In this article, we’ll deep-dive into focal length and discuss how it can help you improve your photography and save you a lot of money. Let’s get started.
What Is Focal Length?
We’ve all heard the term focal length before and understand that lenses are available in such standard offerings as a fixed 35mm focal length or maybe as a zoom lens of 16-35mm. The combinations of focal lengths are nearly countless.
But to understand what focal length is, you’ll need to know a couple of terms first. This will help us understand how focal length is calculated.
Image Sensor
The image sensor (the camera’s sensor) captures light from the lens and converts it into an electrical signal. This information is processed and transformed into an image.
Point of Convergence
If you look at both ends of a camera lens, you’ll notice that your reflection in the glass is upside down. This is the effect that happens at the initial stage of processing an image.
When the lens is attached to the camera and ready to take pictures, then the image is flipped back to the right side up position inside the lens to continue the second half of the journey to the imaging sensor. The intersection at which the image is effectively flipped back to its normal view is called the Point of Convergence.
The Simple Focal Length Formula
Focal length is the distance in millimeters from the point of convergence to the imaging sensor (when focused to infinity). So if we have a 35mm lens, the approximate distance from the point of convergence, the lens’s optical center, to the camera’s sensor is 35 millimeters.
Now that we know what focal length is, let’s move into a different scenario and another formula.
Equivalent Focal Length
The equivalent focal length considers the lens’s focal length and the camera’s sensor size. If you have a 35mm format camera, you don’t have to worry about any new formula. That’s because the equivalent focal length is based on the 35mm format, which is called a full-frame camera.
Let’s take a look at how focal length is affected when we consider the size of the sensor.
Sensor Size and Crop Factor
There are numerous sensor sizes for cameras. Most are smaller than full-frame cameras, although there are much larger sizes. However, if we’re talking about any sensor size other than full-frame, then you’ll need to consider the crop factor too.
This means that there’s a magnification effect, a cropped view when you use a camera system with a smaller sensor. So what we have 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 an APS-C Canon system with a 1.6 crop factor and a 35mm lens, the equivalent focal length would be 56mm.
We’ll summarize what this means next.
Focal Length and Field of View
Keep in mind that no matter what your camera’s sensor size, your lens’s focal length 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 the equivalent focal length.
The field of view simply means how much of the world your lens captures at varying focal lengths.
Are you confused still? The video below sums up most of what we’ve discussed so far and provides some helpful visual examples.
Let’s discuss one more aspect of focal length as to how it relates to the depth of field. This may be useful for those who love to create the bokeh effect.
Focal Length and Depth of Field
The focal length of your lens also affects the depth of field in your image. In general, the larger 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 below video comparing an 85mm lens to a 135mm lens.
The relationship between focal length and the depth of field may not concern photography hobbyists, but it’s worth understanding when it’s time to purchase a new lens.
How Focal Length Affects Your Lens Purchases
Understanding the basic principles of what we’ve covered will help you immensely when it comes time to shop for new camera gear. When you factor in the focal length, crop factor, and depth of field, among other things, you’ll have a better understanding of what camera and lenses you need to purchase to shoot the kind of photos that you wish to create.
Wildlife Photography and Subjects That Require Telephoto Lenses
If you’re a wildlife or sports photographer, for example, you may be caught up in the marketing campaign of the latest full-frame camera system. But do you really need a more expensive full-frame camera, not to mention the much more expensive lenses and accessories required to make use of one?
The short answer is no, especially if you’re on a budget and can’t afford a full-frame system. There are significant advantages to using an APS-C camera system with a 1.5 or 1.6 crop factor. And professional photographers take advantage of this too. Why?
Because an APS-C camera kit will effectively increase your telephoto reach 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! To capture this field of view on a full-frame lens equivalent would cost a lot more money.
Understand Focal Length and Save Money on Camera Gear
The importance of learning about focal length can’t be overstated. If you’re a passionate photographer, you should know the craft of photography inside and out. Mastering your understanding of focal length will certainly 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!
