Photoshop by the Numbers

Photoshop by the Numbers

"Photoshop by the Numbers" is a series of articles about everything related to numbers in digital photography and retouching.

Photoshop is full of numbers. Resize an image, or change its resolution, and you have to enter numbers. The Unsharp Mask filter asks you to provide three numbers; a Levels adjustment five. Bit depth can be 8, 16 or 32: numbers. Open the Info panel, move the mouse over your image and you see many numbers. A color cast can be corrected "by the numbers".

A digital image is nothing but a rectangle of colored pixels, where every pixel is represented by a set of numbers. Any manipulation of the image - lightening, sharpening, neutralizing, blending, etc. etc. - means that some arithmetic is applied to those numbers.

"Photoshop by the Numbers" studies "the numbers of a photograph": their meaning, relevance and relationships.

I investigate and explain RGB values, but also LAB, CMYK and HSB color spaces, color profiles, 8-bit vs. 16-bit vs. 32-bit, the effect of adjustments, masks and blend modes on pixel values, image size and resolution and much, much more.

List of articles

Possible numbers for skin color

Exposure and Contrast pt.6 - The Auto options

Both Curves and Levels have an Auto button. There is more behind this button than you might think. This article explores the options.

Exposure and Contrast pt.5 - Using the eyedroppers

Multiple Photoshop adjustments have eyedroppers. They are supposed to simplify basic exposure and color fine-tuning. This article explains all you need to know about them.

Exposure and Contrast pt.4 - Exposure

The Exposure adjustment in Photoshop hides a few secrets. I try to unlock them all, but a few mysteries remain. Beware, a lot of math in this article.

Exposure and Contrast pt.3 - Curves

Curves may be a bit intimidating for beginners, but it is way more powerful than any other contrast/exposure adjustment. This article shows why.

Exposure and Contrast pt.2 - Levels

I explain how Levels works, what it can and cannot do in comparison with the Brightness/Contrast adjustment. Some surprises await!

Exposure and Contrast pt.1 - Brightness/Contrast

This article explains Photoshop's Brightness/Contrast adjustment in great detail, especially the difference between the legacy and the current implementation.

Channels pt.3 - The powers and blessings of Apply Image

Definitely the most powerful channel operation is Apply Image. This article explains why, and illustrates it with a few interesting examples.

Channels pt.2 - Channel Mixer and Calculations

Photoshop supports a number of channel operations. In this article, I explore Channel Mixer and Calculations. Channel Mixer seems the most useful of the two.

Channels pt.1 - The basics

Who hasn't struggled with the concept "channels"? I explain what they are, and illustrate some of their characteristics.

The Unsharp Mask filter pt.4 - Why "Unsharp Mask"?

Why is Unsharp Mask called Unsharp Mask? There is a reason, and in this article I show in precise detail what it is.

The Unsharp Mask filter pt.3 - High radius, low amount sharpening

An interesting effect results from a high Radius and low Amount setting on the USM filter. I compare it, among others, to a global contrast curve.

The Unsharp Mask filter pt.2 - The parameters in practice

Amount, Radius and Threshold - this article makes some suggestions how to set Unsharp Mask's parameters for optimal sharpening results.

The Unsharp Mask filter pt.1 - The basics

How can you sharpen an edge that is already sharp? The Unsharp Mask filter does it, and this article explains how.

Histograms pt.2 - The other color modes

Convert your image to CMYK, LAB or Grayscale, and Photoshop shows histograms for it. This article outlines how they work, and questions their usefulness.

Histograms pt.1 - Basics and numbers

Most retouchers know what a histogram is. I explain its mechanics in detail, and encounter a few small irritators along the way.

Bit Depth pt.3 - 32-bit, HDR and tone mapping

This article not only shows how to construct a 32-bit image file from multiple exposures, but also how to convert it back to 8-bit.

Bit Depth pt. 2 - The magic of 32-bit coding

For some imaging needs, 16 bits per channel are not enough. 32-bit color coding has enormous potential in terms of dynamic range.

Bit Depth pt.1 - 8-bit and 16-bit compared

Should we edit in 8-bit or 16-bit? Before answering that question, I investigate what the 8 extra bits mean.

Color Profiles pt.2 - The pitfalls

Working with different color profiles, especially the wider ones, involves a few risks. It's important to be aware of them.

Color Profiles pt.1 - The basics

Using a few simple examples, this article explains why you need a color profile, and how you can switch between different profiles.

Image Resolution pt.3 - How many megapixels?

How many megapixels does a good quality print really need, taking into account a normal viewing distance? The answer may surprise you.

Image Resolution pt.2 - Why resizing?

This article explains what happens when you resize an image in Photoshop. How does it impact file size, PPI, print quality?

Image Resolution pt.1 - The basics

What is resolution? This article explains the basics and shows how Photoshop's Resize Image works.

Color Correction by the Numbers pt.5 - The final judgement

The numbers are correct, yet the image doesn't look quite right. How to proceed? This article provides an answer.

Color Correction by the Numbers pt.4 - Some miscellaneous techniques

A photo with a bad cast, an oversaturated original and a sorely faded print are the subjects of study in this article.

Color Correction by the Numbers pt.3 - The tricky mixed lighting

How to correct a color cast that is different on either sides of a photo? It's remarkably simple once you realize what needs to be done.

Color Correction by the Numbers pt.2 - The case of the blue shadows

Shadow areas in a photograph often have a blue cast. This article discusses two ways to correct this.

Color Correction by the Numbers pt.1 - The basics

Incorrect colors can best be adjusted by checking color numbers. This article introduces the technique by going through one example image.

Numbers and Colors: How they relate

Can we guess a color from its RGB, LAB or CMYK values? Or can we estimate the RGB, LAB or CMYK numbers from a color? To some extent: yes we can.

The color yellow: names, properties and a surprising discovery

Theory says: no dark yellow exists. But is it really true? The human vision susprises us again.

Some thoughts on color names

How do we define colors by their names? Is RGB's green the same as LAB's green? This article proposes some alternative names for LAB's opposing colors.

HSB: The ultimate split into Hue, Saturation and Brightness

The HSB way of color coding splits color into hue and saturation. Yet, it is not so special as a color space. This article explains why.

The Specialties of the LAB Color Space

The LAB color space has some very interesting properties. It is considerably different from RGB and CMYK. We examine its mechanics.

CMYK and its relationship with RGB

CMYK was designed as a coding for printer ink. This article examines its properties. It shares some properties with RGB, but there are differences too.

Color Coding and the RGB Color Space

In digital image files, colors need to be coded as numbers. We have a look at the most common way of doing that: in RGB.