An introduction to focal length in photography

Most people have a basic understanding of focal length. If you see a 35mm lens, you know that’s the focal length. You can even get a pretty good idea of ​​what you’ll see when you hold the camera up to your eye. But there is much more than that.

In this article, we’ll dig deeper into focal length and discuss how it can help improve your photography and save you a lot of money. Let’s start.

What is focal length?


Close up of camera lens

We have all heard the term focal length before and understand that lenses come in standard offerings such as a 35mm fixed focal length or perhaps a 16-35mm zoom. The combinations of focal lengths are almost innumerable.

But to understand what focal length is, you need to know a few terms first. This will help us understand how the focal length is calculated.

Image sensor

The image sensor (the sensor in the camera) captures the light from the lens and converts it into an electrical signal. This information is processed and transformed into an image.

Focal point


Focal length chart

If you look at both ends of a camera lens, you will notice that your reflection in the glass is upside down. This is the effect that occurs at the initial stage of processing an image.

When the lens is attached to the camera and ready to shoot, the image is flipped up inside the lens to continue the second half of the journey to the imaging sensor. The intersection at which the image is effectively returned to its normal view is called the point of convergence.

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The simple focal length formula

The focal length is the distance in millimeters between the focal point and the imaging sensor (when focused at infinity). So if we have a 35mm lens, the approximate distance between the focal point, the optical center of the lens, and the camera sensor is 35 millimeters.

Now that we know what focal length is, let’s move on to a different scenario and another formula.

Equivalent focal length


Close-up focused on the camera lens

The equivalent focal length takes into account the focal length of the lens and the size of the camera’s sensor. If you have a 35mm format camera, you don’t have to worry about a new formula. This is because the equivalent focal length is based on the 35mm format, referred to as a full frame camera.


Let’s see how focal length is affected when we consider sensor size.

Sensor size and crop factor


Camera Sensor Size Comparison Chart

There are many sensor sizes for cameras. Most are smaller than full-frame cameras, although there are much larger sizes. However, if we are talking about a sensor size other than full frame, you will also need to consider the crop factor.

This means there is a magnifying effect, a cropped view when using a camera system with a smaller sensor. So what we need to do is multiply the focal length of the lens by the crop factor to arrive at the equivalent focal length in 35mm terms.

For example, if you have a Canon APS-C system with a crop factor of 1.6 and a 35mm lens, the equivalent focal length would be 56mm.

We will summarize what this means next.

Focal length and field of view


Ruler and pencil on the table

Keep in mind that no matter what size sensor your camera has, the focal length of your lens doesn’t actually change. A 35mm lens is a 35mm lens on any camera. However, what changes is the field of view when considering the crop factor and equivalent focal length.

Field of view simply means what part of the world your lens captures at different focal lengths.

Are you still confused? The video below summarizes most of what we’ve discussed so far and provides some helpful visual examples.

Let’s discuss another aspect of focal length as it relates to depth of field. It can be useful for those who like to create the bokeh effect.

Focal length and depth of field


Photo showing shallow depth of field

The focal length of your lens also affects the depth of field in your image. In general, the longer the focal length, the shallower the depth of field when the aperture is at its widest setting.

There’s a great example of this in the video below comparing an 85mm lens to a 135mm lens.

The relationship between focal length and depth of field may not concern photography enthusiasts, but it’s worth understanding when it’s time to buy a new lens.

How focal length affects your lens purchases


Focused camera lens

Understanding the basics of what we’ve covered will help you immensely when it comes time to buy new camera equipment. When you consider focal length, crop factor, and depth of field, among other things, you’ll have a better understanding of the camera and lenses you should buy to take the kind of photos you want to create. .

Wildlife photography and subjects requiring telephoto lenses

If you’re a wildlife or sports photographer, for example, you may get caught up in the marketing campaign for the latest full-frame camera system. But do you really need a more expensive full-frame camera, let alone the much more expensive lenses and accessories needed to make use of it?


The short answer is no, especially if you’re on a budget and can’t afford a full-frame system. Using an APS-C camera system with a crop factor of 1.5 or 1.6 has significant advantages. And professional photographers also benefit. Why?

Because an APS-C camera kit will effectively increase your telephoto range 1.5 or 1.6 times more than a full frame camera. This means that a 70-200mm lens on a Canon APS-C would have the equivalent focal length of 112-320mm! Capturing this field of view on a full-frame lens equivalent would cost significantly more.

Understanding focal length and saving money on camera gear

The importance of learning focal length cannot be overstated. If you are an avid photographer, you must know the art of photography inside and out. Mastering your understanding of focal length will definitely make you a better photographer in the long run.

If anything, an understanding of focal length could save you thousands of dollars when it’s time to upgrade your camera and lenses. It’s a win-win for photographers!


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About the Author

Stewart C. Hartline