Modern Classic SLR Series
Canon A-1 - Camera Operation
- Part IV

 

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The Basics of Exposure

Note: This section contains some photographic term and definition which might be used throughout this site. In the PIM site, I did compiled a glossary section to help those who might need help. Further, thiere is another Back-to-Basic section which also can be used as supplement.

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The amount of exposure is a matter not only of the amount of light allowed to strike the film but also of the amount of time the light is allowed to strike the film. The camera has two mechanisms to control exposure, the diaphragm and the shutter. The diaphragm consists of blades which open and close to certain size openings called apertures. The size of the aperture determines the amount of light which will fall on the film. The various size apertures are indicated by a set series of numbers called f/stops or f/numbers. Each f/stop represents some amount of light that is allowed to pass through the lens. The smaller numbers are called large f/stops while the larger numbers are called small f/stops. This is because the larger numbers represent smaller apertures and allow less light to pass through the lens. Each time vou move from one f/stop to the next smaller f/stop (larger number the amount of light allowed through is exactly halved. In effect, the amount of exposure itself is also halved. Using f/2 as a standard, the amount of light reaching the film will change according to f/stop as indicated below:

 

f/stop

1.2*

1.4

1.8*

2

2.8

3.5*

4

5.6

8

11

16

22

Brightness ratio

3

2

1 1/4

1

1/2

1/3

1/4

1/8

1/16

1/32

1/64

1/128

* Half f/stops.

The largest f/stop on the lens is called the lens maximum aperture. The smallest f/stop on the lens is called the lens minimum aperture. The maximum and minimum apertures differ according to the lens. The maximum lens aperture is important because it indicates the largest amount of light that the lens will transmit. Sometimes the maximum aperture is a half-f/stop rather than a full f/stop on the aperture scale.

AT Dial.jpg
The length of time that the light is allowed to strike the film is determined by the shutter speed. The relationship between shutter speeds is like that between f/stops.

As you move from one shutter speed to the next higher shutter speed, exposure is halved. Your image is actually exposed by the opening of the shutter. Shutter speed can be controlled on the A-1 by turning the AT dial. Otherwise the camera controls it. On the other hand, aperture can be controlled either by turning the AT dial or the lens aperture ring to the desired f/stop. Otherwise the camera controls the aperture. Now suppose that you have the camera set for a certain exposure value (EV), say the exposure you get with f/4 at 1/60 sec. There are certain other combinations of aperture and shutter speed which will give you the same amount of exposure as that above. Some of these combinations in this example include f/5.6 at 1/30 sec. and f/2.8 at 1/125 sec. You simply move up and down the f/number and shutter speed scales. It is not important to know this relationship when you use the A-1 in an AE mode. The camera will do the figuring for you. But it is an important photographic fact which you may find useful when you use the camera manually or in correcting exposure.

AE Photography AE photography means automatic exposure. The camera is selecting either shutter speed or aperture or both for you. The most obvious, unusual feature of this camera is the absence of the shutter speed dial which has been replaced by the AT dial. This dial allows you to set either shutter speed or aperture for any of five different AE modes. Perhaps the A-1 is the only camera in the market prior to the Canon T-90 some 9 years later that use input dial to change the aperture value without having to turn the aperture ring.

Through-the-lens full aperture metering

Shutter-speed priority AE
Aperture priority AE
Programmed AE
AE flash photography
setlens.jpg

Through-the-lens stopped-down metering Stopped-down AE

ln shutter-speed priority AE, you set the shutter speed, and the camera automatically selects the necessary aperture according to lighting conditions for correct exposure. Shutter-speed priority AE is applicable to most subjects but especially useful in action photography. ln aperture priority AE, you set the aperture while the camera automatically selects the necessary shutter speed for correct exposure. It is convenient for the various forms of still photography in which depth of field is of importance. The camera automatically selects both aperture and shutter speed in the programmed AE mode. This mode is helpful when your only goal is correct exposure and neither depth of field nor control of movement are very important in the picture. It is the easiest mode for the beginner. In stopped-down AE, you set the aperture manually on the lens aperture ring while the camera automatically selects a shutter speed for correct exposure. This is a big advantage in specialized areas of photography such as photomacrography and photomicrography. AE flash photography is possible with the Canon Speedlites 155A or 199A. In this mode, the camera sets both aperture and shutter speed automatically, eliminating any worries about guide number calculations and flash synchronizing shutter speeds.

Setting the Lens for AE Photography The first four AE modes described above i.e. shutter-speed priority AE, aperture priority AE, programmed AE and AE flash photography, are performed with full aperture metering thr'ough the lens. This means that the diaphragm is fully open during metering for easiest viewing and operation.

setlens2.jpg
For correct operation in these four full aperture metering AE modes, only an FD lens can be used and the lens aperture ring must be set to the "A" mark.

To do this, simply hold in the EE lock pin on the lens while turning the aperture ring from the minimum aperture to the "A " mark. This can be done either before or after the lens is mounted on the camera.
Note: Illustration here is using an older version of FD lens with chrome ring, the newer FDn is using a black aperture ring instead. At the "A" mark, the aperture ring is locked and cannot be turned to any other setting. Reverse the procedure to disengage the aperture ring from the "A" mark. Note: Instead of an "A" mark, some earlier FD lenses have a small green circle on the aperture ring for AE photography. Its use is the same as that of the "A" mark, and the procedure is the same as that described above.

AE Mode Selector The AE mode selector has two possible settings, Tv (
time value) and Av (aperture value). When the AE mode selector is switched to Tv, the camera is set for the shutterspeed priority AE mode. You set the shutter speed and the camera selects the aperture. When switched to Av, the camera is set for aperture priority. Here you set the aperture, and the camera automatically selects the shutter speed.

avtv.jpg
The AE mode selector click stops at for Tv and at O for Av.


AT Dial This dial is works in conjunction with the AE (Automatic Expsoure) selector.
When the AE selector is set at Tv , it shows the shutter speed scale. When the AE selector is set to Av, it shows the aperture scale. The AE mode selector has two possible settings, Tv (time value) and Av (aperture value). When the AE mode selector is switched to Tv, the camera is set for the shutterspeed priority AE mode. You set the shutter speed and the camera selects the aperture. When switched to Av, the camera is set for aperture priority. Here you set the aperture, and the camera automatically selects the shutter speed.

AT Dial.jpg
The AE mode selector click stops at for Tv and at O for Av.

To operate: first slide down the AT dial guard (which is designed to prevent accidental movement of the dial. Depending on the setting of the AE selector, turn the AT dial until it click stops at the exact aperture or shutter speed desired so that the setting is aligned with the index mark. The figure set on the AT dial has priority in determining the exposure. If, for example, you have set a shutter speed on the dial, the camera balances the aperture against the shutter speed you have selected until exposure is correct.

atdialset.jpg
If, on the other hand, you have set an aperture on the dial, the camera selects the necessary shutter speed for correct exposure.
Note: The AT dial will not turn past the highest and lowest limits of each scale.


dial_s.gif
Shutter Speed Scale on the AT Dial There is a choice of 16 click-stop settings rangirlg from 1/1000 (sec.) to 30 (sec.) on the AT dial's shutter speed scale.

Note: Above the highest speed of 1/1000 is the "P" for programmed AE. Below the lowest speed of 30 is the "B" setting. The dial will not turn past the "P" or the "B".

The slow shutter speeds from 2 (sec.) to 30 (sec.) are marked in orange while shutter speeds from 1 (sec.) to 1/1000 (sec.) are in white. The white numbers on the scale are the reciprocals of the true shutter speeds so that a white 2 indicates a speed of 1/2 sec. while a white 1000 indicates a speed of 1/1000 sec. Be careful not to confuse the slower orange range with the faster white range. The "B" (Bulb) setting is intended for exposures longer than 30 sec. When the shutter speed is set at "B", the shutter will remain open as long as you press the shutter button. However, AE photography is
not possible at the "B " setting. The aperture must be set manually. Remember that it is very important to use a tripod and a cable release, preferably with a lock, in long exposures to reduce the chance of image blur. Also, since long exposures present a real drain on the battery, it may be wise to carry a spare. The A-1 has a hot shoe and PC outlet for an X-synch (flash synchronization) at 1/60 sec. (the position). This shutter speed need not be considered, however, when the A-1 is used with specified Canon flash . Please note: Intermediate settings on the shutter speed scale cannot be used. However, if you accidentally set an intermediate shutter speed, the camera will automatically select the next lower or higher shutter speed as the effective shutter speed, and that shutter speed will be displayed when you take an exposure preview. Note: If you mistakenly try a time exposure on "bulb" while the lens is still at "A", exposure will take place at the lens minimum aperture.

Aperture Scale on the AT Dial The aperture scale on the AT dial is calibrated in f/stops ranging from f/22 to f/1.2. The f/1.2 setting is indicated by a dot. The dial click-stops at both full and half f/stops. It will not turn past the largest and smallest f/stops. The maximum aperture indicated on the AT dial is large enough for any Canon lens.

Dial_a.gif

Note: Since the smallest aperture that can be set on the AT dial is f/22, if you want to set an aperture smaller than f/22, switch to stopped-down AE and manually set the aperture on the lens aperture ring.


Under certain conditions in the aperture priority AE mode, the maximum and minimum apertures on the lens have priority over the aperture set on the AT dial. For instance, if the aperture vou have set on the AT dial is laraer than the maximum aperture of the lens in use, the camera will match the shutter speed to the lens maximum aperture rather than the aperture vou have set on the AT dial for correct exposure. Thus, if you are using an FD 50mm f/1.8 S.C. lens with the AT dial set to f/1.4, the camera will use f/1.8 as the aperture setting. In this case, the maximum aperture of the lens will be displayed in the viewfinder when you take an exposure preview regardless of the setting on the AT dial. On the other hand, if you have set an aperture on the AT dial smaller than the minimum aperture of the lens, the mage will be exposed at the lens mininiuin aperture but the shutter speed will not be adJusted accordingly, resulting m over-exposure. ln this case, the aperture set on the A'1 dial will be displayed in the digital readout.

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