It’s really about Magnification
We know some truths about Depth of Field (DOF). We know there are 3 things (actually 4 but the forth is controversial and not all agree and it would only apply if you switch cameras in between shots)
Anyway, there are 3 things that affect Depth of Field
- Distance to Subject
- Focal Length
- The Larger the aperture (lower f/ number),the shallower the Depth of Field
- The closer we are to our subject, the shallower the Depth of Field
- The longer the Focal Length, the shallower the Depth of Field
and of course the inverse of all would be true
All of these are facts, all are true. Except sometimes absolutes don’t fit into how we actually do things.
There is no doubt that aperture affects DOF. We really don’t have to worry about that. But the other two, we do. An interesting thing happens and for most of us it is a unconscious thing we are used to doing: When we change focal lengths…we move.
Think about it, You, have a 40mm lens on and you are taking a headshot of someone (The Subject) we use our knowledge and say to ourselves, Oh wait I want a shallower DOF, I’m going to put my 200mm Lens on. OK It’s on, let me just step back so I frame that person the same way…. and I GOT THE SHOT!
But look at what just happened there. We didn’t do one thing, we did two. We changed focal length and we also MOVED. So, we didn’t do one thing that affects DOF we did TWO!
And the funny thing is, they directly counteracted each other so the actual effect was NIL. The DOF did not change.
OK, I know. Prove it
This image was shot with a 40mm lens at f/4, 1/1600, ISO 100 on Camera Flash. The subject (Frosty) was 1’10” from the Film Plane
Using my Simple DOF phone App here is the DOF for the Image
This next Image was shot with a 200mm lens at f/4 1/1600 ISO 100 On-Camera Flash Shot 9’ 2” from the Film Plane
This is the DOF from the DOF App
As you can see, the framing of the Subject is exactly the same, the DOF is the same (sometimes there are slight variations but within a reasonably close amount) Both are 1.71” (Something that can change, depending on the combination of length and distance, is the distribution of DOF behind and in front of the plane of focus)
Now, I also know what you are saying, “Besides Frosty’s size, those two images don’t look anything alike!! ”
Well true, while the DOF is the same we have two wildly different Fields of View between the two lenses. Ah, you say. But Even the DOF does not look the same. Well that is simply an illusion. Since all you are seeing is the background magnified in the second image shot with the 200mm. The so called “Telephoto compression” But if we zoomed in on the 40mm we see the amount of Blur is essentially the same (remembering that each lens has different Bokeh)
This is zoomed in to the background on the 40mm shot
So this shows us a few things. That the DOF is the same but we can use different lenses for different effect. If we were doing an Environmental Portrait we may want to use a 24-40mm lens. Still have the shallow DOF but have a better idea of the environment. The image taken zoomed in to 200mm would be better if you really had a distracting background you wanted to get rid of you could see less of it zoomed in and still maintain a shallow DOF for isolation.
But one trick you can use by going to a longer focal length since it has an “apparent” if not actually, shallower DOF is you can use a Higher f/ stop on your lens for more DOF on your subject (getting their eyes AND nose within the field of focus). But still look isolated from the background
This one was shot at 200mm f/8.0. 1/400 ISO 100 On-Camera Flash from 9’2”
This is the DOF app
If you look close, we doubled the DOF and put more of Frosty in Focus but still had him isolated in a shallow DOF
Frosty is about the size of a Human Head. So you can see how shallow the DOF would be if you were shooting that tight on someone’s head. If you moved back in both examples (less Magnification)to a full body length the DOF would be a lot deeper. If you increased the aperture you would get more in focus, decreasing the aperture would yield less DOF (shallower)
So, what is the take away here?
The more an object fills the frame, the shallower the Depth of Field will be
When we frame a subject equally regardless of focal length or distance, it is said they have equal Magnification and that is how we should look at DOF. The higher the magnification of our subject, the shallower the DOF. It’s as simple as that instead of thinking about two things, (Focal Length and Distance to Subject) we just think about one.
When we know that, we also know that the only true way to change the DOF (keeping the same magnification) is by changing our Aperture. The wider the aperture (lower number) the shallower the DOF. The tighter the aperture (higher number) the deeper the DOF.
All things being equal, a Headshot will have a shallower DOF than 1/2 Body than a full body. Use THAT to determine your aperture for the effect you want. i.e shoot wider open on a Full Body shot than a Headshot because a Full body shot is less magnified thus has a deeper DOF to begin with
Hope that helps