Why You Should Know What Focal Length Means
Knowing what the focal length means, especially in relation to your camera, is very important when it comes to buying lenses. This post will leave you well informed with the correct information at to what the lenses do, which ones are right for you, how to use them creatively, and all the technical speak you’ll need.
Section 1 – What does it actually mean?
The focal length of your lens essentially determines how ‘zoomed in’ your photos are; the higher the number, the more zoomed your lens will be.
It is often misunderstood that the focal length is measured from the front or rear of the lens, when in reality it’s the distance between the point of convergence in your lens to the sensor or film in your camera. Take a look at the diagram below that explains this.
Section 2 – Different focal ranges and what they’re used for.
Ultra Wide Angle 14-24mm
These lenses are often considered specialty items and the range is not often included as part of a kit lens. They create such a wide angle of view that they can look distorted as our eyes aren’t used to seeing that sort of range. They’re often used in event and architectural photography for getting a lot into a photo when shooting in a confined space. Wide and ultra wide lenses are about putting yourself in the middle of it all, not just about getting the whole of a scene in. These lenses are not particularly suitable for portraits as they enhance the perspective so much that the facial features can look unnatural.
Wide Angle 24-35mm
This is where you’ll find most kit lenses for full frame cameras will start at. 24mm is roughly the point at which the distortion that appears to stretch the side of the image stops appearing unnatural. They are used widely by photojournalists for documenting situations as they are wide enough to include a lot of the context and still look realistic.
It’s in this range at about 45-50mm that the lens will reproduce what our eyes will see (excluding peripheral vision). I personally like to use this range when shooting on the street or in situations with friends in a close setting such as at a dinner table or the pub. A standard lens such as a 50mm f1.8 is an excellent, inexpensive addition for a camera and will provide excellent results. Prime lenses (lenses with a fixed focal length – can’t zoom) will always provide better results than your kit lens as the lens is built with a single purpose in mind. It does one job well rather than multiple jobs poorly.
Mild Telephoto 70-105mm
This range is often where kit lenses will stop and you’ll start to get into the range of telephoto lenses and portrait primes (around 85mm). This is a good range for portrait lenses as the natural perspective of the lens will separate the face from the background without completely isolating the face.
Lenses in this range are often used for distant scenes such as buildings or mountains. They’re not suitable for landscapes because of the way they will flatten the perspective of a scene. Lenses higher than this range are mostly used for sport and animal photography.
Section 3 – How does the focal length affect the perspective of a photo?
I’ve tackled most of this in the previous section, but to give you a better idea of how the focal length affects the perspective of a photo, I’ve taken 4 photos of the same subject at different focal lengths and compared them below. The subjects (3 soup cans) are kept in the same position (about 10 inches apart from one another) in every photo. It’s worth noting that these photos are shot with a crop sensor so the actual focal length will be higher then listed – something I explain below in Section 4.
To say that it’s the focal length that is changing the perspective is however, quite misleading. You see, it’s actually the distance from the subject. The focal length is an indicator to the distance from the subject, so they stay framed mostly the same, it’s because the focal length is getting longer (zooming in), as the camera moves further away. Remember, it’s the distance from the subject, and the focal length is just used to compensate for this.
Section 4 – What about my crop sensor?
Shooting on a crop sensor has what’s known as the ‘crop factor’. This essentially means that any full frame lenses (EF, FX, etc.) that you put onto a crop sensor body will have a cropping effect. The actual factor is approximately 1.6. In real terms, this means that if you to shoot at 35mm, the actual result you’ll get will be close to a 50mm image.
The way this works is demonstrated in the diagrams below. What you’re effectively doing is zooming in on an image and avoiding the widest parts of the scene.
Even lenses built for crop cameras such as the EF-S range and DX range will still have this effect, that’s because lenses are listed by their actual length, not their field of view. These lenses will not work on a full frame body without a heavy vignetting effect as the image will not project onto the whole of the sensor.