I want to know about the relationship between focal length and aperture [duplicate]

Question

This question already has an answer here:

• Why does my aperture setting change as I zoom on my DSLR kit lens? 2 answers
• How does changing focal length affect depth of field? 5 answers

I have Nikon D7000 and I know the basic relation of shutter speed, aperture, and iso.

All I want to know is focal length and aperture relation.

As I zoom in the camera the f/stop increases (e.g. it starts 3.5 and then to 5.6)

Some one told me that having large aperture gives more blurry background. (ie: f3.5 will give me more blur than f5.6, if I am not wrong)

However, when I zoom my lens to 140mm minimum f value available becomes 5.6 and it gives me more blurry background than f3.5 at 18mm

Why is this?

Answer 1

You are correct in that a lower/wider aperture produces greater background blur. However, there are 3 things that help introduce more background blur - 1) the aperture setting, 2) subj-to-background distance, and 3) lens focal length. So, to get the most beneficial setting for background as possible, you would choose your lowest possible aperture, get the subject as far away from the background as possible, and shoot at the longest focal length possible.

In your scenario, you are working with a variable aperture zoom lens where the max aperture changes as you zoom in. Although your aperture does go from a wider to narrower aperture, you are also going from 18mm (ultrawide angle) focal length to 140mm (telephoto). Looks like the settings at 140mm/f5.6 produce more background blur than your 18mm/f3.5 setting. The signficantly increased focal length is producing more background blur than the 18mm setting.