The Camera Lens

The purpose of a camera lens is simple: to collect light, control its magnification, focus and depth of field, and then project the light onto the film plane.

To get started, let's talk about a part of the exposure process that’s not always part of the camera lens but can be found in the camera body--the Shutter.

The shutter is a curtain that is in front on the film plane and opens for a very precise amount of time. Controlling the amount of light hitting the film is the first part of properly exposing film. But a shutter does one other thing, it controls motion. By speeding up or slowing down the shutter you can blur the motion happening within the image, or bring the motion into sharp focus.

We’ve all seen the high-speed photos of bullets fired out of a gun barrel just as it comes out or a lightning bolt as its hitting tree. A very fast shutter is necessary to capture those images.

There are two types of shutters in cameras. A Focal Plane Shutter is built into the camera body, just in front of the film. The focal plane shutter has two main drawbacks: you cannot use electronic flash at very fast shutter speeds, and the shutter is noisy as it opens.

The second type is the Leaf Spring Shutter. It is quieter to operate, but more importantly, you can use the flash at any shutter speed. If your shutter is not Synchronized with your flash you’ll get a shutter drag that looks like parts of the photo is missing.

The Aperture is the next thing we will talk about in learning about the camera lens. The aperture in the lens acts like the pupil in your eye as it controls how much light is allowed to enter. The aperture in a lens does that but we can control something here that we can’t in our own eyes.

We can set the f-stop on a camera lens and therefore control the Depth of Field within the image. f-stop is the term we use to indicate how large an opening there is in the Diaphragm or aperture. At F -2.8 the aperture is wide open in most camera lenses and at F -22 it’s a very small opening. The smaller the opening the more depth of field you have.

A camera lens is called slow or fast and you’ll pay a lot more for a faster lens. This just refers to how far a lens f-stop opens up to. An example of this is an f- 2 is a faster lens then a f-2.8. A lens that’s at f-1.8 is a very fast and expensive camera lens.

You can see these very big ,mostly white in color lens on a football field. A Photojournalist really needs that speed as they hand-hold or at most use a Monopod to keep the camera still. In the poor lighting of a sporting event, they must take professional quality images such as those used in a magazine like Sports Illustrated.

I’ve talked about both Shutter and Aperture individually, but you need to set them both correctly to give your film the right amount of light to expose it properly. You can change ether shutter speed or the aperture to stop fast action in a photo or change the depth of field to suit your needs, but you have to remember that if you change one you must change the other in combination to balance the exposure. I’ll cover the topic of Exposure later in this web sight or click on the link for a definition in the Glossary.

This brings us to the next topic and that is the Focal Length of a camera lens. Here’s what that means. Focal length is the measurement between the lens and the Focal Plane or the spot the lens is focusing on. Confused? Well, just think of it this way. Focal length is the magnification of the image as it appears on the film and the angle of view is the amount of the scene shown on the film.

Camera Lenses are available as a Normal Focal Length Lens , a Long or Telephoto Lens, a Short or Wide Angle Lens, and a Special Purpose Lens like the Fisheye Lens or Macro Lens.

Choosing a Camera Lens

What do you want to do with your camera? This question will tell you what lens you’ll need to capture those images. What type of camera should you buy? What film format do you want to use? Digital or film? What accessories will you need?

Something else you need to remember is that the focal length of most digital cameras is shorter because the CCD image sensor is smaller then a 35mm piece of film that measures 1 1/2”x 3/4”. The smaller the sensor the shorter the focal length needs to be to cover it. This is a potential problem if your camera has an interchangeable lens. Follow the manufacturers recommendations when buying an interchangeable camera lens.

When buying a Medium Format (2 1/4x 21/4 – 2 3/4x 2/14 )or Large Format(4x5 5x7 8x10) camera, the focal length of those lenses are longer then a 35mm camera.

OK--back to the lens for 35mm cameras. Here is an explanation of the lenses that I mentioned a few paragraphs ago.

Normal Focal Length lens or a 50mm lens. This is the lens that sees a scene like the human eye does. These lenses are faster then longer lens and more compact and lighter to carry.

Short or Wide angle Lens or 24mm-28mm. This lens will show you more of the scene then a normal lens at the same distance. Wide-angle lens will have large depth of field at most F-stops. Wide-angle lenses show a lot of distortion of perspective, which can be used to create interesting composition in your photographs.

Long Lens or Telephoto Lens 28-200mm 50-300mm There is almost no limit here. This lens combines a range of lens in one. It enlarges a scene and narrows the view you’ll see though the lens. These lenses are slower and heavier and carry a higher price tag..

Special-Purpose Lenses A few of the many of these lenses are the Fisheye lens 8mm-15mm. This lens has a very large depth of field as well as an angle of view. Its wide-angle distortion effect is striking as an image.

The Macro Lens is like putting a microscope on your camera. You need a tripod and good lighting to take advantage of this lens. Its fun to use on your backyard flowers.

A Soft Focus Lens is called a portrait lens because portrait photographers use them to make their clients look younger by softening the facial wrinkles. Yes folks, that’s how they do that!

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Want to know more? Continue the Photo Technique Mini course--return to Cameras or learn about Film. Return to Photography or HOME.