I recently purchased a Canon 6D....

[PhotosByNeva]

I wanted to upgrade from my Rebel. I have several lenses already and noticed that when I use them on my 6D there is a round black ring around the outside edges. Upon reading it seems that the Rebel has a "cropped frame" and the 6D has a "full frame". Does this mean that I have to get all new lenses to use with my 6D, and make sure they are EF?

[ScottyP]

Yes. I am surprized you were able to even mount an EF -s lens. Do not do that anymore as they are not meant to go together and in some cases you can damage the camera or the lens by doing so.  

You need lenses for full frame Canon cameras. Either Canon EF lenses or 3rd party lenses made for Canon full frame mounts.   The different manufacturers (Sigma, Tamron, etc) use different letters to designate FF vs. crop lenses.  

[Waddizzle]

I have found a way to cheat and mount an EF-S lens on a full frame body.  I used to use an extender and close-up filters on my Rebel as an inexpensive way to explore macro photos.  After I purchased a 6D, I noticed that the extender was EF mount, but the bayonet to mount a lens had both a red dot and a white one, just like my Rebel. 

Sure enough, with the extender attached, I could mount an EF-S lens without issue.  Of course, I could see the entire image circle.  But, I thought I was pretty clever just the same.

[ebiggs1]

"I have several lenses already ..."

Are any of these Canon brand lenses?  It is possible for the rear of the lens to strike the mirror.  It can be cracked if EF-S lenses are "modified" to fit on a 6D.  Sigma and Tamron lenses will bolt right up and will cause the black corners.

The "S" in EF-S stands for short focus.  That is why they have pin to make mounting impossible.

[TCampbell]

Do these lenses screw on to the front of existing lenses?   Or do they mount directly to the camera body?

The 6D is indeed a full-frame sensor body and it requires Canon EOS "EF" lenses (without the "-S" suffix)   It can also use the EOS TS-E and MP-E specialty lenses.   You can use third party lenses if they are designed for use for Canon EOS full-frame bodies.

[PhotosByNeva]

They screw on to the camera body. I didn't do enough research before my upgrade because I have several lenses that fit my older DSLR Rebel and I guess I have to buy all new lenses for my 6D. I just paid over $600 for a new Tamron Piiezo Drive 16-300 mm lens that won't work on my Rebel. I get two shots and it shuts down my camera, same problem with the 20D I had. It works on my 6D just fine except it isn't a full frame lens apparently. 

[ebiggs1]

The specs clearly state, 

"Tamron Di-II lenses are engineered expressly for digital SLR cameras with image sensors commonly referred to as APS-C, measuring approximately 24mm x 16mm. Rendering an ideal image circle for APS-C sensors, Di-II lenses also include properties to optimize digital imaging performance."

So that is the problem with that lens.  Not all third party lenses are compatibile with all Canon cameras.  Even the ones that are designed to work on a FF and/or APS-C, may not work correctly.  Most newer versions do, however.  Sometimes, try before you buy is a good rule.  Plus third party lenses do not have the altered mount to prevent you from mounting the wrong lens as you have found out.  Without modification of the mount, you can not mount a Canon brand ef-s lens on a 6D.

[PhotosByNeva]

So what you are saying is even though Tamron, Sigma, or other brands that state that they will specifically fit certain Canon Cameras, they don't?

I had good luck with the Tamron that was on my 20D and the Sigma that I bought for my Rebel, they have been good lenses. I always read plenty of consumer reviews before I buy. I do not always have the option of "trying" before I buy. How would I go about doing that? Renting?

I appreciate your input on this matter. I have sold a few of my pictures and I wanted a better quality, newer camera. Couldn't always afford the Canon lenses, so the less expensive alternative worked, just tried not to go "cheap". I am attending a workshop in the spring with a professional, I am excited and hoping to really get a lot of good experience and learning. I do better with the hands on experience.

Thanks again!

Neva

[PhotosByNeva]

I do have extenders that I bought to experiment with macro. I LOVE them! I have had so much fun exploring the tiny world in BIG pictures. Have you photographed an ants eyes before? lol
Maybe I could try that.
One thing, what is the significance of the red or white dot, because I did notice that one of my lenses has a white dot.

[PhotosByNeva]

I do have some Canon lenses. I just took the 35-50mm Canon lens that was on my original Rebel (actual 35mm film camera) and put it on my 6D. I took a couple pictures with it and it seems to be doing fine, no black edges.
Sigma DC 18-250 mm Macro HSM
Bower 35-500mm Fixed Focus Lens with Canon Adapter (I haven't tried this lens out yet on either camera)
Zeikos AF Macro Extension Tube Set (3 pc): I have used this but not with AF as they seem to work best with manual focus.