At first it might seem confusing how depth of field changes when using your camera but of course it’s a logical phenomenon that you can control.
What is depth of field?
If you’re new you may be wondering what “depth of field” means. It is essentially the part of the picture that is in focus. When your camera focuses on something it moves the depth of field until the subject is in focus.
In the image below the butterfly is in focus and everything else is not, this is an example of narrow depth of field.
How do we control depth of field?
Depth of field can be made deeper or shallower in two ways, by changing the aperture of your lens, or by changing the field of view of the lens.
A wider aperture has a thinner or shallower depth of field and conversely a smaller aperture has a deeper depth of field. So if you want more of your image in focus you should use a smaller aperture.
The other way to change the depth of field is to change the field of view of the lens. This can be done in two different ways depending on the whether you’re using a zoom lens or not. The rule is a smaller field of view has a smaller depth of field. For example, when you use a zoom lens, if you stand in the same place and zoom all the way in you will reduce the depth of field and increase the background blur (assuming the aperture stays the same on your zoom lens). Conversely, if you zoom out while standing in the same place you will increase the field of view and increase the depth of field as well which will lower the background blur.
Framing is key to depth of field
If you are using a zoom lens, and let’s say you zoom in, but you also move back to achieve the same framing you had when zoomed out, you will have the same depth of field. This is because depth of field is defined by the frame of view of the lens not the focal length!
If you don’t have a zoom lens you can zoom with your feet. Field of view for a lens is defined at the plane of focus. So if you focus on something closer to you it effectively narrows the field of view and reduces the depth of field therefore increasing the background blur. Of course the opposite is also true, if you focus on something far away the depth of field will be increased.
There are some other aspects to depth of field and background blur that you may want to learn but for now just remember, you can control depth of field with the aperture and the field of view of the lens.
Good luck and happy shooting!