The basic concepts of controlling depth of field with aperture, focal length, and distance are fairly easy to grasp, and we covered those topics in part one of this series, but now it's time to dig a little deeper.
In this segment, we talk about perceived depth of field, how sensor size and angle of view can affect your ability to get the depth of field you want, and using depth of field to rack focus.
There's a lot of confusion surrounding these topics, but a little common sense, and of course a bit of math and science can help us break it all down.
When it comes to depth of field, one thing that's helpful to understand is that there is a difference between the mathematical result of depth of field, and how a viewer perceives depth of field. Let's look at an example.
In this shot, we're using a 24mm lens at f4. Our subject is 2 feet away. Now here's the same shot taken with a 105mm lens at f4. To get the subject to fill the frame in the same manner, we had to move to 9 feet away. Let's look at the images side by side. Aesthetically, the depth of field looks deeper in the 105mm shot, but in actuality, the 24mm shot has a depth of field of .5 feet, while the 150mm shot has a depth of field of .52 feet. But because the longer lens is magnifying the background more, the result appears more blurred. The takeaway here is that focal length has more of an impact than the distance between your camera and the subject when it comes to perceived depth of field.
Now let's talk about how sensor sizes and angle of view come into play when attempting to control depth of field.
You'll commonly hear that cameras with bigger sensors get better depth of field. While the sensor itself doesn't change the depth of field that's being produced by the optics in your camera, The size of the sensor influences the focal length of the lens on a given camera. And of course the focal length influences depth of field, and the field of view. The field of view is the portion of the scene that your sensor is actually seeing, and it directly affects how far you have to be from a subject to attain the framing you want. This can definitely affect your camera's ability to control depth of field.
The term crop factor or focal length multiplier is commonly thrown around when talking about DSLR style cameras. Basically, the focal length listed on a DSLR lens is based on a full frame 35mm sensor. But what happens when the sensor is smaller than that? That's where crop factor comes in.
Let's say this is the circular image captured through a 50mm lens that is based on the 35mm film standard, and this is a full frame sensor. Now let's take a look at an aps-c sized sensor using the same lens. You can see that due to the smaller sensor, our 50mm lens has a much smaller field of view. Crop Factor is the number that you multiply by the lens focal length to figure out the effective focal length of a particular lens on a particular camera.
Let's take a look at this shot, taken with a full frame 5d Mark III, with a 50mm lens at f1.2, 15 inches from the gearshift. This produces a 24.9 degree field of view, and has an extremely shallow depth of field. Now let's mount the same lens to a 1D mark IV, and leave the camera in the same position. Now, the focal length is still technically 50mm, but the 1D mark IV has a smaller sensor, and therefore has a 1.3x crop factor. This means the effective focal length has changed from 50mm to 65mm, and the field of view has changed from 24.9 degrees to 19.3 degrees.
The important thing to remember here is that the actual focal distance is not changing. The depth of field at this distance is exactly the same as the full frame sensor, but we'll need to move the 1D back in order to attain the same field of view.
And we know that increasing the distance between our subject and our lens will increase the depth of field. This is why full frame cameras are a bit better at achieving depth of field.
The crop factor for DSLR cameras is easy to find, and it's easy to see how it affects a shot. But the relationship of Video Cameras and DSLR cameras is a bit tougher to discern. You won't find any crop factors listed on a camcorder, and that's because they're not based on the traditional 35mm film standard.
This JVC camcorder has 1/3” sensors. As we stated earlier the smaller sensor dictates that the focal distance of the lens needs to be smaller. The actual focal length of this lens is 4.1mm to 94.3mm. The full frame sensor equivalent of this is 29mm to 667mm. Essentially this is the equivalent of a 7.2x crop factor.
Having such short focal lengths drastically affects the field of view.
If we set the actual focal length of the JVC to 50mm, our effective focal length is 360mm, and we have a 5.5 degree horizontal field of view, compared to a 39.6 degree field of view of a full frame 5d mark III at 50mm.
So to keep the 50mm focal length and get the equivalent framing on the JVC, you'll have to move way back, which of course, increases the depth of field.
This shot was taken with the JVC at the actual focal length of 50mm at f2.8. In order to attain this framing, we had to place the camera 17 feet 4inches away from the tape measurer. Because we had to move so far back from the subject, we've got a deep depth of field.
Here's the same shot with our actual focal length set to 7mm which gives us a 50mm equivalent field of view. We are now just 3 feet from the tape measurer at f2, but of course with such a short focal length we've still got deep depth of field.
Here's the same shot with a full frame 5dmarkIII from 36 inches away at f2. You can see that the depth of field is far shallower. Let's take a quick look at the shots side by side.
The combination of short focal lengths and narrow field of view explains why traditional video cameras are very good at keeping everything in focus, and present more challenges when attempting to get shallow depth of field.
Now let's talk about the creative use of depth of field. One of the main purposes of achieving shallow depth of field is to direct your viewers' attention to specific portions of a scene. Using a rack focus, you can shift the viewers' attention in the middle of a scene.
A rack focus refers to changing from one point of focus in a shot to another. This is particularly effective with a really shallow depth of field. Let's take a look at an example.
In this scene, we want to have the gear shift in focus which is 16 inches from the lens. The xylophone is 18 inches behind, and way out of focus. We're shooting with a 50mm lens at f1.2 on a full frame camera. We can easily rack focus and make the gear shift completely blurry.
Now, this is fairly easy to achieve with a f1.2 50mm lens on a full frame camera, but what about achieving this with a camcorder that has a much wider lens.
In this case, we're shooting at f2, and from the same distance. Our JVC is set to 20mm, but because our lens is so wide, the xylophone that was extremely blurred in the shot taken with the 5D mark III is now barely out of focus.
The solution here is to simply move the xylophone further out of the focused area. In this shot, we moved the xylophone back over 6 feet from it's original position to get a similar amount of blur as the 5d. Alternatively, we could move the camera way back if we have the room and zoom in as much as possible.
Learning to work with the strengths your camera has is an essential step to improving your shooting skills. Knowing how aperture, focal length, distance, and sensor size influence your camera's ability to control depth of field can help you get the shots you visualize on the page, onto the screen. Thanks for watching.