45 mm is where your portraits come from — and it is also the lens’s optically weakest setting. That tension is real: f/5.6 gives you the only meaningful background blur this lens can produce, while f/8 gives you its best sharpness. Knowing which you want is the skill.
69 mm equiv. f/5.6 wide open f/8 sharpest MFD 35 cm
Where sources disagree, honestly
One lab test calls 45 mm wide open “hardly useful” and wants f/8 to f/11. Another review rates central sharpness at f/5.6 as very good. A forum consensus lands in between: visibly soft past about 30 mm, but by f/5.6 sharpness across the field is remarkably uniform. Some of this is sample variation — kit lenses vary more than premium ones — and some is criteria: one is measuring against a lab threshold, the other is looking at photographs.
The defensible position
15-28 mm is genuinely excellent and can be shot wide open with confidence. 33-45 mm is the soft end, and stopping down to f/8 visibly helps. Verify it on your own copy — a ten-minute test that also teaches you what stopping down does. Then decide per photograph: f/5.6 when separation matters more, f/8 when detail does.
Three portrait recipes, by distance
| Recipe | Setup | Frame and depth of field | Watch out |
|---|---|---|---|
| A — best blur this lens can give | 45 mm, f/5.6, subject at 1.0-1.5 m, background 5 m or more behind | Head and shoulders. Depth of field about 10.6 cm at 1 m, 24 cm at 1.5 m | Focus the near eye and use eye detection — 10 cm of depth punishes a sway |
| B — moving subjects | 45 mm, f/5.6, subject at 2 m, background as far as possible | Upper body, or a small child full length. Depth of field about 44 cm | Enough tolerance for a child who will not hold still |
| C — the fake telephoto | 45 mm, f/5.6, subject at 1 m, you positioned so the background is across a street or open sky | The most separation the lens will ever produce | The trick is geometry, not glass |
Assignment: the same face, two apertures
At 45 mm and 1.2 m, photograph a friend at f/5.6 and at f/8, with a background at least five metres behind them. Compare at 100 percent: f/8 has more detail in the eyelashes, f/5.6 has a softer background. Decide which you prefer for that photograph. There is no permanent answer, and forming an opinion is the point.