Mythbusters: Focal Length Range on Zoom Lenses
I'm going to start a new series of posts on photography myths. There are a number of persistent beliefs, understandings, and nuggets of conventional wisdom that have one thing in common:
They're not true.
Or they're misrepresented or oversimplified, leading others to misunderstand or misapply them. Of course, I'm not the only person in the world to notice them and they've been addressed on other photography sites too. But I think I'd like to add my voice to the mix to further bolster (even if by a miniscule amount) the body of factual information on these topics floating around on the 'Net.
The first one I'll tackle is focal length range on zoom lenses. A lot of people are really fixated on it. As in, "What's the point of having a tiny 11-20mm focal length range?" which is a criticism that's been levied at the newly announced Tamron 11-20mm for Fujifilm X mount, as a very recent example.
Spoiler: 11-20mm isn't bad at all, actually.
You'd only make that criticism if you hadn't thought about, or were ignorant of, the math. But even a very well-known, supposedly experienced, photography blogger, who probably gets more site visits in a week than I get in a year, had the same complaint with the Sigma 18-35mm, which he moaned had a "silly zoom range". Spoiler: It doesn't. And that dude should know better.
Now, I'm not trying to defend any particular lens because I don't much care what other people like or don't like. But I do want to address the mathematical reality.
You see, focal length has its uses when comparing lenses but it's not a very good way to assess zoom range. Angle of view is the right measurement because it directly tells you how much of the scene in front of you will be captured by the lens. Focal length just tells you the distance between the optical center of the lens and the sensor. There is of course a correlation between focal length and angle of view, but it isn't a linear correlation – in other words, as you zoom, the focal length and angle of view both change, but not at the same rate.
An example will make it really clear why that matters.
A 18-35mm lens ranges in angle of view from 66.6° at 18mm to 37.3° at 35mm. That's a 29.3° range, which is not shabby at all. The following graph shows the range of angles of view of an 18-35mm lens. Imagine you're at the center of the circle and taking a photo at a subject placed at the top of the circle using an 18-35mm zoom lens. The colored sections show how much of the view in front of you will be in the photo at the two ends of the zoom range. The light tan slices are the available range in angle of view the lens provides from fully zoomed in to fully zoomed out.
Nobody thinks of the classic 70-200mm full-frame lens as having a constricted zoom range because it has 130mm of focal length range. But the story is different when you look at the range of angle of view. A 70-200mm full-frame lens has an angle of view range of 18.5° (on APS-C the range would be even less than that). As you can see from the graph below, the difference in angle of view between 70mm and 200mm isn't that much. And it's significantly less than difference in angle of view between 18mm and 35mm. The 18-35mm lens actually has a greater zoom range, even though its focal length range is less.
At the shorter end of the focal length spectrum, small differences in focal length result in big differences in angle of view. Or put another way, 9mm of range on a wide angle zoom is a much bigger deal than 9mm on a telephoto zoom.
As crazy as it may sound, when you're talking about zoom range, not all millimeters are created the same.