extention tubes

[ifurlong]

Hi all, I am wondering, Ive heard that extention tubes are not the way to go as far as to getting 1:1 and greater magnification in that they cause image degredation. This confuses me, why, well first there is no glass in an extention tube and second the new 65mm canon macro works exactly like a variable extention tube, so if it herus the image why would they do this. So is this conjecture about image degredation false or am I missing something, thanks, Ian.

[Hatem Eldoronki]

As far as I know, extension tubes affect the aperture value of the lens,. They increase the focal length, thus a lesser amount of light reaches the sensor. This would make the image really dark, and vignetting would occur too, hence the image degradation.
I posted a request for samples of a teleconverter with a 75-300mm lens, but got no replies so far (which makes me believe I'm right..)

[ifurlong]

mcouper, that sounds reasnable but then how do you explain how the 65mm macro works, since it gets its magnification from a variable extention tube.

[CyberDyneSystems]

Mcouper is 'mostly" correct. There is no vignetting at all. There is a reduction in aperture value but it is very minimal. (the 10D's as well as any automatic SLR will do the TTL metering so no compensation on your part is neccesary)

I do not think the Focal length is altered,. but by moving the lense further from the focus plane, it allows a closer distance to the subject. ( i could be wrong on that one,. as far as focal length,. ? )

They contain no glass at all so the image quality is not effected in any way. They do not effect autofocusing in any way that I can see.

I do not have a 75-300mm lens to post an example,. but I have used the rings succesfully with a 50mm f/1.8, Sigma 28-300mm, and Sigma 70-200mm f/2.8

They work very well and for the cost I think they are a must have. You can use them with ANY of your lenses, so the flexibilty and combinations is unlimited.

I have the Kenko three ring set and highly recomend it.

[kfong]

I also have the Kenko 3 ring set and am using in conjunction with a 50mm f/1.8 for photographing surface mount components on pc boards, as well as duplicating my old slides. I'm quite happy with the results.
The only thing to watch out for is that the depth of field is quite narrow. So use a small aperture (large f number) with a tripod and plently of lighting.

Ken