Fixed Aperture vs. Variable Aperture Lenses

I responded to a comment on one of my posts, and my response ended up being the size of a blog post, so I’m just going to turn it into one! Please note that the title of this post should actually, technically be “Fixed Maximum Aperture vs. Variable Maximum Aperture,” as I will explain in a second.

If you are getting into dSLR cameras and lenses, you may have noticed that some lenses have a fixed maximum aperture, while others have a variable maximum aperture.  This is spelled out in the name of the lens.  For example, the Canon EF-S 17-55mm f/2.8 IS USM lens has a fixed maximum aperture of f/2.8 at all focal lengths, while the Canon EF 28-135mm f/3.5-5.6 IS USM has a variable maximum aperture which ranges from f/3.5 to f/5.6, depending on which focal length you are using.  (the EF vs. EF-S means that EF lenses can be used on any Canon dSLR, while EF-S lenses are designed for, and can only be used on Canon dSLRs with 1.6x cropped sensors, including all Rebels, 50D, 60D, 7D, T2i/550D, but not the full frame 5D.  IS means image stabilization.  USM means ultrasonic motor, and means the lens has a high quality, rapid, and quiet motor for auto focusing.) The term fixed aperture usually does not mean that the lens only has one aperture setting you can use, but rather that is a common way of saying it has a fixed maximum aperture. So you can change the aperture of a “fixed aperture” lens and set it anywhere from its maximum aperture, possibly f/2.8, to its minimum aperture, perhaps f/32.

Barbes drummer face
Barbes, Brooklyn, NY

With variable aperture lenses, the largest, maximum aperture you can choose when you zoom to the telephoto end will not be as wide open as the largest aperture you can choose at the wide angle end. For example with the 28-135mm f/3.5-5.6, with the lens set at the focal length of 28mm (the wide end), you can use the f/3.5 aperture setting. But with the lens zoomed to 135mm, the widest aperture you can use is f/5.6. This will slightly affect the amount of background blurring – or foreground blurring in the image above, and will decrease the amount of light entering the lens.  Wider, larger apertures like f/2.8 or f/3.5 blur the background the most, which helps to create dramatic images.  The reason not all lenses have fixed apertures is that they require more sophisticated internal parts and mechanisms, such as more lens elements, which thus makes them very expensive (and heavy), so variable aperture is a compromise in order to offer more reasonably priced lenses.

Barbes sax hands
Barbes, Brooklyn, NY

Also, the wider apertures (f/2.8, f/4) are best for low light situations because they allow more light to enter the camera and thus allow you to select a fast shutter speed that won’t blur the image while hand-holding the camera. If you are typically working outside, this shouldn’t be too much of a concern, but if you work indoors or in low light, lenses with wide apertures like f/2.8 or f/1.4 are desirable.

Now, why is f/2.8 called a large aperture and f/22 a small aperture?  2.8 seems like a smaller number than 22, right?  No, f/2.8 and f/22 are fractions.  So if f were to equal 1, a slice of pie that is 1/2.8 of the pie is a bigger piece that a slice that is 1/22 of the pie, right?!  So f/2.8 is a large aperture, which means a large opening, which lets in lots of light all at once, but which then causes objects not in the plane of focus, such as the background, to be blurry.  f/22 is a small aperture, a small opening which lets in just a little light.  But everything from near to far is in focus, like when you squint to see a street sign clearer!  (The letter f in the fraction stands for the focal length of the lens.)

Please leave a comment, ask a question.  Let me know what has been helpful, and what you’d like to read more about.

For additional posts about lenses see Best Lenses for Travel and Humanitarian Photography and Why You Shouldn’t Buy the Kit Lens.

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