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Photoshop Technical Essentials | Lesson 1: Understanding Pixels and Image Size In Photoshop

So there we were thinking that digital photography was the simplest thing in the world. No more complicated dark-room developing, just upload pictures to your computer and print away! Well, nearly, but not quite. Digital photography is beautifully simple, but you have to grasp a few simple facts about pixels, resolution and image size before you can start creating your own prints. In lesson 1 on 'Photoshop Technical Essentials' I'll bring you up to speed...

What are pixels?

Pixels are the building blocks of your photos. Open an image in Photoshop and zoom in really close (by holding Cmd/Ctrl and tapping "+". You'll start to see lots of little squares - those are pixels.

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3200% zoom view!

Each pixel only contains a small amount of colour information. But side-by-side, millions of little pixels make up complex colour tones and transitions.

Whilst Photoshop has some features that do not relate to pixels (vector images and 3D objects), it is essentially pixel based, and most features do relate to pixels.

Loads of cool photographic adjustments and effects can be made to pixels, which makes Photoshop a really powerful application. But the trade off with pixels is that they place limits on the size of prints you can create (pixel images are 'resolution dependent').


Pixels and image size

The more pixels an image contains the larger it can be printed. The number of pixels within an image, or the level of detail, is called resolution. Resolution is measured in ppi. You may have noticed ppi in Photoshop, it stands for pixels per inch (the number of pixels that fit within the space of 1 inch).

By lowering resolution (ppi) you increase the possible print size. By increasing the resolution you decrease the possible print size. That's obvious, right? There are only so many pixels to go around, and the more of them within each inch, the smaller the overall print size will be.


What's an acceptable ppi?

It's important to understand there is no hard and fast law about 'acceptable' image resolution. Trust your own eye when you see a print. So, with that caveat (or cop-out!) out of the way, 300 ppi is a common standard for high quality prints. 300ppi is seen as the benchmark for commerical images. So if you are planning on printing your photos to sell, set ppi to 300 in Photoshop.

In Photoshop, go to the Image menu and select Image Size. You'll see the number of pixels along the horizontal and vertical edges. Times these 2 numbers together for the total pixels in the image. You'll also the resolution and image size details under the 'Document Size heading'. Uncheck the 'Resample Image' box and enter 300 ppi in the Resolution field.

Did you see what happened to the width and height measurements when you did that? They shrunk (if they didn't, be sure you've unchecked Resample image). Now enter a really low number in the Resolution field, like 50. Notice how the size measurements jump up. At 50 ppi, pixels would be clearly visible; this is called 'pixilation'.

But I want bigger prints than 300 ppi gives me!

Dont' worry if you want to make bigger prints than a resolution of 300ppi allows. It shouldn' be a problem. 300ppi is really only essential when images are going to be hand held, or viewed from about arm's length. These tend to be smaller prints anyway.

Let's say you've shot wedding images for a client. They want an album produced, or a collection of 6x4 prints. These should always be printed at 300 ppi to ensure good quality detail.

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300 ppi for smaller, hand-held prints

But then the couple ask for a large poster sized print for a wall in their home. Think about it, this is never going to be held by hand. It's going to be viewed from across the room at a few metres distance.

So the same rules don't apply. You can drop the resolution in order to increase print size. I'm very happy printing at 240 ppi for large posters. The difference in detail between 240 and 300 ppi for a large poster is imperceptible at typical viewing distance.

The same principle applies if you need to print a really massive canvas for an office space, or a road side advert (where a ppi of 100 or less would be acceptable). Viewing distance increases with image size. People step back to see the image clearly, and slight imperfections in resolution disappear.

There is never any need to tick the Resample Image box. But just try it now as an experiment. You'll see that increasing the resolution also increases the total number of pixels, allowing image size to remain the same.

In other words, instead of packing more of the available pixels into each inch and reducing image size, Photoshop 'creates' new pixels out of nowhere, 'guessing' what they should look like based on the surrounding ones. This is detrimental to image quality, unless done with expensive enlargement software like Genuine Fractals.


What about dpi?

Unfortunately, two important areas in printing digital photos have very similar names: dpi and ppi. But they are totally different things. We've been looking at ppi, pixels per inch in Photoshop. Dpi means 'dots per inch' and is a printer setting for the number of dots of ink the printer places on the paper within the space of 1 inch.

So they're not to be confused. Most printers enable a considerably higher number of dots per inch than the pixels per inch we will have in our photos. At the very least, ensure that your printer is printing the equivalent dpi to ppi. 300ppi images should be printed with at least 300dpi, normally quite a lot higher.



So that's lesson 1 of the Photoshop Technical Essentials section wrapped up! I hope you realize how pixels, resolution and image size are actually very simple indeed. You should be able to print very successfully now. It's simply a trade-off between resolution and size, with the caveat that larger sizes are less demanding on resolution.

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