It can be argued that zoom lenses rule in the photography world. You can go from shooting portraits one minute to still life the next without ever having to change a lens. The beloved Canon 24-105 f/4 seems to be one of the go to lenses for social outings and sometimes, weddings; videographers even love it due to the constant f/4 aperture. It, like most modern DSLR zooms, seem great until you switch it to manual focus.
In an ideal world, you manual focus a zoom lens at the longest focal length and then zoom to the desired focal. If you have had any experience with DSLR zoom lenses, you probably know that they do not work this way. Most modern lens are varifocal and change focus at different focal lengths. Few lenses like the Canon 17-40 f/4 and the Nikon 24-70 f/2.8 AF-S are parfocal; the focus remains constant throughout the zoom range.
Lens design is always a series of trade-offs; if money, size and weight are no object, you can build a lens that will be parfocal, fast and have high quality optically. It’s incredibly difficult to design a cost effective parfocal lens. It is more important for manual focus lenses to be parfocal because a well functioning auto-focus can quickly adjust the lens to keep it in focus. Auto-focusing zoom lenses have relatively few and loose focusing cams in order to make auto-focusing possible with weak focusing motors. This, to a large degree, is why the majority of all zooms made for DSLRs are varifocal.
Parfocal lenses are most useful for video work where it would be awkward to keep having to pull focus as your zoom changes. High-end broadcast camera lenses and some cinema lenses are traditionally parfocal. There are a few DSLR lenses that work great for video like Canon’s 70-200 f/2.8 Non-IS and the Panasonic 7-14 f/4.
Varifocal lenses should be considered as a handful of primes, each requiring its own separate focus. Parfocal lenses are “true” zooms and can be considered as one lens. So, should you get a parfocal lens or not? In most cases yes, if the lens has an optical and ergonomic advantage over the varifocal counterpart. If this is not the case, it is pointless because of modern auto-focus systems. Your money would be better spent elsewhere.
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