Some lenses have an aperture range in their name rather than a single, fixed maximum aperture.
I’m going to use the example of the Sigma 60-600mm f/4.5-6.3 DG OS HSM Sports lens here, but there are plenty of others that also have an aperture range in their name. It’s just one of several confusing things about how lenses are named.
When you look at a lens like the Sigma 60-600mm f/4.5-6.3 DG OS HSM Sports, you’ll notice a range of numbers where the aperture should be (the f/4.5-6.3 part).
In the industry, this is known as a variable aperture zoom. It’s one of the most common naming conventions in photography, but it’s also one of the most frequently misunderstood.
This particular issue applies only to zoom lenses, and only to some of them at that.
Most of the time, lens manufacturers use the convention of including the lens’s maximum aperture as part of the name. So you might see lenses such as a 24-70mm f/2.8 or 200mm f/4. But sometimes there’s more than one aperture.
In this example, the f/4.5-6.3 part of the lens’s name refers to the maximum aperture. So this lens has a maximum aperture of f/4.5 to f/6.3, depending on the zoom. When zoomed out, the maximum aperture is f/4.5. When zoomed in, the maximum aperture is f/6.3.
You could be forgiven for thinking that it means that it can only shoot between f/4.5 and f/6.3, but that’s not what it means. This range only indicates the widest possible opening (maximum light-gathering ability) at the respective ends of the focal range. It doesn’t in any way prevent you from using smaller apertures like f/8 or f/11.
The why
So why do manufacturers design lenses with a variable aperture? It comes down to the unavoidable reality of optical physics.
An f-stop isn’t just an arbitrary setting; it is a literal mathematical ratio of the lens’s focal length divided by the diameter of its entrance pupil (the physical opening letting light in). When I look at that Sigma 60-600mm, achieving f/4.5 at the 60mm end only requires an entrance pupil of about 13.3mm across. That’s easy enough to engineer.
But if Sigma wanted to build a constant aperture lens that maintained that exact same f/4.5 light-gathering ability all the way out to 600mm, they could, but the physics dictate they would need an entrance pupil pushing 133mm across. That requires a front element of solid, highly corrected glass well over 5 inches wide. A constant 600mm f/4.5 zoom wouldn’t just be astronomically expensive to manufacture; it would be physically massive and practically impossible to handhold. And no-one would actually buy it.
But this same math applies to the cheap, compact kit lenses you see bundled with entry-level cameras, like a standard 18-55mm f/3.5-5.6. While an 18-55mm wouldn’t become physically gigantic if it had a constant f/3.5 aperture, the glass elements would still need to be noticeably larger, and the optical formula much more complex to correct for aberrations at that wider opening.
By allowing the maximum aperture to gradually ‘slide’ down — whether to f/6.3 on a super-telephoto or f/5.6 on a standard zoom — lens designers can keep the size, weight, and cost of these lenses incredibly low, making them infinitely more practical for everyday shooting.
The hidden variable: Macro lenses and “Effective” aperture
There is at least one notable exception where a lens’s maximum light-gathering ability changes, but you won’t see a variable range printed on the barrel. It happens with true macro lenses — like the Nikon 105mm f/2.8 macro lenses I use regularly. I kept running into this issue was became curious about it before I figured out what was going on.
From the name, you know this lens has a maximum aperture of f/2.8. But when you rack the focus in to shoot a 1:1 macro close-up, your camera will suddenly tell you your maximum aperture is f/3.3, or even f/4.8.
It turns out this isn’t a defect; it’s a physical phenomenon known as “effective aperture” (historically called the bellows factor). To achieve 1:1 magnification, the optical elements inside the lens have to move significantly further away from the sensor. The physical aperture blades aren’t actually closing down, but because the light has to travel a greater distance down the lens barrel, it spreads out and loses intensity by the time it reaches the sensor.
If you are a Nikon shooter, this is a quirk you have to get used to. Nikon bodies actively calculate this physical light loss and display the true, working effective aperture on your screen. If you shoot with a Canon macro lens on a Canon body, it will stubbornly display “f/2.8” at minimum focus distance, even though the exact same physical light loss is happening behind the scenes.
Things worth knowing
- As I said, this only applies to zoom lenses. And only some zoom lenses. It’s a result of the optical design.
- Higher-end lenses (i.e., more expensive and, often, faster) often have a constant maximum aperture through the zoom range. For example, the Sony FE 24-70mm f/2.8 GM II is a zoom lens, but the maximum aperture of f/2.8 is available at both 24mm and 70mm.
- The minimum aperture isn’t included in the name — that’s something that you find on the spec sheets or on the aperture ring or by using it. But the same kind of optical design decisions can affect the lens’s minimum aperture. The Sigma lens I referred to above is a good example because its minimum aperture shifts between f/22 and f/32 according to the zoom setting of the lens.
