Some Handy Tables

On this page, I include some handy information related to photography, often in tabular form.

When Is a Lens Normal?

The usual rule of thumb given for a "normal lens" is that its focal length is close to the diagonal of the film format.

This definition gives rise to questions all by itself. If the criterion for normalcy relates to the appearance of the photographic image, shouldn't the focal length be proportional to the vertical dimension of the film format, so that subjects of the same kind could fit in the frame?

I think the obvious answer is that the diagonal was chosen for reasons of practicality; a lens of a given design also has a certain angular coverage, so the image it forms has a circular extent. Therefore, for a given focal ratio, the same lens design can be used as a normal lens with a given focal ratio for a camera of any size or format, just scaled up as appropriate.

But in practice, this rule isn't always strictly followed; the typical normal lenses for some film formats have been larger or smaller than might be expected from this rule of thumb.

Here is a table of a few examples:

The Kodak Medalist, with its 90 mm by 60 mm image format, has the same 3:2 aspect ratio as a typical 35mm film camera with its 36 mm by 24 mm image format. If one compares the sizes of the two formats, the ratio is as 5 to 2. So a 50 mm lens for the typical 35mm film format would be equivalent to a 125 mm lens on a Kodak Medalist.

The obvious reason that comes to mind is that since the Kodak Medalist was a rangefinder camera, naturally a longer focal length was used for 35mm SLR cameras, because the reflex mirror kept the lens farther away from the film, and so a longer focal length was needed to avoid the need for a lens with a more complicated retrofocus design.

But the very first Leica cameras, without interchangeable lenses, used a 50 mm lens, and so did other 35mm cameras without interchangeable lenses, like the Argus Model A, even though they were not of a reflex design.

So perhaps it was the fact that the Kodak Medalist camera had a larger format which was the reason; a shorter focal length was chosen because the larger format already made the lens larger and heavier and thus more expensive.

Exposure

The simplest film cameras, without large and expensive lenses, and without complicated controls and adjustments, could only be used to take pictures of objects some distance away from the camera... and on a bright, sunny day.

Or maybe indoors to, with help from a flashbulb.

But with fancier cameras, with lenses with focal ratios like f/3.5 or so instead of f/11 or f/16, with knobs to turn to set the shutter speed... exposure becomes something that needs to be set correctly. At least that was true for the many years in which such cameras existed, but they were still manual, not automatically setting the correct exposure themselves.

On a typical camera, successive settings offered for exposure times differ by one-third of a stop; that is, one has to move three settings from a given setting to give one that offers twice as long an exposure time, or half as long an exposure time.

The sensitivities of film that are offered for sale also differ by what is termed one-third of a stop.

But on a camera lens, the settings for the iris diaphragm, the "stops" to which that term mentioned above returns, that are usually marked - f/1.4, f/2, f/2.8, f/4, f/5.6, f/8, f/11, f/16 and f/22, each in succession admits only one-half the light of the previous stop. The numbers refer to a linear dimension of the iris diaphragm, and thus they change in proportion to the square root of two, with the area that is opened, and thus the light admitted, changing by a factor of two.

There is nothing wrong with that; there may be less space to write numbers on a camera lens than there is on the exposure time dial on the camera. But what is a pet peeve to me is that while film sensitivities and exposure times both go in steps of a third of a stop... the typical camera lens, in addition to having a detent, a click stop, at each of the numbered stops shown... also has one halfway between each pair of numbered stops. Instead of two detents, so that the steps offered for all three factors of exposure are consistent in size.

Here is a table intended to make it easy to calculate equivalent exposures.

How do you use this table?

Each row of the table begins with what I've termed a "brightness number". Following that, there is given an exposure time, an aperture setting or f-stop, and an ASA rating for film.

Each of those three factors determines how sensitive the camera will be to light. The brightness numbers are in units of one-third of a stop. If you reduce the exposure time by one-third of a stop, but use a film that is one-third of a stop more sensitive, these two changes will balance out. So any combination of film and camera settings where the total of the brightness numnbers of the three factors of exposure time, f-stop, and film ASA is the same will yield a correct exposure under the same lighting conditions.

So we just need a way to relate these totals to lighting conditions.

And that requires this table:

Bright sun       32
Hazy sun         35
Cloudy bright    38
Cloudy dull      41

These are the appropriate totals for the brightness factors given an average subject, lit from the front, at a normal distance.

If one is taking a picture of a very bright subject, like a beach, a seaside, or a landscape covered in snow, subtract six from the required total.

Subtract three for a particularly light-colored subject, add three for a dark-colored subject.

Add three if the subject is lit from the side, and add six if the subject is lit from the back.

When the sun is near the horizon (within 20 degrees), add three.

Add three for close-up photographs (within 4 feet or 120 cm), subtract three for photographs of distant subjects (including those only somewhat distant if they have little shadow detail).

Similar tables appear in many introductory books on photography; this one is based on information from a U.S. Navy course in journalism.