Lens Language

Why you bought a SLR camera

One of the reasons why you bought a SLR camera was because the lens comes off. You know this a great feature because it means the vision of your camera is nearly unlimited. Change the lens and it becomes a completely different camera. But what do the different lenses do? What do all the numbers and letters mean?

Photography has been around for over 100 years and has developed its own language related to the complex physics of lenses and the other parts of a camera. Like any strange language, the techno babble of lenses can be confusing and make it difficult to get to the knowledge you want. Understanding the mathematics and engineering of lens design isn't necessary to enjoy photography and the fun of using different lenses.

This article was written to help explain the language of lenses from a "what does it do" viewpoint instead of a technical view.

What is a lens?

A SLR camera lens is not really one lens at all - but a tube of multiple lenses of different shapes and sizes all working together. These are called elements. The elements work together to bend and direct light coming in the front of the lens into a precise composition out the back. The composition of light out the back of the lens is what your camera sees and makes a picture of.

Different sizes and types of elements assembled in different ways make different pictures. One lens might be designed to see wide angles while another might be designed to look far into the distance. There are many different kinds of lenses because there are many different kinds of photography.

The lens you want to use will depend on what kind of photography you want to do. Part of the fun of an SLR camera is being able to change lenses and change the kind of photography you do.

Zoom lenses are a very popular lens design. They are so popular that many people think that all SLR cameras have always used this kind of lens. Zoom lenses were invented after many years of photography using fixed lenses, often called "primes." Prime lenses are still a very important part of photography because of the uncompromising picture quality they produce.

Zoom lenses are special because the elements inside can move. By moving the elements, the composition of light out the back of the lens changes - so the lens can be adjusted to give different pictures. Zoom lenses are great because they can give you the equivalent vision of several lenses in one package.

Focal Length

The focal length of a lens is measured in millimeters. A 50mm lens has a focal length of 50 millimeters. A 70-200mm zoom lens can be adjusted to different focal lengths - from 70 to 200 millimeters. Focal length is an important attribute of a lens because it affects what the picture will look like; but, unfortunately, it's not obvious what the millimeters mean. There are many, great, technical explanations of focal length on the Internet. In this article, however, the explanation will stay within the "what does it do" definition.

Lower focal lengths produce pictures with wider angles. Higher focal lengths produce pictures with narrower angles.

You can imagine focal length like this. Make a circle with your thumb and finger. Close one eye and peep through the hole. Look at what you can see and imagine a picture made of everything inside the circle. Now move the circle closer to or further from your eye. You just changed the focal length. With the circle closer to your eye (lower focal length) you could make a wider-angle picture. With the circle further from your eye (higher focal length), you could make a narrower-angle picture. Imagine the resulting picture being the same size regardless of how close or far you hold the circle from your eye. A lower focal length would produce a picture with a broad view. A higher focal length would produce a picture of the same size but of a selected, narrower view.

A lens focal length from 28mm to 50mm is generally though of as the view of the world you see with your eyes.

Lenses with focal lengths lower than 28mm are commonly called "wide angle."

Lenses with focal lengths greater than 100mm are commonly called "telephoto."

Ranges in between are "mid-range" or "medium distance" lenses.

What about 35mm lenses?

It's unfortunate that SLR cameras were for decades (correctly) called 35mm cameras while lens focal length was also measured in millimeters. It's unfortunate because, today, it has resulted in some confusion. Most SLRs are no longer described as "35mm" cameras. What does the mysterious 35mm mean?

The 35mm measurement is not related to lenses or focal length at all. It's a measure of the size of film used in film-based SLR cameras. In today's world of digital-based SLR cameras the 35mm description isn't applicable. But it sometimes comes up (confusingly) when comparisons are made between film-based cameras and digital-based SLR cameras. There is no such thing as a "35mm Digital SLR camera". There are 35mm film SLR cameras and digital SLR cameras.

Differences in field of view

One common comparison between film-based and digital-based SLR cameras that you might hear is regarding their different fields of view. This is sometimes called "crop factor" or "magnification factor" and is often described with terms like "1.5X" Imagine attaching the very same lens to a film SLR and then to a digital SLR. There are technical differences between a film SLR camera and a digital SLR camera that would result in the pictures made to be different from one another. Staying within our "what does it do" definition; digital SLR cameras have a narrower field of view than film SLR cameras. That means the picture taken with the digital SLR would appear to have been made with a lens with a higher focal length.

When someone describes a "crop" or "magnification" of "1.5X" they mean that the picture with the digital camera would appear to have been taken with a lens with a focal length 1.5 times greater compared to the film camera picture.

This change in field of view that came with digital cameras brought good and bad changes for photographers. On the good side, lenses on digital cameras became narrower in view and more magnifying. This was good for photographing distant subjects. However, wide angles were harder to obtain, so the digital "crop factor" was bad for photographing wide scenes. Photographers needed new lenses for their digital cameras to get the same wide views they used to.

Full Frame

You may hear some cameras described as "full frame" This is refers to a digital SLR camera design with a field of view closer to that of a film-based SLR camera. It is of particular interest to photographers who enjoy wide-angle photography.

Aperture

The aperture of a lens is a mechanical device that controls how much light passes through. Adjusting how much light comes into the camera is one of the controls for adjusting exposure. (Shutter speed is the other control.) The aperture is adjusted in increments commonly called "stops" or "f-stops". The name "f-stop" comes from the way aperture increments are written starting with an "f" like this: f/2.8, f/5.6 or f/8. The "f" stands for the focal length of the lens - you'll notice that f-stops are actually written as mathematic fractions. However, understanding the math isn't necessary to understand how to use the aperture.

One of the important attributes of any lens is it's "best" aperture - how much light can it pass through to the camera when opened as wide as possible. While it might seem backwards, lower aperture f-stop settings allow more light through the lens. So a lens with a best f-stop of f/2.8 would be considered better than one with a best f-stop of f/4.

Lenses can all be set to aperture f-stops smaller (less light) than their rated "best" aperture. Most lenses can be set as small as f/22 or f/32.

In general, a zoom lens with a best aperture of f/2.8 is as best as they come. There are many exceptions to this, especially in fixed focal length, prime lenses that may have best apertures of f/2, f/1.8. f/1.4 and even f/1.0.

The best aperture of a lens is also commonly used to describe the "speed" of a lens. An f/2.8 lens might be said to be "faster" than an f/4 lens. The lens is not literally faster; the description comes from one of the advantages of using a lens with a low f-stop aperture - faster shutter speed.

Modern, automatic cameras select and adjust the lens aperture for you when the camera is set to fully automatic operation. Your camera will pick the most suitable aperture it needs to make the picture. When using a "faster" lens with a lower f-stop aperture, your camera has more options for keeping the shutter speed fast to avoid blurry pictures. You can also take advantage of the aperture f-stop of a lens by using your camera in "aperture priority" mode or fully manual mode.