Equivalence

Albert Pedrosa
Photo Mania

ONE of my favorite lenses is the 50mm 1.4. I couldn’t afford the 1.2L, so I have to stick to this lens. I’m well versed with this lens that even without pointing my camera to the scene, I can tell how much angle it can cover. Since it’s a prime lens, I know exactly when to step back and move forward even before I take a peek through the viewfinder.

When I decided to upgrade my system to full-frame, suddenly, I couldn’t figure out my 50mm anymore. I expected this to happen since a full-frame sensor allows you to see a wider view compared to your cropped sensor. But what I didn’t realize was how much adjustment I have to make in order to foresee the result. I felt like having a 35mm lens.

Few years ago, I read an article about equivalence. I know how crop factor works, but this article enlightened me and gave more than I expected. This article couldn’t have existed if we were still in the film era, where the 35mm film was a standard. It’s a bit technical, but we all know that photography is half-art and half-technical, right?

The APS-C, which is common among crop sensors, is smaller than the full-frame with a crop factor of 1.6. This means that you have to multiply your focal length by the crop factor to get the right focal length view. For example, a 50mm lens mounted on an APS-C sensor will give you an equivalent of 80mm focal length. The 100mm will give you 160mm focal length. Easy, right?

Now let’s complicate it a bit by adding the aperture value and depth of field. The aperture f/5.6 is read as focal length divided by the aperture value. If your focal length is 50mm, you need to divide it by the aperture value, which is 5.6. The result is 8.93mm which defines the circumference of the opening in your lens.

A 200mm focal length at f/5.6 is equivalent to an opening of 35.7mm in circumference. Therefore, even if you have the same aperture between 50mm and 200mm, the depth of field at higher focal length will be narrower, since the size of the opening is bigger. The rule of depth of field is the bigger the hole, the shorter the DOF.

This explains why at a higher focal length, you get a blurred background even at f/5.6. To add more to the twist, aperture value like f/5.6 in 50mm and 200mm, which defines the size of the opening, has no difference in terms of exposure.

If you want to talk more of this topic, feel free to discuss this in www.grp.ph/forum or email me at photomania.sunstar@gmail.com. Keep on shooting, everyone!