Photoshop - CS5

Lesson 10: Dimensions (1)

10/94 Lessons 

Pixels, Resolution and PPI

Let me first explain what pixels are.

A photo or a scanned image consists of millions of blocks, these blocks are called pixels.

Each pixel in a photo has a position, color and contrast.

The quality of your pictures is determined by the number of pixels. The more pixels, more details are displayed and the smoother the picture will be.

So you take a picture and on scanning the image, always choose the highest resolution, you can still do anything with it in Photoshop.

What is resolution now? Resolution is the number of pixels per inch of your photograph.

What is an inch? And inch is about 2.5 cm.

So when your photo has a resolution of 72 ppi means that every inch has 72 pixels.

For those who not know, ppi stands for pixels per inch and is used as a measure for the number of pixels in a row in your photo description.

When you calculate this per inch ², this is 72 times 72, and your picture has 5184 pixels per inch ².

When the image increases by more than 500%, the pixel grid of the image is visible. If you want to disable this temporarily, click the Ctrl + H on your keyboard.


In this section we talk about the image size and how we can customize it for different purposes.

As I said in the previous section, take pictures with the highest possible resolution and change it later in Photoshop.

If you have a digital camera, it will take photos at 72 ppi.  If you want to print photos on your inkjet printer, for example, then the ppi of the image must be increased to a minimum of 300.

Just click “Image” in the menu bar and select “Image Size” in the drop-down menu.

The dialog “Picture Size” is divided into three sections.

The first section “Pixel Dimensions” (1), gives us the number of pixels in the picture.

The second section “Document Size” (2), only has to do with printing of the photo.

And in the third section, there are a number of options (3).

I have the resolution set to 300 pixels / inch and the unit of measure is inches (section 2), this makes it a little easy.
When I set the paper width of 20 inches, it is quickly calculated that I have 6000 pixels across a width of 20 inches. Do you follow? 300X20 = 6000.

Same for the document height, 300 pixels / inch document height is 15 inches = 4500 pixels.

For an image of 20×15 inches, I then have 6000 times 4500 pixels = 27 million pixels.

When the options “Constrain Proportions” and “Resample Image” in section 3 are selected, if I change the height or width in section 2 or 3, the rest of the options change automatically.

The first “Constrain Proportions” is that any alteration of width or height, the ratio between the two will be retained.

The second “Resample Image”, ensures that the number of pixels in the picture will be adjusted with any alteration of the resolution.

In other words, if you increase the resolution, there are more pixels on the photo and if you reduce the resolution, pixels are reduced.

It is therefore important that both options are selected when changing the resolution.

If the resolution of the image is increased, which is true in this case (from 72 ppi to 300 ppi), you will see at the top of the dialog that the size (in MB) of the photo, is significantly increased (from 20.3 to 77.2).

To print the picture, it is fine, if you place it on the Internet, this is too high.

The top option “Scale Styles”, is just as important when working with “Layer Styles” in the picture. We have not seen this, so this is not applicable.

If the resolution of the image is changed, pixels are added or removed. We select the method that Photoshop will use for this, in the lower box of section 3.

Just click the downward pointing arrow and choose one of these options:

The explanation from Adobe:

Nearest Neighbor

A fast but less precise method in which the pixels in an image will be duplicated. This method is for use with illustrations containing edges that are not anti-aliased, so preserves hard edges and a smaller file. The disadvantage of this method is that it can produce jagged effects, which become visible when an image is distorted or scaled or when you perform multiple manipulations on a selection.


A method that adds pixels by averaging the color values of surrounding pixels. It produces medium-quality.


A slower but more precise method based on an examination of the values of surrounding pixels. Bicubic uses complex calculations and produces smoother tonal gradations than Nearest Neighbor or Bilinear.

Bicubic Smoother

A suitable method for enhancing images on the basis of Bicubic interpolation, but designed to produce smoother results.

Bicubic Sharper

A suitable method for reducing the size of an image based on Bicubic interpolation with enhanced sharpening. This method maintains the detail when you resample. With Bicubic Sharper, some areas of an image are too sharp, try Bicubic.

Phew, I’m already nervous when I started to read.

My explanation:

Nearest Neighbor

This option takes a pixel, multiplies it by the degree of resolution of the picture.So you have a photo with a dpi of 100 and change this to 300 dpi, copies one pixel to 9 pixels, that is 3X3.


This option is completely useless.


This option is useful. It compares multiple pixels, and then adds a new pixel. This is also the option you will need to use in most cases.

Bicubic Smoother

This option does the same as the previous one, this only makes the transition, after the addition of the pixels, smoother. You possibly use this when you have a lot of noise in your picture.

Bicubic Sharper

Ditto the previous one, this only makes the transition between the pixels harder.

We continue in the Next lesson..

You've completed Lesson 10