Bonus Topic: Lenses and Physics

There are certain optical properties that can’t be circumvented and that means that lenses will often be larger and heavier than we wish. To gather lots of light, you need lots of glass, especially when covering a ‘full frame’ sensor. When something seems to good to be true it likely isn’t. Here’s an example: Sigma produces an 18–35mm f1.8 lens. It’s an optical marvel and pretty well universally loved. Many photographers want the “shallow depth of field” look however, and an f1.8 lens on a cropped camera doesn’t look as shallow as it would on a full frame camera. This has to do with where you’d put the camera relative to the subject, but please don’t worry about that at the moment. So that Sigma 18–35mm f1.8 lens is going to look, depth-of-field-wise, closer to a f2.8 lens on a full frame camera. So essentially we are left with a 37mm to 52mm f2.8 full frame equivalent. Not a terribly exciting specification as there are many decent 24–70mm f2.8 lenses already in existence.