Defective EF 35-80 mm Lens?

[shaval]

Here's a pic. Manual Focus and Auto Focus. Manual focal lenght zoom.
If he was refering to how the aperture changes while zooming (focal lenght) then the lens is not defective.
But if he says that the lens aperture changes while autofocusing, I don't think that's possible.

 

[SoCalTiger]

Here's a pic. Manual Focus and Auto Focus. Manual focal lenght zoom.
If he was refering to how the aperture changes while zooming (focal lenght) then the lens is not defective.
But if he says that the lens aperture changes while autofocusing, I don't think that's possible.

^ I think this is what is happening.

 

[unpopular]

Shaval - inside the lens there is an encoder which is operated by several brush contacts which in turn sends data to the camera. These brush contacts slide along the encoder when you you adjust the lens. I am not sure if the focus position is related to the camera or not, but focal length position is.

It is possible, though very unlikley, that their is a defect in this mechanism which would cause the encoder to report back to the camera an incorrect focal length, and thus also an incorrect aperture. The user would only see the aperture change.

However, this is very, very unlikely and given that your tests indicate that the lens is functioning properly I am guessing that the buyer either just wasn't happy with the lens' performance or was not familiar with how variable f-ratio zooms are supposed to function.

 

[TCampbell]

Here is the link to Canon's page on that lens: Canon U.S.A. : Support & Drivers : EF 35-80mm f/4-5.6 III


It does auto-focus ... it has a switch on the side to switch between auto & manual focus modes. It also zooms... but as is the case with all zoom lenses on a DSLR camera, YOU twist the zoom ring to perform the zoom (there's no electric motor responsible for zooming like there is on a point & shoot camera.)


Your first answer to the post correctly described that when you have a "variable focal ratio zoom" lens such as this one, the maximum aperture at the shortest focal length setting will be larger than the maximum aperture available at the longest focal length. This is completely normal and not a defect.


You would only notice the behavior when attempting to use a low f-stop (e.g. trying to use f/4 and then zooming to 80mm would force the f-stop to change to f/5.6). If you set a higher f-stop (e.g. set the f-stop to... say, f/8, then it won't change as you zoom because all focal lengths can support f/8 on that lens.)

As they've already returned the lens, I'm guessing they later decided they just didn't want it -- or were unfamiliar with the industry standard notation used to describe a "variable focal ratio" lens (f/__-__) where the two values in the blanks represent the lowest f-stop available at the shortest focal length and longest focal length respectively.