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Fixed aperture vs variable aperture zoom lens

As a total beginner, it can be confusing to understand the meaning of F/3.5, F/5.6, F/2.8, etc. written on DSLR camera lenses. Some lenses have two F numbers, while others have only one F-number. So what's the difference between these two lenses? How do they work when changing in-camera settings?
Lenses with single F-number value like F/1.8, F/2.8, F/4, F/5.6 are known as fixed aperture lenses. Fixed aperture lenses are not to be confused with fixed focal length lenses, fixed focal length ones are called prime lenses.  Prime means the lens will only have one focal length, and all the zooming in and out part is done by the user moving physically. On the other hand, lenses with multiple values such as F/3.5-F/5.6, F/3.5-F/4.5, etc. refer to variable aperture zoom lenses. In simple words, these lenses change their widest aperture values when zoomed in or out.

variable aperture vs fixed aperture lens

Example of fixed aperture prime lenses:
Canon EF-S 24 mm F/2.8
Nikon AF-S Nikorr 85 mm F/1.4G
Nikon AF-S DX 35 mm F/1.8G
Canon EF 50 mm F/1.8 STM

Variable aperture zoom lens: F-number changes when zoomed in or zoomed out without user's permission even in Manual mode. The first value is for the lowest zoom, another one is for maximum zoom mentioned on the lens. Example: 
Canon EF-S 18-55 mm EF-S F/3.5-F/5.6
This lens will not allow the camera aperture value of less than F/3.5 at 18 mm focal length. When zoomed into the scene at 50 or 55 mm, the value of F/3.5 will change to F/5.6 due to lens optics limitation. This means in-camera aperture value will stick at a minimum of F/5.6 when zoomed in. In terms of the exposure triangle, this means less light is available to take photos. In terms of background blur (bokeh), this means lesser blur (there are other factors too). Few examples of well known variable aperture zoom lenses include: 
Canon EF 70-300 F/4-F/5.6L IS USM
Canon EF 75-300 F/4-F/5.6 III USM
Canon EF-S 55-250 F/4-F/5.6 IS STM
Canon EF-S 10-18 mm F/4.5-F/5.6 IS STM

Fixed aperture zoom lens: Zoom in or zoom out, the value of F-number will stay the same. Simply, the zoom will have no effect whatsoever on the F-number value unless the user changes it by themselves inside the camera. Few examples of well known fixed aperture zoom lenses include: 
Sigma 17-50 EF-S F/2.8 EX DC OS HSM
Sigma 18-35 f/1.8 DC HSM Art
Canon EF 70-200 F/2.8L IS II USM
Canon EF 24-70 F/2.8L II USM
Tokina AT-X Pro DX II 11-16 F/2.8
Tokina AT-X 11-20 F/2.8 Pro DX
Tokina AT-X 16-28 F/2.8 Pro FX
Canon EF 16-35 F/4L IS USM

Fixed aperture vs variable aperture lens:
Fixed aperture lenses are better as the user does not have to think about aperture getting changed just because focal length was varied. This sometimes leads to poorly exposed shots. Also, fixed aperture makes life a lot easier when shooting a fast-paced event like a wedding, sports, etc. Let's talk about a few points comparing both with each other: 
Sharpness: Fixed aperture lenses are in general more sharp than variable aperture zoom lenses. Also, most of the fixed aperture lenses are made with professional grade quality glass which naturally produces finer images.
Cost: Fixed aperture lenses are more expensive as compared to the equivalent variable aperture zoom lens. Some of the variable aperture zoom lenses are also made professional grade quality glass which makes them more expensive than equivalent lower quality glass variable aperture zoom lens.
Weight: Fixed aperture zoom lenses are heavier as compared to variable aperture DSLR lenses. This is because a lot more amount of glass is used to manufacture fixed aperture lenses.

Hope this post will help you understand your camera lenses in a better way. Share the photography love by sharing this post. :) 


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