Safe speed and stabilization

Learning goal

You will be able to choose the shutter speed that prevents shake when shooting handheld, according to the lens you use, the sensor and your own hand. You will explain why the safe shutter speed rule depends on focal length; you will apply the rule with the equivalent focal length on cameras with small sensors; you will measure your own hand's limit with a simple test; you will turn the gain of image stabilization in stops into shutter speed; you will tell what stabilization solves and what it can't; and you will know that lengthening the time to brighten a face in dim light opens the door to subject movement on every skin.

The question

Late in the afternoon you are on a ferry pier. You have set a long zoom lens to 200 millimeters and are photographing the attendant holding the rope of a ferry coming in from the opposite shore. The camera has chosen 1/60. The wide frame you took with the same hands, in the same place, at 35 millimeters was sharp; this frame is shaky from start to finish.

The same hand, the same time. Which speed is safe when shooting handheld? And why does this speed change from lens to lens? In this lesson we will talk about where that old, rough rule comes from, how it is applied on today's cameras, how to measure your own limit, and how many steps stabilization, which shifts the sensor or the lens, gains you.

Context

In Lesson 8.4 we met an old measure of photographers: when shooting handheld, the shutter speed shouldn't be longer than one over the focal length. At 50 millimeters at least 1/50, at 200 millimeters at least 1/200. In Lesson 3.6, on the other hand, we talked about the grip, the breath and pressing the shutter button slowly. These two lessons gave both the cause of shake and its solution on the side of the body.

This lesson looks inside the rule. Why does the rule depend on focal length? On a camera with a small sensor, or with a very high-resolution sensor, is the same number enough? What does image stabilization add to this calculation? If instead of just memorizing the rule you know its logic, you can quickly find your own limit with every new lens and every new camera.

Simple explanation

Why does the rule depend on focal length?

When your hand shakes, the camera turns through a very small angle. This angle is independent of the lens; your hand is the same hand, the turn is the same turn. But how much room this small turn takes up in the image changes with the lens. A wide-angle lens fits a large part of the scene into the frame, and a small turn becomes a small shift within the frame. A long lens, on the other hand, fills the frame with a narrow slice of the scene, and the same turn becomes a large shift within that narrow slice.

A 200-millimeter lens magnifies the image four times compared with a 50-millimeter lens; it also magnifies your hand's shake four times. To keep the same sharpness, you need to bring the time down to a quarter. The safe shutter speed rule, often called the reciprocal rule, is a rough summary of this proportion: when the focal length doubles, the safe time halves. The rule is a starting point set in the film era by looking at small prints; it isn't a law, but a first guess.

How do the sensor and resolution change the rule?

The number in the rule was meant for a sensor the size of a 35 mm film frame. On cameras with smaller sensors, the same lens sees a narrower angle; the measure of this narrowing is called the crop factor. On common small-sensor cameras this factor is about 1.5 or 1.6. A 50-millimeter lens on such a camera gives the angle of a 75- or 80-millimeter lens; this is called the equivalent focal length. You apply the rule with this number: at 50 millimeters, not 1/50, but about 1/80.

Resolution enters the calculation too. On a very high-resolution sensor, when you magnify the photo and look, even shake too slight to be visible in a small print can be made out. That is why many photographers apply the rule leaving a margin of double: at 200 millimeters, 1/400 instead of 1/200. With a zoom lens, calculate the rule not with the shortest number written on the lens but with the focal length you are using at that moment; when a 70–200 zoom is opened to 200, the speed for 70 isn't enough.

How do you measure your own limit?

The rule gives everyone the same number, but hands aren't the same. Tiredness, cold, a heart beating faster after a run, a heavy lens change the limit. The way to measure your own limit is simple. Photograph a still object, for example a sign with writing on it, handheld and with your usual grip. At the same focal length, start from 1/250 and lengthen by one step each time, going down to 1/8, and take five frames at each time.

Then magnify the frames and count: for each time, how many of the five frames are sharp? The time where the number of sharp frames drops noticeably is your practical limit for that lens and that hand. The result is usually a probability, not a precise line: something like five out of five at 1/60, three out of five at 1/30, one out of five at 1/15. That is also why, for an important frame, pressing the shutter button several times in a row helps raise the chance of a sharp frame.

What does stabilization solve, and what can't it?

Image stabilization detects the camera's small turns and makes a counter-move against them. This counter-move can happen in two places: in optical image stabilization (OIS), a group of glass elements inside the lens shifts; in in-body image stabilization (IBIS), the sensor itself moves. Some systems use both together. Manufacturers give the gain in stops, and this is turned into shutter speed with the stop logic you learned in Lesson 8.4. A gain of two stops means doubling the safe time twice: where 1/100 is needed, you can shoot at 1/25. These numbers are best results obtained under measurement conditions; don't trust all of them without testing with your own hand.

Stabilization only balances the camera's movement. It doesn't touch the subject's movement: a camera stabilized at 1/25 draws the pier sharply, but still makes a walking person's legs flow. That is why lengthening the time to show a face brighter in a dim place is a trap. A photographer who wants to catch the detail of a dark-skinned face in the dark, or one trying to brighten a light-skinned face left in backlight at sunset, may lengthen the time and lean on stabilization; the face softens the moment it talks or laughs. When photographing people, choose the time according to the subject's movement, and gather the light with the aperture, with ISO, or with a light you add to the scene.

Visual evidence

You go back to the pier. The camera has a small sensor; the crop factor is 1.5, so 200 millimeters gives an angle of about 300 millimeters. By the rule, around 1/320 is needed at least; taking the high resolution into account as well, you go up to 1/500, and because the light is dropping you raise the ISO. The stitching on the glove of the attendant holding the rope and the weave of the rope are sharp.

Then you turn on stabilization and bring the time down to 1/60. The pier's iron bollard, the rust spots on it and the name of the ferry behind are sharp; stabilization has balanced your hand's shake. But just at that moment, the face and legs of a dark-skinned street vendor who walks into the frame carrying a tray of sesame bread rings have flowed. You bring the time back to 1/250 and raise the ISO one more step; in the next frame the vendor's smile and the sesame seeds on the tray are sharp. Stabilization had taken away the camera's shake; the vendor's walk, though, only a short time could stop.

Common mistakes

Summary

Your hand's shake turns the camera through a small angle, and a long lens magnifies this turn in the image; that is why the safe shutter speed rule ties the time to one over the focal length. On a small sensor the rule is applied with the equivalent focal length, at high resolution with a margin, and your own limit is measured with a simple test. The gain of image stabilization in stops lengthens the safe time, but doesn't stop the subject's movement; when photographing people, on every skin, instead of lengthening the time to brighten the face, you need to gather the light with the aperture, ISO or an added light.

Mini assignment

In this assignment you will measure your own hand's limit and test the gain of stabilization.

1. Photograph a still sign handheld, with stabilization off, taking five frames at each time from 1/250 to 1/8, and write down the number of sharp frames for each time.
2. Repeat the same test with stabilization on, and find by how many steps the time where sharp frames drop has lengthened.
3. If you have a zoom lens, do the same test at its shortest and longest focal lengths and compare the two limits.
4. In a dim place, photograph someone talking with stabilization on, first at a long time, then at a short one; compare the difference in the face.

Check yourself

• Did your own hand's limit come out shorter or longer than the number the rule gives?
• With stabilization on, by how many steps did your limit lengthen?
• Did the face of the person talking soften at the long time, even with stabilization on?

Glossary

Mini quiz

1. Why does the same hand need a shorter time at 200 millimeters than at 50 millimeters?

• A) Because a long lens gathers less light
• B) Because a long lens magnifies the image, and the hand's small turn, four times
• C) Because long lenses have no stabilization
• D) Because a long lens lowers the ISO

2. On a camera with a crop factor of 1.5, with a 50-millimeter lens, what time is needed at least by the rule?

• A) About 1/80
• B) 1/25
• C) 1/50
• D) 1/15

3. Where 1/100 is needed, what time does stabilization with a gain of two stops allow?

• A) 1/400
• B) 1/200
• C) 1/25
• D) 1/50

4. In a dim place, why is it a problem to lengthen the time to 1/15 and trust stabilization to brighten a dark-skinned speaker's face?

• A) Because stabilization doesn't work on dark skin
• B) Because the white balance goes wrong
• C) Because the aperture stops down by itself
• D) Stabilization balances the camera but doesn't stop the talking face's movement

Answers

1. B — The hand's turn is the same, but because a long lens fills the frame with a narrow slice, it magnifies that turn four times in the image.
2. A — The equivalent focal length is 50 × 1.5 = 75 millimeters; the nearest common time is 1/80.
3. C — Each stop doubles the time: from 1/100 to 1/50, from there to 1/25.
4. D — Stabilization only balances the camera's shake; to freeze the face, the time is chosen according to the subject's movement, and the light is gathered another way.

Summary card

• A long lens magnifies the hand's shake too: safe time ≈ 1 / focal length.
• On a small sensor multiply by the crop factor; at high resolution leave a margin.
• Measure your own limit: five frames at each time, how many are sharp?
• Stabilization: each stop doubles the time; it balances the camera, it doesn't freeze the subject.
• When photographing people, choose the time by the movement; on every skin, gather the light with the aperture, ISO or an added light.

Bridge to the next lesson

In this module we always put sharpness on one point: an eye, a face, a moving subject. But in a landscape, with a rock in front, a village in the middle and mountains behind, a single point isn't enough; you want everything from the front to the horizon to be sharp. Where should sharpness go so that the foreground and the mountains stay sharp at the same time? The next lesson talks about the hyperfocal distance, where focusing at the right distance makes the slice of sharpness stretch to infinity.