By Inna Sumra M. A. in Photography, Cr. Photog., M. Photog., ARPS (UK).
One of the most important characteristics of a lens is its focal length. It is first item about your lens that you need to know. The focal length of a lens is defined as the optical distance from the point where the light meets inside the lens to the camera’s film or the sensor.
Why is the focal length so important?
Before the arrival of digital cameras people used 35 mm film cameras. On each roll of the film there were 24 or 36 frames. After the film was exposed in the camera and developed by the lab, each frame created an image that was 36 mm wide and 24 mm tall.
As you will remember from your math class, diagonal of a 36×24 mm rectangle is 50 mm. Any lens with focal length equal to the diagonal of this photographic frame is called a normal lens. Since the diagonal of the 35 mm frame is 50 mm, a lens with 50 mm focal length is called a normal lens for a 35 mm camera.
And a normal lens i.e., the lens with focal length equal to the diagonal of the frame, will have an angle of view of 45 degrees.
When you double the focal length the angle of view becomes half. So, a lens with a longer focal length let us say, 100 mm, will have an angle of view that would be half as much, that is 22 degrees. This type of lens is called a telephoto lens.
And when you half the focal length the angle of view doubles. So, a lens with focal length of 25 mm will have an angle of view of 90 degrees. This type of lens is called a wide-angle lens.
Why is the sensor size important?
In the start 35 mm digital cameras had a sensor that was 36×24 mm. That is the same size as the 35 mm film frame. But with the advances in technology, it became possible to squeeze more light gathering pixels onto the sensors and make the sensors smaller. This allowed the manufacturers to reduce the size of the cameras and the lenses.
The next generation of sensors, APS-C (Advanced Photo Systems – Classic) sensors reduced the size of the sensor to 25×16 mm. The diagonal of this sensor is 30 mm. A lens with focal length of 30 mm will produce an angle of view of 45 degrees. Therefore, a lens with 30 mm focal length will be a normal lens for this sensor.
What is a crop factor?
A crop factor is a ratio of the dimensions of a sensor as comparted to the dimensions of a 35 mm camera. To calculate that you divide the diagonal of the 35 mm sensor, which is 50 mm, by diagonal of the sensor of your camera. If you were using a camera with APS-C sensor, that diagonal will be 30 mm. So dividing 50 by 30 will give you 1.6. And that is the crop factor for the APS-C camera.
It is very important that you find out what sensor your camera has. Crop factor for your lenses will depend on the sensor size of your camera.
Why do I need to know the crop factor of my camera?
A 50 mm lens on a 36×24 mm camera (aka full frame sensor) will act like a 50 mm lens. On an APS-C sensor the same lens, with focal length of 50 mm, would act like an 80 mm lens, (50 mm multiplied by the crop factor of 1.6). And on a camera with four thirds sensor the same lens will act like a 100 mm lens on a 35 mm camera (50 mm multiplied by crop factor of 2).
So, when you select the lens for your camera you must know the crop factor for your lens. Take the focal length of the lens on a 35 mm camera and multiply it with the crop factor of your camera. That will be the focal length equivalent of the lens on your camera.
What are the prime lenses?
Prime lenses have a fixed focal length. You cannot zoom a prime lens.
A standard lens on a 35 mm camera has a focal length of 50 mm. That is a prime lens. You could buy prime lenses with a focal length of 15, 25, 50, 85,100, 135, 200 mm etc.
What is the maximum aperture of a lens?
When you are photographing in low light, a lens needs to allow more light to pass through the lens to create an image. The maximum amount of light a lens can allow to go through, depends on the diameter of the hole through which the light travels in the lens. That is called the aperture. Smaller that number bigger the hole or the aperture. The aperture is marked on the lens by f numbers like 1.4, 2, 2.8, 4, 5.6, 8, 11, 16, 22 etc. These are commonly called f-stops. These numbers are calculated mathematically so that f1.4 allows twice much as light to pass through as f2, which allows twice as much light to pass through as f2.8, which allows twice much light to pass through as f4, and that allows twice much light to pass as f5.6 and so on.
So, when you buy a lens with maximum aperture of f1.4 compared to f2, that lens will allow you to produce an image in half as much light as f2 lens. Therefore, a lens with large aperture i.e., a smaller f-stop number, will need to have a larger diameter. It will allow the lens to gather more light. And it will be a heavier and more expensive lens.
As an example, a Canon 50 mm 1.8 lens sells for about $100, Canon 50 mm 1.4 lens for $400, Canon 50 mm 1.2 for $1400. And as the f-stop number goes down the price goes up and the weight goes up. A Canon 50 mm f1.8 lens weight only 0.6 pounds and f1.2 lens weighs 1.2 lb.
In short, a prime lens will have the sharpest image, it will have no zoom capability and it will allow you to work in lower light than zoom lenses. The price and the weight will vary according to the maximum f-stop of the lens you choose.
What are zoom lenses?
A zoom lens allows you to change the focal length of the lens by zooming in or out. You will be able to get a waist to head shot of the subject or zoom out and photograph the person from head to toe. To achieve the same effect with a prime lens you will have to move further back or change the lens. So, a zoom lens is very convenient. It is a handy tool to have.
Zoom lenses are offered in various focal lengths. Here are the three most popular focal lengths. A 16-35mm for wide angle photography, 24-70mm for normal photography and 70-200mm for telephoto type of photography.
If you can only afford one consider 24-70mm.
Zoom lenses also come in different maximum apertures. Some of the larger zoom lenses can be heavy. Check the weight of the lens before you buy it.
Some zoom lenses have a very wide focal length range. Let us say from 18-400mm. As you increase the focal length range of a zoom lens, its ability to work in low light get worse and worse. Therefore, it requires more and more light to produce the image.
With the zoom lens you will sacrifice some quality but gain the convenience. I suggest that you stick with a zoom lens.
What is a macro lens?
A macro lens allows you to get extremely close to the subject. You can photograph flowers, wedding rings or any other small object you wish. These are prime lenses and generally expensive.
Any other points to consider?
If you plan on doing a lot of low light photography, consider a prime lens.
Buy the lens with the widest aperture you can afford. It would be better in low light and generally it will have better image. However, such a lens will be more expensive and heavier. Make a compromise according to your budget.
If you don’t want to carry multiple prime lenses or spend a lot of time changing the lenses consider a zoom lens.
If you can only afford one zoom lens, consider a zoom lens like 24-70mm. It will be a small and convenient lens to have.
Consider the size and weight of the lens when making your choice.
Research lenses within your budget to get the best results for your money.
How to read the specifications of a lens?
If you are looking online and trying to figure out which lens is better, you need to know how to read the specifications of the lens.
Here is an example. It is just designed to show you how to read the specifications of a lens.
Your Brand 18-400mm f/4-8 Lens for Canon EF
Key Features
- EF-Mount Lens/APS-C Format
- 28.8-640mm (35mm Equivalent)
- Aperture Range: f/4 to f/32
- Autofocus Motor
- Image Stabilization
- Moisture-Resistant Construction
So, it also tells you that this is a Your Brand lens made for a Canon Camera.
It tells you that this lens is an EF-Mount. EF is a Canon format. So, this lens is made for a Canon EF Mount Camera with APS-C senor.
It tells you the minimum, 18 mm and maximum 400mm, focal length of the lens for a 35 mm or a full frame camera.
It tells you that largest aperture of the lens varies from f4-8. They assume that you know that change of aperture, from f4 to f8, depends on how much you zoom the lens. In real life it will be pretty much close to f8 unless you are totally zoomed out.
How much difference does the maximum aperture make? A lot. Let us say if the same lens was offered in f2.8. As an example: Canon EF 300mm f/2.8L.
Going from f2.8 to f4 will require 2x the light as f2.8.
Going from f4 to f5.6 will require 4x the light as f2.8.
Going from f5.6 to f8 will require 8x light as f2.8.
Basically, every f-stop that is a bigger number, allows half as much light to go through the lens, as the previous f-stop. So f8 allows half much light to go through as f5.6. In other words, f8 will requires twice much light as f5.6 to produce the picture because it allows only half the light to go through. But as the aperture gets smaller the picture gets sharper. We will explore this in another blog in more detail.
They have already calculated the focal length range of this lens for an APS-C camera 28.8-640mm (by multiplying the focal length of 18 – 400 with a crop factor of 1.6).
The lens has autofocus capability.
The lens has image stabilization built in.
That gives you a way to compare different lenses.
What else should I check?
Make sure you have enough storage space to carry the camera, lens and other gear. Buy a bag to allow you to carry one or two more lenses.
Read reviews of different lenses to find what will work best for you. Some of the reviewers are pros. Some would be doing a lot of nit-picking. Read a lot of them. You will get the idea. Don’t buy the cheapest lens. Cheaper lenses will have optical elements that are made out of plastic instead of glass. And more expensive lenses will have a coating on the glass to reduce the glare.
Avoid lens with plastic barrel construction. They won’t last as long as the metal barrel lenses and the quality of construction will be not as good as the metal barrel.
Good luck.
Thank you for reading.
