You may have heard professionals talking about the right focal length and how choosing anything less than the ideal length can adversely affect the background of your photos. Now, why does the wrong focal length affect your background? To understand this we need to first have an understanding of what each of the focal lengths tend to do.
But before that we need to understand what focal length is. Focal length is often wrongly presumed to be the length of the lens barrel. As a matter of fact the size of the lens has nothing to do with how optically ‘long’ it is. Focal length is the distance between the optical center of the lens and the sensor when the lens is set to focus at infinity.
The longer that distance is, the shorter is the angle of view of the lens. Angle of view is the extent of the scene that you can see through the viewfinder when you have a lens mounted in front. A longer lens will see further into a scene and the angle of view cuts down dramatically. A wide angle lens will see almost everything in front of it and thus the angle of view is much more pronounced. A fish-eye lens, e.g. can see 180 ° in front of it and sideways.
Using a wide angle lens
A wide angle lens is one that is less than 50mm. More of a 35mm or less. Mind you this takes into consideration that the camera you use is either a 35mm film camera or an equivalent full-frame digital SLR camera.
A wide angle lens has a greater angle of view. A greater angle of view has the advantage of capturing a vast amount of the scene that is in front of the camera. That, however, has a disadvantage as well. When you shoot a subject with a wide angle lens it tend to get lost in all that background clutter.
To compensate you need to step forward, so that you can make a tighter crop. But that has the effect of distorting the features of the subject that are closer to the camera. Shorter the focal length more is the distortion.
Another thing you will notice is that the background and the foreground tend to get mashed into one. Everything is flattened out and this happens when you shoot from a distance.
Using a standard lens
A standard lens, again considering that you are using a full-frame / 35mm film camera is one that has a focal length of 50mm. Arguably, at that focal length, the angle of view is the same as the human eye. This is why street and journalistic photographers often shoot with a 50mm lens, because it helps them to capture a scene very much the same way they see it with their own eyes.A standard lens will give a ‘normal’ perspective. Nothing will be exaggerated and or out of place.
Using a telephoto lens
A telephoto lens is anything that has a focal length of more than 50mm. Although, strictly speaking there are various sub-classification of tele-lenses depending on their focal length – short tele, medium tele and super-tele; so giving a generalized name to all lenses beyond 50mm as just tele is incorrect.
When you shoot with a tele-lens something like 200mm or longer, the lens will capture only a thin slice of the scene in front of it. If you look closely you will notice that the lens has the effect of ‘pulling’ things in front.
There is more subject and background separation if you look closely. Also, the facial features are also in proper perspective. No distortions whatsoever.
So does the use of the right focal length the only reason why some images appear to be just right while others appear skewed and distorted? There is something else at play, which may not have been obvious to you so far; and that is subject to camera distance.
The hidden story in all this – Subject to camera distance
The subject to camera distance changes in each of the above situations. Let’ start with the tele-photo lens. Let’s say that you are shooting with a 200mm lens and from a distance of say 15’. You have a nice subject to background separation and all the other goodies. Notable among these is the fact that the background gets sucked in.
Now mount a wide angle lens. The wide angle lens gives you a greater field of view. So, in order to maintain the same composition as the tele lens, you will be forced to zoom with your feet and come closer to your subject. Once you do that the subject to camera distance reduces. With it the background gets pushed out.
So, it is not only the focal length that governs whether the background gets sucked in or pushed out, but also the distance between the camera and the subject.