What’s a portrait lens?
Why does no one know what a portrait lens is?
When you hear explanations of portrait lenses, they usually revolve around focal lengths or background blurriness or sharpness. These are all partly the reasons but they mostly missed the point, which is strange because the answer has existed almost since the birth of photography.
What IS a portrait lens? Let’s bust some myths and get to the real answer.
Can any lens can be a portrait lens?
Some careers have been built on using non-traditional lenses for portraits. Bill Brandt and Platon used wide angles, and paparazzi have to use really long telephotos out of necessity. Pictures like these stand out because they are exceptions to the rule.
If we want flattering portraits, something a client will pay for and be complimented by and be happy with, we’ll need to narrow the selection down.
You can take a portrait with a short lens to give a less flattering perspective, or with a slow zoom if you bump up the ISO. You can do whatever you want.
All lenses can take a portrait, but not all lenses are portrait lenses.
Prime lenses are portrait lenses because they're sharper than zooms?
That was the case in the 1980s when most zooms were trash but not anymore. The quality of modern zooms is exceptional. But that might not be for the best.
Until recent history, a sharp lens was considered unflattering for portraits, because “this tends to hardness and gives an unpleasant effect” (Portrait lenses). Softer renderings of the human face were preferred, and that’s why soft-focus lenses existed. For a flattering portrait I'm usually nudging the clarity down in post production, unless it's an intentionally dramatic portrait. Rembrandt would have pushed the clarity slider up and Da Vinci would have slapped his hands.
Still, there has always been a down-side to zooms, maximum aperture is the compromise. There’ll always be a prime out there that’s faster than your zoom lens.
Are portrait length lenses are of a certain focal length?
Almost. We’re getting closer now.
Focal length is just how much of a screen is visible. Its almost irrelevant once you start thinking in terms of linear perspective.
The laws of perspective for photography work a lot like the inverse square law of light. Most of the dramatic changes take place at short distances. A picture taken 12 inches from the face looks very different than one taken from 6 feet away. At 12 inches the nose is relatively close to the camera and the ears are relatively far away. At 6 feet both the nose and the ears are practically the same distance from the camera.
By the time you take a headshot from 6 feet away, you are in the world of diminishing returns. You could further reduce diminution of facial features by standing 10 feet away, or 100 feet away, but the difference between these distances are much less dramatic than when you are really close in.
Distance matters but the focal length of the lens doesn't change a thing other than how much the subject is magnified before it hits the sensor. I use an 85mm from 6 feet away for headshots, and a 35mm from the same distance for ¾ or full length shots. Because they are at the same distance from the subject, the perception of perspective is exactly the same.
You can use any focal length for a portrait, but shorter lenses used close up won't be as flattering as longer ones from further back, and very long lenses mean you'd have to stand too far back, which you might not have room for and it also prevents you from naturally interacting with the sitter.
That’s the important thing - the camera’s distance from the subject. Too close, the perspective is extreme because the nose and back of the cheeks are relatively far away from each other. 6 feet back and you are at a natural conversation distance. This is the sweet spot because it’s the perspective we get of a person when we are having a conversation with them.
Before we move on this is really important. I can use a fisheye at 6 feet and crop in. Notice its the same perspective as an 85mm lens. This shows its the distance not the lens that’s important. BUT when I crop in, we lose quality because we have so few pixels left. That's why using an 85 is so important- the optical crop exploits all the quality your sensor has to offer.
Portrait lenses have wide apertures to blur out the backgrounds?
We are really close now. We are right but for the wrong reasons. This one dates back to the birth of photography. Large format cameras, super low iso and small aperture lenses.
It’s the opposite of today's problem. People saw front to back sharpness as a flex, and bokeh with a narrow depth of field was a problem you bought expensive equipment to solve.
So blurry backgrounds aren't the reason portrait lenses exist, not until very recently anyway. Preventing blurry moving subjects was the reason.
Let’s pretend we are in a Victorian photo studio. No electronic flash available yet. Just light pouring in from the studio skylight. We've got an f15 lens made by Chevalier. Our dry plate has an iso equivalent of 2. Not iso 200, iso 2.
We aren't getting full sun, but to simplify things I'll use the sunny 16 rule that days we’ll have to expose for half a second. The literature says photographers were exposing for 30 minutes, likely because it wasn’t full sun, or the chemistry was even slower or perhaps even due to reciprocity failure. Even at ½ a second, that's going to produce a blurry photo of the subject so much as breathes. With a child there's no chance of getting a good picture.
We would gladly trade some image quality and some hard earned cash just for a faster lens. So petsvel comes along and gave photographers an f4 option, 4 stops brighter than what they had before. That gets us to a 1/30th second shutter speed - now we don't need to tie anyone down anymore! As long as they don't make any sudden movements.
Wider apertures let us use faster shutter speeds in natural light conditions. It’s at the expense of a narrower depth of field, but we've all convinced ourselves that blurry backgrounds are a good way to get subject separation anyway.
