The aperture of a lens determines the amount of light that passes through the lens to the film or digital sensor. It's a diaphragm of overlapping leaves that is either controlled by an external ring on older lenses or set by controls on the newer cameras which sets the lens electronically.
In the case of older cameras and lenses, turning the ring one way opens up the lens so there's no obstruction to incoming light. Turning it the other way closes down the lens until only a small circular opening is left for the light to pass through.
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| Aperture openings |
With newer cameras, buttons or a dial on the camera will let you set the appropriate aperture. However, unless there's a depth of field preview button, you may not see any change. The camera will close the lens down to the value you've set just before it takes a photo.
f-Stops
On all lenses, aperture is calibrated using what are called f-stops (in fact, the terms are almost interchangable). Along with the focal length of the lens, the maximum aperture is usually used to specify a lens. By that I mean that if the lens is described as an f/2.8 or an f/4 lens, for instance. The actual range of f-stops varies between lenses but the sequence is always the same:
f/1, f/1.4, f/2, f/2.8, f/4, f/5, f/5.6, f/8, f/11, f/16, f/22, f/32, f/45, f/64
Maximum apertures can be "non-standard" - that is, not in the above list, so it's common to see short focal length zoom lenses with wide open settings of f/3.5. Longer focal length zooms may have maximum values of f/4.5.
Standard lenses, sometimes known as "fixed" lenses (i.e. they don't zoom), tend to have higher maximum f-stops than zoom lenses. On film SLRs (Single Lens Reflex), the standard 50mm lens that comes with the camera will have a wide open setting of f/2, f/1.8 or even f/1.4.
Regardless of the focal length of the lens or the diameter of the lens, f-stops denote relative apertures. - they actually specify the ratio of the diameter of the aperture to the focal length of the lens.
Let's take an example. On a 135mm telephoto lens (good for isolating something in the distance), an aperture setting of f/4 means that the physical diameter of the diaphragm opening is 33.75mm. How did I get this figure? Divide the focal length of the lens by the f-stop. So, 135 / 4 = 33.75.
On a 28mm lens (great for wide landscape photos) with its aperture set to f/4, the size of the opening will be 7.5mm (28 / 4 = 7.5).
Here's what's neat about this system: The amount of light that can pass through a lens decreases proportionally with increasing focal length - basically, the longer the focal length, the less light gets through the lens. Using the aperture system outlined above allows you to change lenses, keep the same f-stop between lenses and still get the same exposure. If you keep all the camera/lens settings the same (aperture, shutter speed, film speed) your exposure will remain the same and the only thing that changes is what you see through the lens - more or less of the landscape you're photographing.
If you're mathematically minded, you probably noticed that each successive f-stop is the square-root-of-2 (approx. 1.414) times the previous f-stop.
Decreasing your f-stops double your exposure. For example, if your exposure is 1/125 sec at f/5.6 and you change your aperture to f/8 need to double your shutter speed to 1/60 sec. If you changed to f/8, you'd need to double your shutter speed again to 1/30 sec.
Likewise, increasing your f-stops halves your exposure for each f-stop. So, changing from f/4 to f/2.8 would mean halving your shutter speed from 1/125 sec to 1/250 sec.
Simple Rules
So, two simple rules of thumb to get the same exposure are:
• double your shutter speed for each decrease in f-stop
• halve your shutter speed for each increase in f-stop
Why is Aperture Important?
Depth of Field is the answer.
Understanding Camera Exposure Modes
Almost every digital camera on the market makes it easy to take quick-and-dirty snapshots using
an automatic exposure mode. Automatic exposure is great much of the time, but I hope that you
will sometimes want to get a little more creative.
Not all cameras provide manual or semi-manual exposure settings; if yours does not, then you might want to
think about upgrading at some point in the future to a more full-featured camera. Nowadays most point and shoot cameras has basic user controlled settings like sports, portrait, flower etc...
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| Camera dial control |
For SLR/DSLR here’s what each of these settings does, and when you would want to use them:
Automatic
In this mode, both shutter speed and aperture settings are selected by the camera to match the current lighting. Some digital camera automatic modes try to select the fastest shutter speed possible in order to minimize camera shake when you take a picture, while most choose something in the middle, a compromise between
speed and depth of field. There’s generally nothing you can do to change the settings that the camera chooses when set to fully automatic, except for adjusting the exposure compensation (EV) dial to over- or underexpose the scene.
Program or P
The program mode (usually indicated by the letter P on your camera’s dial or LCD display) is similar to an automatic mode. Although the camera selects both the aperture and shutter, you can generally modify the camera’s selection by turning a dial or pressing a button. The effect: you can increase or decrease the shutter speed, and the camera will adjust the aperture to match. This is a good compromise between fully automatic operation and manual selection. Use this mode if you don’t want to worry about devising your own exposure values, but still want some say over the shutter speed or aperture.
The program exposure mode is often the best all-around setting for your camera. In this
mode, the camera chooses a good exposure setting, but you can turn a dial to tweak the
shutter speed. The camera will instantly compensate by changing the aperture setting,
keeping the overall exposure the same.
Shutter priority
This setting is usually indicated by the letter S on your camera’s mode dial or LCD display (Tv for Canon). Using this mode, you can dial in whatever shutter speed you like, and the camera accommodates by setting the appropriate aperture to match. This mode is ideal for locking in a speed fast enough to freeze action scenes, or slow enough to intentionally blur motion.
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| Slow motion effect |
Aperture priority
This setting is usually indicated by the letter A on your mode dial or LCD display (Av in Canon). Using this mode, you can dial in the aperture setting you like, camera accommodates by setting the appropriate shutter speed. Use this mode if you are trying to achieve a particular depth of field and you don’t care about the shutter speed.
This is my personal favorite setting where I can focus on adjusting aperture openings for creative purposes. for example if I'm shooting landscape, I normally set my aperture to f8 to f16 depending on the lighting condition. The purpose is get the view focused from foreground to background..like this..
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| Onepoto Domain, North Shore |
This settings also very handy in portrait or macro photography where you need to blur your background to highlight your subject like this..
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| Chess |
Manual
The manual mode (typically indicated with an M) is like an old-style noncomputerized camera. In manual mode, you select the aperture and shutter speed on your own, sometimes with the help of the camera’s recommendation. This mode is best used for long exposures or other special situations when the camera’s meter is not reliable like this shot..where you need to set your aperture and shutter speed (normally in bulb mode)
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| Long exposure |
Now that you know what your camera’s various exposure modes are for, you can think about using them when you encounter unique photographic situations. Every situation is a little bit different, but here are a few general guidelines that can get you started.
Thanks for reading and please don't forget to subscribe, I'll be posting some great tips and tricks in my next post..
Cheers
Andre
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