A variable aperture zoom is a zoom lens whose widest aperture gets smaller as you zoom in. A lens marked f/3.5-5.6 opens to f/3.5 at its shortest focal length and only to f/5.6 at its longest, which is one and a third stops less light exactly where you most need fast shutter speeds.
Most zooms sold with cameras, most travel zooms and almost every compact camera lens work this way. The shift is harmless once you know where it bites: when you shoot wide open, in manual exposure, with manual flash, or while recording video. This page shows how to read the marking, why the f-number rises even though the opening inside the lens gets bigger, what your camera does as you zoom, and a simple way of working that makes the whole thing a non-issue. The zoom lens page covers zooms in general; this one stays on the aperture.
Reading f/3.5-5.6 on the lens
The marking gives two maximum apertures: the first for the shortest focal length, the second for the longest. A 24-105mm f/4-7.1 opens to f/4 at 24mm and only to f/7.1 at 105mm. On the barrel or the front ring the same thing is often written as a ratio, 1:3.5-5.6, which means the same thing. A zoom marked with a single number, such as 70-200mm f/4, is a constant aperture zoom: f/4 at every focal length.
- The numbers are limits, not the only settings. f/3.5-5.6 does not mean the lens works only between f/3.5 and f/5.6. It means it cannot open wider than those values. f/8, f/11 and f/16 are available at every focal length.
- The gap between the numbers is what matters. It tells you how much light you lose by zooming all the way in with the lens wide open.
- The values assume focus at a distance. On some lenses, macro lenses especially, the camera shows a smaller maximum when you focus very close.
Light falls with the square of the f-number, so the loss in stops is twice the base-2 logarithm of the larger number divided by the smaller. Here is what the common markings cost, with the shutter speed the camera must switch to if ISO stays put.
| Marked range | Light lost from wide end to long end | 1/250 at the wide end becomes |
|---|---|---|
| f/2.8-4 | 1 stop | 1/125 |
| f/4-5.6 | 1 stop | 1/125 |
| f/3.5-5.6 | 1 1/3 stops | 1/100 |
| f/4-6.3 | 1 1/3 stops | 1/100 |
| f/4-7.1 | 1 2/3 stops | 1/80 |
Not sure how a jump from f/3.5 to f/5.6 adds up to more than a stop? Understanding f-stops shows how the numbers on the scale map onto halvings and doublings of light.
The maximum changes in steps, and early
The two numbers on the barrel are only the ends. In between, the camera reports the maximum in small steps, usually third stops, and it does not wait until the end of the range to start closing. Read off one 16-85mm f/3.5-5.6 zoom, the widest settings run like this:
Two things stand out. First, half the total loss has gone by 35mm, about a quarter of the way through the focal length range, so “I’m only zoomed in a little” is no protection. Second, the lens sits at its long-end maximum for the whole last stretch of the range. If you want one aperture that holds everywhere, the second number on the barrel is the widest you can choose.
Why the f-number rises while the opening grows
The f-number is the focal length divided by the diameter of the entrance pupil, the aperture as you see it when you look into the front of the lens. To keep the same f-number as focal length doubles, the entrance pupil has to double too.
A variable aperture zoom does make the pupil bigger as you zoom in. It just does not make it big enough to keep up. On the 16-85mm in the chart:
| Focal length | Widest aperture | Entrance pupil (focal length ÷ f-number) | Pupil needed to hold f/3.5 |
|---|---|---|---|
| 16mm | f/3.5 | 4.6mm | 4.6mm |
| 35mm | f/4.5 | 7.8mm | 10mm |
| 50mm | f/5 | 10mm | 14.3mm |
| 85mm | f/5.6 | 15.2mm | 24.3mm |
The opening at 85mm is more than three times as wide as at 16mm, yet the f-number has risen from 3.5 to 5.6, because the focal length grew more than five times. So the common picture of “the hole shrinking” as you zoom is wrong: the hole grows, the f-number rises, and the light reaching each part of the sensor falls.
Holding the f-number costs glass. A 70-200mm f/2.8 needs a pupil of about 71mm at 200mm (200 ÷ 2.8), which sets the size of the front elements and the weight of everything that carries them. A 70-300mm f/4-5.6 needs only about 54mm at 300mm (300 ÷ 5.6), though it reaches farther. That gap is why variable aperture zooms are smaller, lighter and cheaper to make, and why they are the norm on small cameras and long-range zooms. The zoom lens page draws these openings to scale.
What your camera does as the aperture closes
The camera knows the maximum aperture at every focal length and shows it in the viewfinder. What it does when you zoom in with the lens wide open depends on the exposure mode.
- Aperture priority or program: What happens as you zoom in wide open: The f-number rises by itself to the new maximum; the shutter speed lengthens to match, or ISO rises if Auto ISO is on.
- Shutter priority: What happens as you zoom in wide open: Your shutter speed holds and the camera uses the widest aperture now available; if that is not enough, it warns of underexposure, or raises ISO if Auto ISO is on.
- Manual, fixed ISO: What happens as you zoom in wide open: Nothing compensates, so the picture darkens by the stops in the table above.
- Manual with Auto ISO: What happens as you zoom in wide open: ISO rises to hold the brightness; aperture and shutter stay where you put them.
In aperture priority the change is easy to miss, because the brightness holds. What slips is the shutter speed, and at the long end, where a small wobble is magnified most, that is exactly where camera shake starts to show. In manual mode, zooming darkens the picture only if your aperture is wider than the long-end maximum. Set f/5.6 or smaller on an f/3.5-5.6 lens and the exposure does not move when you zoom. What happens when you zoom back out differs between cameras: some return to f/3.5, others stay at f/5.6. Check yours once.
Lost light has to be paid for somewhere: lock an exposure in the tool below, close the aperture by a stop and a third, and choose whether the shutter speed or the ISO pays for it.
Two smaller side effects come with the darker long end. Most cameras focus with the lens wide open or close to it, so autofocus has less light to work with and may hunt sooner in dim rooms. And an optical viewfinder gets visibly dimmer as you zoom in; an electronic one simply raises its own gain.
The long end in low light
The design takes light away at the end of the zoom where you need it most. A long focal length already calls for a short shutter speed to hold the camera steady and to stop a moving subject, and that is where the variable aperture zoom is darkest.
The numbers in that photo show the scale of it. Holding 1/200 at f/5.6 instead of f/2.8 costs two stops, so ISO 4000 becomes ISO 16000. At f/6.3 it would be about ISO 20000. Compare a brighter scene: ISO 400 at f/2.8 becomes about ISO 2000 at f/6.3. Each of those stops shows as noise and lost dynamic range.
Stabilization helps with still subjects by letting you hand-hold a slower shutter speed. It does nothing for a moving one: a bird or a child moves just as far during 1/60 whether the camera is steady or not. For moving subjects in dim light, the honest options are higher ISO, getting closer and zooming out to win back the wider aperture, or a faster lens.
Want the camera to raise ISO for you without letting the shutter speed slide? Auto ISO explains the minimum shutter speed setting and how to choose an ISO ceiling.
Background blur: zooming in usually gives more, not less
A common belief is that because the f-number rises, a kit zoom gives less background blur at the long end. Usually the opposite happens. For a distant background, the size of the blur relative to your subject depends on the diameter of the entrance pupil, not on the f-number alone. Frame a person the same size at 16mm f/3.5 and then, stepping back, at 85mm f/5.6, and the pupil grows from 4.6mm to 15.2mm. The background blur is about three times larger at the long end, and the narrower view also shows less of the background behind the subject.
What is true is narrower: at the same focal length, f/5.6 blurs less than f/2.8 would. Framed the same, a variable aperture zoom at 200mm f/5.6 gives a softer background than it does at 18mm f/3.5, but not as soft as a 200mm f/2.8 lens. If you want the most blur a variable aperture zoom can give, zoom in, open the lens fully, get close to your subject and keep the background far away. Depth of field and bokeh go further into what shapes the blur.
Flash: why the subject gets darker as you zoom
The brightness of a flash-lit subject depends on aperture, ISO, flash power and distance. Shutter speed plays no part, so the camera cannot make up the lost light with a longer exposure.
Manual flash. The power is fixed, so zooming in darkens the subject by the stops the lens loses. Think of it as reach. A guide number is the distance multiplied by the f-number that gives a correct exposure at full power at a stated ISO. With a guide number of 28 in meters, the flash reaches 8 meters at f/3.5 (28 ÷ 3.5) and 5 meters at f/5.6 (28 ÷ 5.6).
Automatic (TTL) flash. The camera measures the flash through the lens and raises the power to cover the smaller aperture, so within range you see no change. The limit arrives sooner: once the flash is at full power the subject goes dark, and before that each shot takes longer to recharge.
The background. In manual exposure with TTL flash, the flash restores the subject but nothing restores the ambient light, so the room behind your subject drops by the lost stops as you zoom in. If that balance matters, choose an aperture both ends of the zoom can hold.
Want to work out how far your own flash reaches? Guide number shows how flash power, aperture and distance trade against each other.
Video: why zooming during a take shows
In stills, a change of exposure between frames is invisible. In video it happens on screen. Zoom during a take with the lens wide open and, in manual exposure, the picture darkens as the aperture closes. In automatic exposure the camera chases the loss, usually with ISO because the shutter speed is normally fixed for video, and the brightness dips and recovers. Either way the exposure pumps, and it looks like a fault.
- Set the aperture to the long-end maximum or smaller before you record: f/5.6 or smaller on an f/3.5-5.6 lens.
- Set ISO, lighting or ND filter for that aperture, not for the brighter wide-end value.
- If you cannot spare the light, do not zoom while recording. Reframe between clips.
Need the rest of the exposure set for moving pictures? Camera settings for video covers the 180-degree shutter rule, ND filters and ISO for video.
A way of working that makes the shift harmless
Nearly every problem with these lenses comes from treating the wide-end aperture as if it were available everywhere. This routine removes the surprise.
- Know your long-end maximum. It is the second number in the marking: f/6.3 on an f/4-6.3 lens.
- When the light allows, use that aperture or smaller through the whole range. Exposure, flash balance and depth of field then behave the same at every focal length. In good light, at f/8 or f/11, the variable aperture makes no difference at all.
- Turn on Auto ISO. When you do open the lens fully at the wide end, ISO absorbs the lost stops as you zoom in, so the shutter speed does not have to.
- Set a minimum shutter speed for the long end. Choose it in the Auto ISO menu for your longest focal length, where shake and subject movement are magnified most. Or shoot manual with Auto ISO and set the shutter speed yourself.
- Glance at the f-number after every zoom. The value from a moment ago may no longer be true.
When the light is too low even for that, brace the camera and lean on stabilization if the subject is still, raise ISO if it is moving, or zoom out and walk closer, which gives back the wider aperture.
Variable versus constant aperture zooms
A constant aperture zoom keeps the same maximum at every focal length, typically f/2.8 or f/4. It avoids every complication above, at the cost of size, weight and price.
| Variable aperture zoom | Constant aperture zoom | |
|---|---|---|
| Maximum aperture | Shrinks in steps as you zoom in | Same at every focal length |
| Manual exposure while zooming | Changes if set wider than the long-end maximum | Holds |
| Manual flash and video while zooming | Darken toward the long end if wide open | Unaffected |
| Low light at the long end | Weakest where shutter speed matters most | As good as at the wide end |
| Size, weight and cost | Smaller, lighter, cheaper to make | Larger, heavier, costlier |
Two cautions keep the comparison honest. Constant does not mean brighter: an f/2.8-4 zoom lets in more light at its wide end than a constant f/4 zoom and the same at its long end. And at f/8 in good light a variable aperture zoom gives up nothing. The gap opens only when you need the widest aperture at the longest focal length.
Weighing a zoom against a fixed focal length lens with a much wider aperture? Prime lens vs zoom lens sets out what each one is good at.
What “variable aperture” means on a phone
On phones the phrase means something else. Most phone cameras have a fixed aperture: the opening never changes, and the phone controls exposure with shutter speed and ISO alone. A phone sold as having a variable aperture has a main camera whose opening physically changes, either between two sizes or through a range.
That has nothing to do with zooming. It is the phone gaining the aperture control larger cameras have always had: wide open for dim light, smaller for bright light or for close subjects that need more depth of field, such as a document or a plate of food. Because phone sensors are small, the change in background blur between settings is modest. And when a phone zooms by switching from its main camera to a telephoto camera, the telephoto usually has a smaller aperture. That looks like a variable aperture zoom from the outside, but it is a jump between separate fixed lenses. Optical versus digital zoom explains what a phone does between those lenses.
Common mistakes with variable aperture zooms
- Setting a manual exposure wide open at the wide end, then zooming. The long-end frames come out dark. Fix: set the long-end maximum from the start, or turn on Auto ISO.
- Trusting the aperture from a moment ago. You plan around f/3.5 and find at the long end that the camera is at f/5.6 and the shutter speed has fallen below what you can hold. Fix: read the f-number after each zoom.
- Letting aperture priority choose a slow shutter speed at the long end. The pictures are blurred by shake or movement. Fix: set a minimum shutter speed in Auto ISO.
- Thinking the camera is broken because the aperture changed by itself. It closed to the widest value the lens can manage at that focal length. Fix: choose an aperture equal to or smaller than the second number.
- Zooming out to get more background blur. On these lenses the long end usually blurs the background more. Fix: zoom in, open up, get close.
- Expecting manual flash to carry as far when zoomed in. Fix: raise flash power or ISO, move closer, or hold one aperture throughout.
- Zooming during a video take with the lens wide open. The brightness pumps on screen. Fix: stop down to the long-end maximum before you press record.
Try this: map your own zoom in ten minutes
- Point the camera at an evenly lit plain wall. Set manual mode, a fixed ISO and the widest aperture at the wide end. Adjust the shutter speed until the meter reads zero.
- Zoom to each focal length printed on the barrel. At each one, write down the f-number the camera now shows and take a picture without touching anything else.
- Note where each step happens. Most lenses reach the long-end maximum well before the end of the ring.
- Now set the long-end maximum aperture, correct the shutter speed once, and repeat the zoom.
How to judge it: in the first set, the histogram should slide left step by step as the f-number rises, and the darkening should match the stops in the table near the top of this page. In the second set the histogram should not move at all. Keep the list of focal lengths and f-numbers in your camera bag; it is the map of your lens.
Want to check how well you can read the numbers printed on a lens? The lens specifications quiz asks about aperture markings and the other figures on a spec sheet.
Variable aperture questions
What does f/3.5-5.6 mean on a lens?
It gives the widest aperture at each end of the zoom: f/3.5 at the shortest focal length and f/5.6 at the longest. Smaller apertures such as f/8 or f/11 are available at every focal length.
Why does my aperture change when I zoom?
You have set an aperture wider than the lens can manage at the longer focal length, so the camera closes it to the widest value available there. Choose an aperture equal to or smaller than the second number on the lens and it will stop changing.
Is a variable aperture lens a bad lens?
No. It trades light at the long end for a lens that is smaller, lighter and cheaper to make, and that says nothing about how sharp it is. In good light or at middle apertures the changing maximum makes no difference; it limits you in dim light at long focal lengths and when you zoom during video.
Can I keep the widest aperture when I zoom in?
No. The lens cannot form a large enough entrance pupil at the long end. A wider aperture there takes a different lens, or a shorter focal length and a step closer.
Does the light loss show up in the exposure meter?
Yes. The camera meters through the lens, so the meter always sees the light that actually gets through. In automatic modes it compensates for you; in manual with a fixed ISO the meter needle simply moves toward underexposure as you zoom in.
Is variable aperture on a phone the same thing?
No. On a phone it means the main camera can physically change the size of its opening, where most phones have one fixed aperture. It is unrelated to zooming.