I'm a portrait photographer. So I get asked a lot about what cameras and lenses the portrait photographer should be using. And while I'm a true believer that any lens can be a portrait lens, what we're looking at here is trying to find the 'ideal' lens that should be the first one you reach for when doing portrait work. So what we'll examine here is the 'why' behind the lens of choice for the majority of portrait work. This is a lens that will routinely work in a variety of situations and produce outstanding results. Let's get starting with some of the key features to look for in your choice.
1: FOCAL LENGTH. This is always step one. A good starting point is to take the diagonal measure of the film or sensor your camera uses and multiply it by two. For example, the film diagonal of a medium format square camera is about 80mm. The diagonal of the film is used to determine the focal length of a 'normal' lens. I believe the reasoning is that a lens of that length gives us a 'normal' perspective when made to 8 x 10 and viewed at arms length, but don't quote me. In the above instance, a typical lens length for portraits is 160mm. And it seems that all makers of square format lenses just happen to make a 150mm lens that all portrait photographers own. Translating that to the 35mm format, we'd come up with a 100mm lens, and it also seems that for years 35mm camera makers have made a 105mm lens. Coincidence? I don't think so.
Now you can see that if we look at the 'perfect lens' for digital, our sensor size must be taken into consideration. If it's a full frame camera, then we can use the same thinking that applies to the 35mm format. If the sensor is only half the size or less, that must be taken into consideration. For example, the Nikon D 80 camera chip is about two thirds the size of a 35mm frame, so the lens 'conversion factor' compared to 35mm is 1.5. That means that a 100 mm lens on a 35mm camera becomes a 150mm lens on a camera with a 1.5 conversion factor. Now that I've taken the time to explain the relationship between film or sensor size and focal length, let's just stick with 35mm format and you can do your own interpolating later depending upon what camera you own.
Now that that's out of the way, what's the reasoning behind the longer than normal focal length for typical portraits? There are actually a few reasons for it. First, there is perspective. The perspective you have in a photo is dependent upon how close you are to the subject, not the lens. Take a few images on a tripod of the same subject and keep changing the focal length. While some images will have more in them than others, the perspective remains the same. However, to include the same subject at the same size means moving the camera. Walk up to a friend, get two inches from his nose, close one eye, and look. You're seeing what a wide angle sees in close. Back off six feet and look through a paper towel tube, there's your long lens. So a longer than normal lens allows us to keep some distance between ourselves and our subjects, giving us a pleasing perspective. And by keeping that distance, our subject is also more comfortable than if we shove a camera up his nostrils, so the longer length now accomplishes two objectives - a pleasing perspective and subject comfort.
Now we're going to throw in another nice feature - angle of view. The longer the focal length, the narrower the angle of view. For example, a 28-200mm lens has an angle of view of about 74 degrees to about 12 degrees. Naturally, the longer the lens, the less is included in the image, and the easier it is to isolate your subject on a background and crop out distracting elements. Is this all starting to seem quite logical? Good, let's look at some other factors.
2: LENS SPEED. In studio portraits, lens speed is usually not that critical, since you may be shooting at f/5.6 or smaller. Outside, on location, and for weddings, you want the fastest lens you can get. No matter what your shooting aperture, it's always nice to have it as bright as possible so both your eyes and the camera can focus better.
3: ZOOM LENSES. It used to be that prime focal length lenses were much better than zoom lenses because they don't have to make the many compromises needed in the design of a zoom. While that's still true, lens technology is now so advanced that the gap is very tiny. So in practical terms, a quality zoom will fit the bill for most photographers. A zoom offers many focal lengths and allows you to crop the image in camera so that you can maintain optimum image quality. One other note, if at all possible, try to buy a lens that is a continuous f/stop as opposed to those that vary with focal length, it will make your life easier and your exposures more accurate when manually metering.
3: OTHER FACTORS. Here are a couple of other factors to consider in your search for the perfect lens. Based on my limited testing, if you're digital, buy the lenses made for digital cameras. Film and sensors do not focus the same way, I see a considerable difference using digital lenses. Another consideration is the blade shape of the lens diaphragm. Some are designed to make more of a circle, so your out of focus highlights assume a pleasing, circular shape. Also, a Vibration Reduction lens will cost you more but also allow you to shoot handheld about two stops more than the standard issue. If you do mostly outdoor and location portraits, I'd suggest it.
4: THE PERFECT LENS. Taking all of the above into consideration, I'll take a couple of stabs at the 'perfect' lens. For 35mm and full frame digital, there are 70-200mm f/2.8 lenses with VR that seem pretty close to perfection. There are also 55-200mm digital lenses that have variable apertures, pick them if cost is an issue. A shorter and lighter lens, like a 28-75 or 80mm, is nice to have when you are working in tight quarters or when the weight of the longer lenses becomes an issue. On my wish list for the future? A 50 to 300mm VR f/2 lens that you can throw in your pocket and buy for $400. Read this three years from now and you probably won't thing it's so ridiculous!
SHOOTING F/STOPS. While the lens you use has f/stops from about f/2.8 to f/22, for portraiture you'll probably settle on a couple of favorites. Since not all of you have studios, let's look at how you'll be shooting outdoors.
My zoom lens goes to f/2.8 but most of my shooting is done between f/2.8 and f/5.6. I find that these apertures will in most instances give me the 'look' I want. They are open enough to throw backgrounds out of focus, but small enough to keep my subject in focus.
Do your own practice shooting to find your favorites!
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