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.