Manual cameras / ASA setting

<p>I cant seem to find the answer to this one. What exactly, mechanically, happens to the camera when you adjust the ASA dial on a mechanical camera?</p>

<p>Usually it moves a do-hickey so that the meter reads a different exposure..... of course the do-hickey and the linkage are different for every brand/model of camera.</p>

<p>So it dosnt really do anything but fool the user into changing an exposure control? Then its worthless for exposure compensation in an all-manual old camera?</p>

<p>The way I see it, changing the ISO is the same as changing aperture or shutter speed (except it won't change depth of field or stop motion). If you change ISO from 200 to 400, you've effectively changed exposure by one stop. If you change from 100 to 200, you've again changed one full stop. Correct me if I'm wrong, please.</p>

<p>Dennis, yup you are right.<br>

<br /> charles Not true, you can use it for exposure compensation, what do you think you are doing when you move the compensation dial on something like the Pentax ME Super? You can do the same thing without fiddling with the ASA dial, set the camera so that the meter shows that it is proper exposed and then shift the aperture or shutter speed 1 or 2 stops depending on what you are trying to do. Done.</p>

<p>It simply allows either more or less light to reach the metering cell depending on which direction you move it. Many of the leaf shutter fixed lens rangefinders actually do this mechanically. As you turn the ASA dial, it is moving a plate with different sized holes to eiither a larger or smaller hole that corresponds with the position of the metering cell.</p>

<p>If we're talking here about the ASA dial on a meterless, manual camera, it's just a reminder dial to keep track of what film you've put in it. It's coupled to nothing at all. You'll find one, for example, on the base of a Nikon F., or under the rewind crank on a Miranda F. It's entirely unrelated to the dial on a meter head.</p>

<p>Mechanically, nothing really happens (in an SLR). If your camera has a built in meter, then the ASA dial adjusts a variable resistor which allows the voltage generated by the cds cell to raise or lower the metering needle according to the sensitivity that you've set it for. If there is no meter, then it is as has been said, a reminder.</p>

<p>In a metered manual camera, in some way it adjusts the metering circuit to expose properly for that speed of film. It may be a switched resistor, it may be a variable resistor, it may make a mechanical adjustment (on many match needle or center-needle cameras). For instance, on the Topcon Super D, it changes the anchor point of the chain that "adds" the shutter speed and aperture to rotate the "fixed" part of the meter. On most classic Pentax cameras, it is a variable resistor.<br>

I don't know, but on the fancy pattern metered cameras like the Nikon FA, it may be digitally encoded (say in gray code) to go to a small microprocessor via some number of input pins.</p>

<p>Thanks to all. My question is adequately answered. I should have explained why I ask. I am using an old Canon TX for fun, with max shutter of 1/500, so I was curious about exposure compensation if over-exposed (and no faster shutter speed to help out). So now I know it would do nothing to change the ASA dial in my case. Again, thanks for the responses. :)</p>

<p>Unless that manual camera is truly manual, like a Leica, in which case it simply keeps track of the ISO/ASA being used.</p>

The Canon TX has a meter, Charles, and changing the ASA will indeed do something to the reading it produces.<br>If you don't use that meter, it doesn't matter. If you do however, it could indeed provide the compensation you are after.

<p>There are several methods of coupling the ASA setting to the exposure control system.<br>

On many viewfinder cameras, there is a rotating disk in front of the meter cell (photocell) which lets more or less light passing through. Sometimes there is a variable aperture in front of the meter cell (Yashica Electro 35G series, Olympus SP).<br>

On cameras with a rather electronic exposure control system, in most cases the ASA setting turns a variable resistor.<br>

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<p>I am using an old Canon TX for fun, with max shutter of 1/500, so I was curious about exposure compensation if over-exposed (and no faster shutter speed to help out). So now I know it would do nothing to change the ASA dial in my case. Again, thanks for the responses. :)</p>

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<p>You need to set the ASA to your film speed when you load your camera, you can't use the ASA dial as you would the ISO on a digital camera. If you are over exposed and you are at 1/500 then you need to adjust the aperture to a higher setting, if that is already at its maximum (say at ƒ16, 1/500 with ASA800 film loaded) then you need to install an Neutral Density filter to reduce the exposure.</p>

<p>I've used a Canon TX since around 1977 and it's still a great solid camera with an accurate averaging meter. With the ASA film speed set properly, you use the meter as follows with FD lenses:<br>

The meter is always on, so use a lens cap whenever you're not shooting, to avoid undue battery drain. <br>

Mercury cells no longer exist, so use a Wein cell (PX625 type), or a Size 675 hearing aid battery (with or without O-ring that you can buy in a hardware store---the O-ring helps center the small 675 battery). These are both zinc oxide - air batteries with a relatively short life in use.<br>

Adjust aperture and/or shutter speed until the indicator needle goes through the center of the circle-on-a-stick.</p>

<p>Charles, I think what you want to understand is that changing the ISO/ASA setting on the camera only adjusts the lightmeter calibration. The shutter speed and aperture will still be as-indicated--but if you're using the lightmeter to guide your shutter/aperture settings, the meter will give you different readings based on ASA setting.</p>

<p>This is easy to verify. Adjust shutter & aperture to indicate 'correct' exposure in the viewfinder. Then increase ASA value 1 EV (double the value, for example, change ISO 200 to ISO 400). Note that in the viewfinder, exposure is no longer correct. At this point, if you either close the aperture 1 EV (e.g. change from f/5.6 to f/8) OR increase shutter speed 1 EV (half the shutter speed, e.g. change from 1/125 to 1/60) you'll see that correct exposure is again indicated. </p>

<p>The effect of this is that when the meter is centered, you'll get 1 EV less exposure than before you adjusted ASA. With a fully-manual (no autoexposure) camera like a Canon TX, there's limited reason to do this from shot to shot--more likely you might adjust the ASA slightly for the entire shoot, for example, if you've found through trial & error that the camera tends to underexpose slightly to your taste. For shot-to-shot changes with manual exposure, you're probably better off just using the match needle display in the viewfinder to deliberately add/remove exposure for the given lighting/scene. If the camera doesn't feature match needles but instead has a single indicator for 'correct', you might first set to 'correct' (centered), then adjust one or two click-stops up or down from there as needed.</p>

<p>As an aside, if you want to use your camera in a situation where at the fastest shutter speed it still over exposes, experiment with neutral density filters. They allow you to use older cameras with longer shutter speeds with faster modern films.</p>

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<p>If you are over exposed and you are at 1/500 then you need to adjust the aperture to a higher setting, if that is already at its maximum (say at ƒ16, 1/500 with ASA800 film loaded) then you need to install an Neutral Density filter to reduce the exposure.</p>

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<p>Above, I should have called the batteries zinc-air.</p>

<p>On a film camera, it does exactly what exposure compensation would do. In fact, on auto-exposure Nikons, it is no coincidence that exposure compensation and ASA setting are on the same dial, and turn the same shaft internally.</p>

<p>Both exposure compensation and ASA setting change the sensitivity of the meter. If you are manually setting the shutter speed and aperture, then merely changing the meter sensitivity doesn't change exposure -- it just changes the meter's recommendation for exposure, but you've got to adjust shutter speed or aperture in order to follow the meter's recommendation.</p>

<p>On a camera with auto exposure, when set on auto, changing the meter's sensitivity will cause the meter to adjust shutter speed and/or aperture, depending on the way the auto exposure works.</p>

<p>On a film camera, the only things that REALLY matter are the actual sensitivity of the film, the shutter speed, the aperture, and the lighting. The meter sensitivity only matters inasmuch as it may influence the shutter speed or aperture.</p>

<p>On a film camera, it does exactly what exposure compensation would do. In fact, on auto-exposure Nikons, it is no coincidence that exposure compensation and ASA setting are on the same dial, and turn the same shaft internally.</p>

<p>Both exposure compensation and ASA setting change the sensitivity of the meter. If you are manually setting the shutter speed and aperture, then merely changing the meter sensitivity doesn't change exposure -- it just changes the meter's recommendation for exposure, but you've got to adjust shutter speed or aperture in order to follow the meter's recommendation.</p>

<p>On a camera with auto exposure, when set on auto, changing the meter's sensitivity will cause the meter to adjust shutter speed and/or aperture, depending on the way the auto exposure works.</p>

<p>On a film camera, the only things that REALLY matter are the actual sensitivity of the film, the shutter speed, the aperture, and the lighting. The meter sensitivity only matters inasmuch as it may influence the shutter speed or aperture.</p>

<p>On a film camera with a meter, look at the roll of film you are going to us, find the ASA speed of the film (It's called ISO today but it's the same thing as ASA), then set the ASA setting on your camera to the same of the film. That's all there is to it.</p>