F-Stops and Creative
Cats and owls can see in dim light better than we can, in part, because the lenses of their eyes allow in more light. We could say the "speed" of the lenses in their eyes is greater or better than ours.
We define lens speed as the maximum amount of light that can pass through the lens to end up on the target.
However, it's generally not desirable to transmit the maximum amount of light through the lens, so we need a way of governing the amount.
Like the pupil of an eye automatically adjusting to varying light levels, the iris of the camera lens controls the amount of light passing through the lens.
Under very low light conditions, the pupils of our eyes open up almost completely to allow in maximum light. Conversely, in bright sunlight the pupil contracts in an effort to avoid overloading the light-sensitive rods and cones in the back of the eye.
In the same way, the amount of light falling on the light-sensitive target of a TV camera must be controlled with the aid of an iris in the middle of the lens (shown above on the left).
Too much light will overexpose and wash out the picture; too little will cause the loss of detail in the darker areas.
We can smoothly adjust an iris from a very small to a large opening. We refer to the specific numerical points throughout this range as f-stops. The f-stop designation is often replaced by T-stops for transmission stops, especially in professional circles. Although T-stops are technically more accurate, the difference is normally not significant. For these modules we'll continue with the more traditional f-stop designation.
The "f" stands for factor. An f-stop is the ratio between the lens opening and the lens focal length. More specifically, the f-stop equals the focal length divided by the size of the lens opening.
f-stop = focal length / lens opening
This math explains the strange set of numbers used for f-stop designations, as well as the fact that the smaller the f-stop number the more light the lens transmits.
That's worth repeating: the smaller the f-stop number the more light the lens transmits.
1.4, 2.0, 2.8, 4.0, 5.6, 8, 11, 16, 22
<== more light less light ==>
Occasionally, we see other f-stops, such as f/1.2, f/3.5, and f/4.5. These are mid-point settings between whole f-stops, and on some lenses they represent the maximum aperture (speed) of the lens.
The figure at the right compares f-stop sizes.
We've noted that the speed of a lens is equal to its maximum (wide-open) f-stop. Here, f/l.4 is the speed of the lens.
Opening the iris one f-stop (from f/22 to f/16, for example) represents a 100 percent increase in the light passing through the lens. Conversely, "stopping down" the lens one stop (from f/16 to f/22, for example) cuts the light by 50 percent.
Put another way, when you open up one stop, you double the light going through the lens; when you stop down one stop, you cut the amount of light going through the lens in half.
So how will you use this knowledge?
Cameras with automatic exposure controls use a small electric motor to automatically open or close the iris in response to varying light conditions.
Makers of professional cameras print f-stop settings on the lens barrels and sometimes in viewfinder displays. (Note the f-stop settings in this photo.) It's important for professionals to understand and be able to work with the f-stop concept.
Not wanting to trouble unsophisticated consumers with such things as f-stops, manufacturers of consumer cameras don't show the numbers, and exposure adjustments are automatic. However, depending on circumstances, the camera may not set the iris at the best setting.
In this photo, automatic exposure adjustment has not provided the best video. In a scene that contains areas brighter than the main subject matter -- in this case, the window -- automatic circuitry will generally result in dark (underexposed) video and muted colors.
As we will see, savvy videographers who are stuck with this automatic feature on a camera need to know how to "influence" or override the automatic exposure. Not only can that result in better image exposure, but it can also provide control over such things as depth of field (discussed below).
This problem repeatedly shows up in amateur videos and the work of beginning videography students. In future modules we'll cover different approaches to solving this problem.
Depth of Field
We define depth of field as the range of distance in front of the camera that's in sharp focus.
Theoretically, if we focus a camera at a specific distance, only objects at that exact distance will be what we might consider completely sharp, and objects in front of and behind that point will be, to varying degrees, blurry.
In actuality, areas in front of and behind the point of focus may be acceptably sharp. The term acceptably sharp is subjective. A picture doesn't abruptly become unacceptably blurry at a certain point. The transition from sharp to out of focus is gradual.
For practical purposes, we've reached the limits of sharpness when details become objectionably indistinct. This will vary with the medium.
The range of what is acceptably sharp in standard NTSC television (SDTV) is greater than that of HDTV. In the latter case, the superior clarity of the medium more readily reveals sharpness problems.
Depth of Field and F-stops
The larger the f-stop number (that is, the smaller the iris opening and the less light let in), the greater the depth of field.
Therefore, the depth of field of a lens we set at f/11 is greater than the same lens set at f/5.6, and depth of field at f/5.6 will be greater than at f/2.8.
Except for extreme close-ups, depth of field extends approximately one-third of the way in front of the point of focus and two-thirds behind it.
The drawing on the right illustrates this range.
Depth of Field
And Focal Length
Although it's commonly (and erroneously) stated that depth of field depends on lens focal length, this is not the case. The reason, which is rather technical, was explained most recently in a January, 2009 article in Videography. As we've noted before, you can find more information in Focal Length and F-Stop Myths.
Although depth of field appears to be related to lens focal length, it's only an apparent relationship.
As long as the same image size is maintained on the target, all lenses of similar design set at a specific f-stop will have about the same depth of field, regardless of focal length.
A wide-angle lens appears to have a greater depth of field than a telephoto lens because sharpness problems in the image created by the wide-angle lens are compressed and therefore not as apparent.
If you enlarge a section of image area from the wide-angle shot -- a section exactly equal to the image area created by the telephoto lens -- you'll find that the depth of field is about the same.
Earlier, a Popular Photography magazine test had concluded the same thing: "If the same subject size on the film is maintained, depth of field is relatively unchanged no matter what the [lens] focal length."
Wide-angle lenses (or zoom lenses used at wide-angle positions) are good at hiding a lack of sharpness, so they're a good choice when accurate focus is an issue.
Of course, when you use a wide-angle lens setting, you may need to move much closer to the subject to keep the same size image. But by moving in, you've lost the sharpness advantage you seemingly gained by using the wide-angle lens in the first place.
With a telephoto lens (or zoom lens used at a telephoto setting), focus must be much more precise. In fact, when zoomed in fully at maximum focal length, the area of acceptable sharpness may be less than a few inches (20mm or so), especially with a wide aperture (low f-stop number).
This can represent either a major problem or a creative tool.
In the latter case, it can force the viewer to concentrate on a specific object or area of a scene. (Our eyes tend to avoid unclear areas of a picture, and they're drawn to sharply focused areas.)
In the case of this picture the photographer backed up and used a telephoto setting so that the foreground and background areas were thrown out of focus. This is also called selective focus. We'll talk about this creative tool later.
Focusing a Lens
The following discussion assumes you are using a camera with a manual focus control, or, in the case of a camera with automatic focus, that you can turn off this feature.
It might seem that focusing a lens is a simple process of just "getting things clear." True, but a few things complicate the issue.
It's probably obvious at this point that you should focus the zoom lens after first zooming into a close shot (using maximum focal length). Since focusing errors will be the most obvious at this point, focusing will be easier and more accurate. Once focused, you can zoom back the lens to whatever focal length you need.
If the scene includes a person, you'll want to focus on the catch light or gleam in one eye for two reasons: a person's eyes are normally the first place we look, and this small, bright spot is easy to focus on.
Note the extreme close-up of the woman's eye in the camera viewfinder in the photo on the right.
If you don't first zoom in to focus, but try to focus while holding a wide shot, you'll inevitably find when you later zoom in the picture will go out of focus. (This will suddenly greatly magnify the focus error was wasn't noticeable before.)
In the photo on the left above, the woman (in focus) is sleeping. When the phone rings, the focus shifts to the phone (on the right).
As she picks up the phone and starts to talk, the focus shifts (racks) back again to bring her into focus.
To use this technique,
to rehearse your focus shifts so that you can manually
rotate the lens
control from one predetermined point to another. Some
videographers temporarily mark the points on
the lens barrel with a grease pencil. After locking down
the camera on
a tripod, they can then then shift from one predetermined
point to another as needed.
With most camcorders, you can turn auto-focus on and off. For the following discussion, we'll assume the auto-focus is turned on.
Auto-focus can help in following moving subjects. However, you will encounter problems unless you fully understand how it works.
Most auto-focus devices assume that the area you want in sharp focus is in the center of the picture. The auto-focus area (the area the camera will automatically focus on) is in the green rectangle in this photo.
Remember the rack focus sequence discussed above? Since the area you want to focus on does not remain in the center of the frame, auto-focus would not be useful.
Note in the photo below that the center area is correctly focused (thanks to auto-focus), but the main subject is blurry. Of course, the goal was the opposite.
To make this scene work with auto-focus, you could pan or tilt the camera to bring the main subject into the auto-focus area, but this would change the composition in a way that you may not want.
Some camcorders allow you to center the subject matter in the auto-focus zone and then lock the auto-focus on that area. Once that's done you can reframe the scene for the best composition.
One camcorder attempts to track the photographer's eye movement in the viewfinder and shift focus accordingly. When you (as photographer) look at the woman in this case, the camera would focus on her -- but then as soon as you looked at the building in the background, the camera would shift focus to that point.
Auto-focus systems have other weaknesses. Reflections and flat areas with no detail can fool most of them. Most also have trouble determining accurate focus when you're shooting through such things as glass and wire fences.
Finally, auto-focus devices -- especially under low light -- can keep readjusting or searching for focus as you shoot, which can be distracting.
For all these reasons, professional
videographers typically turn off auto-focus
and rely on their own focusing ability. The
only exception may be a chaotic situation in which there
time to keep moving the subject matter into focus
As we've noted, focus errors not discernible in SDTV can be obvious in images from high-resolution digital (HDTV) cameras. As we've also noted, small HDTV camcorder viewfinders make critical focusing difficult.
manufacturers are experimenting with electronic
approaches" for HDTV lenses. There are various
approaches and at this point it's too early to
practical they might be in day-to-day HDTV
Most zoom lenses have a macro setting that enables the lens to attain sharp focus on an object only a few inches, or even a few millimeters from the front of the lens.
Although lenses differ, to reach the macro position on many zoom lenses, the photographer pushes a button or lever on the barrel of the lens to allow the zoom adjustment to travel beyond its normal stopping point.
Many newer lenses are continuous focus lenses. You can smoothly and continuously adjust these internal focus lenses from infinity to a few inches without manually shifting the lens into macro mode.
Videographers often forget about the macro capability, but it offers many dramatic possibilities. For example, a flower, stamp, or portion of a drawing or snapshot can fill the TV screen.
A tripod or camera mount is a must in using the macro setting. Not only is depth normally limited to just a few millimeters, but unintentional camera movement is greatly exaggerated.
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