Have you ever wondered how camera brands manage to build a zoom lens with a constant aperture from wide angle to tele? Well, I have. Almost all camera manufacturers have cameras with lenses that just sound too good to be true. Tony Northrup makes a case in the video below, how camera brands fool you. Tony backs up his theory with multiple formulas that make absolutely no sense to me (I’m a complete math-nobrainer…), but Tony does a great job explaining it in an understandable way. This video is a bit long at 37 minutes, but it does give you something to think about.


Every photographer should be familiar with the crop factor and how it influences the framing  of your image. If you aren’t, we have some great articles on this topic here and here.

Here is what Tony Northrup argues: Whenever you use a lens with any given focal length, you have to multiply that focal length by the crop factor because the focal length that is written on your lens is usually equivalent to full-frame 35mm sensors. Nothing new.

BUT, the aperture is the focal length divided by the diameter of the opening at the front of the lens. So whenever you have to change the focal length because of the crop factor, the aperture has to be changed as well.

Let’s take a look at an example: if you have a 100mm lens with a 25mm front opening you have a lens with a maximum aperture of f/4. Makes sense, doesn’t it?


When we change the 100mm in our calculation above, we of course have to change the aperture as well, because the aperture doesn’t describe the size of the front opening, the aperture describes the RELATIONSHIP between focal length and front opening.

And here is the problem, according to Tony: Many camera manufacturers “forget” to change the maximum aperture in their advertising material.


Take a look at this one: the Lumix 12-35mm f/2.8 is advertised as the equivalent to a 24-70mm f/2.8 lens. Let’s just do the calculation:


If you divide 70 by 12.5, you don’t get 2.8. Instead what you’re getting is 5.6. That proves that the Lumix 12-35mm lens would be a 24-70mm f/5.6 lens equivalent to 35mm Full-frame.


Have you ever thought about this? Do you agree with Tony and his claim?

[via Tony Northrup, images via screenshots]