Maximum aperture

Learning goal

You will be able to read a lens's maximum aperture and weigh what a wide-aperture lens will gain you and what it will cost you. You will define a lens's light-gathering power as its widest aperture; you will calculate in stops and shutter speed the difference between f/1.8 and the f/5.6 at the telephoto end of a typical lens that comes with a camera; you will see the power of a wide aperture to melt the background and its limits; you will know how thin the slice of sharpness is at the widest opening and how accurate focus needs to be; and in a dim place you will use this gain in light in a way that keeps two faces of different skin tones sharp together.

The question

One evening a birthday is being celebrated at home. The lights are off, and the only light is the candles on the cake. When you shoot with the zoom lens that came with your camera, the camera either chooses a very long time and the moment the child blows comes out blurry, or raises the ISO so much that the faces turn into a grainy surface. A friend hands you a small lens: "Try this, f/1.8." In the light of the same candles, the child's face, puffed-out cheeks and the flames of the candles are clean and sharp.

Why is an f/1.8 lens the best first investment? The lens many photographers first recommend to beginners is often of this kind: a prime lens with a wide aperture at a relatively affordable price. In this lesson we will talk about the reasons behind this recommendation, its numbers, and the sides of it that don't hold for everyone.

Context

In Lesson 12.3 we saw that thanks to their simple structure, prime lenses can offer a wide aperture in a small body. In Lesson 12.5, too, we talked about how on small sensors the slice of sharpness deepens for the same frame. This lesson combines these two pieces of knowledge and looks at one of the most often asked questions in choosing a lens: what is a wide-aperture lens worth?

A lens's widest aperture is called its maximum aperture, sometimes written maximum aperture / lens speed; many photographers call such lenses a fast lens, because they allow a shorter shutter speed. The zoom lenses sold with cameras, of the kind called a kit lens, start around f/3.5 at the wide end and come down to around f/5.6 at the telephoto end. An f/1.8 prime lens, on the other hand, goes several stops beyond this. Seeing in numbers what this difference means makes the decision easier.

Simple explanation

How much light does f/1.8 gain?

Stop calculations double the light at every full step: going f/5.6, f/4, f/2.8, f/2, each step is one stop. f/1.8 is a little wider than f/2; the difference between f/5.6 and f/1.8 is about 3.3 stops. This means about ten times more light falling on the sensor.

You can spend this gain in one of three ways. At the same ISO you can shorten the shutter speed by about three stops: where the kit lens asks for 1/15, you shoot at 1/125, and the face of the child blowing freezes. At the same shutter speed you can lower the ISO by three stops: you shoot at around 800 instead of 6400, and the grain drops noticeably. Or you strike a balance between the two. The movement in the scene and how much grain you can put up with decide which one you choose.

How does it melt the background, and what is the limit?

As the opening grows, the sharp zone thins and the rest softens. At f/1.8, while a face stays sharp, the wallpaper, shelves or lights behind can turn into soft patches. This is a powerful way to separate a subject from busy surroundings, and one of the reasons many people turn to these lenses.

But how much the background softens doesn't depend only on the aperture. The distance to the subject, the distance between subject and background, the focal length and the sensor size decide this result too. Even if you photograph someone standing against a wall at f/1.8, the wall doesn't soften enough; moving the subject a few steps away from the wall is often more effective than opening the aperture one more step. And not every photo needs a melted background: in a frame where a place or a setting is being told, the background is part of the story.

What is the cost of a wide aperture?

At the widest opening the slice of sharpness is very thin. With a 50-millimeter lens on a full-frame camera, at f/1.8, on a face photographed from a meter and a half, the sharp zone is about ten centimeters deep. This isn't much more than the difference between the tip of the nose and the ears. If focus falls on the nose or the eyebrow instead of the eye, the eye softens; that is why, with a wide aperture, focus precision, that is sharpness landing exactly on the wanted point, becomes much more important.

Many lenses give a slightly softer and slightly darker image at their widest opening, especially at the edges and corners of the frame; when stopped down a step or two, sharpness increases. This wide-open softness reminds you that you don't always have to use wide-aperture lenses at their widest opening. And not every photographer's needs are the same: someone photographing landscapes in daylight often works around f/8 and rarely uses the gain in light of f/1.8; someone photographing distant athletes or birds first needs a long lens.

How do you protect more than one face in dim light?

In a dim place, a wide aperture's gain in light lets you photograph faces at a lower ISO and so keep the skin's detail. The fine detail of a brown cheek half in shadow in candlelight first breaks into speckled grain at high ISO, then flattens under the camera's cleaning process; on light skin, the skin's texture can become artificially smooth. The gain in light reduces both problems at once, on every skin.

But if two people are in the same frame and stand at different distances from the camera, the thin slice of sharpness at f/1.8 can soften one of them. In such a frame, either you bring the two faces to about the same distance from the camera, or you stop the aperture down a step or two and deepen the slice of sharpness. Even f/2.8 gives two stops more light than the kit lens's f/5.6, and often gives enough depth to keep two faces sharp together.

Visual evidence

You go back to the birthday table. At the cake is a dark-skinned little girl with curly hair, and her light-skinned, silver-haired grandmother with her cheek leaning against the girl's. When you shoot with the kit lens's widest opening at the telephoto end, at 1/30 and a very high ISO, the girl's face is grainy and her lips, moving as she blows, are slightly blurred; the lines on her grandmother's cheek have turned into a smoothed, flat surface.

You put on the 50-millimeter f/1.8 lens and first shoot at its widest opening: the girl's eyes are sharp, but her grandmother's face is slightly soft, because it stayed a few centimeters behind. You stop the aperture down to f/2.8; sharing the two stops you gained over the kit lens, you shoot at 1/60 and ISO 3200. Now both faces are sharp: the girl's puffed-out cheeks and the glint of the candlelight on her eyelashes, and the fine lines at the corners of her grandmother's smiling eyes. The walls and decorations of the room behind are soft, warm patches. The same candles, the same faces; what set the frames apart was the light the glass let in and the decision you made about the opening.

Common mistakes

Summary

Maximum aperture is a lens's widest aperture; an f/1.8 lens gathers about 3.3 stops, that is, around ten times more light than a kit lens's f/5.6 at the telephoto end, and this gain can be spent on a shorter shutter speed or a lower ISO. A wide aperture softens the background, but the distance between subject and background also decides the result; at the widest opening the slice of sharpness is very thin and focus precision gains importance. This lens many photographers recommend doesn't suit every need; in dim light it helps keep detail on every skin, and for more than one face, stopping the aperture down a little often keeps both faces sharp together.

Mini assignment

In this assignment you will measure the gain and the cost of a wide aperture in your own frames.

1. In a dim room, photograph the same scene with the widest aperture you have and at f/5.6, at the same ISO; compare the shutter speed the camera chooses.
2. Photograph a person first against a wall, then a few steps away from the wall, with the same aperture, and compare the softness of the background.
3. Photograph a face at the widest opening and check by magnifying whether sharpness fell on the eye or on the nose.
4. Photograph two people standing at different distances from the camera first at the widest opening, then at f/2.8 or f/4, and compare the sharpness of the two faces.

Check yourself

• How many stops shorter a shutter speed were you able to use in the dim room with the wide aperture?
• Did moving the subject away from the wall soften the background more than the aperture did?
• In the two-person frame, regardless of skin tones, were you able to find the aperture that kept both faces sharp together?

Glossary

Mini quiz

1. About how many stops of light difference are there between f/1.8 and f/5.6?

• A) One stop
• B) About 3.3 stops
• C) Six stops
• D) No difference

2. In a dim place where the kit lens asks for 1/15, about what shutter speed does an f/1.8 lens allow at the same ISO?

• A) 1/125
• B) 1/8
• C) 1/15
• D) 1 second

3. If the wall behind someone standing against it doesn't soften enough even at f/1.8, what do you do?

• A) Raise the exposure compensation
• B) Stop down the aperture
• C) Move the person a few steps away from the wall
• D) Change the white balance

4. To photograph sharply together, in dim light, a dark-skinned child and her light-skinned grandmother standing at different distances from the camera, what do you do?

• A) Stay at the widest opening and focus on the nose
• B) Stop the aperture down to around f/2.8 and raise the ISO a little if needed
• C) Switch to the kit lens's narrowest opening
• D) Turn off the flash and lengthen the shutter speed to one second

Answers

1. B — From f/1.8 to f/5.6 there are about 3.3 stops; this means about ten times the light.
2. A — About three stops: 1/15, 1/30, 1/60, 1/125.
3. C — The softness of the background also depends on the distance between subject and background; as distance grows, softness grows.
4. B — f/2.8 deepens the slice of sharpness enough to cover both faces and still gathers two stops more light than the kit lens.

Summary card

• Maximum aperture = the widest aperture; f/1.8 is ~3.3 stops (~10 times) brighter than f/5.6.
• Spend the gain on a short shutter speed or a low ISO.
• To melt the background, move the subject away from the background.
• At the widest opening the slice of sharpness is measured in centimeters; put focus on the eye.
• In dim light detail is kept on every skin; for two faces, stop down a step or two.

Bridge to the next lesson

If you look more carefully at the first photo you took at f/1.8 in the birthday frame, you may see that the corners are a little darker than the middle, that a thin purple fringe appears at the edge of the candle flame, and that with a wide angle the room's straight door frame bends slightly. Why are the edges of the photo curved and dark? The next lesson talks about these small flaws of lenses, where they come from, and how most of them are corrected with software today.