Aperture & depth of field
What f-numbers mean, how depth of field really behaves, and worked examples on your two lenses.
- Read f-numbers and predict their effect on light and depth of field
- Explain the three things that set how much is in focus
- Use the calculator to plan focus before you shoot
Aperture is the opening in the lens, and it does two jobs at once: it sets how much light passes, and it sets how much of the scene is sharp. That second job — depth of field — is one of your most expressive controls.
Reading f-numbers
The f-number is a ratio, which is why it runs backwards from intuition: smaller numbers are bigger openings. f/2.8 is wide open and bright; f/16 is a small opening and dim. Each full stop — f/2.8 to f/4 to f/5.6 — halves the light. Your Tamron opens to f/2.8, your Sony to f/4; both close to f/22.
What sets depth of field
Three things decide how much is in focus:
- Aperture — wider (f/2.8) gives shallower focus; narrower (f/11) gives more.
- Focal length — longer lenses render shallower depth at the same aperture.
- Distance — the closer you focus, the thinner the sharp zone becomes.
Stack all three toward “shallow” — 70mm, f/2.8, up close — and focus becomes a sliver. Stack them the other way — 10mm, f/8, focused far — and nearly everything is sharp.
A worked example
A head-and-shoulders portrait at 70mm, f/2.8, two metres holds only about 9 cm of depth. That is enough for both eyes but not for the ears — which is exactly the look that lifts a face off its background. Now compare a landscape at 10mm, f/8: focus a metre or two in and everything from your feet to the horizon is sharp. Same camera, opposite intentions.
Dial the calculator to those two cases and watch the in-focus band collapse and expand. The scene view is drawn to scale, so you can see how little a fast portrait actually holds.
Diffraction: the far limit
Closing down keeps buying depth of field until, past about f/11 on this sensor, diffraction starts to soften the whole image. For maximum sharpness in a landscape, f/8 is usually the sweet spot; reach for f/11 only when you need the extra depth more than the last touch of bite.
In the field
Photograph a friend at 70mm and f/2.8, focusing on the near eye. Then, without moving, shoot the same frame at f/8. Compare how much of the face and background sharpens. You have just watched aperture rewrite depth of field.