How Do You Get Everything Sharp? Focus Short of Infinity
By the BrainSnail editorial team. How these articles are written and checked, and how to tell us when one is wrong.
Focusing a lens at a particular distance makes everything from half that distance to the horizon acceptably sharp. Focusing on the horizon itself wastes half of that range.
Why anything other than the focused plane is sharp at all
A lens focuses exactly one distance onto the sensor, and everything nearer or further is technically out of focus, projecting each point as a small disc rather than a point. What makes photography possible is that a disc small enough is indistinguishable from a point at normal viewing size, so a range of distances reads as sharp even though only one is truly focused. That range is the depth of field, and it is not a property of the lens alone but depends on how large the picture will be viewed and from how far away, which is why the whole calculation rests on a judgement about the viewer.
What the setting achieves
Focusing at the right distance exploits an asymmetry in how the range extends:
- •The sharp range extends further behind the focused plane than in front
- •Focusing on the horizon puts everything beyond it to waste
- •Focusing nearer brings the far limit out to the horizon anyway
- •The near limit then falls at half the focused distance
- •That is the largest total range the aperture can produce
- •Focusing any nearer loses the horizon entirely
What it depends on
The distance is not a fixed number and changes with three things, which is why published tables are always specific to a combination. A wider aperture shortens it, so stopping down extends the sharp range and moves the setting closer. A longer focal length lengthens it substantially, which is why a wide-angle lens can hold everything sharp from a metre or two while a telephoto cannot. And a larger sensor or film format lengthens it for the same angle of view, which is why large format photographers work at very small apertures. Diffraction sets a limit on stopping down, since beyond a point the whole image softens.
The judgement hidden in the calculation
Every published table and every marking on a lens barrel rests on an assumption that is rarely stated, which is how large a blur disc counts as sharp. That figure is chosen by convention, traditionally as a fraction of the frame diagonal corresponding to what an average eye resolves in a print of moderate size viewed at arm's length. Enlarging the image further, or viewing it closer, or examining it on a high-resolution screen at full size, all make the same disc visible, at which point the depth of field the table promised is not there. That is why photographers who once trusted lens markings find the same settings unsatisfactory on modern sensors.
Where it is actually used
The technique belongs to situations where there is no time to focus or no way to. Street photographers set the lens in advance and shoot without focusing at all, which is why manual lenses carry depth of field scales on the barrel showing the near and far limits for each aperture, and the practice of setting those marks is old and still taught. Landscape photographers use it to hold foreground detail and distant hills together in one frame. Fixed surveillance and scientific cameras are set this way and left. Modern autofocus has not removed the need, since a camera focusing on a distant subject still discards near sharpness that a different setting would have kept.
The takeaway
Only one distance is truly focused, and a range reads as sharp because small enough blur discs are indistinguishable from points at normal viewing size, which makes the whole calculation depend on how the picture will be viewed. The sharp range extends further behind the focused plane than in front, so focusing short of the horizon still reaches it while adding near sharpness down to half that distance.