Getting background blur out of a kit lens

The measured truth

Blur is measurable as the diameter of a background point rendered on the sensor. Roughly 0.05 mm is where blur becomes noticeable, 0.1 mm reads as clearly separated, and 0.2 mm and up is creamy. Here is what your lens can actually do at 45 mm and f/5.6:

Subject at	Background 3 m	5 m	10 m	Infinity
1.0 m	0.252 mm	0.303 mm	0.341 mm	0.379 mm
2.0 m	0.062 mm	0.111 mm	0.148 mm	0.185 mm
3.0 m	—	0.049 mm	0.086 mm	0.122 mm

Three conclusions follow, and they are worth more than any lens purchase:

1. Subject distance dominates. 3 m to 1 m multiplies blur roughly fivefold against the same background.
2. Background distance has diminishing returns. Pushing a background from 2 m to 5 m gains about 60 percent; from 5 m to infinity only about 25 percent. Get it past five metres, then stop worrying.
3. Stop trying to blur at 15 mm. Use the wide end for context and drama instead.

The techniques that make this lens look expensive

1. Get close, and put the background far away. If you take one thing from this whole module, take this.
2. 45 mm, f/5.6, subject at 1 to 1.5 m. The lens’s best-looking configuration, even though it is its worst on a test chart. You are photographing a face; the corners are defocused by design.
3. Choose smooth backgrounds, avoid sparkly ones. A shaded wall, distant foliage or open sky renders beautifully. Fairy lights and dappled sun through leaves expose this lens’s harshest trait.
4. Backlight everything. Sun or window behind the subject gives rim light and separation without aperture. Pay for it with a 52 mm hood and a hand shading the front element.
5. Live at the ends of the zoom. 15 mm is optically superb and dramatically wide; 45 mm is your only real separation. The middle is optically fine but visually the most ordinary — it looks like what every phone already produces.
6. Get low, or get high. A wide lens at adult chest height produces the most generic image possible.
7. Fill the wide frame with foreground. At 15 mm, empty foreground is the commonest failure. Put something 30 to 50 cm from the lens — a hand, a railing, a table edge. It focuses to 13 cm, so you have enormous freedom.
8. Wait for the light. This lens renders good light exactly as well as an expensive one does.

Zoom, or move your feet?

Zooming changes how much you see. Moving changes what things look like — the relative size of near and far, and how the background sits behind the subject. They are not interchangeable, and treating them as if they were is the commonest reason beginner photographs look flat.

Move your feet when	Zoom when
You want background blur — the only powerful lever you have	You physically cannot move: across a canal, behind a barrier, on a train
The background is wrong. Two steps left removes a lamppost from behind a head; no focal length will	Moving would break the moment — street, or a child concentrating. This is the kit zoom’s real superpower for family photographs
You want depth: walking closer to a foreground rock at 15 mm makes it huge and the mountain distant	You have already chosen the right distance and just need to trim the frame
You are photographing a face. Distance sets proportions; zoom only sets crop	You need the light at the wide end — 1 1/3 stops for stepping closer
You want the photograph to feel intimate. Viewers can tell whether you were in the scene	Video, where the power zoom does smooth in-shot moves no ring can match

Assignment: same subject, two ways

Photograph one person so their head fills the same fraction of the frame twice: once at 15 mm from about 0.7 m, once at 45 mm from about 2 m. Compare them. The wide version has a stretched face, an enormous background and no separation. The long version has natural proportions, a background a fraction as wide, and visible blur. Nothing changed but distance and focal length, and the two photographs are unrecognisably different.