An online book of Common Sense Photography, by Rhett Stuart

What Do I Need to Know About Camera Lenses?


Think about one of your eyes. It has 2 clear lens elements; the front lens is the cornea which is fixed, and the flexible lens just behind the pupil which changes shape as we focus. There is a fluid filled cavity between the cornea and the lens. The eye itself is filled with a clear jelly like fluid called vitreous humor. These eye parts all work together to focus the light on the light sensitive rod and cone cells that are located on the back of the eye.

Camera lenses are similar. Lenses need multiple elements, or pieces of glass, for a really clear focus because of the complex ways light reacts.

Lenses come in many focal lengths. The focal length measures how long the lens is. The longer the focal length the more the lens resembles a telescope.

The easiest way to remember focal lengths is to think of how wide the angle of view is. The angle of view is how much we can see of the picture as we look through the camera and lens. These photographs were taken at the Lewis Ginter Botanical Garden in Richmond, Virginia.

Standard lenses are called “prime” and don’t zoom. They usually produce a sharper image then zoom lenses, although some expensive zoom lenses have image quality that rivals “prime” lenses. Prime lenses are usually 24mm, 35mm, 50mm, 85mm, 100mm, 135mm, 200mm, 500mm, and 1000mm. 50mm lenses usually produce the sharpest images at the largest apertures for the best price.

Lenses are usually sharper in the center of the picture than at the edges. The center is called the “sweet spot” of the lens. Some lenses work better then others at keeping the image sharp at the edges. The aperture setting also affects the sharpness of a photograph (explained later in apertures).

More expensive lenses can make a dramatic difference in the quality of your photographs. I have a 70-200 f/2.8 zoom lens that makes things look almost 3 dimensional. There is a reason for that high price tag! However, it takes a good photographer to create a good photograph. The more expensive equipment just makes the photo that much better. Untrained photographers can get poor results with very expensive equipment!

Better lenses have a “ring” motor to focus the lens that goes all the way around the glass elements. They are much faster and more reliable. Canon and Nikon both offer lenses with this feature. These lenses also have the benefit of being able to instantly manually override the focusing in case the lens is having a hard time focusing.

One problem with lens design is called flare. Flare is caused by sunlight hitting the lens directly. That is why lens hoods are used sometimes; they stick out in front of the lens to shade the front of the camera lens from the sun hitting it. Flare is bright spots, sometimes several, as the image of the sun bounces around in the camera lenses. Lenses are coated with inorganic salts and other material about 1/millionth of an inch thick to prevent flare. Other lens problems are due to different colors of light bending at different angles. Blue light bends more then green light, and green light bends more then red light while traveling through the lens elements.

Your eye has coatings also: Tears on the outside, clear fluid between the cornea and lens, and the clear jelly like vitreous humor filling the inside of the eye, between the lens and the eye light sensor cells.

Modern lenses, especially zoom lenses, are extremely complicated and require the use of computers to design them. Lens designers have to work around a lot of light principles and physics to try and make as clear of an image as possible. A good lens results from the best decisions of a lot of compromises. With lens design, as one problem is fixed it causes a problem with something else.

Did you know you don't even need a lens? There are still old fashioned cameras available called pin hole cameras. Instead of a lens there is a small hole. If you are ingenious you could make one yourself! Of course the resulting photograph won't be sharply focused. Pin hole cameras work by making the Circles of Confusion smaller (Circles of Confusion are explained in the next section under aperture).

One problem with a lens is water and dust getting into it. Some lenses are weather sealed to keep out the dirt and moisture. This can greatly extend the life of your lens! Lenses often get dust in them as they zoom in and out, sucking in and forcing out air. Some lenses focus internally so this is less of a problem.

Old fashioned large format cameras (they used large 5x7" and 8x10" sized negatives) have what is called tilt and shift focus. This is used to overcome the distortion of objects as they get farther away. Like the parallel lines of a railroad track which appear to join together in the distance, the angles of buildings and houses do the same thing. Tilting the lens away from the camera body and film plane with a special attachment corrects this distortion. The need for tilt and shift has been almost eliminated because tilt and shift adjusting for perspective can be done with Photoshop® on a computer without the need for special equipment. Getting straight lines with buildings is useful with architectural photography.

The one thing tilt and shift focus lenses can still do that Photoshop® can’t do is to tilt down and focus on something very close and keep distant objects in focus also. This could be done in Photoshop®, but would require some careful and time consuming editing!

I'll ignore explaining the details of focal length physics. Photographers concentrate on visualization, or seeing a picture in their mind and then trying to create it with the camera equipment. Thinking about the details of focal length hasn’t helped me take better pictures so I’ll skip that. Only light physics that are useful in helping to produce a good photograph are explained here. Getting too technical with physics and equipment performance is a distraction from getting good photographs, and makes it hard to abstractly visualize good photographs! Good photographers can get good results with average equipment. The equipment can just make it easier, and produce a sharper image.

Usually the lenses with an aperture of f/2.8 and larger are the nicer lenses. They are more expensive for a reason (better image quality), and are usually better built. They are heavier since more lens elements are needed with the larger apertures. Canon designates their higher quality lenses with an “L”.

When I get a new lens, I like to use it exclusively for several days in order to “get to know it”. I get a good “feel” for its abilities and limitations, and the angle of view. I once used an 85mm f/1.8 lens for a whole year, including a cross country road trip. I have a really good idea of what it is capable of! Also, it is interesting to try and be creative with just one lens. If I need a wider angle of view, I just back up. It also made me wish for a very wide angle zoom lens which I now use quite a bit (17-40mm 4.0).