What is a Constant Aperture Lens

In Uncategorized by Jim Harmer13 Comments

What is a constant aperture lens?

Learning about aperture is one of the core fundamentals of photography, but it can also be confusing with all those numbers on especially on the barrel of your lens. Lenses come in variety of flavors but we’re going to concentrate on constant or fixed aperture lenses versus variable aperture lenses.

Let’s look at the Nikon 70-200mm f/2.8 lens. It’s a constant or what is also known as a fixed aperture lens. How can you tell? It states it is an f/2.8 lens – it’s a single number. A variable aperture lens would be the Nikon 18-200mm f/3.5-f/5.6 lens where two f-stops are listed on the lens.

Now what does all that mean?

The Nikon 70-200mm lens has a focal range of 70mm at the short end and can zoom out to 200mm. By having that single f/2.8 number, it can maintain a small aperture as you zoom in and out. You can shoot at f/2.8 at 70mm, at 100mm, at 150mm as well as 200mm and everything in between. Also keep in mind that apertures are fractions – the larger the number, the smaller the opening in the lens and the smaller the number, the larger the opening.

With a constant or fixed aperture lens, the aperture functions independently of the lens focal length. The barrel of the lens does not extend or retract when the focal length changes.

A variable aperture lens means that it will shoot at f/3.5 at the shorter end of the focal length – at 18mm and change to f/5.6 at the longer end or at 200mm. It’s the maximum aperture the lens can open up for each end of the zoom range. Generally, variable aperture lenses are less expensive. Many manufacturers bundle them in a package deal when you purchase the camera body. That’s usually a good deal particularly for someone new to photography. You get a camera body and a decent lens for a good price. The manufacturer takes some of the guess work out of the equation. It can be hard enough for a consumer to figure out what camera to buy let alone what is a good starter lens.

Once you gain a comfort level in photography, the next step is to upgrade to a constant aperture lens. These lenses are usually heavier, better constructed and more expensive but it’s a higher quality of glass in lens. The benefit is that you will have the larger aperture all throughout the focal range – and more light means you will be able to shoot in low light situations.

But don’t think that constant aperture lens can only shoot at one aperture, say f/2.8. They are able to switch to all the apertures that are normally available – the entire range of apertures all the way up to f/22 or f/32.


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Jim Harmer

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Jim Harmer is the founder of Improve Photography, and host of the popular Improve Photography Podcast. More than a million photographers follow him on social media, and he has been listed at #35 in rankings of the most popular photographers in the world. He blogs about how to start an internet business on IncomeSchool.com..

Comments

  1. Hi Jim,

    I have a Canon EF 70-200 f 2.8 L IS lens. Although it is a constant aperture lens, in low light situations my EOS 70D in auto mode shoots at a lower ISO at the 70mm range than the 200, even on a uniform subject such as a wall. This indicates that I have more light at 70mm than 200. Am I missing something here, or is the effective aperture changing a little?

    Thanks

    1. You have less light because you are seeing less of the wall, less surface reflecting light into the camera when you zoom in. The aperture hasn’t changed, your framing has. Now if you zoomed in AND walked backwards until you framed the same amount of wall as you saw at 70mm then the camera would use the same ISO, given a constant aperture.

    2. Are you getting the same shutter speed?

      If you’re in auto or aperture priority, the camera will select your shutter speed for you, and the acceptable speed is based on your focal length, to avoid camera shake. The shorter you go, the slower your shutter speed can go. You get around one and a half stops of difference between 70 and 200, which I’m guessing the camera will handle as two stops of ISO. So you’d get 1/80s @ 100 ISO for 70mm and 1/320s @ 400 ISO for 200mm, for example.

      The specific numbers and ISO will change depending on your camera and light levels. Some cameras can handle jumps in ISO of less than a stop, some can’t.

  2. You wrote about fix apertures

    “But don’t think that constant aperture lens can only shoot at one aperture, say f/2.8. They are able to switch to all the apertures that are normally available – the entire range of apertures all the way up to f/22 or f/32”

    If I have a wide angle lens lens 21-35mm f/2.8 for example, can I still take a photo without blurring the background and, what would be the f range ?

    I am planning to buy a wide angle lens for a Sony SLT A77 II. I am not as much interested in telephoto (zoom lenses) as I am in wide angle photography.

    Any recommendations?

    Thank you for your input

    Zav

    Thank you for your input.

  3. you wrote:

    “But don’t think that constant aperture lens can only shoot at one aperture, say f/2.8. They are able to switch to all the apertures that are normally available – the entire range of apertures all the way up to f/22 or f/32.”

    With a wide angle lens 21-35mm f/2.8 can I take photos without blurring the background? And if so, what is the f range?

    I am planning to purchase a wide angle lens for a Sony SLT A77 II. Interested mostly in wide angle photography and less in telephoto.

    Any suggestions?

    Thanks for your input

    Zav

    1. I believe the A77-m2 has an APS-C sensor … your 21-35mm lens wont be “wide” in the conventional norms. BTW, check the samyang / rokinon series : they have some good but not-so-costly lenses

  4. Can Fixed Aperture lenses reach f7 or f20 or are they stuck at say f2.8?

  5. Thanks for this. The last paragraph of your article answered the question I have been searching the net for weeks to understand. IE “But don’t think that constant aperture lens can only shoot at one aperture, say f/2.8. They are able to switch to all the apertures that are normally available – the entire range of apertures all the way up to f/22 or f/32.”

  6. Gentlemen
    Greetings
    I need some assistance, here is my problem, i need to shoot low light music gigs, have a 600d cannon with a 50mm canon f.1.8 lens this works well but was restricted with angle and distance,I also have a 18 to 250 which works well but not in low light
    I have since bought a canon 7 D body my question is the zoom from the 18 to 250 and but the low light from the 1.8 50mm lens i have a few questions.
    What will give me the 18 to 250 zoom with a fixed low aperture, this i need for low light music gigs and music events, sorry for all the questions but i am a new to photography
    Thank you kindly
    regards

  7. Hi, I have a Cannon 2ti. I’m looking to get an affordable fixed aperature lens under $500 What one do you suggest ?

  8. Hi, does a 70-200 still have a constant f2.8 with different settings such a Scene Selection or Shutter Priority….without being in the Aperture Priority mode?

  9. Thanks for this explanation, it’s been really helpful, particularly the last paragraphy about not just being able to shoot in only one F stop but being able to go higher if you want. I have just purchased the Sony PZ 18-105 G with constant f4 but was having trouble getting my head around certain aspects. Thank you again, really helpful! 🙂

  10. You can’t zoom OUT from 70 to 200. It is: “and can zoom in to 200mm” in the 3rd text paragraph.
    Furthermore: fixed aperture and constant aperture are 2 different terms: Constant is what is described here, fixed means: it CANNOT change. So it will always be the same and could also be a prime lens. This is very likely an old lens.

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