Pedrosa: Understanding Focal Length

Albert Pedrosa

Photo mania

WHEN you buy your new camera, especially the semi pro down to entry-level, a bundled lens is normally attached. Typically these lenses are called kit lens. Often times, these lenses are belittled if not, it becomes an excuse for not getting a good photo.

I’ll discuss more about lens quality in my future articles.

The typical lens bundled with entry levels are 18-55 and 28-135 for semi-pro cameras.

The figures are actually what you call as the focal length. I bet everybody knows that. But do you know how those figures were derived? Did you also know that the aperture is relative to focal length? When do you start calling a lens as telephoto?

You’d probably ask if knowing these things would make your photos look better. Maybe not, maybe yes. What I can assure you is you’ll be able to understand and recreate on how your photos came out well. You’d also understand your equipments better and use them accordingly.

Focal length is actually derived from the distance between the focal plane and the main lens. The focal plane is represented by the sensor. The smaller the focal length, the closer the lens gets to the sensor and the angle of view also widens. Of course, the longer the focal length, the farther the lens goes from the sensor and therefore your view will get narrower.

Sometimes we treat focal length as a means of zooming in and out of the subject so we don’t have to move from a comfortable position. The choice of what focal length to use is imperative especially when deciding on the composition of the shot. A longer focal length will compress the distance between your foreground and background. It will also show tighter background.

When you use a wide angle lens or smaller focal length, you’ll get a more perspective photo. The background will look farther and will cover a wider area. One of the reasons why landscape photographers uses this lens is it allows you to tell a story by separating the dimensions and showing you more elements to add to the story.

Aperture on the other hand is written as f/value. “f” stands for focal length. If you’re shooting using a focal length of 50mm and an aperture value of 5.6, then the area of your aperture is 50/5.6 = 8.92sq/cm. The smaller the aperture value, the smaller the divisor which gives you more diameter.

That’s the reason why a 70-200, f/2.8 lens has bigger diameter than the f/4 version.

When do you call a lens as telephoto? Typically, a lens that is more than 50mm is considered a telephoto lens. However, technically speaking, a lens is considered telephoto when the physical focal length of the lens is shorter than the labeled focal length of the lens. Therefore, the label says 100mm but if you measure it, it’s actually shorter.

The resulting angle of view is still the same even if it’s shorter. Manufacturer are able to do this by adding a magnifying lens to correct the difference. This method is used to produce shorter lens for better handling. Most of the long lens today are telephoto.

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