Article 3 of 4 in the Film Vs. Digital Series 

Apertures are relatively standard across all lenses and measurements transfer from film to digital all the same. The aperture number correlates with the size of a lens “opening” and is often called the “f-stop”, or just “f” followed by a number as in “f2.8 or f3.5”. Essentially, the aperture dictates how much light enters the lens and is transferred to the film.

Contrary to how most people think, the SMALLER the number of the aperture, the LARGER the aperture opening and the more light you get. We often call lenses with large apertures, “bright” lenses.

Because of the necessity for larger glass elements on bright lenses, you will generally experience that lenses with large apertures (1.8, 2.8) are both more expensive, and of higher quality than lenses with smaller apertures (4.0, 5.6). Sound backwards still? Just remember, smaller number = more light.

Here’s a general reference chart for common maximum apertures and their uses:

f1.4 – Very bright, night photography, low light photography in natural light, sports.
f2.8 – Bright, low light photography with little or no artificial light, sports
f5.6 – Less bright, stick with daytime photography unless you’re using a flash.

Another creative use for different aperture settings is depth-of-field, but we’ll talk more about that later… onto ISO.

Like aperture, ISO is another measurement of ability to capture light. But unlike aperture, where a lens is phycially letting more light in, a higher ISO simply means that the same ammount of light is captured faster. We can also call the ISO measurement, light “sensitivity”. Films and digital sensors that are more sensitive, capture light faster than those that are less sensitive.

With all mainstream digital SLR models having “adjust on the fly” ISO capability, lens apertures are becoming less of a concern to digital SLR consumers. When set to an ISO of 1600, a camera can capture 3x the amount of light when compared to a camera shooting at ISO 200. Be aware however, that a high ISO often translates to more digital noise or “grain”, and that unsightly grain can limit the size at which you are able to reproduce the photograph.

The Good – Even with a small aperture lens, bumping up your ISO can increase performance in low-light or on quick-moving subjects.
The Bad– High ISO often results in more digital “noise” and can mar the quality of a photograph. That f2.8 lens is sounding better and better.

Read the next article in this series: Using Film Lenses on Digital Cameras

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