Depending on the aperture used, you can either have everything in focus, or just the main focal point with a blurred background. Blurring the background is particularly useful for portraits, flowers, birds and animals etc., while everything in focus suits landscapes, seascapes and astrophotography.
How does this work?
Aperture in photography is measured in F stops. The f-stop is a ratio of the lens focal length to the diameter of the pupil. It’s why the aperture is written as f/N where N is the diameter of the pupil. As an inverse number, the larger the N, the smaller the aperture actually is. The smaller the F stop, the shallower your depth of field will be. Larger F stops gives you a greater depth of field. The confusion, or apparent contradiction, comes with small F stop is big opening, while large F stop is small opening.
A light bulb moment for me, came when I was reading my favourite photography magazine. The photographer in the article (sorry I have forgotten his name) said that while this did confuse a lot of people there was a simple way of making sense of it. “Small number F stop = small/shallow depth of field. Big number F stop = Big/deep depth of field“. No longer was I wrestling with small number, big depth of field, big number big depth of field. Suddenly it all made sense!
So What Is Depth of Field?
Depth of Field is how sharp or blurred the background is in your images. For landscapes you would want F11 or above, focus on the middle distance to have everything in focus. For flowers, portraits etc., you would want F7 or below, and you focus on the subject. This isolates your subject and giving you the blurred, shallow depth of field.
The image below was taken at 5.6, but because he was a distance from the background, it still blurred it as much as the F4 bird that was much closer to the tree branch behind him. Ideally you put some distance between the subject and the background (don’t pose people against a tree or fence.. put them at least 2 metres from it).
Letting in the Light
Another reason for low F stops is to allow more light in. The aperture, is controlling the amount of light entering the sensor, so a lower F stop means you can capture images in a dark situation, with a faster shutter speed. A lower F stop will let more light in, and with less noise, than a high ISO. However the trade off is a shallower depth of field. If you want deep depth of field and light.. then you really need a flash… and thats an whole other topic!
Next time you are out shooting, try changing the aperture setting and shoot at f/3.5, f/8 and f/16 to see the difference…..
with thanks to Pio Marcelaine from PioPics for extra images