36 with only 72 pixels per inch, it would be very pixelated.  To increase the resolution:

  1. Make sure Constrain Proportions is on
  2. Turn off Resample Image
  3. Change the resolution to your ideal print setting

Now you can see that the width of the image has changed to 7.2 inches wide.  Note that the pixel dimensions have not changed – we have not added or subtracted pixels because Resample Image was turned off.

How big could I print this image?  It depends on the printer’s minimum resolution and whether I trust Photoshop to “Resample” the image by creating new pixels and trying to guess what they should look like.  (I usually don’t!)  I could probably push this image to a resolution of about 200 or so to get a larger print that still looks good. Consult your photo lab when printing larger prints to be sure you have enough information.

We’ve almost completed our journey to simplifying aspect ratio, resolution, cropping and resizing.  Any questions?

Want more information like this?  Take one of Jodi’s online Photoshop classes or Erin’s online Elements classes offered by MCP Actions.  Erin can also be found at Texas Chicks Blogs and Pics, where she documents her photography journey and caters to the Photoshop Elements crowd.

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Understanding Resolution in Photography