While it seems simple, understanding the zoom lens on your camera makes a big difference in how you can make your shots turn out. The common understanding of a zoom lens is that it lets you make objects farther away look larger in pictures. That's true, but it's really doing much more than that.
Terminology
Zoom lenses are typically referred to by four different numbers such as a typical 18-55mm f/3.5-5.6. We'll get to the f/3.5-5.6 numbers later. For now, we're interested in the 18-55mm number. This is the zoom range of the lens, also know as focal length. At it's widest or "zoomed out," the focal length is 18mm. At it's tightest or "zoomed in," the focal length is 55mm.
18-55 is generally considered a "normal" zoom range for these types of cameras. Anything with smaller numbers (eg 10-24mm) is typically considered a "wide-angle" while larger numbers such as a 55-300mm are typically considered "telephoto."
Compression of Field and Field of View
While zooming in or out does make the object you're focusing on appear larger or smaller, the rest of the scene changes too. Specifically, how near or far away objects appear to each other changes and how much of the background you can see changes.
Let's look at an example.
When zoomed all the way out to 18mm, the statue of Emily Dickinson can fill the frame from top to bottom. You can see a good expanse of background behind her -- you have a large/wide field of view. But also note how the trees and buildings in the background look far away. You can just barely make out a building over her right shoulder. When zoomed out, the background is not very compressed. In fact, we're zoomed out far enough that the background seems expanded -- objects in the background appear farther away and smaller than they really are.
The building in the back right appears to be almost smaller than the statue. This presents the statue as being larger than it actually is and almost dynamic. When zoomed out, whatever is closest to the camera seems larger than objects farther away. This is a great technique to use with subjects that are otherwise boring or don't stand out from the background - zoom out, get closer, and fill the frame. However, be careful when taking a picture of a person at eye level zoomed out. Their head can appear larger and their body smaller as it is farther away.
For the second shot I stepped back and zoomed in about half way to 35mm. This is close to what the eye is used to seeing (the eye has a focal length right around 30mm measured on this camera). Note that the statue covers about the same portion of the picture since I stepped back.
You can see right away that less background is visible. The building on the right of the picture is half cut off, and you can see less of the tree on the left of the picture. The field of view is narrower.
The background is also more compressed. Now the buildings and trees in the background appear to be much closer to the statue. The building on the right that appeared small now looks appropriately sized. By using this focal length, the image appears as expected with objects the appropriate distance to each other and in the proper proportions. It's a very useful length to shoot at regardless of subject.
For this shot I stepped back even farther and took the picture at 55mm or all the way "zoomed in." The field of view is now very narrow. The building on the right is almost out of frame.
The background is also very compressed. You can see how the tree from the first shot that looked so far away now appears to be very close to the statue -- almost as though the branches are over her head. You can also clearly see the building to her right (left of the picture) now as it appears much closer. The building that was small in the first shot is now almost out of frame.
Zooming in primarily helps put the emphasis of the image on the object, not the background. This can be very useful if there is a distracting background like a crowd. Alternatively, you can zoom in to bring the background in to play. Had we used a telephoto lens at say 200mm we could have made the building on the left side of this picture look even larger and framed the statue against that.
In Use
If you can't get close to a subject you may be forced to zoom in, or when you can't stand far enough back from a subject you may have to zoom out. But, if you are able to compose the shot as you'd like, try setting the focal length and moving your feet to get the appropriate shot. Here are some general tips for when zoomed in or zoomed out might be helpful.
Wide-Angle/Zoomed Out/Short Focal Length/18mm-
Good For:
- Landscape shots where you want to capture a wide angle of view
- Still life shots where you want to create "tension" by making the object look larger than the background
Bad For:
- Portraits or family shots as people can look distorted
Normal/Medium Zoom/Normal Focal Length/30-35mm
Good For:
- Almost any shot as it provides a similar image to the eye
- "Set it and forget it" and then moving your feet
- Portraits where you want a similar to real life look
Bad For:
- Shots where there is little inherent excitement and wide angle tension or background compression would help
Telephoto/Zoomed in/Long Focal Length/55mm+
Good For:
- Sports, pet, or other subject shots where you want the subject to be the true focus of the image
- Shots where you want to "set" the subject in a distant background by compressing it forward (graduation shot with student on a field and a building in the distance and you make the building look larger)
- Portrait shots that provide a slightly more pleasing compression than a normal view
Bad For:
- Composition-wise it's hard to find a drawback, but there are other concerns (see aperture and shutter speed).
Personal Notes
My best suggestion is to set a zoom at a focal length then walk around and take pictures. Force yourself to move versus changing the focal length. It gives you a great appreciation for some of the things that can be done. Wide-angle shots are the most under used by people who first get a zoom lens.
Technical Notes
There are a couple BIG technical notes here.
First, zoom lenses are difficult to design. As such, you may experience some image issues, although none are likely to be noticeable unless you look. For instance, when zoomed out you may see some curvature to the image (barrel distortion) and the edges may be darker than the middle (fall off or vignette). Zoomed in you may see the opposition distortion (pincushion) and also fall off/vignette.
Second, and this is the big one, focal lengths are sometimes referred to by "35mm equivalent" numbers. Only top end digital cameras have sensors as large as 35mm film was. These are often referred to as "full-frame" cameras. Most DSLRs have sensors about 2/3rds the size of a "full-frame" sensor and are referred to as APS-C size.
For the same field of view and compression of background, a smaller sensor requires a smaller focal length. So, APS-C camera lens focal lengths are about 2/3rds that of full-frame (FF) camera focal lengths. But, because so many photographers grew up with 35mm film, those focal lengths are standard in their minds. They may refer to the kit 18-55mm lens as a 27-82 (18-55 X 1.5).
This obviously breeds confusion, but it does allow for a standard. Few camera companies use exactly the same size sensor, so referring to 35mm equivalent focal lengths allows easy comparison.
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