Aperture and depth of field
Open up one stop and you get twice the light and a thinner slice of focus. The only real hurdle is that f-numbers run backwards, so start there.
1. Why bigger f-numbers are darker
An f-number is focal length divided by the diameter of the entrance pupil. The opening sits on the bottom of the fraction, so f/2 is a bigger hole than f/8 and lets in more light.
Full stops go f/1.4, 2, 2.8, 4, 5.6, 8, 11, 16, 22, and each step to the right halves the light. Every other number doubles because light follows the area of the hole, and area follows the square of the diameter, so a 1.4x change in diameter halves the light. Remember the 1.4x and you can rebuild the rest.
Trading aperture against shutter speed and ISO is covered in the exposure triangle. This piece is about depth.
2. Three variables set depth of field
The fast lens gets the credit for blur, but it is one of three controls, and distance outweighs it.
| Variable | Direction | Weight |
|---|---|---|
| Subject distance | Closer means shallower | Largest. Halve the distance and depth of field drops to roughly a quarter |
| Aperture | Smaller f-number, shallower | Large. At a fixed distance and focal length, depth scales roughly with the f-number |
| Focal length | Longer is shallower | Large, but tangled up with distance |
Sensor format and final print size count too. The same framing on a smaller sensor gives deeper focus; multiply the f-number by the crop factor, so f/2 on APS-C behaves like f/3 on full frame. A file that looks sharp on a phone can look soft a metre wide.
3. Hyperfocal: one focus for a landscape
Focus at the hyperfocal distance and everything from half that distance to infinity is acceptably sharp, rocks at your feet and far ridge alike. The number shifts with focal length, aperture and format. In the field:
- Punch format, focal length and aperture into a depth-of-field calculator on your phone.
- No app? Focus on the nearest thing that has to be sharp, then a touch beyond it, stop down to f/8–f/11, and zoom into playback to check the foreground.
- Don't just focus at infinity. The near ground goes soft every time, and the near ground is where a landscape gets its depth.
"Focus a third of the way in" is roughly right at wide focal lengths with nothing very close to the lens, but it's a rule of thumb. When the shot matters, check at 100% on playback.
4. Diffraction: why f/22 looks soft
Stop all the way down and the whole frame goes soft. Light passing through a small opening diffracts, spreading each point into a tiny disc, and the smaller the opening, the bigger the disc.
- Smaller sensors and denser pixels show it sooner. On full frame it usually becomes visible past f/11 and is obvious by f/16 and f/22.
- It creeps in gradually. Stopping down to f/16 to get the depth you need is usually worth it: a frame that's a little soft everywhere beats a foreground that's plainly out of focus.
- Most lenses peak about two stops down from wide open. To find out for your own lens, shoot a brick wall or a bookshelf at every stop and compare.
5. Rough starting points by subject
| Subject | Usual range | Why |
|---|---|---|
| Single half-length portrait | f/1.8–f/2.8 | Shallow, but both eyes still land in the sharp zone |
| Two or three people | f/4–f/5.6 | They are never on one plane; the depth has to cover all of them |
| Group in rows | f/8 | More rows need more depth; curve the rows slightly too |
| Landscape | f/8–f/11 | Enough depth before diffraction starts to bite |
| Night sky | Wide open, or one stop down | Light is so scarce that depth hardly matters |
| Macro | f/8–f/16 and still paper thin | At those distances depth is measured in millimetres; more means focus stacking |
Treat these as places to start. Decide first what has to be sharp, work out how much depth that takes, and only then pick the f-number.
6. FAQ
Is a lens sharpest wide open?
Usually not. Most lenses have softer corners and more aberrations wide open and peak about two stops down. If the shot needs the light or the separation, shoot wide open anyway. Sharpness is one trade among several.
I opened up and the background is still busy. Why?
Almost always distance. You are too far from the subject, or the subject is too close to the background. Depth of field responds to distance more than to aperture, so get closer and pull the subject away from the background before you reach for the aperture.
Is an f/1.2 lens worth it?
Over f/1.8 it buys you a little more than one stop, often for double the price and weight. Wide open, depth can be so thin that one eye is sharp and the other isn't. Worth it if you want exactly that look. If you just want a cleaner background, moving your feet does more.
Is phone portrait mode the same as a fast lens?
No. The phone estimates depth and blurs the background in software, so hair, the arms of glasses and railings give it away, and the falloff is harsher than optical blur. At phone-screen size it looks fine; enlarge it and the difference shows.