What is Focal Length?
Focal Length is the measurement of the optical center of the lens to the film plane. Lenses come in different “focal lengths,” and more and more, you see mostly zoom lenses with a variable focal length (i.e., 18-55mm). A full extended zoom lens in this example would be 55mm. Zoomed down, the lens is 18mm.
What does the focal length change?
Let me show you 5 photographs that all have the same exposure but different focal lengths. All shots were taken from the exact same spot with the same camera body.
Does Focal Length Impact Perspective?
You may have heard about this statement quite often, the focal length that you use goes a long way in capturing different perspectives for your images. The statement is both right and wrong at the same time. While focal length does not change perspective per se, it does change how the subject is represented.
The Right Focal Length for an Image
You may have heard this statement – certain scenes warrant a certain focal length. What is meant by that is the right focal length is important to capture a certain scene in a proper way (not necessarily a unique way). For such scenes using the wrong focal length can be disastrous. E.g., if you are shooting portrait images, using anything wider than 70mm can be catastrophic.
Wide-angle lenses give you a bigger field of coverage. To fill the frame for a portfolio shot, you will need to step in close to the model. Additionally, wide-angle lenses tend to accentuate the size of an object that is close to the lens.
What happens as a combination of these factors is that the nose of the subject (which is closer to the lens) appears larger, while the rest of the face appears slender, a rendition which no client will like! Alternatively, you cannot hope to shoot a sweeping panorama using 300mm telelens. You will need an ultra-wide-angle lens for that.
General-purpose kit lenses such as the 18-55mm or the ubiquitous 50mm standard prime are great for everyday photography. But when it comes to shooting anything from the waist up whilst filling the frame, the best focal lengths are usually the ones between 100mm to 135mm.
Background compression is one of the types of perspective distortion that happens more with telelens than with wide-angle lenses. Let’s take an example. Let’s say you take an image of a scene at 16mm. Now, you take another image of the same scene at 50mm. For the next two images, you use focal lengths 100mm and 200mm, respectively. Now, analyze the four images side by side.
You will notice that more of the background appears to be sucked into the image for each progressively longer focal length while the foreground appears pushed back. In other words, the background and the foreground appear to be compressed together. This effect is ideal when you want to incorporate more of the background in your images while getting a close-up of the main subject.