Is Zeiss 28-85 Parfocal?

[gnegel]

Does anyone know if the CZ 28-85 zoom is parfocal? By that I mean will it stay in focus when you change the focal length. I found it difficult to focus at the 28mm setting, so I tried focussing at 85mm, and then zooming back. However, the focus seems to be slightly off then. Has anyone else noticed this? Checking the focus by zooming seems like such a useful feature.

 

[wilsonlaidlaw]

On a somewhat similar lens my CZ VS 35-70, the lens was close to = parfocal until I snet the lens to have the focus distances adjusted by Contax = agent to give true readings. When it came back, it was a long way out on = parfocal and the distances had gone the other way. I made a big fuss and = surprise, surprise -when it came back not only were distances spot on, so was parfocal. Wilson

 

[mike_nunan]

I may be wrong on this, but I understood that in optical design terms it's very hard to avoid a small amount of varifocal behaviour when zooming, and for that reason it is usually recommended by manufacturers that you focus at the same focal length you intend to shoot at. That's certainly what the Canon literature recommends (I don't have any Zeiss zooms so I can't check the literature, but maybe someone else can chip in?)

HTH

-= mike =-

 

[rico]

Greg,

My copy is almost parfocal, but not quite enough to be useful. Mr Laidlaw's report about parfocal tuning is fascinating: I thought such behavior was inherent to the lens design.

 

[wilsonlaidlaw]

Rico, I suspect you may well be correct that parfocal behavior is built into = the lens. However, for the nearest to manufacturer's optimal performance, I think it may be necessary to have all the adjustments (there seem to = quite a few adjustment screws under the rubber sleeve, the main one of which has = two adjustment parameters) spot on. I have just checked again on my VS = 35-70 and maybe I am just lucky but, if you zoom the barrel very carefully, = the focus zeros on my RX do not change. This works down to about 2 metres = below which there is a very small change. Wilson