Hi All

This is again George Mathew. This time I am here with my new gear 28-105mm EFII USM. I just bought it and found some thing new(for me) in that. Here are my questions.

1. I can see a Distance Scale Window in it. What is the actual use of it. I know that it is related to focusing. But want to know the technical details of it.

2. Also what are those "Infrared Index" and "Distance Index"? What it means?

3. What is meant by "Infinite-Focus" compensation mark. This I found in the Instruction manual. It says some thing in relation with Temperature. I just know that "Colour Temperature" is also one factor in exposure. Dont know much about it.

4. I read in a review that this lens has the capability of Full time Manual focus overriding. It said we can override the focus achieved by the camera (in AF mode) just by turning the focus ring with out changing it to MF mode. But I could not find any thing in the instruction manual. Is it true? Can we do it?

Thanks in Advance George

-- George Mathew (george_mathew2k@yahoo.com), December 28, 2001

Answers

1) This shows the distance that the focused subject is from the film plane. It is useful in doing hyperfocal and depth-of-field calculations.

2) The infrared index shows where the lens is in focus if using infrared film. This varies from the "normal" focus distance because of the different wavelength of infrared.

3) This is the fact that the lens will focus beyond infinity. The reason to allow this is that at different temperatures (heat, not colour) the materials in the lens expand and contract, causing the infinity focus-point to change. Allowing the lens to focus to what, in some situations, is beyond infinity will, in other situations, just allow the lens to get to infinity.

4) Put the camera into 1-shot AF mode, and get the camera to focus the lens onto your subject. Once focus is locked, and the ready indicator has come on in the viewfinder, turn the focus ring, and you will find that it is possible to adjust the focus, and the camera will still allow you to fire the shutter.

-- Isaac Sibson (isibson@hotmail.com), December 28, 2001.

Full time manual focus (FTM,as canon named it) also allows you to manually fine-tune focus in servo mode. Usually,only lenses with ring-type usm have FTM.

-- legnum (legnum212@email.com), December 29, 2001.

I'm not sure what Legnum's trying to get at, unless he's just trying to confuse the issue. FTM does not work with servo focus mode. As soon as you manually readjust the focus the camera refocuses back to what it was before. So if you want to use FTM, stick to 1 shot focus mode.

-- Jim Strutz (j.strutz@gci.net), December 29, 2001.