An Introduction to Focal Length


The focal length of the lens you choose can greatly affect your photos – much more than just determining how much you can zoom in on your subject. However, before we get into those kind of details, there are some important things to know about how a certain focal length reacts differently when used on specific cameras – this is very important if you're in the market for a new lens.

Focal Length and Sensor Size

The sensor size of your camera has to be taken into consideration when you're picking out what kind of lens to use. Most digital SLR cameras work on what is referred to as a “crop sensor”, which means that it is smaller (cropped) than a full-frame sensor of 35mm (the film standard size). Advanced digital SLRs have full-frame sensors so it is possible to have one, although it will be a more expensive camera when compared to those with a crop sensor. Most photographers will find themselves working on a crop sensor that is about 22mm x 15mm in size in comparison to a full-frame sensor, which is about 36mm x 24mm.

So what does this mean for your focal length? If you think literally, a 100mm lens on a full-frame is going to capture much more of your viewing area than if it were mounted on a crop sensor. A larger sensor size = more in your field of view; you're working with a bigger “canvas” so to speak to paint your picture onto.

For wide angle lenses, this is a definite plus since you can capture more of your surroundings with less distortion from using a wider lens on a crop sensor.

For example, a 24mm lens on a crop sensor isn’t exactly super-wide format, which is important for landscape photographers. However, mount it on a full-frame sensor and you can capture images like this one below:

Photo by Christopher O'Donnell

Notice that the distortion here is minimal – if I used a wider lens on a crop sensor (say 15mm) to get the same image, you'd have much more fish-eye distortion and exaggerated distances.

There is a calculation to determine what a certain focal length on one sensor size will look like when mounted on another – this is known as your “crop factor”. Basically, you’ll multiply the focal length of a lens on a crop sensor by a certain number and it will give you the focal length needed to capture the same field of view on a full-frame sensor. The “number” varies slightly between camera brands – 1.6 for Canon and 1.5 for Nikon, Pentax, and Sony since they have slightly larger sensor sizes.

Let’s say you have a 50mm lens on a full-frame sensor. If you want that same field of view on a Nikon crop sensor, you’ll need a wider lens of 33mm.

So what I did here is take the focal length of my full-frame (50mm) and divide it by the crop factor for a Nikon (1.5), which gave me 33mm. If I wanted to switch it around and calculate what focal length is needed on a full-frame sensor, I’d multiply the focal length by the crop factor instead of divide.

There is much more on the subject of full-frame vs. crop sensors, but for now just know that your sensor size will greatly affect your field of view for a given focal length. Keep this in mind when shopping for a new lens, especially if you’re looking to purchase one at the extreme ends of the spectrum (super-wide or telephoto lenses) as photo examples on a crop sensor will look very differently on a full-frame.

Exaggerated and Condensed Distances

So now that you know how your field of view can change between sensor sizes, you should also be aware of how your focal length will affect the distances and proportions of your image.

Simply put, wide angle lenses have the ability to exaggerate distances, and telephoto lenses will compress your distances. When I refer to “distances”, I am talking about the relationship between you (the camera) and your subject, and the subject to it’s surroundings (foreground, background, and anything in between).

When you get more familiar with your lenses and the effects they have on your subjects, you’ll use this to your advantage, like I did in the image below:

Photo by Christopher O'Donnell

This photo was taken at 300mm, and is actually a 7 (or 9, maybe 11?) photo pano stitch. Instead of spending all that time carefully stitching my images together seamlessly, why didn’t I just take out my wide angle lens and capture it in one photo? I could definitely achieve the same perspective.

There were a couple different reasons why I did this, but my main reason was to compress my distances – especially the distance between the first hill and the yellow lily bokeh in the background. If I used a wide-angle lens, those lilies would be way off in the distance.

However, in my photo below, I used a wide angle lens (24mm) on a full-frame camera. You can see how the distances are exaggerated as there was only a few inches between the foreground dandelion and the ones behind them, and the treeline was less than 30 feet away.

Photo by Christopher O'Donnell

Here are some more examples of the exaggerated distances you can achieve with a wide angle lens:

Russian metro
Photo by Éole

Two Cows
Photo by Martin Gommel

heavenly bandit
Photo by Jrtrippins

And here are some examples of compressed distances found when using a telephoto lens:

Photo by Martin Gommel

Love the One You're With
Photo by code poet

Late October on Lake Michigan
Photo by mic stolz

There is much more to discuss about focal length – like how it can affect shutter speed and camera shake – but we’ll save that for another article. For now, consider these two important tidbits of information and envision how you can use them to your advantage in the field.

Read more great articles by Christopher O’Donnell at his blog or follow him on Facebook.

About Author

I'm a professional landscape photographer living on the coast of Maine. Through my work, I like to show a vantage point that is rarely seen in reality; a show of beauty, emotion, and serenity. Feel free to visit my website.

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