i want to buy a mid-to-tele maybe EF70-210 or EF80-200 but both have a rotating front end so using a PL filter is problematic i know those lens with IF or RF do not cause such problem but they more expensive,is there any alternative? also why circular polarizer is recommended for SLR?

-- albert (fibrebundle@hutchcity.com), July 05, 2001

Answers

The 100-300 USM doesn't rotate the front filter ring, but sure enough, it cost's more.

I'll tell you what I've read about circular polarizers though. A Cirular polarizer is needed for most modern SLR metering systems and all/most SLR autofocus systems. It seems they already use a polarised/semi-silvered mirror in front of the AF & meter systems. Using one on your lens often polarizes the light in ways contrary to the internal ones, yeilding over exposure or AF failure. If you turn a linear polarizer in the right position, they work fine, but that is often not the position you want to use it at.

A circular polarizer, has an additional layer behind the polarizing layer that undoes the objectionable part of the effect of the first layer. They also only polarize in one direction. If you reverse the filter there is no polarizing effect. In use, they behave the same way as the linear types.

That's the non-technical explaination. There are a lot more technical versions of this to be found at other photo sites. Maybe someone else will chime in here with better data.

-- Jim Strutz (j.strutz@gci.net), July 05, 2001.

Actually, the rotating front end only matters when you are shooting quick shots. If you have a few seconds it's not hard to focus, adjust the CPL, and then shoot. Consider it as part of the process of "composing" the image. I'm not sure how useful it is to use a CPL for "grab" shots anyway, where the focusing distance is going to be changing.

-- Gary Berg (Gary_Berg@bigfoot.com), July 06, 2001.

Actually, if you can find the 70-210/3.5-4.5 USM (not sure if it's still in the line-up, but should be available used at least) you'll have a lens with ring USM, FTM, and a non-rotating front element. Build quality is nearly identical to 100-300 USM. Jim

-- Jim Simon (jsimon724@aol.com), July 06, 2001.