A variable aperture zoom is a zoom lens whose widest aperture gets smaller as you zoom in. The range is printed on the lens: one 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, a loss of one and a third stops. Any aperture that both ends can reach, such as f/8, stays put through the whole zoom.
The zoom lens guide compares constant and variable designs for anyone choosing a lens. This page is about living with the one you have: how to read the marking, why lenses are built this way, what happens to exposure, flash and video when the aperture shifts, and a way of working that makes the shift harmless. It ends with what the same words mean on a phone.
Reading the aperture range on the lens
The marking gives two maximum apertures: the first for the shortest focal length, the second for the longest. A zoom marked 24-105mm f/4-7.1 opens to f/4 at the 24mm wide end and only to f/7.1 at the 105mm telephoto end, with smooth or stepped values between. On the barrel the same thing is often written as a ratio, such as 1:3.5-5.6.
- The numbers are limits, not settings. They say how wide the lens can open at each end. Any smaller aperture is yours to choose.
- The widest setting may not last long. On many lenses the maximum starts to close soon after you leave the wide end. Zoom slowly and watch the f-number in the viewfinder to learn the pattern of yours.
What matters is how many stops separate the two numbers, because that is the light you lose by zooming all the way in with the lens wide open.
| Marked range | Light lost from wide end to long end | A shutter speed of 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 |
The last column assumes a fixed ISO, with the camera lengthening the shutter speed to hold the exposure. The f-stop scale explains how the numbers map onto stops.
Why zooms are built with a shrinking maximum aperture
The reason is geometric. The f-stop is the ratio of focal length to entrance pupil diameter. The entrance pupil is the aperture opening as it appears when you look into the front of the lens, which is not the same as the physical hole between the blades. To hold a constant f-stop as focal length increases, the entrance pupil has to grow in proportion.
The arithmetic shows the cost. A 70-200mm f/2.8 needs an entrance pupil of about 71mm at 200mm, because 200 divided by 2.8 is 71. The front of the lens must be at least that wide, which forces large, heavy glass and a barrel strong enough to carry it. A 70-300mm f/4-5.6 needs only about 54mm at 300mm, even though it reaches farther, and ends up much smaller and lighter.
A variable aperture zoom still enlarges its entrance pupil as you zoom in, just not enough to keep up. The 24-105mm f/4-7.1 has a pupil of 6mm at the wide end and about 15mm at the long end. Holding f/4 at 105mm would take about 26mm.
The lost light buys three things:
- Size. Smaller elements mean a narrower, shorter barrel that balances on a small camera.
- Weight. Glass is heavy, and its weight rises quickly with diameter.
- Cost. Large elements are harder to make accurately and need stronger mechanics to move them.
That is why the design is common in consumer and travel zooms, the standard zooms sold with cameras, long-range superzooms and the built-in lenses of compact cameras. The optical trade is real and unavoidable.
What happens to exposure as you zoom
The camera knows the maximum aperture at every focal length and shows it in the viewfinder. What it does next depends on the mode. Each row assumes the lens is set wide open at the wide end and then zoomed in.
| Mode | What happens as you zoom in |
|---|---|
| Aperture priority or program | Shutter speed lengthens to match the smaller aperture, or ISO rises if Auto ISO is on |
| Shutter priority | Your shutter speed holds and the aperture opens as far as the lens allows; beyond that the camera warns of underexposure or raises ISO |
| Manual, fixed ISO | Nothing compensates, so the picture darkens by the stops in the table above |
| Manual with Auto ISO | ISO rises to hold the brightness |
Modern cameras adjust exposure automatically as you zoom, so in aperture priority or program the change is easy to miss until the shutter speed has fallen too far. In manual mode, zooming changes the aperture and therefore the exposure unless you compensate with ISO or shutter speed.
The opposite case matters most. Set an aperture the lens can reach at every focal length and nothing changes when you zoom: f/8 on an f/3.5-5.6 zoom is f/8 from one end to the other. Only apertures wider than the long-end maximum are affected. What the camera does when you zoom back out differs between models, so check yours once.
Two side effects follow. Most cameras focus with the lens wide open, so at the long end autofocus has less light and may hunt more in dim conditions. An optical viewfinder also gets dimmer as you zoom in.
The long end in low light, and depth of field
The slow long end reduces low-light ability where it is most needed. A 200mm focal length already demands a relatively short shutter speed for sharp handheld pictures. If the maximum aperture there is f/6.3 where a brighter lens offers f/2.8, you must raise ISO by more than two stops to keep that speed: ISO 400 at f/2.8 becomes ISO 2000 at f/6.3, with the penalty in noise and dynamic range.
In-body stabilization helps with static subjects by letting you hold a slower shutter speed steady. It does not freeze motion: a bird or a child moves just as far during the exposure.
Depth of field shifts too. At the same framing, f/5.6 or f/6.3 keeps more in focus than f/2.8 or f/4, so backgrounds at the long end are less blurred than the focal length alone suggests.
Flash exposure when the aperture shifts
Flash is more sensitive to this than daylight, because the brightness of a flash-lit subject depends on aperture, ISO, flash power and distance. Shutter speed plays no part, so the camera cannot cover 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. With a guide number of 28 in meters, the flash reaches 8 meters at f/3.5 and 5 meters at f/5.6.
Want to work out flash reach for your own unit? Guide number explains how guide number works and its practical applications.
Automatic (TTL) flash. The camera measures the flash through the lens and raises the power to make up for the smaller aperture, so within range you see no change. The limit arrives sooner, though: 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. Zoom in and the room behind the subject goes darker by the lost stops. If that balance matters, choose an aperture both ends of the zoom can hold.
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. On an f/3.5-5.6 lens that means f/5.6 or smaller for the whole clip.
- Set ISO or lighting for that aperture, not for the brighter wide-end value.
- If you cannot spare the light, do not zoom while recording. Reframe and record a separate clip.
Need the rest of the exposure set for motion? Camera settings for video covers the 180-degree shutter rule, aperture for video, ND filters and ISO for video.
A working method: one aperture, Auto ISO and a shutter speed floor
Most problems with these lenses come from treating the wide-end aperture as if it were available everywhere. This routine removes the surprise.
- Find the 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.
- 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 shutter speed floor. Use the minimum shutter speed setting in the Auto ISO menu and choose it for the long end, where shake and movement are magnified most. Or use manual mode with Auto ISO.
- Read the display. Glance at the f-number in the viewfinder after every zoom; the value from a moment ago may no longer be true.
Not sure which values to give Auto ISO? Auto ISO: how to set it up and when to use it covers the minimum shutter speed, choosing the maximum ISO ceiling, and Auto ISO in each exposure mode.
When the light is too low for the long-end aperture, brace the camera and rely on stabilization if the subject is still, raise ISO if it is moving, or zoom out and move closer, which gives back the wider aperture.
Constant aperture zooms compared
A constant aperture zoom offers the same maximum aperture at every focal length. The familiar examples are the 24-70mm f/2.8 and 70-200mm f/2.8 zooms that working photographers rely on, along with lighter f/4 versions. They are heavier, larger and considerably more expensive than variable designs, and they avoid every complication above.
| Variable aperture zoom | Constant aperture zoom | |
|---|---|---|
| Maximum aperture | Shrinks 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 | Darkens toward the long end | 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 build | 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 a variable aperture lens used at f/8 in good light is at no disadvantage from its changing maximum. The gap opens only when you need the widest aperture at the longest focal length.
Weighing up which zoom suits you? Zoom lens: versatility in a single lens covers common zoom ranges and their uses, image stabilization in zoom lenses, and choosing a zoom.
What “variable aperture” means on a phone camera
On phones the phrase means something else. Most phone cameras have a fixed aperture: the opening never changes, and exposure is controlled by shutter speed and ISO. A phone described as having a variable aperture has a main camera whose opening physically changes, either by switching between two sizes or by moving small blades 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, stopped down in bright light or for close subjects that need more depth of field, such as a document or a plate of food.
Two things keep expectations realistic. Phone sensors are small, so the change in background blur between settings is modest. And when a phone with several cameras zooms by switching from its main lens to its telephoto lens, the telephoto usually has a smaller aperture. That resembles a variable aperture zoom, but it is a switch between separate fixed lenses.
Common mistakes
- Setting a manual exposure wide open at the wide end, then zooming. The frames at the long end come out dark. Fix: set the long-end maximum aperture from the outset, or turn on Auto ISO.
- Assuming the aperture from a moment ago. You plan around f/3.5 and find at the long end that the depth of field is deeper than expected and the shutter speed has dropped below what you can hold. Fix: read the f-number in the viewfinder after each zoom.
- Letting aperture priority choose a slow shutter speed. Long-end pictures are blurred. Fix: set a minimum shutter speed in Auto ISO.
- Expecting manual flash to carry as far when zoomed in. Fix: raise flash power or ISO, move closer, or work at one aperture throughout.
- Zooming during a video take with the lens wide open. The brightness shifts on screen. Fix: stop down to the long-end maximum before recording.
Try this
Take fifteen minutes to map your own lens. Point the camera at an evenly lit plain wall. In manual mode with a fixed ISO, zoom to the wide end, set the widest aperture and adjust the shutter speed until the meter reads zero. Take a picture, then zoom to each focal length marked on the barrel without touching any other control and take a picture at each, writing down the f-number the camera shows. Review the frames and watch the histogram slide left as the aperture closes. Then set the long-end maximum aperture, correct the exposure once and repeat. This time the histogram should stay where it is.
Frequently asked 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?
Because you have set an aperture wider than the lens can manage at the longer focal length, and 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 bad?
No. It trades light at the long end for a lens that is smaller, lighter and cheaper to make. 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 limit is physical: 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.
Can I use a variable aperture zoom for video?
Yes. Set the aperture to the long-end maximum or smaller before recording and exposure will hold through a zoom.
What is a variable aperture on a phone?
A phone camera whose opening can physically change size, where most phone cameras have one fixed aperture. It is unrelated to zooming.
Related guides
- What is aperture in photography?: how the opening controls light and depth of field.
- Understanding f-stops: the scale behind the two numbers on the lens.
- Zoom lens: zoom ranges, stabilization and choosing a zoom.
- Auto ISO: the minimum shutter speed and ISO ceiling that make these lenses easy to use.
- Manual mode: setting exposure yourself, including manual with Auto ISO.
- Guide number: how aperture sets the reach of a flash.
- Camera settings for video: keeping exposure steady while recording.