The focal plane is the flat surface inside your camera where the lens forms a sharp image, and it is where the sensor (or film) sits. It matters because it has a partner out in the scene, the plane of focus: a flat sheet, parallel to the sensor, on which everything is equally sharp. Once you see focus as a sheet rather than a point, a lot of soft pictures stop being a mystery.
Look at the chain above. It runs away from the camera at an angle, so it passes through that sheet in one place only. The links where it crosses are crisp, and the links in front and behind fall away into blur. Nothing was wrong with the focus. The plane was simply flat, and the chain was not lying in it.
Focal plane, plane of focus, depth of field: which is which
- Focal plane (image side, inside the camera). The plane where the lens brings light back together into a sharp image. When you focus, the lens moves so that the sharp image lands exactly on the sensor. Strictly, a lens has a different image plane for every subject distance; “the focal plane” in everyday use means the one the sensor occupies.
- Plane of focus (subject side, out in the scene). The flat sheet of the world that the lens is focused on. Every point on it is imaged as a point on the sensor. This is what you are choosing when you focus.
- Depth of field (subject side). The slab around the plane of focus that still looks acceptably sharp in the final picture. It is usually deeper behind the plane than in front of it. How thick it is depends on aperture, distance, focal length and how big the picture will be viewed, all covered on the depth of field page.
- Depth of focus (image side). The matching tolerance at the sensor: how far the sensor could be from the true image plane before things look soft. It is tiny, a fraction of a millimeter.
- Focal-plane shutter. Not a plane at all, but a shutter named after where it sits: two curtains just in front of the sensor. More on it below.
On a normal camera the focal plane and the plane of focus are both flat and parallel to each other, and the lens links one to the other.
Focus is a flat sheet, not a point
When you put an autofocus point on someone’s eye, it feels as if you are focusing on that eye. You are really choosing a distance. The camera sets the plane of focus at that distance, measured straight out from the sensor, and that plane extends across the whole frame. A leaf at the left edge of the picture, a sign at the right edge, the eye in the middle: if they are the same distance from the sensor plane, they are all equally sharp.
This is why a flat subject photographed square on can be sharp from corner to corner at a wide aperture, as the fence above is at f/2.8. Depth of field is thin at f/2.8, but the fence does not need any depth: all of it sits on the plane. Turn the camera so the same fence runs away at an angle, and only a vertical strip of it would be sharp, like the band of links on the chain.
Two consequences follow, and both surprise people:
- Distance is measured along the camera’s axis, not in a straight line from the lens. Something near the edge of the frame is farther from the camera in a straight line than something in the middle at the same depth, but it is just as sharp, because it lies on the same plane.
- The plane cuts through everything at that distance, not only your subject. The ground, a wall, a tabletop or a lawn will show a sharp band wherever the plane passes through them. That band is a useful clue when you check a photo: find it and you know exactly where the camera focused.
In the mink photo the camera focused on the animal’s face, but look at the rock. A narrow strip of granite at the same distance as the mink is just as sharp, and the rock in the foreground is soft even though it is closer and in plain view. At 200mm and f/2.8 the slab of sharpness is thin, so the plane is easy to see.
Focus on one figure in the row below, then change the aperture: the sharp zone grows and shrinks around the distance you chose, and figures nearer and farther blur by how far they sit from it.
Want to know how thick that slab is for your lens, aperture and distance? The depth of field calculator gives the near and far limits in front of and behind the plane.
Lining the plane up with your subject
Since you cannot bend the plane on a normal lens, the skill is arranging the camera and subject so the important parts fall on it, or near it.
Flat subjects: square up the camera
Paintings, documents, quilts, murals, the front of a building, a shop sign, a fence: if you want them sharp edge to edge at a wide aperture, make the sensor parallel to them. Center the camera on the subject and keep it level with no tilt up or down. On a tripod, a mirror held flat against the subject helps: when you can see the lens reflected in the middle of the mirror through the viewfinder, the camera is square. Even a few degrees off shows up at f/1.8 close in, as one side going soft.
Angled subjects: decide where the band goes, or deepen it
A row of bottles, a railing, a line of fence posts or a face turned three quarters away all cross the plane at an angle. You get a sharp band wherever they cross it, with soft areas either side. You have three choices: put the band on the part that matters (usually the nearest eye), stop down so the slab of depth of field is thick enough to hold the rest, or move so the subject is more square to you. Stopping down never turns the plane. It only makes the slab around it thicker.
Groups: a straight row is already on the plane
A common piece of advice says to arrange a group in a curve “so everyone is the same distance from the camera”. That treats focus as a sphere around the camera, and it is not one. A straight row of people standing square to the camera is already on a single plane, so if one face is sharp, all of them are.
The curve can even work against you. Put a group on an arc 3 meters from the camera, spread about 1 meter either side of center along the arc, so the people at the ends stand near the edges of the frame of a 50mm lens on a full-frame camera. Measured along the axis, those end people are only about 2.84 meters away, roughly 16 centimeters in front of the plane. At f/1.4 the near limit of depth of field at 3 meters is about 2.86 meters, so those end faces sit just outside it. At f/2.8 the near limit moves to about 2.73 meters and the curve no longer matters. A gentle curve is fine for posing, but sharpness does not need it. More on arranging people is on the group photography page.
Focus and recompose: the plane turns with the camera
Focusing with the center point, holding focus, then turning the camera to frame the shot is a long-standing habit, and the single-shot autofocus mode is built for it. The catch is that the plane of focus is fixed to the camera, not to the subject. When the camera turns, the plane turns with it and stays at the same distance along the new direction of the lens.
Here are the numbers. Focus on an eye 1 meter away, then turn the camera 15 degrees to place the eye near the edge of the frame. The plane is still 1 meter out along the new axis, but the eye is now only 1 m × cos 15° = 0.966 m along that axis. It sits 34 millimeters in front of the plane.
Whether that matters depends on how much depth of field there is in front of the plane. With an 85mm lens at f/1.4 on a full-frame camera at 1 meter, the whole depth of field is about 11 millimeters, and only about 5 of those lie in front of the plane. A 34 millimeter miss is six times too far. The table shows how far you can turn before the eye leaves the zone of sharpness, for a few common setups at 1 meter.
| Lens and aperture (full frame, subject at 1 m) | Total depth of field | Sharp zone in front of the plane | Largest turn before the eye goes soft |
|---|---|---|---|
| 85mm at f/1.4 | 10.6 mm | 5.3 mm | about 6° |
| 85mm at f/2.8 | 21.3 mm | 10.5 mm | about 8° |
| 50mm at f/1.8 | 41.1 mm | 20.1 mm | about 11.5° |
| 50mm at f/4 | 91.4 mm | 43.6 mm | about 17° |
| 35mm at f/2 | 94.7 mm | 45.1 mm | about 17° |
The amount the plane moves is the distance multiplied by one minus the cosine of the angle: at 1 meter, about 4 millimeters for a 5 degree turn, 15 for 10 degrees, 34 for 15 degrees and 60 for 20 degrees. It grows with distance, but depth of field grows faster, which is why focus and recompose is harmless for a full-length portrait at 5 meters and costly for a head-and-shoulders portrait at 1 meter with a fast lens.
The fix is to stop turning the camera after focusing. Move the active autofocus point onto the eye, or use eye detection if your camera has it, and compose first. When you do recompose, keep the turn small or stop down.
Still getting soft pictures after fixing the focus? How to take sharp photos works through camera shake, subject motion and the other causes in order.
The focal plane mark on your camera
Most interchangeable-lens cameras, and many fixed-lens ones, have a small symbol on the top plate: a circle with a line through it, often near the mode dial or the hot shoe. The line marks the position of the sensor, which is the focal plane. It is there because lens focus distances are measured from it.
- The distance scale on a lens and the minimum focus distance in its specifications are measured from the focal plane mark to the subject, not from the front of the lens.
- Working distance is the gap from the front of the lens to the subject, and it is always shorter. On a typical 100mm macro lens that focuses down to about 30 centimeters, the front of the lens ends up only about 15 centimeters from the subject at life size, because the camera body and the lens itself take up the rest. That is why a macro lens can still shade the subject or scare an insect. The macro photography page covers the practical side.
- Measuring focus by tape. In film and video, a camera assistant who sets focus by distance hooks a tape measure to a stud or mark at the focal plane and measures to the actor, so the number matches the lens scale. Still photographers do the same for repeatable studio and product photography setups, or when focusing by scale with manual focus.
Depth of focus: the tolerance at the sensor
Out in the scene, depth of field can be meters deep. Behind the lens, the matching tolerance is tiny. For subjects at ordinary distances, depth of focus is roughly 2 × N × c, where N is the f-number and c is the circle of confusion, about 0.03 mm for a full-frame sensor.
| Aperture | Approximate depth of focus (c = 0.03 mm) |
|---|---|
| f/1.4 | 0.08 mm |
| f/2.8 | 0.17 mm |
| f/5.6 | 0.34 mm |
| f/8 | 0.48 mm |
| f/16 | 0.96 mm |
At f/1.4 the sensor has to sit within about 0.04 mm either side of where the image forms. The whole band is thinner than a sheet of printer paper, and it explains three things photographers notice. Fast lenses are unforgiving of tiny focusing errors. A lens mount, adapter or sensor that is very slightly out of parallel can make one side of the frame softer than the other, most visibly at wide apertures. And a lens whose sharp image is slightly curved rather than flat (an aberration called field curvature) gives soft corners wide open that usually sharpen when you stop down, because the thicker depth of focus covers the curve. The tolerance grows at close range: multiply by (1 + magnification), so at life-size macro it doubles.
Tilting the plane, and why shift does not
On a normal lens, the lens and the sensor are parallel, so the plane of focus is parallel to both. Tilt the lens relative to the sensor and the plane of focus tilts too. The rule that describes it, the Scheimpflug principle, says that the sensor plane, the lens plane and the plane of focus all meet along one line. Tilt a little forward and you can lay the plane of focus along a field of flowers from your feet to the horizon at a moderate aperture, or tilt the other way to make only a thin strip of a city sharp, the “miniature” look.
This needs a tilt-shift lens or a view camera. Shift, the other movement those lenses offer, slides the lens sideways or up and down to change the framing without tilting the camera, which keeps the verticals of a building parallel. It does not change the plane of focus at all. The tilt-shift photography page shows both movements at work.
Need more depth than any aperture or tilt can give, as in a close-up of a whole flower? Focus stacking shoots the plane at several distances and merges the sharp parts.
The focal-plane shutter
The shutter in nearly every interchangeable-lens camera is a focal-plane shutter: two curtains that travel across the frame just in front of the sensor. The first curtain opens to start the exposure and the second follows to end it. At slow speeds the whole frame is uncovered at once. At fast speeds the second curtain starts before the first has finished, so the exposure is made through a moving slit. The mechanical shutter page explains the curtains in detail.
Two everyday limits come from this design. A flash burst is far shorter than the curtains’ travel, so it can only light the whole frame at speeds where both curtains are fully clear at the same moment. The fastest such speed is the flash sync speed. And because the slit travels, a very fast subject can be recorded slightly later at one side of the frame than the other. With modern mechanical curtains that lean is small. The strong skew people see in fast pans and propellers usually comes from the sensor reading out row by row with the electronic shutter, which is slower than the curtains on most cameras; rolling shutter covers it.
Common mistakes with the focal plane
- Thinking the focus point is the only sharp place. Everything at that distance from the sensor plane is sharp, across the whole frame. Use that: put two subjects at the same distance and both will be sharp at any aperture.
- Calling depth of field the focal plane. The focal plane is at the sensor. Depth of field is the slab around the plane of focus in the scene. Mixing them up makes the rest of the subject harder to follow.
- Shooting flat subjects at a slight angle. A painting or document shot a few degrees off square goes soft down one side at wide apertures. Square up, or stop down to f/8.
- Curving a group for sharpness. A straight row facing the camera is already on one plane. Curve the row for the look, not for focus, and stop down if the curve is deep.
- Focus and recompose close up with a fast lens. A 15 degree turn at 1 meter moves the plane 34 millimeters. Move the autofocus point instead.
- Measuring minimum focus distance from the front of the lens. It is measured from the focal plane mark, so you will be closer to the subject than the number suggests.
- Stopping down to fix a tilted subject, or expecting shift to change focus. Stopping down thickens the sharp slab without turning it. Only tilt turns the plane.
Try this: find the plane on your kitchen table
Ten minutes, any camera that lets you choose the focus point (most phones let you tap to focus).
- Line up five similar objects, such as mugs, cans or books standing on end, in a straight row on a table. Stand about 1 meter away. Use your widest aperture, or a phone’s main camera, and get as close as framing allows.
- Square on. Face the row so it runs straight across the frame. Focus on the middle object and take a picture.
- Angled. Turn the row about 45 degrees so it runs away from you. Focus on the middle object again and take a picture.
- Recompose. Go back to the square row. Put the center focus point on the left object, hold focus, and turn the camera until that object sits near the right edge of the frame. Take a picture.
- Move the point. Keep the same framing, but move the focus point (or tap) onto that left object without turning the camera. Take a picture.
Judge the results zoomed in to 100 percent. In the square-on shot, all five objects should be equally sharp. In the angled shot, the middle object is sharp and the others soften the farther they are from it, in both directions. Compare the last two: if the recomposed frame is softer on that object, you have seen the plane turn. For a bonus, lay a tape measure on the table pointing away from the camera, focus on one number, and read off how many centimeters stay sharp in front of and behind it.
Focal plane questions
Is the focal plane the same as the plane of focus?
No, though they come as a pair. The focal plane is inside the camera, where the sensor sits and the image forms. The plane of focus is out in the scene, the flat sheet the lens is focused on, and it moves every time you refocus.
Where exactly is the focal plane on my camera?
At the surface of the sensor. On most cameras it is marked on the top plate with a circle crossed by a line, and the line shows its position. Lens distance scales and minimum focus distances are measured from there.
Does a phone camera have a focal plane?
Yes. Its lens forms an image on a small sensor in exactly the same way, and it has a plane of focus out in the scene. The sensor is so small that depth of field is usually deep, so the plane is hard to notice until you shoot very close. The blur in portrait mode is added by software from a depth map, so its “plane” is an estimate rather than optics.
Why is the middle sharp but one side soft?
Most often the subject was not square to the camera, so one side was nearer the plane than the other. If a flat target shot square on still shows it, the cause may be field curvature (soft corners that improve when stopped down) or a lens element or mount slightly out of line (one side soft at every aperture). Depth of field settings rarely explain a one-sided softness.
Is focus and recompose ever safe?
Yes. The plane moves very little for small turns, and the error is easily covered at longer distances or smaller apertures. In the table above, a 50mm lens at f/4 at 1 meter tolerates a turn of about 17 degrees, and at 3 meters the same lens and aperture tolerates nearly 30.
Can I change the angle of the plane of focus without a special lens?
Not optically. A normal lens keeps the plane parallel to the sensor. You can imitate a tilted plane by blurring parts of a photo in editing, or extend sharpness with focus stacking, but a real tilt needs a tilt lens or a camera with movements.
Want to set sharpness from your feet to the horizon without tilting anything? Hyperfocal distance places the plane so the depth of field just reaches infinity.