The ultimate challenge when using vintage photographic cameras is that they usually predate the automatic exposure era. The problem of determining exposures has to be faced by the photographer alone, and this is one of the reasons why I love them. As I began photographing with the Retinette I thought about getting a light meter. At first I even took one of our dSLRs with me to measure, and then transferred shutter speed and aperture settings to the Retinette, but this was kind of using a can(n)on to kill a fly... ;) Then I came across Fred Parker's ultimate exposure computer, which was exactly what I was looking for: a (relatively) easy way to determine exposure without the need of a light meter, using nothing more than your eyes and your brain.
The main idea is to estimate the amount of light falling into a scene. This is the only input required: everything else is known or can be decided creatively. The amount of light is measured by a magnitude called exposure value (EV). Our eyes and our brains are indeed perfectly capable of estimating exposure value, one just needs a little practice and some reference points. For example, a typical scene in bright direct sunlight has an exposure value of EV15. A direct light in heavy overcast or an area in open shade with clear sunlight needs EV12. The light with a clear sky one hour after sunrise is EV9-11. A home interior in artificial light is EV5-7. And so on.
(Retinette IA, Fujicolor Superia Reala 100, ISO 100/21°)
Once the exposure value has been estimated, determining the aperture and shutter speed is quite easy if you use the sunny f/16 rule as an anchor point. This rule states that EV 15 (corresponding to bright direct sunlight) requires approximately the reciprocal of the ISO speed for the shutter at f/16. If you are using ISO 100, then 1/125 @f/16 is the nearest setting; for ISO 200 it would be 1/250 @f/16 and for ISO 400 it would be 1/500 @f/16.
(Flexaret VI, Kodak Tri-X 400, ISO 400/27°)
Both shutter speed and aperture (f-number) scales are calibrated for each stop to be exactly one EV point. This makes using the sunny f/16 rule very easy. Suppose we want to shoot a scene at EV12 and we have ISO 200. Now EV15 would mean 1/250 @f/16, but we want to go 3 stops down to EV12. Now we could go three units down in any dimension: keep shutter speed at 1/250 and set aperture to f/11 (-1EV), f/8 (-2EV) and finally f/5.6 (-3EV = EV12, our target), or you could keep aperture at f/16 and reduce shutter speed to 1/125 (-1EV), 1/60 (-2EV) and 1/30 (-3EV = EV12, again our target). A combination of both is of course possible: 1/125 @f/8 (-1EV in shutter speed and -2EV in aperture) and 1/60 @f/11 (-2EV in shutter speed and -1EV in aperture) would both expose correctly for EV12.
In this way, you can always decide an aperture first (for depth of field considerations) and then determine shutter speed and ISO for the scene. Or start with the shutter speed (to freeze a moving subject, for example) and determine aperture and ISO later. The ISO speed scale is calibrated in EV units as well. The step from one ISO speed to the next is exactly 1 EV. For film users like myself ISO speed is something that you cannot change easily from picture to picture, but it is no problem if you use a digital camera.
(Praktica BMS, Prakticar 50mm 1:1.8, Fuji Neopan 100 Acros, ISO 100/21°)
I find it very useful to learn the most usual ranges of aperture and shutter speed scales by heart (*), because cameras (especially the older ones) do not always stick to the standard EV stops, especially on technically challenging matters like maximum aperture or maximum shutter speed.
(Hapo 66-E, Kodak Tri-X 400, EI 800/30°)
I admit that it might look difficult at first, but it is true that with a little practice you will be estimating exposures in no time. This way you can avoid having to rely on light meters, which have many problems and pitfalls of their own. Of course there is the danger of estimating EV wrong in the first place, but with time and practice one gets better and better (**). I often discover myself thinking about the EV of scenes that I see around me in the bus, in the street, in a restaurant, ... and even though I do not take a picture, determining which ISO, shutter and aperture would I use.
(Flexaret VI, Fujichrome Velvia 50, ISO 50/18°)
I am convinced that meterless exposure is a very healthy exercise to train your eye for the light which is, indeed, what every photographer should aspire to.
(*) These are the agreed standards for shutter speed and aperture/f-number. Each step to the left represents +1 EV and each step to the right represents -1 EV.
(**) the exposure latitude of the different types of film or sensors that you use, that means how "forgiving" it is, can also help when you make such mistakes. Color slide film is the least tolerant one, exposure has to be estimated quite precisely (maximum 1-2 EV error).