The manufacturer tells you the ’35mm equivalent focal length’ of the lens but then claims its absolute aperture. Here’s an example. Let’s say the camera has a 1? sensor, which is a 2.7X crop factor. If they claim it has a 24-70mm full-frame equivalent lens, it’s actually a 9-28mm f/2.8 lens. OK, fair enough.
But then they calculate the aperture at the actual focal length (not FF equivalent). So the lens is a 9-28 f/2.8 aperture is 28mm divided by 2.8 = 10mm. That’s the actual size of the aperture. But if you then list it as a 24-70mm, well, a 10mm aperture at 70mm = f/7 (same math, different direction). It’s not a huge problem, but it makes you think that the lens might act like a 24-70 f/2.8 lens on your SLR and it won’t. ~ Roger Cicala
Roger’s post is an informative and entertaining read. After reading this, I think I am reconsidering “needing” a Fuji X100F ($1300 new) for travel/walkabout photography. I love the design of the camera, and it has just the right size and looks to make me feel like an invisible tourist while walking around. However, I would prefer a normal focal length and I think I can achieve the same result with a Fujinon XF27mm f/2.8 pancake lens ($450 new) which has a ~41mm full-frame focal length.
I can have my pancake and shoot it too!