The camera lens has come a long way since Venetian nobleman Daniello Barbaro invented the first camera lens in 1568. That may seem hard to believe but it’s true. His work on the single element Plano-convex lens was published in La Pratica della Prospectiva in the same year. Barbaro’s Plano-convex lens greatly improved the performance of the pinhole camera which dated back to the 5th century BC; of course those images weren’t recorded on film. The first recorded images didn’t happen until the summer of 1827 when Joseph Nicephore Niepce produced the first photographic image using the camera obscura also known as the pinhole camera. Now, with that little bit of historical trivia behind us, let’s dive right into the topic of lens speed, f-stops, shutter speed and depth of field.
What do we mean by lens speed?
On the front of every fixed focal length lens you will find an alpha-numeric string of characters that represents the lens speed i.e. f1.2. This number represents the maximum aperture of the lens and is equal to the ratio of the lens’ focal length and the diameter of the actual lens (f#=fl/d). On a zoom lens you may see the following ED18-55mm 1:3.5-5.6GII, these are actual numbers off of one of my Nikon lenses. If you’ll think back to my first article in this series you will recall that the ED stands for Extra-Low Dispersion glass that solved the problem of axial or longitudinal aberration. The 18-55mm indicates that my lens is a zoom lens with a short focal length of 18mm and a maximum focal length of 55mm. The 1:3.5-5.6 indicates the lens speed at both its minimum and maximum focal lengths. The lower the numbers the faster the lens, so this particular lens isn’t a really fast lens but its exceptional quality more than makes up for what it lacks in speed. As a rule, lenses rated at f1.4 or lower are considered fast lenses. A fast lens is especially important if you take a lot of available light picture under low light conditions but remember that it’s a tradeoff between speed and quality.
f-stop number, what do they mean
The f-stop number represents the actual size of the lens aperture and is equal to the ratio of the focal length of the lens to the diameter of the lens aperture opening. The size of the iris (aperture) opening is inversely proportional to the f-stop number-the bigger the number, the smaller the iris opening. The f-stops on a typical lens might be f2.8, 4.0, 5.6, 8, 11, 16, 22. The iris or aperture of your camera lens, like the iris of the human eye, controls the amount of light entering the camera.
Shutter speed settings
While the f-stop setting determined how large an opening the light has to enter through, the shutter speed setting determines how long that opening remains open for the light to enter. Typical shutter speed settings on a modern, high-quality lens might be 1, ½, 1/30, 1/60, 1/125, 1/250, 1/500 and 1/1000 second. Once again the amount of light entering the camera is inversely proportional to the shutter speed.
Aperture and shutter speeds are directly related
F-stops and shutter speeds are directly related and can be used together to maintain proper exposure. If the original exposure settings were f22 at 1/125sec but you need to stop some really fast action you could shoot at 1/1000sec and open up the lens to f11. You increased the shutter speed by two stops and open the lens by two stops, keeping the balance for a perfect exposure.
Depth of field scale
The depth of field scale appears on most manually adjustable lenses and appears on the lens’s focusing ring and tell you how far in front of the focus and how far behind the focus point that object will be in focus. To use the depth of field scale
· Focus on the subject
· Set the f-stop
· Locate that f-stop on both sides of the center line on the depth of field scale, the
· Locate both the near and far in focus distances on the focus ring directly in front of the selected f-stop numbers.
Depth of field is directly proportional to the f-stop numbers, the larger the number, the greater the depth of field. Knowing how to use depth of field settings is especially important when photographing fast moving sports where you have maintain wide areas in clear focus at all times. Most good SLR cameras allow you to pre-focus on the center of the area where the activity will take place and then lock that point so constant focusing is one less thing that you have to worry about. Locking the focus in allows you to concentrate on getting those award winning photos that you are looking for. We’ll talk more about pre-focusing techniques in another article.
In my next article I will cover ASA/ISO settings and how you can use them to get the pictures that you want even if the only illumination in the room is a single candle.
