How f-Stops Work When You Change Focal Length

A 50mm at f/4 has a physically larger opening than an 18mm at f/1.8, yet the wider-angle lens is the faster of the two. That contradiction sits at the center of how apertures actually behave once you start swapping focal lengths.

David Bergman answers a viewer named Steve in an episode of Ask David Bergman on Adorama, and he starts from the ground up before circling back to the specific comparison. Aperture is the adjustable opening inside your lens, formed by blades that create an iris much like the one in your eye. The f-number you see written as f/2.8 or f/5.6 is a fraction, with the focal length on top and the f-number as the denominator. Divide the focal length by that number and you get the diameter of the entrance pupil, the apparent size of the opening when you look through the front of the lens. That single equation explains why the numbers seem to run backwards, since a smaller denominator produces a larger opening.

Bergman makes the point concrete with two lenses that share the same maximum aperture. The Canon RF 50mm f/1.8 has an entrance pupil of about 28 mm, while the Canon RF 135mm f/1.8 works out to roughly 75 mm, over two and a half times larger. Both still deliver the same exposure at f/1.8. The longer lens gathers more total light but spreads it across a larger projected area, while the shorter lens concentrates less light over a smaller area, and the two effects cancel out. That balance is why you don't have to adjust exposure when you change focal length at a fixed f-number.

The f-stop sequence itself carries information most people never stop to unpack. Each full stop, running through f/1.4, f/2, f/2.8, f/4, f/5.6, f/8, doubles or halves the light reaching the sensor. The spacing looks strange until you remember the aperture is a circle, and a circle's area grows with the square of its diameter. To double the area, you increase the diameter by about 1.4 times, which is where that repeating multiplier in the numbers comes from. Understanding this turns the aperture ring from a memorized list into something you can reason through on the spot.

Here's a way to use all of this the next time you're comparing lenses on a spec sheet. Terms like "fast" and "slow" only describe the maximum aperture, never the focal length, so a fast lens is one with a wide maximum opening such as f/1.4 or f/2.8 that lets you push shutter speeds higher in dim light. That's the deciding factor for concert stages, dark arenas, and anything where you're fighting to freeze motion. Focal length changes your framing and your working distance, but it doesn't make a lens fast or slow on its own. Keeping those two ideas separate saves you from the exact trap Steve fell into, and it tells you which number to weigh when the budget forces a choice between reach and speed.

That brings the answer back around to Steve's pair. Run the math and the 50mm f/4 has an entrance pupil of 12.5 mm against 10 mm for the 18mm f/1.8, so the longer lens does have the bigger physical opening. But the 18mm at f/1.8 lets in roughly 2.3 stops more light, enough to jump from 1/500 of a second to over 1/2,000 at the same exposure. The wider maximum aperture wins, and it wins by a wide margin.

Watch the full breakdown in the video above to see Bergman walk through each calculation with the lenses in hand.