Photography is by far one of the best mixes of science and creativity that I know of.
It is no surprise hence that we photographers need to learn about science every now and then, working behind the scenes to produce a good image.
The concept of focal length is one such instance.
It is the basic yardstick used to denote ALL camera lenses. I am pretty sure you have seen the notation many times. It looks something like below:
- A single value represented in millimeters (mm), e.g., 50mm. This is for fixed focal length lenses or primes. Or,
- A range of values, e.g., 18-105mm. This is for variable focal length lenses or zoom lenses.
Since you are already here, what do you say? Should we delve into this a little deeper and cement our understanding of this crucial bit of information that would serve you till the last image you take?
I assure you this is so much easier than you currently think it is.
Physics Of The Focal Length
First, let us get clear about what science has to say about the focal length. And before you start wondering, NO you do not need to understand any of this.
Are these concepts interesting and insightful? Well maybe, if you are up for it.
Are they absolutely necessary for you to grasp the essence of focal length and make good images with that knowledge? Not at all.
They are good to know and learn about but not crucial at all. For our purposes at least.
Circling back now.
The focal length of a lens is defined as follows:
The focal length is the distance between the center of a convex lens or a concave mirror and the focal point of the lens or mirror — the point where parallel rays of light meet or converge.
Let’s break down the definition bit by bit.
First, a convex lens is one that has bulging outer walls. In other words, it looks like this.
Remember this part, we will have to come back to it later. I want to point out something about it.
The next important term is the focal point. Well, let me explain it this way. When light passes through a convex lens, parallel rays of light passing through it converge to a single point. This point is called the focal point of the lens.
It is the properties of this particular lens (convex) which help converge the rays of light in this way.
I hope you see it now, that in our case, the distance between point A and B in the diagram above represents the focal length of the lens.
Once again, it is the distance between the center of the lens and the point where all light rays converge after passing through the lens. Please don’t confuse the focal length with the PHYSICAL length of the camera lens itself.
Why Do I Need To Know This?
The focal length of any camera lens represents two very important aspects:
- Angle of View
- Degree Of Magnification
The angle of view is simply the amount of the scene you can see through the camera once you put on the lens. If you have ever used a zoom lens you know exactly what I am talking about.
Say you put on the 18-55mm kit lens. The focal length of the lens as mentioned varies between the range of 18mm to 55mm.
Once you have the lens attached to the camera, you will notice that when the lens at 18mm focal length, a large part of the scene is visible through the viewfinder. As you increase the focal length by twisting the lens barrel (zoom in), the extent of the scene you can view gets smaller and smaller.
Longer the focal length of the lens, the narrower the angle of view and vice versa.
The degree of magnification works hand in hand with the angle of view we just talked about. Just think about it once. Say you employ a lens with a long focal length, e.g., 200mm. Since the lens has such a long focal length, it is inevitable that it would have a very narrow field of view. This means that a very small part of the scene would be stretched across the entire frame. There you have a highly magnified image.
Longer the focal length of the lens, the greater the degree of magnification achieved and vice versa.
I would strongly advise you to focus (pun intended) on what the focal length of a lens does to the image even if you do not understand the underlying principle of physics associated with it.
Okay, now that we have a basic understanding of what the focal length is and how it affects our images, let’s get on to the next stage.
I don’t think I need to make you aware of the plethora of camera lenses available in the market today. To understand these different lenses, we photographers have categorized them into smaller groups based on their focal lengths. Because that is the one thing you are sure to find on ANY lens.
One important thing you should know before you progress further is that the focal length of any lens is independent of the camera you use it with. You may have already heard about image sensor sizes in cameras and how they affect the effective focal length of the lenses.
This is just to make you aware that a 50mm lens stays a 50mm lens irrespective of the fact whether you use it with a Full frame (35mm equivalent), Crop format, or a Micro Four Third (MFT) camera body.
If you are wondering why, I strongly suggest you read: Here Is Why You Do Not Need A Full Frame Camera | Physics Speaks. This should more than clear your understanding of the nuances of the issue at hand.
Since a 50mm lens on a full-frame sensor camera body closely mimics the field of view of the human eye, we photographers have conveniently tagged it as the standard/normal focal length.
Lenses with focal lengths lying roughly between 50 to 60mm have very little distortion and are hence widely used for portraiture, nature photography, and photojournalism.
Wide Angle Lenses
Lenses with focal lengths between 18 to 50mm are generally termed as wide lenses since they offer surprise, surprise, a wide-angle of view. In other words, you can photograph a huge extent of the scene using these lenses.
Smaller the focal length, the wider the field of view the lens offers and vice versa.
There is, however, one downside to wide-angle lenses. Since they have to bend a lot of light into a very small area on the sensor, some lenses tend to distort the image to a noticeable degree. This makes straight subjects such as buildings, trees, etc. look bent and crooked.
The wider you go, the more distortion it creates.
On the other end of the standard/normal lenses are the telephoto lenses with longer focal lengths. This is generally in the range of 60 to 200mm.
Yes, you guessed it right, lenses in this category are used to shoot subjects at a distance. It is no surprise hence that birds, wildlife, and sports photographers all use these lenses.
Apart from the ability to shoot a faraway subject, telephoto lenses have a special trick up their sleeve – lens compression.
Lens compression, in simple words, is the ability of the lens to be able to make great distances look closer than they actually are. It compresses the space between two elements of the scene and makes them look closer.
I know this may sound trippy, so here have a look.
The rule of thumb is wider the lens, the more exaggerated the distances are and vice versa.
It is because of this amazing ability to compress long distances and delicious bokeh that the 70 to 200mm focal length is loved universally. It is truly the swiss army knife of the photography world. Nearly anything looks good when shot using this lens. Portraits, nature, product photography, all come alive.
Lenses with focal lengths smaller than roughly 15mm are called ultrawide.
The field of view for lenses of these focal lengths are so wide that the image sometimes literally comes out as a circle rather than a regular rectangular shape.
Some lenses such as the 8mm fisheye have an astounding field of view which is more than 180 degrees. This means that the lens can see behind the lens, albeit a small range.
As the name suggests, these lenses have a really long focal length and are used to photograph subjects that are a great distance away from the camera.
Yes, you can shoot the moon, other planets, celestial objects, the International Space Station, or distant galaxies with it.
Any lens with a focal length in excess of 500mm falls in this category.
Fixed/Variable Focal Length
Apart from the length of the focal distance. Camera lenses could also be classified on the basis of whether or not they have fixed focal lengths or variable focal lengths.
If this sounds too alien, let me make you a little more comfortable.
Variable lenses are simply zoom lenses and lenses with fixed focal lengths are what we call prime lenses. See, I told you, you already knew them.
As you already know zoom lenses cover a range of focal length rather than a single value. This makes them extremely versatile and robust for a wide scope of use.
The range of focal length a specific lens covers is generally always depicted on the lens barrel itself.
Take, for example, the 18-55mm kit lens that most new cameras come with. The 18mm here is the widest the lens goes and the 55mm is the longest.
For the extra flexibility that a zoom lens offers, it is also nearly always more expensive than a prime counterpart. This is because a zoom lens requires a lot more optical engineering and material to construct. Although primes are still leading in terms of image quality, zoom lenses have been making quite a case for themselves lately.
These are lenses with fixed focal lengths. They are a lot simpler to build since they do not have to deal with any of the optical gymnastics that zoom lenses have to deal with. And they are also generally smaller in size and cheaper in price.
A key advantage prime lenses have over their zoom counterparts is in the department of aperture size.
The f-number or f/stop as you may remember is not an absolute number but a fraction, which changes with an equivalent change in the focal length. Since prime lenses have a fixed focal length they tend to also have smaller aperture values.
For example the famous Nifty-Fifty, 50mm has a small aperture value (large aperture hole) at f/1.8. It is hard to find such small aperture values on even costly zoom lenses.
Owing to these humongous aperture holes, prime lenses are absolutely at home when it comes to shooting in low light. Letting in a ton of light is just a flick of a dial away.
There is potentially just one thing that you could call a con. If zooming with your feet is not an option and the situation also demands a regular change in focal lengths, switching lenses frequently may be a problem. Not only will it require time which you may not have during the shoot, but it also may let in dust inside the camera if you decide to change lenses at inopportune times.
That is all folks. That is all you need to about focal length and how it affects your images.
Just one last thing that I want you to understand before we conclude this post.
Remember how I asked you to mark the point where I said that a convex lens converges all parallel rays of light into one point? By that definition, it may falsely lead you to believe that the entire lens barrel contains just one lens but that is NOT true.
Most camera lenses, even prime lenses consist of multiple pieces of lenses of various strengths and shapes. There is no ONE lens.
These pieces of lenses are called lens elements and each one of them has specific functions to perform.
I used the example of a convex lens because ultimately all of the lens elements together have to function as a convex lens which converges the rays of light coming in through the lens onto the imaging sensor.
That is all.
If you have any further questions, please post them below and I will be happy to answer them.
Keep shooting beautiful!