There is a very good wikipedia article that goes into great detail regarding optical apertures, how it works, and other info. In this blog post we will narrow our focus just to how aperture affects depth of field.
One of the photographer’s most important creative tools
Knowing how aperture affects depth of field, you can therefore use that information so you create the photo look that you want.
Depth of field is the distance between the nearest and the farthest objects that are in acceptably sharp focus as an image is projected through the lens onto the photo sensor.
In the images below, I will show you how the depth of field changes as you increase the F-stop value. The lens I am using goes from F/2 to F/22.a
Now for the images
In this first image, I am in Aperture Priority mode. My focus center point is directly in the middle of the orange. I have set the ISO value to 400 and will leave it there throughout the series of images to illustrate the effect aperture also has on shutter speed. More later on why I did this.
To start out, I am using an aperture F-stop value of F/2.
At aperture F/2, camera automatically selects 1/90th of a sec shutter speed. Because of the shallow depth of field, only the orange is in focus.
At aperture F/4, camera automatically selects 1/20th of a sec shutter speed. This is a fair amount slower to get in more light, needed as a result of closing the aperture down. Notice how the depth of field has grown.
At aperture F/5.6, camera automatically selects 1/10th of a sec shutter speed. This is half the speed af the above photo. Notice how the depth of field has grown even more.
At aperture F/8, camera automatically selects 1/6th of a sec shutter speed. Again almost half the speed of the above photo. Notice how the depth of field has grown even more. Now the middle objects are pretty sharp.
At aperture F/11, camera automatically selects 1/3rd of a sec shutter speed. Again half the speed of the above photo. Most of the objects are now in focus.
What is the relationship between aperture and depth of field?
In the above series, I could keep going up to F/22 and eventually have all the objects and the background completely in focus. I actually did shoot an image at F/22 and the camera automatically chose 2 full seconds as the shutter speed.
I locked the ISO speed so you could see the effect aperture has on shutter speed. Those 2 settings are very inter-related and this relationship in important to keep in mind.
In most scenario’s you won’t be setting the ISO at 400 and leaving it there, so you wont see shutter speeds that slow.
If you do want to set the ISO at a fixed number, be sure to use a tripod to reduce camera shake and blur as the shutter speeds get slower.
Wrapping it up
I hope this helps your understanding of how aperture affects depth of field!
Happy photo hunting!