Zooms vs. Varifocals
I've been shooting with Canon since the early 1970's. Back then, there were no zooms for any brand of still camera, and all lenses were manual focus. In fact, there was no automatic exposure. Through the lens manual metering was state of the art.
When zoom lenses first started to appear in the late 1970's, there were two designs: true zooms, and varifocal. A true zoom would hold focus as focal length was changed. Varifocals had to be zoomed first, then focused afterwards.
This being the manual focus era, standard practice was to zoom in to maximum telephoto (there were no wide angle zooms in the beginning), set focus, then pull back to compose. Zoom lenses were far more popular than varifocals because zooming in to focus allowed much more precise focus, same as magnified Live View today.
My belief is today's auto focus zoom lens designs are primarily varifocals, not actual zoom lenses. When auto focus became the norm, maintaining true zoom performance became irrelevant on still lenses. Other design objectives became more important, like faster apertures, wide angle zooms, etc.
I've never bought into the true value of auto focus, and all these cameras with millions of program modes in some ways obfuscate photographic fundamentals and, in the quest to simplify everything for people who don't know what they are doing, everything has become overly complicated. After all, real audio guys never use auto gain (levels) and real cinematographers manually set everything like ISO, shutter speed, aperture, etc. The first thing all these online cinematographer tutorials coach DSLR filmmakers how to do is to turn off all the in-camera enhancements.
We all know that modern lenses are optimized for auto focus, thus the short throw non-mechanical focus rings and lack of useful depth of field scales. It stinks for video, and makes a clear space in the market for Zeiss manual focus primes. Not to mention crazy stuff like PL mount conversions for true cine glass (too much trouble and expense for H.264, in my opinion).
My question is this: can anyone identify true zooms within the modern major still lens lines? I'm not satisfied to use the camera's auto focus confirmation. What makes sense is to zoom in and make an actual creative decision about plane of focus, then pull back to the final composition. Because that's what shallow DOF is all about.
My specific interest are the Canon L series zooms, the three that I own being 16-35mm f/2.8 (Series I), 24-70mm f/2.8L, and the 70-200mm f/2.8 IS (Series I).
When you say true zooms- you're really asking which lenses have manual focus marks? or?
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No, I am saying lenses that hold focus while focal length is changed. In other words, something you can zoom during a take, and focus holds, even at large aperture w/shallow depth of field.