How do I get a blurry background without an expensive lens?

Move your subject further from the background and get closer to your subject. Distance is the strongest factor and it costs nothing.

What f-number should I use for portraits?

On a camera with a real aperture, f/1.8 to f/2.8 gives strong background blur. The exact number matters less than the distance between subject and background.

Is portrait mode the same as a real shallow depth of field?

No. Most phone lenses have a fixed aperture, so portrait mode measures the scene and blurs the background in software. It is useful but it can misjudge edges like hair.

How do I blur the background in a photo?

That soft, creamy background behind a portrait has a name — shallow depth of field — and it is not an effect added afterwards. It is decided while you shoot.

What is actually happening

A lens can only focus at one distance at a time. Everything nearer or further than that distance is slightly out of focus. How quickly that blur builds up as you move away from the focus point is called depth of field.

Shallow depth of field means the blur builds fast: your subject is sharp and the wall behind them melts. Deep depth of field means it builds slowly: the subject and the mountain behind them are both sharp.

Three controls, in the order that matters

1. Distance between subject and background. The single biggest factor and it costs nothing. Move your subject two steps away from the wall and the wall blurs. This works on every camera and every phone.
2. Distance between you and the subject. Closer to the subject means shallower depth of field. This is why close-up portraits blur so easily.
3. Aperture. The opening inside the lens, written as f-numbers. Small number = big opening = more blur (f/1.8). Large number = small opening = less blur (f/16).

Most people reach for aperture first. Start with distance instead — it is free, it is stronger, and it works on the phone in your pocket.

The f-number is backwards on purpose

f/1.8 sounds small but means a wide opening. f/16 sounds big but means a narrow one. The f-number is a ratio, not a size, so it runs the other way. Everyone finds this confusing at first and then stops noticing it.

Wide open also means more light reaches the sensor, so aperture changes brightness and blur at the same time. That is why it sits next to shutter speed and ISO — change one and the other two have to answer.

What phones can and cannot do

Most phone lenses have a fixed aperture. It physically cannot open or close. So a phone produces background blur by measuring the scene and blurring it in software — this is what portrait mode is.

It is genuinely useful and it is genuinely a simulation. It gets edges wrong on hair, glasses and leaves, because it has to decide where the subject stops. Knowing that tells you when to trust it and when to move your subject away from the wall instead.

Lensmera shows you the depth-of-field band as you turn the aperture wheel, and it says plainly when your device is simulating rather than opening a real iris. The principle is the same one you will use on a real lens later.