Advanced? lens buying terms?

[DB83]

I understand the basics with lenses, but there are a few terms/abbreviations that I have come across that I do not quite understand:
CA
barrel distortion
vignetting
using tape for certain pins

Can anyone explain?

 

[WTF?]

CA stands for chromatic aberration, which is the bluish kind of halo that appears sometimes where theres a sharp transition between light and dark

barrel distortion is usually found at the edges of wider angle lenses, its where it like...curves everything outwards, an example of extreme barrel distortion:
http://upload.wikimedia.org/wikipedia/commons/thumb/b/b4/Fisheye_photo.jpg/600px-Fisheye_photo.jpg that was taken with a fisheye lens, which intentionally uses this effect for a more unique image, but in most lenses the effect is much more subtle, and undesired.

vignetting is where the image darkens in the corners.

and i dont know what you mean by "using tape for certain pins"

hope i helped!

 

[DB83]

yes, you did. I'll have to look for chromatic aberrations in pictures.
And as for the tape over the pins, it has something to do with the auto-focus system. But that's about as much as I know about it.

 

[Big Mike]

Don't be concerned with the 'tape over the pins'. It's a 'hack' that some people have done, to limit the communication between camera and lens. I think there is a very limited set of circumstances where this is a good idea...so it's rather unlikely that it's something you would have to (or want to) do.

 

[KmH]

Chromatic aberration - Wikipedia, the free encyclopedia

Barrel distortion is often accompanied by pin cushion distortion at the other extreme of a zoom lenses focal length.

 

[DB83]

Don't be concerned with the 'tape over the pins'. It's a 'hack' that some people have done, to limit the communication between camera and lens. I think there is a very limited set of circumstances where this is a good idea...so it's rather unlikely that it's something you would have to (or want to) do.

What purpose does this serve?

 

[Big Mike]

The issue (I believe) is that when you use a teleconverter with a 'slower' lens (a lens with a smaller maximum aperture), the autofocus may not function properly. The reason being that the camera wants at least F5.6 for the AF to work....and teleconverters steal some light. In some cases, the camera recognizes that there is a TC attached (it goes between the lens and the camera) and if it also sees that the lens doesn't have a large enough aperture, it disables the autofocus.

By taping the pins, you limit the communication to the camera, so that it doesn't know what the maximum aperture of the lens is...thus allowing AF to try to focus. It may or may not have enough light to actually work well, but at least it's not automatically disabled.

 

[Dwig]

CA = Chromatic Aberation: There are two distinct types of CA. Both involve the displacement of the image in different portions of the spectrum, much like the colors created by a prism or in a rainbow. The first form is classical lens based chromatic aberation. This affects both film and digital and is the form explained in the Wikipedia link provided in another post. The second is a purely digital issue and occurs in almost all digital cameras to some degree. It is a sensor based aberation that occurs in the micro lenses that are part of the sensor system. The degree that these lenses produce CA is affects by a range of characteristics in the primary camera lens.

Pincushion Distortion: This is one aspect of the more general "rectilinear distortion" in which straight lines are not reproduced as straight. When parallel lines bow outward at the center, like the staves in a barrel, it is referred as barrel distortion. When they bend inward it is pincushion distortion. This is in no way related to the perspective "distortion" often seen with wide angle lenses where parallel lines remain straight but don't remain parallel. This latter "distortion" is not technically a form of optical distortion. A more proper term would be "perspective alteration".

Vignetting: This refers to a form of uneven brightness across the image where the center is brighter than the corners. It occurs to some degree in all lenses, though it is generally insignificant in telephotos. It is generally the biggest problem for the designers of very fast (wide aperture) lenses and wide angles, especially when the wide angle is also a zoom. Vignetting can also be causes, or significantly increased, by inappropriately small lens attachments or overly deep lens hoods.

 

[DB83]

good stuff. thanks to all who have educated me.