Focal length comparision

How focal length affects background objects in our photos

You may or may not be familiar with how focal length affects how close we get to objects we want to photograph. Many may more commonly know this as a zoom. But in actual fact the word zoom refers to the fact that we can adjust the focal length of the lens.

What is focal length?

When we take a photo, light passes through the lens and falls upon the focal plane (sensor or film). In an optical zoom the distance between the lens and the focal plane is known as the focal length, as illustrated here:

Example of camera lens focal length

The longer the focal length, the more the scene or your subject will be magnified. The standard field of view for a digital SLR, or 35mm SLR film camera is 50mm. This is equal to what we see with our eyes. Anything below 50mm = wide angle. Anything above 50mm = telephoto

This diagram helps to illustrate this:

Focal length and angle of view guide
© Alhovik | Dreamstime.com

As you can see, the shortest focal length (fisheye) has the widest angle of view. What falls upon the sensor is what is seen between those two outer points. Therefore:

Increase the focal length: the angle of light narrows, and thus the narrower the field of view. So essentially you are compressing the scene and reducing how much of it falls upon the sensor, effectively isolating a part of it. Think of it as cropping part of the photo away.

Decrease the focal length: the angle of light widens, and thus the wider the field of view. So you are increasing how much of the scene falls upon the sensor.

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The following photos illustrate this.

Photo at wide angle:

rapeseed field in Brnik with Kamnik Alps in the background, Slovenia.
Rapeseed field in Brnik with Kamnik Alps in the background, Slovenia.

When the focal length is increased the angle of light will be reduced and only this much of the scene will fall upon the sensor:

rapeseed field in Brnik with Kamnik Alps in the background, Slovenia.

So the zoomed image will then look like this on the sensor:

rapeseed field in Brnik with Kamnik Alps in the background, Slovenia.

But how does focal length affect background objects?

Often people talk about how long focal lengths can be used to render the background more out of focus, which is great for portrait and wildlife photography. As you can see from the focal length chart above, the illustrator has suggested that anything from telephoto to super telephoto focal lengths are used for things like portraits, children, sports, birds etc.

But, something which is often overlooked, is how you can use longer focal lengths for landscape photography and how it can make a dramatic impact on your background objects.

Wide angle and telephoto effects for landscapes

Wide angle makes foreground objects larger and distant objects smaller, telephoto has almost the opposite effect. When you zoom in you make everything larger.

The following photo was taken with a wide angle. You can see that the foreground grass is large and prominent in the scene, but the church in the background is very small:

35mm

View from Rantovše hill across to Sveti Tomaz nad Praprotnim (church of Saint Thomas) in the Skofja Loka hills, Slovenia.
View from Rantovše hill across to Sveti Tomaz nad Praprotnim (church of Saint Thomas) in the Skofja Loka hills, Slovenia.

For this photo I moved back a bit and used a longer focal length. As you can see, the church is now much larger in the background.

105mm

View from Rantovše hill across to Sveti Tomaz nad Praprotnim (church of Saint Thomas) in the Skofja Loka hills, Slovenia.
View from Rantovše hill across to Sveti Tomaz nad Praprotnim (church of Saint Thomas) in the Skofja Loka hills, Slovenia.
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By moving back to a distance and zooming in, I have made the background church more dominant in the scene.

So you can see here that I have made a dramatic difference to the whole photo simply by changing my perspective, moving further back and zooming in with a longer focal length.

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Watch the video

Here you can watch my YouTube video all about this:

For a more comprehensive look at photography, check out my book here

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