One of the most important reasons why you buy a lens is down to the focal length. This lets you know how close you need to be in relation to the subject you are capturing.
A longer focal length is necessary for objects further away. A smaller focal length captures more of the scene in front of you. To understand focal length in four, easy steps, read on.
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Why Focal Length Is Important
Knowing what focal length means in relation to your camera is very important when it comes to buying lenses. Read this post to find out what different lenses are used for. You’ll see which ones are right for you, how to use them creatively, and all the technicalities.
Lenses are divided into two categories based on whether they can zoom. There are the ones that have a fixed focal length (prime lenses). And there are those that have a variable focal length (zoom lenses). In general, prime lenses are sharper and often have a wider aperture. These are great for low light conditions.
Zoom lenses allow you to use one lens to cover a range of different photography fields. One lens means less time spent searching for and changing lenses. There are advantages and drawbacks to both types of lenses. So, having a mixture of them gives you versatility and power.
We can use the human eye as an example. Its focal length varies between 17mm and 25mm, depending on who you ask and who you examine.
We have an approximate field of view of over 180°. This is different from the 90° angle of view from a lens, which is down to the fact that we have two eyes.
The area that we actively perceive is smaller, similar to a 40-50mm lens. That’s why the 50mm is called the ‘standard’ focal length.
Step 1 – What Does Focal Length Mean?
The focal length of the lens 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.
In reality, it’s the distance between the point of convergence in your lens and the camera sensor. This is the case of simple lens designs (like Double-Gauss). There are also plenty of complex optical designs that work differently. In many cases, that distance is not equal to the focal length.
The key takeaway is that the focal length of a lens is identical to the focal length of a single lens that would provide the same field of view.
Take a look at the diagram below that explains this.
Step 2 – Different Focal Ranges and What They’re Used For
Ultra Wide Angle and Fisheye 14-24 mm
These lenses are often considered speciality items. Usually, they are not included as part of a starter kit. They create such a wide angle of view that can feel unnatural to work with at the beginning. This is down to the lens having to fit more of the scene into film or sensor.
These lenses are not suitable for portraits. They enhance the perspective so much that facial features can look unnatural.
When used for journalistic scenes, they can provide a dramatic, interesting angle. Especially if you’re close enough to the subject.
Rectilinear wide angles project an image in which the straight lines remain straight. Fisheye lenses distort the scene into a spherical shape.
Wide Angle 24-35 mm
This is where you’ll find most kit lenses for full-frame cameras start. 24 mm is the point at which the distortion that appears to stretch the side of an image stops appearing unnatural.
Standard 35mm-70 mm
It’s in this range (at about 45-50 mm) that the lens will best reproduce what our eyes see (excluding peripheral vision). I like to use this range when shooting on the street or with friends in a closed setting. Examples would be at the dinner table or the pub.
A standard lens such as a 50 mm f/1.8 is an excellent, inexpensive addition for a camera. It will provide excellent results. A prime lens will always provide better results than your kit lens, as it is built with a single purpose. It does one job well.
Mild Telephoto 70-105mm
This range is often where kit lenses stop. Here, you’ll start to get into the range of telephoto lenses and portrait primes (around 85mm). This is a good range for portraits as the natural perspective of the lens will separate the face from the background.
It does this without isolating it.
Telephoto 105-300mm, and Above
Lenses in this range are often used for distant scenes such as buildings or mountains. They’re not really suitable for landscape photography because of the way they flatten the perspective of a scene. But they can provide interesting perspectives in that field, too.
Step 3 – How Does Focal Length Affect Perspective?
I tackled most of this in the previous section. Let me give you a better idea of how the focal length affects the perspective of a photo. I took four photos of the same subject at different focal lengths and compared them below.
The subjects (three soup cans) remained 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. This means the effective focal length will be higher than listed.
To say it’s the focal length that changes the perspective is, however, quite misleading. You see, it’s the distance from the subject.
The focal length of a lens is an indicator of the distance from the subject: the images are all framed the same. Differences arise because the focal length is getting longer (zooming in) as the camera moves further away from the subject.
Remember, the distance from the subject is changing the perspective. The focal length is just used to compensate for this.
Step 4 – What About My Crop Sensor?
Shooting on a crop sensor has what’s known as the ‘crop factor’. What you’re doing is zooming in on an image, and avoiding the widest parts of the scene. The diagrams below show the way this works.
Even lenses built for crop cameras such as the EF-S range (Canon) and DX range (Nikon) will still have this effect. This is because lenses are listed by their physical focal length rather than their effective (or equivalent) focal length.
However, these lenses will not work on a full-frame body without heavy vignetting. This is because the image will not project onto the whole of the sensor.
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