Manual focus

Learning goal

You will recognize before the shot the scenes where autofocus falters, and be able to find sharpness by hand in these scenes. You will explain why autofocus gets confused in the dark, behind glass and at very close distances; you will use focus peaking and magnification together while turning the focus ring; in close-up shooting you will try setting sharpness not with the ring but by moving your body back and forth; and in dim light, especially on a face with little contrast, you will know how to confirm on the screen that sharpness has landed on the eye.

The question

On a rainy winter evening, you want to photograph from outside someone reading a book behind a café window covered in droplets. You half-press the shutter button; the lens goes back and forth, then starts again without stopping. Finally it settles somewhere, but sharpness is on a water droplet on the glass. The face inside is soft.

When does autofocus get it wrong? And when it gets it wrong, what do you have left? In this lesson we will talk about the scenes where autofocus struggles, the ways of finding sharpness by hand in these scenes, and using the two helpers on the screen together: peaking, which colors sharp edges, and magnification, which zooms into part of the image.

Context

The basic idea of Lesson 10.1 was this: autofocus doesn't look, it measures; and what it measures is the edges in the image. If there is nothing to measure, or the wrong thing is being measured, the system gets it wrong however advanced it is. In weak light edges fade, in front of a pane of glass reflections and smudges offer sharper edges than the real subject, and at a distance of a few centimeters the slice of sharpness is so thin that even a small twitch loses sharpness.

In Lesson 4.7 we saw an abbreviation in the phone's pro mode: MF, that is manual focus (MF). On cameras this option is usually turned on with a switch on the body or the lens, sometimes from the menu. In manual focus the camera doesn't search for sharpness; you turn the ring on the lens and decide where sharpness falls. This lesson is about making that decision quickly and accurately.

Simple explanation

Where does autofocus falter?

The scenes where autofocus struggles come from the same root: there aren't enough edges, or the right edges, for the measurement. In a dim room, on a street at night or in the back rows of a concert hall, there is little light and the difference at edges is hard to read; the lens searches for sharpness, can't find it, and comes back. A blank white wall, a cloudless sky or a plain fabric give the same result, because there is no line to measure.

Glass sets another trap. A shop window, a bus window or an aquarium's glass: the smudges, droplets and reflections on it are often more contrasty than the subject behind, and the camera takes sharpness there. Wire mesh, branches and cage bars push to the front in a similar way. At very close distance, on the other hand, the problem is different: the pistil inside a flower or an insect's eye sits in such a narrow slice of sharpness that even if autofocus finds a spot, it is often not the spot you wanted.

How do you turn the ring?

In manual focus, the wide ring on the lens changes the focus distance. When you turn the focus ring one way, sharpness goes near; when you turn it the other way, it goes far. Many old or manual lenses have a distance scale in meters and feet on the ring; the lenses of mirrorless cameras, on the other hand, often read the ring electronically, and instead of a scale show a distance bar on the screen, or show nothing at all.

One way to turn accurately is to pass sharpness a little and come back. Turn the ring a little beyond the point where you think the face has become fully sharp, see the image soften again, then come back slowly. The moment of highest sharpness lies between two softenings, and this going and coming back finds it more easily than looking for it by approaching from one direction only. Some cameras also offer a middle way, often called direct manual focus (DMF): autofocus first finds sharpness, and while you keep the shutter button half-pressed, you make the last fine adjustment with the ring.

What are peaking and magnification for?

On a small screen or in the viewfinder, it is hard to tell by eye exactly where sharpness falls. Focus peaking puts a bright color such as red, yellow or white over the sharp edges in the image. As you turn the ring, the colored lines slide back and forth in the image; when the color gathers on the subject's eye and eyelashes, sharpness is there. Peaking is fast, shows the whole frame at once, and helps even with a moving subject.

But peaking works by a threshold: it paints every edge that looks sharp enough. With a narrow aperture or on a small screen the painted area can be wide, and the exact point of sharpness stays somewhere inside this area. That is why focus magnification is the second step. The camera enlarges a small part of the image several times; while turning the ring, you see the eyelashes of the eye, the veins of a leaf or the letters of some writing at close to pixel level. Approach first with peaking, then make it certain with magnification; together, the two are much more reliable than guessing by eye.

How do you confirm sharpness up close, in the dark and on every skin?

In very close shooting, turning the ring is sometimes too coarse. Many macro photographers choose another way: they set the ring to an approximate distance, then bring sharpness to the point they want on the subject by moving their body a few millimeters back and forth. The slice of sharpness here is so thin that this small body movement is a finer adjustment than the ring can make.

In the dark, the edges peaking will paint shrink too. In a dim room, especially on a dark-skinned face or a face in shadow, there may be very few lines on the skin for peaking to catch. Here magnification is more reliable, and the place to look is clear: the small catchlight in the eye, the eyelash line or the edge of the eyebrow. These details are the sharpest lines of the face on every skin. If they still can't be made out, hold a lamp or a phone light to the face and find sharpness, then turn the light off and shoot; as long as sharpness stays on the ring, the light isn't needed.

Visual evidence

You go back to the rain-spotted café window. You bring the front of the lens close enough to almost touch the glass's surface, so that the reflections disappear, and switch focus to manual. Inside, next to a yellow lamp, a dark-skinned woman is reading a book; half her face is in shadow. When you turn on peaking, the red lines first shine around the droplets on the glass. As you slowly turn the ring toward far, the color on the droplets fades, appears on the edges of the book's pages, then gathers on the frame of the woman's glasses.

You magnify the image. Behind the glasses, the small bright point the lamp makes in her eye and the fine line of her eyelashes can be made out; by passing the ring a little and bringing it back, you find the sharpest point. In the frame you take, the droplets on the glass are soft specks of light in the foreground, and the woman's eye and her fingertips resting on the page are sharp. The camera was the same; this time it was the ring and your eye that decided where sharpness would fall.

Common mistakes

Summary

Autofocus gets it wrong when it can't find enough edges to measure, or measures the wrong edge; this most often happens in the dark, behind glass, wire or branches, and at very close distances. In manual focus you find sharpness by turning the ring a little past sharpness and bringing it back, by approaching with focus peaking and making it certain with magnification, and in close-ups by moving your body back and forth. In dim light and on a face with little contrast, looking at the catchlight in the eye and the eyelash line is the way to confirm, on every skin, that sharpness has landed in the right place.

Mini assignment

In this assignment you will photograph three scenes where autofocus falters, with manual focus.

1. Photograph an object behind a shop window or window pane, first with autofocus, then with manual focus after bringing the lens close to the glass.
2. In a dim room, photograph a face by focusing with peaking and then with magnification; look at whether the catchlight in the eye is sharp.
3. Focus on a flower or a small object very close up, leaving the ring fixed and moving your body back and forth.
4. Focus on the same face twice, once passing sharpness a little with the ring and coming back, and once approaching from one direction only; compare which is sharper.

Check yourself

• On the object behind the glass, did sharpness fall in a more correct place than in the frame you took with autofocus?
• Inside the area painted by peaking, were you able to find a sharper point with magnification?
• In dim light, on a face with little contrast, is the catchlight in the eye or the eyelash line sharp?

Glossary

Mini quiz

1. Why can autofocus focus on a smudge on a shop window instead of the subject behind it?

• A) Because the glass blocks the light completely
• B) Because the ISO is too low
• C) Because the smudge or reflection offers a more contrasty edge than the subject behind
• D) Because the white balance is wrong

2. What does focus peaking do?

• A) It puts a bright color over the sharp edges in the image
• B) It sets the exposure automatically
• C) It stops down the aperture
• D) It sharpens the photo afterward

3. When focusing on an insect's eye very close up, which way is often used for a fine adjustment?

• A) Raising the ISO
• B) Setting the ring approximately and moving the body a few millimeters back and forth
• C) Switching to wide-area focus
• D) Lengthening the shutter speed

4. In a dim room, if peaking shows very few lines on a dark-skinned face half in shadow, what do you do?

• A) Change the peaking color and shoot
• B) Set sharpness by guessing and shoot
• C) Go back to autofocus and press repeatedly
• D) Magnify and make sharpness certain by looking at the catchlight in the eye and the eyelash line

Answers

1. C — Autofocus measures the most distinct edge; a smudge or reflection on the glass is often more distinct than the face behind.
2. A — Peaking shows where sharpness falls by coloring the sharp edges that pass its threshold.
3. B — At close distance the slice of sharpness is very thin; moving the body gives a finer adjustment than the ring.
4. D — On skin with little contrast there are few edges for peaking to paint; the eye's catchlight and the eyelashes are the sharpest lines on every skin.

Summary card

• Autofocus measures edges; if there are none, or the wrong edge is in front, it gets it wrong.
• Darkness, glass, wire, branches and very close distances: the scenes for manual focus.
• Turn the ring a little past sharpness, then come back.
• Approach with peaking, make it certain with magnification.
• In dim light and on every skin, look at the catchlight in the eye and the eyelash line.

Bridge to the next lesson

In manual focus, finding sharpness and pressing the shutter button became two separate jobs: the ring in the left hand, the shutter button in the right. In autofocus, on the other hand, half-pressing the shutter button tied both to a single finger. So is it possible to separate these two jobs without giving up autofocus? The next lesson talks about back-button focus, which moves focus to a button under the thumb and leaves the shot to the index finger, and about the scenes where this separation is useful.