Depth of field is one of those technical photography terms that can be tricky to understand. Plus it sounds like it involves math. So photographers generally over-simplify and think of depth of field as being controlled by aperture when in fact aperture is only one of the three components that impacts your depth of field.

Don't worry - this tutorial isn't going to get too technical. And we're not going to do any math. I want to try to keep the discussion very practical and provide pretty photos along the way :).

Like this one -
Depth of field, very simply, is the distance in your photo - both in front of and behind your subject - that is in focus. I love to use a small depth of field to isolate my subject from the foreground and background. The subject pops out of the image and your eye goes straight to it.
The three factors that determine your depth of field are:

1. Aperture
2. Camera to subject distance
3. Focal length

Most photographers know that they can impact their depth of field by changing their aperture. Buying expensive lenses with low aperture capabilities allows you to create images with a smaller depth of field. For this reason and many others I LOVE my 50mm 1.2 and 24mm 1.4. They allow me to get a super-shallow depth of field. You won't be able to do so, unfortunately, with inexpensive kit lenses.

Oftentimes photographers are scared to shoot their expensive lenses wide open (with a small aperture) because they're afraid that they will end up with lots of out-of-focus shots. But if you keep in mind that depth of field also depends on your camera to subject distance and your focal length, you can shoot wide open with more confidence.

For instance, this photo was taken with my 50mm 1.2 at 1.2:
I knew I could shoot with confidence at 1.2 because the subject was so far away from me. You see, the greater the camera to subject distance, the greater your depth of field.

On the reverse side, the smaller the camera to subject distance, the more shallow the depth of field. So when you are shooting portraits up close like this one -
...or detail shots like this one:
...and you use a small aperture (mine was 1.2 in both the above shots) then you have to be super-careful about picking your focus point and being accurate with focus. You can see that in both of the above shots, the depth of field is no greater than 1 inch.

Oftentimes I like to challenge myself and shoot processionals and recessionals at an aperture of 1.2. It generally works out well because I'm not right on top of the subject (the camera to subject distance allows for a decent depth of field), but I also make sure to put the focus point on the subject's torso (a larger area than their head but also on the same plane as it) and track the motion using back button focus (click here for a fuller explanation of this technique). This photo was shot with my 50mm at 1.2 and you can see where I put my focus point:
Focal length is the third variable that impacts your depth of field. The longer the focal length, the shorter your depth of field and the shorter your focal length, the greater your depth of field.

Another one of my favorite lenses is the 70-200 2.8 IS. The long focal length allows me to achieve a shallow depth of field and isolate the subject in the frame. I use it occasionally for portraits of the couple and to get in close during the ceremony or first dance. When using this lens, I don't ever take it off of f2.8. The long focal length does isolate the subject in a shallow depth of field but when using it I am forced to be a good distance from my subject. So even at 2.8, my depth of field isn't super-shallow.

This image was shot with my 70-200mm at 2.8:
If you are shooting with a wide angle lens - the one I use is the 24mm 1.4 - your depth of field is going to be greater (given the aperture and camera to subject distance are the same) when compared to shooting with a lens with a longer focal length. So if you want to isolate your subject in a shallow depth of field, it pays to invest in a lens with lower aperture capabilities. I used to shoot with a 16-35mm 2.8 and while it was a good lens, it was difficult to get a very shallow depth of field due to the focal length and 2.8 aperture.

This image was shot with my 24mm at f1.4. The short focal length was off-set by the fact that I had a small aperture and the camera to subject distance was also small. So I was able to achieve a shallow depth of field and had to be accurate with my focus.
I love using my 24mm 1.4 during receptions. Because I shoot a good deal over my head at receptions (without looking through the viewfinder) using my low-light focus trick, I make sure to give myself plenty of depth of field to work with by setting my aperture between 4.5 - 5.0. The combination of a short focal length, a decent camera to subject distance (4-5 feet) and a higher aperture gives me plenty of depth of field to make sure my subject is in focus even though I'm not looking through the viewfinder. Here's a couple examples of the result:
Ha! In this next one you can even see a reflection of me shooting over my head in the mirror on the left - never noticed that before!
I hope you found this tutorial helpful in understanding how the three variables of aperture, camera to subject distance and focal length work together to determine your depth of field! Let me know if you have any questions! And click here to learn about more resources I offer photographers!
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