Understanding Lenses

By Alan Ross Back to

(Figure 1) 105mm vs. 24mm (Figure 1) 105mm vs. 24mm

There are two primary elements of choosing and using a lens: what focal length to use and where to focus/what aperture to use. Let’s take a look at how these elements affect an image.

How Does Focal Length Affect Your Image?
The only thing that changes when you use a different focal length lens is the cropping of your image!

Optical aberrations aside, short focal length or wide angle lenses do not distort close subjects, and long focal length or telephoto lenses do not compress subject features. What really causes these familiar effects discussed so often in popular texts is a change of perspective: a change in the camera’s physical position relative to your subject. (Figure 2).

When you move in close to a subject, it becomes very large in relation to its background. That over- large nose you get with a wide angle lens portrait is because you’ve probably moved in very close to the subject in order to fill the frame, and the nose, being closest to the lens, is now very large in relation to the ears. This is a matter of your proximity to the subject and has nothing to do with the lens itself. (Figure 1).

When you look at a distant scene through a long focal length or telephoto lens, elements in the scene may appear compressed, almost right on top of each other. Once again, this has nothing to do with the lens, but is simply a matter of the tight framing on the subject. If you put the camera down and frame the scene just as tightly with your hands, the elements of the scene will appear as “compressed” as they did through the lens.

(Figure 2) Different focal length lenses change only the cropping if camera position is unchanged.

What about zoom lenses as opposed to fixed focal- length lenses? Do they help make your choice easier? Well, yes and no. If the optics are up to snuff, a zoom lens can provide a great deal of convenience−it’s a zillion focal lengths in one piece of hardware. But that convenience can lead to overly casual, rather than critical, vision. Imagine a photographer out for a walk. He (or she) comes across a detail or a scene of interest. Camera goes up to eye, hand zooms lens to frame the subject, auto-focus and metering do their jobs, shutter goes click and it’s on down the path. Would the image have been more powerful if our photographer had moved in close to some boulder in the foreground, making it monolithic in relation to the background? Maybe. Or maybe backing up a bit might have let some tree branches frame the scene.

The point in all this is that to maximize the impact of a visual statement it is important to give thought to the image structure first. Is the composition better closer in? Farther back? Up, down left or right?

Once you pick your camera position, then choose the focal length that gives the cropping you want. If your first guess is too tight, use a shorter lens; if it’s too loose, use a longer lens. If you don’t have a lens that is quite right, use one slightly shorter than you would like and crop.

Where do you focus, and how does the aperture affect an image? In a certain way, the opening question should be the other way around. There is a law of physics that governs the relationship between shutter speed and aperture (f-stop). Shutter speeds are pretty easy to understand: 1/60 second is one half as much time as 1/30. F-stops are a little different: f/8 is one half the light of f/5.6, which is half the light of f/4! The point is, for any shutter speed/f-stop combination, one-half the exposure time with twice the light equals the same total amount of light given to the film. 1/60 @ f/4 = 1/30 @ f/5.6 = 1/15 @ f/8.

There is always the inescapable relationship between exposure time and aperture. If you are photographing a sports event, you will likely go with a fast shutter speed and let the aperture fall as it will. This article will focus on aperture as primary.

F-Stop #s vs. Depth-of-Field
A lens can only truly focus on one plane. With a perfect lens that plane would be equally sharp at any aperture−but everything nearer or farther would rapidly become unsharp. Increasingly smaller apertures reduce this apparent unsharpness, increasing what is called depth-of-field. The smaller the aperture (f/16 is smaller than f/4), the greater the apparent sharpness.

In Figure 3, image A is focused approximately on the line of traffic in the foreground. The chain-link fence is way out of focus, as is the distant railing. The wide- open aperture (f/1.4) necessitated a very fast shutter speed resulting in the cars frozen in time. Image B is with the lens stopped down 4 stops (f/5.6). The point of focus was not changed, but the fence is now a good deal sharper, as is the distant railing, but at the now much longer exposure time, the nearby car, while still sharp in focus, is blurred in time. Image C is still focused in the same place but the lens is now stopped down 3 more stops to f/16. The fence now appears to be quite sharp as does the distant railing, but the car is now quite blurred at 1/15 second. It is a total coincidence that the images seem to show the same car. Most fixed-focal length lenses have an engraved scale allowing you to evaluate how much apparent sharpness (depth-of-field) you can get at various apertures. Figure 4 shows a Hasselblad 80mm lens set at f/22. As the lens aperture is stopped-down, the depth of field increases in the proportion of 1/3 toward the lens from the plane of critical focus and 2/3 beyond the plane of focus. Image A shows the lens focused at about 3.3 feet, and at f/22 the depth of field runs from 3 feet away to 4 feet. Image B shows what would happen if we did a landscape with the lens focused on infinity. The image would only be “sharp” from about 17 feet away to distant mountains. If we instead focused at 17 feet (this is called the “hyperfocal” distance) the image would now be sharp from about 9 feet to the mountains (image C).

(Figure 3)
(Figure 4)

There are two ways to plan how to make this work. One way is to choose your aperture first and see how much depth-of-field you get, and the other is to find out what aperture you need to work with and then see how much depth of field you need to work within. Let’s say your camera is on a tripod, and you want as much as possible near-and-far to be sharp. Take the Infin- ity mark on the lens and place it over the engraving for your smallest aperture. The lens is now focused automatically at the hyperfocal distance and you can read the depth of field on the focus scale of the lens. In this example (Figure 4, Image C), f/22 gives you a pretty sharp image from about 9 feet to infinity. Lets say the camera is NOT on a tripod, and you can’t manage to stop down to f/22, but only to f/8. In this case, you would place the Infinity mark over the f/8 index. You would now see that the image would only be sharp from about 20 feet to infinity (See green arrows, Image C).

What if your lens doesn’t have markings? A lot of mod- ern zoom lenses have distance scales, but no depth-of- field markings. If this is the case, you can find the hyperfocal distance by putting the nearest subject and distant subject marks on the lens an equal distance from the central focus mark. If your camera has a “depth of field preview” button, this can be a useful aid in seeing just how much is sharp−or not! But the actual depth of field for any given f-stop will just be a guess.

(Figure 5)

Selective Focus
One bit of fun with f-stops: Sometimes, you can make a stronger statement by limiting how much is in focus. Just leave the lens at its widest aperture. The image on the left was done with a 200mm lens at f/4.5 focused exactly on the near marker, and the image on the right was done at f/22 with the lens set at the hyperfocal distance. (Figure 5).

One last thing I’d like to comment on in this writing: lens quality. Photo gear can be expensive, no doubt about it. Especially at an entry level, the prospect of getting an off-brand lens for a lot less than the brand that has your camera manufacturer’s name on it can be awfully tempting. In these days of computer-aided engineering design a “Brand X” lens can be quite good −but there is an equally good chance that it will not measure up to the quality or durability of a top brand. Do your research and read reviews for a particular lens and if you need to save dollars, look for quality used gear from a reputable source.

Hopefully, all of this will help you have a better understanding of the relationship between your vision, your lens and your results.

About the Author

Alan Ross
Photographer and master printer Alan Ross has been Ansel Adams’ exclusive printer for over 36 years. His experience includes operating a commercial studio with projects ranging from ad campaigns to murals for the National Park Service. Since 1993, he has devoted his energies to his personal work, teaching and work for select clients, including Boeing, Nike, IBM and MCI. His photography hangs in collections and galleries throughout the country and internationally, and he has lectured and led workshops in locations from Yosemite to China. For a more in-depth discussion of Ross’ masking process, visit his website, alanrossphotography.com.