I'm going to now switch from the 5D MkII to the Panasonic GH2 for our test bed. This is simply because the GH2 is the camera I have at my disposal.
F/stop, Depth of Field, and Aperture
The actual part of the camera we are going to explore is the Aperture Ring, which is typically at the rear of the lens, and is marked in f/stops.
The f/stop scale looks something like this:
1.4 2 2.8 4 5.6 8 11 16 22
In this example, f/1.4 is the widest that the aperture can open, lets in the most light, and provides the shallowest area of critical focus.
F/22 is the smallest that the aperture will close on this lens. It lets in the least amount of light, and provides the deepest area of critical focus.
Each full stop up the range results in a reduction of light by half. So, f/2 let in half the amount of light as f/1.4. F/22 let in 1/256 the light as f/1.4.
Right now, this might seem like just a bit of math. So, we're going to look at how it can be applied.
When we see close up shots of a character in a movie, the background is often blurred. This helps us mentally and emotionally separate the character from the surroundings. Not only does it "feel" like how a movie should feel, it is a strong storytelling tool. This is also a contributing factor to why most video has a "non filmic" look and doesn't draw us in as readily as film. Due to the small sensor on the camera and the limited adjustability of the lens, everything in the shot is in focus. There is no focal separation.
This image is taken from a GH2 with a 50mm lens at f/1.4. By now, hopefully you will recognize that this is equivalent to 100mm on a full frame camera. In the following shots, notice the labels on the tags…they indicate the distance of the tags from the camera, and will help demonstrate the limits of the depth of field at f/1.4, which, as I've already mentioned, represents the aperture at its most open position.
Notice that as I focus on objects further away, more of the focusable area becomes longer.
Finally, at the end of our run at 3.5 feet away, enough detail of the 3 foot mark is visible for us to make out its shape.
The tighter objects are clustered in terms of depth, and the further away from the camera they are, the sharper they will be in focus. This is because, the closer to the camera an object is, the shorter the area of sharp focus will be. The farther away an object is, the deeper the area of critical focus is.
The graphics below further illustrate this point. Look how the subject distance, which is the number to the upper right of each of these graphics, affects the total depth of field, which is the area between the two vertical lines in the illustration at the bottom of each of the graphics:
This demonstrates why, as focus is adjusted toward infinity, all of the markers are visible, whereas in the first photo, focused at 1.5 feet, only the first tag was discernable.
The following images demonstrate the same focal points, but at a different f/stop. I've adjusted the aperture ring to f/4, which closes down the aperture by 3 full stops. With the aperture closed to f/4.0, notice how the near and far extremes of the focal depth is affected:
So, to better illustrate the point, look at the difference between f/1.4 and f/4.0 at the same focal length:
To further explore this concept, you can practice with this Depth of Field calculator.
So....
Objects closer to the camera have greater focal separation.
Objects farther from the camera have little focal separation.
Increasing the f/stop decreases focal separation.
In keeping with our understanding that we need to double our distance to subject or use wider lens, we also need to open up the iris twice as much (one stop). The next stop down from f/1.4 is f/0.95. That's a big piece of glass!
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Voigtlander Nokton f/0.95 25mm Lens
With these concepts and calculators, you should be able to determine which lens on your camera will best accomplish a specific 35mm look. However, there is a deeper level to this f/stop business, and it becomes visible as we get into cameras with smaller sensors. We will cover this topic briefly in our next lesson.
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