Trying to choose my all-around zoom.

Most of the 70-200's are parfocal I believe. My Tamron maintains perfect focus through the zoom range in all the testing I've done.

It's great to be able to use the zoom in/pull focus/zoom out technique just like on a camcorder, but I need to get used to the fact that my other lenses don't do it!
 
So just to clarify, are parifocal zooms by their very nature NOT constant aperture?
If so, then if I had to choose, constant aperture would take priority for me. Can't think of too many times I'd have to zoom while filming.
 
I've been very happy with the 17-85 (retails for about 500). Just the amount of sharpness you want for video. Imo pretty much the perfect range, though just a liiiiiiitle wider & faster would be sweet... but then, it would cost more ;)
 
eco_bach said:
So just to clarify, are parifocal zooms by their very nature NOT constant aperture?
If so, then if I had to choose, constant aperture would take priority for me. Can't think of too many times I'd have to zoom while filming.
Click to expand...

it has nothing to do with that
look at the list I linked to, with the parfocal stills lenses: all but one are constant aperture!!
 
Tokina in their early literature claimed the 80-200 f2.8 (manual focus) was parfocal, but it's not. It's difficult & expensive to design a lens like that, and there's not much incentive for still lens manufacturers to do it, with autofocus being so fast. The only reason for parfocal for stills is if the lens was manual focus, and you were shooting birds or something, you could zoom in our out without having to change focus. Only cinema lenses (like Arri or Zeiss) or video lenses are practical for zooming while filming.
 
eco_bach said:
So just to clarify, are parifocal zooms by their very nature NOT constant aperture?
If so, then if I had to choose, constant aperture would take priority for me. Can't think of too many times I'd have to zoom while filming.
Click to expand...

As Norman said there is no correlation between constant aperture and parfocal zoom lenses - except for the fact that both are generally only found on higher quality lenses.

Being Parfocal is not necessarily an advantage because it lets you zoom during a shot. It is beneficial because it allows you to zoom in fully, magnifying whatever you are trying to focus on as well as making DOF shallower so you can be more precise with your focusing. Then you can zoom back out to get correct framing and the focus will stay exactly the same. It is essentially a better, optically optimised version of the focus assist zoom feature.

It is also particularly helpful in events such as weddings. You might want to change framing, knowing you will cut around it it post. With a varifocal lens, that means you need to stop recording (so you can access focus assist), re-frame, engage focus assist zoom, re-set your focus, then re-start recording. With a parfocal lens you just re-frame your shot and you're done (assuming focus was set correctly in the first place).
 
plus, zooming in with the lens itself reduces the DoF, so focusing is actually much easier than with the digital focus assist
 
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