Know Your Gear: Lens Limitations
Lens technology has made incredible leaps over the past few decades. Lenses are more crisp, clean and more dependable than ever before – and often at a reasonable price to boot. Even consumer grade lenses of today do a better job than those available several years ago. As I think back to what was available when I started shooting, I never would have thought my walk-about lens would be an 18-135mm zoom. Such wide ranges zooms were rare, and the image quality was nothing compared to shooting with the primes of the day. But such a lens is now not only pretty good, but it's also quite affordable. However, despite all of the improvements in lens technology, such devices must still adhere to certain laws of physics and there are some things that we will never overcome. While each lens is different, every lens has certain limitations that we need to be aware of. Knowing these limitations will save us time and effort on the shoot, but it also will provide us a medium to work with. At the minimum, every photographer should know the following about their lenses:
- Minimum Focus Distance – Every lens has a minimum focus distance – the minimum separation between the lens and the subject in order to get a tack sharp image. When shooting, if you're too close to the subject, your auto-focus will be hunting for the perfect focus. Check your lens's manual, or look it up on the manufacturer website and memorize this fact. The minimum focus distance could be as close as a few inches for macro lenses, but most non-macro lenses require a much more significant distance. Don't be surprised if the minimum focus distance is a couple of feet.
- Maximum Aperture – The maximum aperture not only affects how much light gets into the camera, but it's a critical factor in determining how well you can hand-hold a shot or how narrow you can get your depth of field. When selecting the best lens for a shot, this is one of the statistics you'll want to mentally compare. My 18-135mm f/3.5-5 lens is a good walk-around lens, for example, but when I want to really dial in the depth of field, I will grab my 50mm f/1.8. Most of you probably already know this information about all of your lenses. If you don't, it's usually printed on the lens. Be aware, some consumer grade zooms have a maximum aperture that changes based on focal length. My zoom is f/3.5 at 18mm, but it drops to f/5 at 135mm.
- Aperture “Sweet Spot” - Even on the best lenses, there is going to be a specific aperture where your lens performs its absolute best: the sharpest image with the least aberrations. For a general rule of thumb, this is usually two clicks from the widest aperture. But that is not always the case. You'll really need to play with your lens or check with a site like PhotoZone to confirm. My 50mm f/1.8, for example, performs best at f/2.4. If you can afford to, try not to go wider than this sweet spot. There will be times where you'll want or need wider, and that's okay…just be aware of the trade off.
- Distortion – Some lenses, particularly the wide angle ones – will show some signs of barrel distortion. Sometimes, this is a desired effect – such would be the case with a fisheye lens. But other times, you'll need to correct for it in post. The barrel distortion is often obvious. It is the slight distortions you really need to be aware of. In short, don't trust the wide angle of your lens.
- Vignetting – Many lenses show signs of vignetting, which is the darkening at the corners, for wide angle shots. This is particularly going to be the case when you're using filters that will further increase the effect. Vignetting isn't usually a problem as it can be corrected in post. But it will cause the focus at the edges and corners to soften. When this is not your desired effect, you will want to take precautions to prevent it. For starters, eliminate any unnecessary filters. You should also learn at what focal length the vignetting is eliminated so you can zoom in accordingly if possible.
- Metering Dependability – Some lenses will meter differently than other lenses. For example, my 50mm f/1.2 (an older fully manual lens) tends to overexpose with any aperture wider than f/4. Knowing this, I can account for it while I'm shooting to save time in the field and unnecessary corrections in post. There isn't a published statistic that will help you determine this limitation as it is directly related to the lens/camera combination. You will really need to shoot with your lenses often to learn about how a lens will meter. If you've got a shiny new camera with a pretty current lens, this is likely not going to be an issue for you.
I'm sure I haven't covered everything and you may have a few things to add to this list. If you do, I would love to hear your thoughts. Just leave a comment below to fill in any gaps that I have left.