Lens: what does this mean?

[PoohJen]

I'm lens-ignorant. Can someone explain to me what the last sentence of this description of the Sigma 18-125 means? TIA!

The Sigma 18-125mm f/3.5-5.6 DC lens was exclusively designed for digital SLR cameras. The image circle (rear of lens) is made to match the smaller dimensions used for the image sensor on digital cameras. As a result these lenses are also more compact and lightweight, and are better matched to digital SLR cameras. This lens has a minimum focusing distance of 19.6" (50 cm) at all focal lengths and is equipped with an inner focusing system. Since the front of the lens does not rotate, a dedicated Petal Shaped Hood and a circular polarizing filter can be easily attached and used (not included).

 

[WillCAD]

Think of the front of the lens like the barrel of a telescope - it moves in and out, but as id does so it does not roatate.

"Petal" hoods are plastic shades that fit onto the front of a lens to shade it from glare, which can cause lens flares. But instead of a hood that is tubular, a petal hood has 4 petals - top, bottom, left, right - that only shade the areas of the lens that the camera's rectangular sensor or film plane actually see.

But since petal hoods are not perfectly round, it's important for the top to remain up, etc., so if a lens were to rotate as you telescope it out, the petals would not line up properly. Every lens I have ever owned (all 4 of them!) has been of the non-rotating type.

Circular polarizing filters are also designed to be turned to a specific direction to filter out glare or reflections in shiny subjects (example: you can align a CP filter to cancel out your reflection in a window, or in the glass of a framed poster or painting). But again, if the lens turns, it takes the CP filter out of alignment, so it's important to have a lens whose end does not turn as you zoom it in or out.

 

[PoohJen]

Thanks, Will, I get it now (I think!). I had originally read that sentence to imply that by not rotating, the lens was somehow deficient, which could be corrected w/ the petal hood and filter.

I see now it meant that such accessories could be used properly on this lens, since it doesn't rotate.

 

[handicap18]

Check out this ebay item. It has a picture of a lens with one of the shades on it to give you an idea. The shade itself will not move when the lens focuses or you zoom in and out.

http://cgi.ebay.com/NIKON-AF-S-NIKK...QQihZ017QQcategoryZ106863QQrdZ1QQcmdZViewItem