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Why Does Aperture Change When I Zoom My Lens?

In How-To by Jeff Harmon9 Comments

The aperture changes as you zoom your lens because the lens does not physically support the widest (smallest number) aperture at all focal lengths of the lens. This is most often something photographers see in very inexpensive lenses.

Congratulations! You are far enough along in your photographic journey to have noticed that your lens doesn’t allow you to set the widest open aperture at all focal lengths. When the lens is zoomed out (at the smallest focal length) you can probably set the aperture to f/3.5 but when you zoom the lens you can only get that aperture opened up to f/5.6 which makes it hard to get enough light indoors.

Your lens is not broken. Nothing is wrong with your camera. Having the widest aperture (small number) change as you zoom is something lens manufacturers do to produce inexpensive lenses (less than $500). It costs more money to manufacture and develop lenses that can have the same wide aperture from zoomed out to zoomed in.

Variable Aperture Lenses

When the widest aperture available on a lens changes as you zoom it is called a variable aperture lens – the aperture capabilities “varies” as you zoom. Lenses that physically allow the aperture to be the same throughout all of the zoom of the lens are called “fixed” or “constant” aperture lenses and they are usually quite a bit more expensive ($2,000+).

You can know if a lens is variable or constant aperture by some of the numbers that you find on the lens or in the description of the lens as you are looking to buy them. Let’s walk through an example using the image you see here.

You may see something like “18-55mm 1:3.5-5.6” written on the front or side of the lens. The “18-55mm” is telling you the focal lengths of the leans. It means when the lens is not zoomed it has an angle of view of 18mm (very wide angle of view that fits more of the scene than what you see with your eyes). When you turn the zoom ring on the lens it will zoom into the scene to a maximum of 55mm (an angle a view that is “zoomed” in more than what you see with your eyes).

The “1:3.5-5.6” looks scary, but now that you are reading this post it won’t be. It combines with the information about the focal length to mean that when the lens is at its widest focal length of 18mm the aperture will open up as wide as f/3.5 and as you zoom the lens to the longest focal length of 55mm the widest aperture you can use is now f/5.6.

If you are shopping online and looking at buying a lens then you will probably see a description more like “18-55mm f/3.5-5.6” which should now make a lot more sense to you of what that means.

Another common Canon lens that has a variable aperture is the 55-250mm f/4-5.6. Can you now decipher what that means?

Are Variable Aperture Lenses Bad?

As with most things in life you get what you pay for and there is a reason lenses are really inexpensive (less than $500). These lenses are often referred to as “kit” lenses because they are the kind that come with your camera. If you know what you are doing you can create incredible images with them, but image quality dramatically increases as you invest some in higher quality lenses.

With these very inexpensive lenses that are variable aperture they do fine when there is plenty of light like shooting outdoors while the sun is out but they don’t have a wide enough aperture to let in enough light to do well with night or indoor photos.

On the other hand, there are also some expensive and high quality lenses that are still variable aperture. You have seen these lenses on the sidelines of major sporting events. Those lenses are either prime (they don’t zoom in and out) or they are variable aperture like the Canon EF 100-400mm f/4.5-5.6L. These are super high quality lenses that are still variable aperture.

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  1. I purchased a Sigma 60/600. It was not inexpensive and it is a variable aperture

  2. Your answer explains what happens. The question was why it happens.

    1. Author

      @Burnel, it is physics. It takes larger lenses and glass elements to allow the lens to have a constant aperture throughout.

    1. Author

      It is a mathematical representation of the aperture being a ratio. All f-stops are ratios measuring the size of the opening created by that aperture compared to the whole circle being open. 1:1 would be an aperture with the whole equal to the size of the opening, stopping nothing.

  3. Pingback: What Is The Difference Between Using A Wide Aperture And Small Aperture? – Erickkasysavane

  4. Do I have a comment!!! Let’s say you have two lenses: one is 50mm, and the other is let’s say 200mm. Both lenses are set at the same f/stop. It doesn’t matter what the f/stop is. But lets say both lenses are set at f/8. It could be f/5.6, f/11 Any f/stop, but the same f/stop. The f/8 on the 200mm lens will be wider. Do you know why? Let me try to make things simple IF I can. It’s called the inverse square law. Most people taking pictures never heard of this. and really you don’t need to know this. If the exposure is 1/250 @ f’/8, then set the shutter on your camera to 1/250 and the aperture on your lens (no matter what focal length lens you have) to f/8. Depending on the focal length, the actual size of the opening (f/8) will vary. I know the reason, but it’s kind of hard to explain.

    Let’s say you have a light on a stand in a reflector. It’s placed one foot from a wall. and in my example, the light covers one square foot. All the light that comes from this light source will cover one square foot. it’s a 100 watts. So 100 watts will cover one square foot. Now when we move the light two feet from the wall, the light will cover four square feet. and each square foot (there’s 4 of them now) will receive 25 watts. And if we move the light back to three feet, the light will cover nine square feet, and each square foot will receive a little less the `10 watts…..and so on. And in order for the exposure to be the same, you must use a larger and larger f/stop (lens opening) so at one foot, the f/stop may be f/16. But at two feet the f/stop would be f/8. and at three feet the f/stop would be f/5.6 So as you can see, the f/stop is getting bigger and bigger the farther away the light source is from the wall.

    Now for the lenses. Both the 50 and 200mm lenses are at the same distance. Let’s say 20 feet. If you’re photographing a person with a 50mm lens, their head will cover a certain area on the film. But now if you use a 200mm lens, the head will cover a larger area on the film. The same light is reflected from the face with any lens. So because the face covers a larger area like the light. you must use a larger opening. So if its a sunny day and you’re photographing a person 1/250 at f/16 it is with any lens.

    Hope i explained it right and you the reader got it.

  5. I always thought that as you zoom the lens in, the aprature becomes larger and not smaller. In your example you say at 18mm, the f/stop is 3.5. But when you zoom in to 55mm, the f/stop becomes 5.6. this is somewhat confusing to me. Let me explain. As I said in my comment above, the physical size of the opening will change when the focal length of the lens changes. I have a 50mm lens, and I am set at f/2. That actually means the physical size is 25mm. 1/2 of 50 is 25. Now if I have a 200mm lens and I’m set at f/2, the actual physical size of the opening will be 100mm. This must be that way because of the inverse square law. So if I’m taking a picture under a certain light and the exposure is 1/125 @ f/2, I set the lens opening at f/2 regardless of the focal length. I let the lens designers (engineers, mathmaticians..etc) worry about the size of the opening.

    Two holes of the same size will let the same amount of light enter. But not when it comes to different size focal length lenses. F/2 on a 200mm lens MUST be larger then the f/2 on a 50mm lens because of the inverse square law. So when you zoom in, the opening should become wider and NOT smaller. But you say in your example that at 18mm the f/stop is 3.5, and at 55mm the f/stop is 5.6. The f/stop should remain the same f/3.5, BUT as you zoom in, that F/3.5 should become wider because of the inverse square law. Can you explain this?

    1. Author


      This is not an article that has anything to do with the inverse square law. This is about variable aperture zoom lenses vs constant aperture zoom lenses.

      Variable aperture lenses do not offer all of the aperture settings throughout all of the focal lengths of the lens. A common example of is the Canon EF-S 18-55mm f/3.5-5.6 IS STM Lens. At the widest focal length of 18mm the lens offers a maximum aperture of f/3.5. As you zoom in to larger focal lengths the lens no longer offers that f/3.5 aperture. At the longest focal length of 55mm the maximum aperture is limited to f/5.6. These lenses are less expensive for manufacturers to produce, which means they cost less for photographers, and as a result lenses like these are often included in the purchase of a camera (“kit” lenses).

      Constant aperture zoom lens offer the same aperture settings no matter the focal length. A common example is the Canon EF 24-70mm f/2.8L II USM lens. The maximum aperture for that lens is f/2.8 at 24mm and all the way through to 70mm. These lenses are more expensive to manufacture, making them much more expensive to photographers, in same cases more than the camera body.


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