The Theory of Nothing

How do lenses work?

How do lenses work?

Scientists study lenses in a branch of physics called optics. A lens is piece of glass ground so that its sides are curved. This curving makes a lens do its thing, which is focus or diverge light. As we've seen before, light waves can be bent, and it is this bending of light that makes a lens useful. I should add here that plastic lenses are now more prevalent than glass ones because they're less expensive, lighter, and easier to manufacture.

A lens can be positive or negative. A positive lens focuses or converges light, while a negative lens diverges light. There is a rather complicated equation that expresses that relationship between the focal length of a lens and the radius of curvature of both the front and the back of the lens as well as the thickness of the lens and its refractive index. It's called the Lensmaker's Equation.

Once people realized the value of a lens they soon found that it could do wonderful things, such as magnification, and if you combine an imaging lens with an eyepiece lens, one can make a microscope. Just imagine how valuable that invention became. Of course, the telescope also uses two lenses, and in some cases a focusing mirror and an eyepiece lens.

One main problem with a lens is called chromatic aberration. This is where the different colors in light focus at different points, causing color blurring. One way to fix this problem is to combine a flat lens that has a negative curve with a positive lens. This double lens stops the chromatic aberration effect.

Other optical problems are barrel and pincushion distortion. These defects happen at the edges of the field of view and makes the image look as if its being squashed or stretched. Again, using multiple lens arrangements can alleviate these types of aberration.

The most familiar use for lenses is in cameras. At first, people found that if you're in a darkened room and punch a tiny hole in the wall, you will see a projection on the wall opposite the tiny hole an upside down and reversed image of whatever is outside. This is what was called a pinhole camera. You might wonder how the image is upside down and reversed, but if you trace the light rays you'll see this is what happens. Remember that light is composed of photon particles that behave in crazy ways.

Add a lens to the pinhole camera--also known as a camera obscura--and you now have a camera, which makes the upside down and reversed image sharper. Obviously, people couldn't carry a room around, so they built a box with a lens. At first, they simply removed a lens cap for a given amount of time to take the picture on slow photographic film or plates. Eventually, as film became faster, they added a shutter, essentially a device that could provide a controlled amount of exposure. Also added was an aperture, essentially a hole that could be sized according to the light available. At this point, we have a standard film camera.

As time progressed, photographers demanded better lenses, and manufactures obliged by combing various lens elements in a single cylindrical container, known as a lens barrel, to accomplish difficult situations such as telephoto, fisheye, macro, and portrait photography, and eventually a zoom lens that did all things with just a twist of the lens barrel. These specialty lenses require multiple lens elements and because of this are much more expensive to make.

Back in the old days, one focused a SLR (single lens reflex) camera by looking through the lens in a viewfinder and turning a focusing ring on the lens. Using a separate light meter, one set the aperture and shutter speed as two operations according to the amount of light and the speed of the film being used.

One then had to remember that the aperture setting is in f-stops, in which the larger the f-stop the smaller the aperture (less light gets in). Smaller apertures (larger f-stops) result in a larger field of focus (range of the closest distance and furthest distance that has sharp focus), while a larger aperture (smaller f-stop) results in a smaller field of focus. In addition, a larger f-stop lets less light into the camera so one has to use a longer exposure (slower shutter speed). It's easy to see how this could get confusing unless one has a lot of experience.

Modern cameras have taken all of those chores away by providing auto-focusing and auto-exposure, thanks to computer electronics. Now, you don't have to think. You can concentrate on the scene composure instead of all those technical things connected to exposure.

I bring this up because lenses are rated as to lowest aperture number at a given focal point (expressed in millimeters), usually the closest. This is important because a smaller aperture lens lets one get better photographs under low light conditions without a flash. This may sound stupid, but flash photography has artistic limits. Obviously, a lower aperture rated lens is more expensive.

I know that you're not concerned about these things when you take a photo with your cell phone, but it's interesting to know about what's going on inside there and it might help you choose what cell phone you buy in the future if photography is your thing.

Thanks for reading.