Friday, April 1, 2016

Telephoto Lenses - A User's Primer

As a young photographer, I was fascinated by telephoto lenses. Perhaps it was from watching too many wildlife shows on PBS. It would be many years before I'd appreciate long lenses for their perspective-flattening qualities more than their ability to photograph at much longer distances.

I was asked to comment on using telephoto lenses in the field.  There are several factors that must be understood it long lenses are to be used successfully.

Variable Apertures: It is less expensive to build a variable aperture zoom lens than one where the aperture remains constant throughout the zoom range. Shown here is my Nikkor 75-300 AF lens, easily obtained for $100 - $130 from eBay at the time this post was written (I paid considerably more when I purchased my specimen from a local dealer a few years ago, but it was still a bargain). Today, the lens is less popular that it was when it was introduced in 1989 because it's an AF (not an AF-S) lens, meaning you'll need to focus manually with the D3xxx and D5xxx series Nikon bodies. Second, its push/pull trombone zooming mechanism (supposedly) "inhales" dust into the lens' interior. And third, the front element rotates while focusing, annoying if you're trying to use a polarizing filter.

Notice the official designation (see photo, above), which means:
  • If you use the lens at 75mm, the widest aperture available is F4.5.
  • If you use the lens ate 300mm, the widest aperture available is F5.6.
Notice that there are three focus indicators: This lens is not a true zoom, but a Variable Focal Length Lens, meaning that at different focal lengths, the plane of focus shifts. This isn't apparent when the lens is used normally, but in the rare instances when you need to know the camera's to subject distance,  you'll need to select the distance index appropriate to the focal length.

To prevent some unexpected aperture changes during zooming, think of the lens as a 75-300mm F 5.6 lens, and set the aperture accordingly.

Telephoto Lenses And Maximum Exposure Time: There was, and is, an old rule regarding the longest possible exposure time for any lens when hand-held. The rule is as follows, with as modification for the APS sized sensors found on most digital SLR cameras:

Longest Safe Shutter Speed = 1 / (focal length of lens * 1.5)
                    = 1/(300 * 1.5)
                      = 1/450 second

At longer exposure times, the effects of the photographer's shakes and tremors become visible. This does NOT take into account your subject's movement. You could properly freeze a Olympic speed walker or a snail race, but not an arrow in flight. This assumes that you're holding the lens properly, with your elbows tucked close to your body and the top of the camera resting against the bones beneath you eyebrows. Oh, the 1.5 is an adjustment because the APS sensor increases the usable focal length of the lens.

Joe McNally posted "Da Grip" a while back. Check it by clicking here.

Vibration Reduction (VR): Vibration Reduction (Nikon), or Image Stabilization (Canon) is an important feature, especially when using longl lenses. Call it magic, but it allows a steadier platform for the shooter working with with longer exposure times.The improvement provided by VR is often expressed in stops. For example, let's say we have a lens that when VR is engaged, it provides a 2-Stop improvement in performance. In the world of long lenses, it means that if your exposure dictates an shutter speed (exposure duration) of 1/125 of a second with VR off, you should achieve similarly sharp results if your exposure time was 1/30 of second, or four times as long. If you gain these two stops, you could shoot at an aperture two sizes smaller for more depth of field, or a lower ISO setting for improved quality.

This is not an absolute: In a discussion on lens choice,  David Ziser reminded us that VR or IS is not a guarantee of sharper images; It simply improves the probability that you image will show less degradation due to photographer tremors.And some reminders: VR must be turned on if it's going to work, it may not work properly when the camera is mounted on a tripod, and decreases battery life.

Now What? If there is a legitimate reason for not setting your aperture and shutter speed manually, I would suggest the following procedure:
  • Set Your ISO: Chose an ISO to start. I would start at ISO 400.
  • Set you camera for Aperture Priority Exposure Automation: This allows for exposure automation with an aperture that you select, and the ability to choose a high ISO value for the corresponding decreased exposure time.
  • Select Your Preferred Aperture: If I were using this lens, I would set it to 1 F-Stop down from 5.6, or F 8.0. My reasoning is that lenses are seldom sharpest when used "wide open" (at maximum aperture), and by stopping the lens down one stop boosts the performance a bit.
  • Make A Test Shot and Check The Exposure Time: After making a test exposure, check to see how the camera responded. If the shutter speed selected is 1/450 of a second or briefer, you're good to go. If not, go back to the first step and up the ISO another full stop, and repeat.
In The Field: When using automated exposure mode, you'll need to use the camera's Exposure Compensation feature to bring the exposure exactly where you want it. Remember that changing the aperture will have no noticeable effect on the overall appearance, as the shutter speed will adjust up or down, following your aperture choices.

One final suggestion:  If possible, get a shooting position where the sun lights the subjects from the front. Fighting for shadow detail with most telephoto lenses is a loosing battle due to physical limitations associated with long lenses.