Photoshop Fundamentals: Changing an Image’s Resolution and Size

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Photoshop Fundamentals: Changing an Images

Resolution and Size

Adapted from Photoshop CS4 After the Shoot

(http://www.wiley.com/WileyCDA/WileyTitle/productCd-0470389869.html)

(Wiley Publishing)

By Mark Fitzgerald

Version: Adobe Photoshop CS4

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Ive met many photographers who dont understand how to change an images size or resolution correctly.

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The problem usually stems from a lack of understanding about image resolution and how it affects image

size.

Caution:Always save a master file with all layers before changing size or cropping. Otherwise it may be

hard to back up and resize the file for a different output use later.

Understanding Resolution

One of the things that can be the hardest to get your head around when you start down the digital path isresolution. This confusion is compounded by the fact that there are two different kinds of resolution in the

digital world. One is dots per inch, and the other is pixels per inch.

. Dots per inch (dpi). This refers to the number of dots per inch that an inkjet printer is capable of

applying to a sheet of paper. It can range from 720 to 2800 and more. The closer these dots are to each

other, the more they blend together forming continuous tones on the print. Naturally, this depends on

the paper thats being printed on. If the paper is porous watercolor paper, the dots soak in and blend jus

fine at lower dpi settings like 720. On glossy photo papers, a higher settingsuch as 1440is needed

because the ink dries on the surface. About the only time people discuss dpi is when they are talking

about a printer.

. Pixels per inch (ppi). This is what is usually discussed when talking about resolution in digital

photography. It refers to the distance between the pixels that make up digital images. Pixels per inch is

an important setting because it determines what digital images look like when theyre displayed and

printed. Sometimes a lower value is desirable, and other times a higher value is preferred.

If you have a file with a resolution of 120 ppi or lower, you run the risk of seeing the space between the

pixels when you print. This causes edge detail, which should be smooth in the print, to look jagged. The goa

is to get the pixels close enough together so that these single dots form continuous tones and l ines. The

illustration below gives you an idea of how this works. As the dots get closer to one another, they begin toform a line. When you zoom out, the individual dots disappear. (Its similar to the idea of getting ink dots

close together on a printer (dpi).

Tip:Keep in mind that some people mistakenly use the terms dpi and ppi interchangeably, saying dpi when

they really mean ppi. If theyre not specifically talking about a printers output, then they probably mean pp

The thing to keep in mind when its time to change resolution is that if you have an image file thats 150 ppi


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Resampling the Image Size

In the previous example, you were asked to turn off Resample Image because Resample Image has a

special function that affects the math involved in resizing an image. When you turned off Resample

Image, the Pixel Dimensions remain fixed when you changed the resolution value. Take a look at what

happens when Resample is turned on:

1. Begin with the file you created in the previous set of steps, and return to its original state using the

History panel.If you closed it, then go back and complete Steps 1 and 2.

Tip: If you still have the Image Size window open from the

preceding example, you can reset it to the settings it had when you

opened it by holding down Alt and clicking Resetwhere the

Cancel button used to be. This works with almost every dialog box

where you see a Cancel button.

2. Choose Image > Image Size (Alt+Command+I/ Alt+Ctrl+I).The Image Size dialog box appears.

3. Leave the Resample Image option selected or select it if it isnt already checked.

4. Change the Resolution to 300.Leave the dimensions at pixels/inch. Notice that the Image Size remains

at 8" 10". What changed was the Pixel Dimensions at the top of the window. They went from 1200

1500 to 2400 3000.

5. Change the Resolution to 600.Now the Pixel Dimensions changes to 4800 6000, but the Document

Size remains fixed, as shown below.


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Something to notice when comparing the two previous illustrations is the little chain icon that appears to

the right of the Document Size settings. This icon indicates that these values are linked. When one value

is changed, the other linked value is affected. In the first illustration, where Resample is off, the Width,

Height, and Resolution settings are all l inked. In the second, where Resample is on, only the Width and

Height values are linked. Resolution is not be affected by changes to these values. This is a great way to

remember which Resample setting to choose.

When Resample Image is turned on, modifi cations to Resolution or Document Size in the Image Sizedialog box affect only the Pixel Dimensions. When you make an images width and height dimensions

smaller, or reduce its resolution, resampling takes pixels from the image. This is called downsampling.

If you make the Document Size larger or increase the Resolution value, resampling adds pixels to the

image. This is called upsampling. You can verify that this is taking place by looking at the file size readou

next to Pixel Dimensions in the previous illustration. The original file size was 5.15 megabytes (M). Now,

with the addition of all the new pixels being added by Photoshop, the file size is 82.4M.

Tip:There are two different ways to measure a photo files size in megabytes. One is the size of the photo

when its open. This size is determined by the pixel dimensions width height three color channels.This is the size shown next to Pixel Dimensions in the Image Size dialog box. The second measurement is

for when the file is saved to disk. Naturally, this varies by the type of file thats saved. For example, a

saved TIFF file is always bigger than a JPEG saved from the same file because the JPEG is compressed. Thi

size is the size shown when you look at the file in your Mac Finder/Windows Explorer.

Keep in mind that adding lots of pixels to an image can affect the images quality. Photoshop is pretty

good at upsampling, but only so much can be done. Lots of guesses need to be made on Photoshops part

when deciding what color to make a new pixel. The quality of the outcome depends on the size and

quality of the original file. When a quality file is used, its easy to double, and even triple, the size of the


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file. However, if you push it too far and try to upsample an image beyond Photoshops capabilities, you

can hit a point of diminishing returns where quality begins to suffer. For projects that require massive

upsampling beyond Photoshops abilities, look to a plug-in like Genuine Fractals by onOne Software

(http://www.ononesoftware.com). This plug-in uses fractal math to accomplish some amazing

upsampling feats.

So remember, if you only want to change the resolution of the file, uncheck Resample. If you need to make

the image smaller or larger, then Resample must be checked. Use the table below as a recap of therelationships you covered in this section:

Using the Correct Image Interpolation Method

Theres one more wrinkle to throw at you before moving on. In the previous examples, you probably

noticed a pop-up menu next to Resample Image in the Image Size dialog box, as shown below.

This box allows you to change the image interpolationthats used when you resample an image. Differen

interpolation settings affect the way new pixels are assigned color based on the pixels that surround

them. There are five options in this menu:

Nearest Neighbor.Fast, but not very precise; best for illustrations with edges that are not anti-aliased

rather than photos. It preserves hard edges.Bilinear.Medium quality results with most types of images.

Bicubic.Slower but more precise; produces smoother graduations than the two previous methods.

Bicubic Smoother.Based on Bicubic Interpolation, but designed for enlarging images.

Bicubic Sharper.Based on Bicubic Interpolation, but designed for reducing image size because it

maintains the detail of the original image.

Changing Size and Resolution Together

Sometimes its necessary to change the document size and resolution at the same time. For example,
http://www.ononesoftware.com/


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suppose you have a file thats sized to 8" 10" at 250 ppi, and you need to change it to a 4" 5" at 300ppi

Making this change with the Image Size command requires two steps. You can carry out both in one use

of the Image Size command:

1. Choose File > New to open the New dialog box; make the file measure 8" 10" at 250 ppi.

2. Choose Image > Image Size (Alt+Ctrl+I).The Image Size dialog box appears.

3. Deselect the Resample Image option, and change the resolution to 300.Notice that the Document

Size changes to 6.67" 8.33" because youre moving the pixels closer together.4. Select Resample with the Bicubic Sharper option from the Resample Image menu, and change the

Document Size Width to 4 inches.

5. Click OK.Now you have a file that is 4" 4 5" at 300 ppi.

Adapted with permission from Photoshop CS4 After the Shoot

(http://www.wiley.com/WileyCDA/WileyTitle/productCd-0470389869.html)by Mark Fitzgerald.

Copyright 2009 Wiley Publishing

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