Understanding Lenses: What is Focal Length?
Mastering focal length is the first step to understanding how a lens works.
The most important information to know when looking for a camera lens is the focal length. Focal length tells a photographer or videographer a lot about how the image is going to look. The shorter the focal length, the wider the angle of view and vice-versa. In the following article we will dissect (hopefully) everything you could ever want to know about focal length. If you have any questions or comments please share in the comments at the bottom of the page.
Lenses are often broken down into 3 different categories wide, telephoto, and standard. A wide angle lens is any lens that is 35mm or smaller. Lenses that are more wide than 24mm can be called ultra-wide angle lenses, but most photographers just call them fisheye lenses. Due to size exaggeration, wide angle lenses are great for shooting landscapes, real estate, and architecture.
Telephoto lenses are any lens with a focal length of 85mm or higher. They are usually very long in length making them easy to identify. Telephoto lenses are generally used to shoot objects that are far away making them ideal for capturing weddings, wildlife, and events.
Telephoto lenses usually have more glass elements inside than wider lenses making them generally more expensive. In fact, you’ve probably seen an expensive telephoto lens at a sporting event. Telephoto lenses can be broken down into two further subtypes: medium telephoto (85-300mm) and super telephoto (300mm+). They usually create a very blurry background making them ideal for isolating your subject…we’ll dive into ‘depth of field’ in a future post.
Standard lenses are any lens between 35mm and 85mm. The most commonly used standard lens is the 50mm prime or “nifty-fifty”, as it’s affectionately referred to by many photo pros. Standard lenses usually have a much cheaper base price than their wide and telephoto counterparts.
These lenses are the Goldilocks of lenses, not too wide, not too telephoto, making them perfect for shooting portraits, medium shots, and general photography. Technically speaking, a lens is considered standard or “normal” if it is close to the diagonal length of the camera sensor in millimeters.
Zoom vs. Primes
Lenses with focal lengths that can change are called zoom lenses and those that remain fixed are called prime lenses. When comparing equally priced prime and zoom lenses, prime lenses usually will produce a better image. This is because zoom lenses require many moving parts that hinder light’s ability to move through the lens. Professional photographers do use zoom lenses for their work (like the Canon 70-200mm), but it’s more typical for high-end productions to use prime lenses, as they let in more light. Lenses that come with a camera (kit lenses) are usually zoom lenses.
What is Focal Length?
Focal Length is not…
- The length of the lens.
- Half the length of the lens.
- The diameter of the lens.
Focal length is the measurement (in millimeters) from the optical center of a camera lens to the camera’s sensor. The optical center is also known as the focal point. For all lenses (including primes) the focal length changes depending on what the lens is focusing on. For example a 50mm lens when focusing to infinity will have a focal length of 50mm, but when focusing on an object 1 meter away the focal length needs to be moved 2.6mm further away from the camera sensor to be in focus. Thus what you thought was a 50mm image is actually a 52mm image.
Not to be confused with a 35mm lens, most high-end cameras have a camera sensor that is 35mm in length. A 35mm sensor is “full-frame“, meaning it uses the entire lens when capturing an image. This 35mm standard was designed to be identical to film cameras which used 35mm film to capture images. So a 50mm lens on a 35mm film camera will act very similarly to a 50mm on a 35mm “full frame” sensor.
However, if you are using a camera that has a smaller sensor than 35mm you are going to experience crop factor. If you’re in the photography or video world than you are probably well aware of crop factor, but for those who aren’t already acquainted, crop factor is a phenomenon in which a lens will act more telephoto than it actually is. So for example, a 100mm lens on a camera with a crop factor of 1.6x will have a similar field of view as a 160mm lens on a full frame camera.
When you read online about cropped sensors you will run across pages and pages talking about how cropped sensors have a focal length multiplication factor. This means that if a camera sensor has a multiplication factor of 1.5x than a 50mm lens will actually have a focal length of 75mm. This is actually somewhat false. As we’ve found out above, the only way for an image to be in focus is for the camera sensor to be a very specific distance away from the camera. If the focal length actually changed from 50mm to 75mm you would have an image that was completely out of focus. Instead, a crop factor is actually decreasing the of angle of view.
The market offers camera adapters that increase the angle of view of 35mm lenses to reduce crop factor. These adapters are called focal length reducers (but we know they actually mean ‘angle of view increasers’). Cameras with a cropped sensor can make shooting wide shots very difficult so be sure to take that into consideration before purchasing a lens.
Remember how we talked about the focal point inside of the lens? This point is the place in which light is directed, but unfortunately light doesn’t always bend perfectly. If you’ve ever shined a flashlight through a glass prism, or seen a Pink Floyd shirt for that matter, than you know that when bent, light will separate into different colors because color waves move at different speeds. This happens in camera lenses too, and most photographers consider it a bad thing. It’s called chromatic aberration. For a digital camera chromatic aberration occurs when blue, green, and red light separate across 3 separate focal points. The result is the skewing of colors around the edges of objects within your picture.
Newer camera lenses have a lens element known as a ‘flint’ specifically designed to focus red, green, and blue light rays onto a single point…but older lenses typically do not. Good quality lenses are those which have minimal chromatic aberration.
Chromatic aberration is worse around the edges of an image frame, so when you are buying a new lens look around the edges in your image for color shifting or “purple fringing”. However, if you are intentionally trying to get a vintage look, try using an older lens with an adapter, you will find plenty of chromatic aberration!
The most important take away is focal length is directly related to the angle of view. There are many more technical things to learn about focal length but the topics discussed in this article are the most important for understanding how it plays into both photography and videography. If you’ve enjoyed this post and are interested in learning a little more about the science of focal length in photography check out the articles below.
Have any questions about focal length? Anything you would like to add? Share in the comments below.