How to use manual focal lengths and focus to make a digital picture

We can use different focal length lenses to change the way a scene looks in the picture, and control how much of the image is in focus. We are going to look at examples made with different combinations, and discuss how to use those effects creatively. As I offer examples, experiment with your own equipment. If possible, frame the scene using different focal lengths and focusing on different points within the frame. I’ll also slip in a few more terms. (No, I haven’t forgotten f/stops, but more tech talk can wait.)

Going Wide While Getting Close

The name wide-angle lens may make it sound like this class of optics is designed for landscapes, but they also have a real ability for working close to your subject and giving your viewer an interesting perspective.

I took the next picture with a 12mm-24mm zoom set to 15mm. Notice how large the horse’s head appears compared to everything else, and that everything in the image appears in focus.

The horse’s head is large because it is so much closer to the camera than the rest of the picture. Wide-angle lenses let us play tricks like this with visual perspective. It also lets us get very close to a subject and still see its surroundings.

If you have a wide-angle lens, or a zoom that goes into the wide-angle range, experiment a bit with framing objects near the edge of the frame and see how it adds perspective to your visual statements.

Wide-angle lenses let you work close to a subject and still keep distant objects in the in background in focus. The wide-angle optical design imparts a relatively large depth of field to the image. Depth of field refers to the distance near-to-far that appears in acceptable focus in a picture.

When we (or maybe our DSLR) focus a picture, a specific point is chosen as the focus point. That is the object that will have the finest optical resolution in the recorded image.

Extreme wide-angles lenses can focus extremely close and still have a large depth of field. This lens can focus as close as one foot to a subject. I focused on the bride’s hands not the horse. The church steeple is in reasonable focus too. The smaller the f/stop the larger the depth of field (remember, bigger numbers mean smaller apertures).

The sunny day let me use a very small f/stop, f/16. With the lens zoomed to 15mm, everything about 20 inches from the camera to infinity was within the depth of focus range. I used the words “reasonable” and “about.” Only one point in the picture is perfectly in focus. Zoom lenses don’t offer a depth of field (DOF) calculator on the barrel, so I am basing the depth of field on what looks sharp in the image.

Many prime lenses do have a DOF calculator, like the one on my 10.5 fisheye, pictured in the next image.

Let’s use it to look more closely at how DOF works.

There are three rows on the scale above the name of the lens. See the solid white oval between the two 8s just above the name of the lens? That is the focus-point indicator.

Right now, the lens is set to a focus point just under one meter. That means that the sharpest object in the picture, the one in perfect focus, is located 0.3 feet from the focal point located in the middle if the lens. (We won’t go into all of the optics involved.) You do need to know the optical trigonometry symbol for infinity to use the calculator. It looks like a number 8 on its side. You can see it in Figure 5.7 just above the number 22 on the left side of the DOF calculator.

This symbol marks the focal point on the distance scale beyond which everything in the scene will appear in focus. That’s why it has a symbol rather than a number. (Remember the focal point is the optical center of the lens and the focus point is the object in the picture you are focusing on.) There are two points marked with the number 22 on the bottom row.

The one on the left marks the farthest point on the distance scale that will be inside the DOF range at f/22. The one on the right marks the nearest point.

everything from seven inches to infinity will look reasonably sharp with this lens when the exposure is made with the f/stop set to f/22. Come in and read the distance markings at the two 16s, and you know the DOF range if you take the picture with the lens set to f/16.