The f-number on your lens tells you how bright your exposure will be, and it says far less about how blurry your background will turn out. If you’ve ever copied a recommended aperture and ended up with a frame that looked nothing like the example, the gap usually traces back to the size of your sensor and how far you stood from your subject.
Cody Mitchell starts with an iPhone, which has an f/1.8 lens, yet a photo of a camera taken on it looks nothing like the shallow depth of field people expect from f/1.8 on a larger camera. Mitchell traces the difference to format size, the physical dimensions of the film or sensor sitting behind the lens. A lens projects a circle of light, and a smaller sensor captures a smaller slice of that circle, which is where crop factor comes from. On Micro Four Thirds, the crop factor is two, so a 50mm lens frames a scene the way a 100mm lens would on a full frame camera.
Mitchell applies the crop factor to aperture as well. A common piece of advice says f/5.6 is the comfortable middle ground between a soft background and everything sharp. That holds up on full frame. On Micro Four Thirds, f/5.6 multiplied by the crop factor of two lands you around f/11 in 35mm equivalent terms, so your “middle ground” frames come out with a very deep depth of field. To match the full frame look, you’d divide instead and shoot around f/2.8. The same approach works for APS-C, where the crop factor is roughly 1.5x on most bodies and 1.6x on Canon, and for medium format, where the factor drops below one.
An f-number is a ratio: focal length divided by the diameter of the opening. A 50mm lens at f/2 has an opening about 25 mm across. On Micro Four Thirds, you’d use a 25mm lens to get a similar field of view, and at f/2, its opening is only about 12.5 mm. The f-number is the same, but the opening is half as wide, which gives a noticeably deeper zone of focus at the same framing. Phone cameras push this to the extreme, with actual focal lengths of just a few millimeters, which is why Apple introduced Portrait mode on the iPhone 7 Plus in 2016 and why every major phone maker now relies on depth mapping to fake background blur in software. The ratio does keep one thing consistent: exposure. f/2.8 at 1/250 s and ISO 400 gives you the same brightness on a phone, a Micro Four Thirds body, or a medium format camera. Exposure settings carry over between systems in the same light, but you still have to work out depth of field for your own camera.
Distance is the other half of the equation. With a smaller sensor, you have to back up to get the same framing, and stepping back deepens your depth of field even when the aperture stays put. Mitchell points out that pushing a phone right into a cluster of flowers makes it hard to get everything sharp, while shooting the same flowers from 10 feet away gets the whole scene in focus with no effort. A plant shot at 50mm on full frame, where the f/5.6 advice works well, makes the same point: stepping back to the distance a Micro Four Thirds camera would need for the same framing deepens the depth of field without any change in aperture. At f/2.8, a tree trunk isn’t entirely sharp up close, while from far away the trunk and nearly everything in front of and behind it are in focus, and at that distance, stopping down all the way to f/32 makes no discernible difference in depth of field. The video also lays out a short exercise: pick one subject, photograph it 10 times with your own camera at different depths of field and distances, and spend about two minutes reviewing the results.
A side-by-side framing comparison of full frame, APS-C, and Micro Four Thirds at 28mm is in the video above.
