What's with a variable minimum aperture?

 

Comparing zoom lenses:  Variable minimum aperture (top) versus fixed minimum aperture (bottom).

When you look at the specifications printed on a lens, it usually denotes brand (Nikon), focal length (single number for prime or range for zoom), and often other tidbits such as type of glass (ED), type of mount (G), build quality (S and N).  Another thing you usually find is the minimum aperture denoted as a 1:___ ratio.  The question is, is it a single number (1:4) or a range (1:3.5-4.5).

All prime lenses have a single minimum aperture, usually lower than zooms of similar focal length.  Zooms may have either a single minimum aperture common to all focal lengths or the aperture may vary depending on what focal length the zoom is operating at.  In general, you will find that more expensive zooms have a fixed minimum aperture while their less costly counterparts have a variable minimum aperture.  It's not only the common minimum aperture that's pricey though, these lenses often have a host of other features making them perform well.  Such features include better glass (low dispersion), faster focusing, and stabilization.

Having a fixed minimum aperture is desirable when shooting in an evenly lit environment and needing to alter focal length.  Shutter speeds, ISO, and aperture values all stay the same meaning that there is no need to continually check exposure settings; just shoot and go.  A very popular zoom is the 70-200 f/2.8.  It typically costs around $3000 CAD, taxes in, but performs brilliantly in sporting environments.  The 70-300 f/4.5-5.6 lens is generally a kit lens which has a greater zoom range but lacks the high quality glass and ultra-fast focusing.  It can be bought new for around $900, all in.  

This is not to say that variable minimum aperture lenses are not good; I have owned quite a few of them over the years and many have been excellent.  I currently use a Nikon 80-400 f/4.5-5.6 S series lens that I love.  I also have a few wide angle zooms, although they all have a fixed minimum aperture.

Lenses that have a wide zoom range often have variable minimum apertures.  This is because it isn't feasible to build a lens that has both a wide zoom range and fixed minimum aperture.  When zoom ranges are around 3x, it is possible to construct one.  The 70-200 is just under 3x while the 80-400 is 5x.  The greater the zoom range, the higher the chance the lens has a variable minimum aperture.

The bottom line is to look for a lens that is either in your price range or performance range.  A fixed minimum aperture is desirable, but not the only limiting factor on making a purchase.  Understand though that fixed minimum aperture lenses often cost more and perform better than their comparable variable minimum aperture rivals.


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