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EF-S to EF conversion

Yakim Peled , Mar 14, 2005; 08:57 a.m.

In his famous article Modifying the Canon EF-S 18-55/3.5-5.6 lens for use on a Canon EOS 10D, Bob gives a warning in big and bold letters: IF YOU PUT THIS LENS ON A CAMERA WITH A LARGER MIRROR, YOU'RE ASKING FOR A DISASTER. On any film based body, or on a 1Ds or probably on a 1D, the mirror will HIT THE LENS AND CAUSE DAMAGE.

My question is: Why?

Once the 18-55 became an EF lens, why can't if function just like any other EF lens which can be mounted on any EOS body? In other words, what makes the modified lens different from any other "naturally born" EF lens?

I know I am missing something - and it is probably very basic - but it simply escape my eyes (and, more importantly, my brain.....). Therefore I appeal to all those people with less restrictive brains :- )) to unveil the veil from my eyes. TIA.

Happy shooting,


Paul Marbs , Mar 14, 2005; 09:02 a.m.

Don't the 1.6x camera's have a smaller mirror which will miss the rear of the lens? The modifications only allow the lens to FIT on the 10D which is a non EF-S camera. Full frame cameras have larger mirrors.

Mike Smith , Mar 14, 2005; 09:07 a.m.

Perhaps the clue is in the wording "a larger mirror".

Bob did not say his conversion was to make the lens the same as an EF lens, only that it would now work on the 10D which has the same crop factor as the Drebel and therefore would benefit from the "S" lens characteristics.

10D presumably has a slightly smaller mirror and hence reduced arc of swing than the Film cameras or 1D digitals.

Bill Muth , Mar 14, 2005; 09:16 a.m.

According reports I've read, during operation, the mirror doesn't simly arc, but also moves back. Here's a quote from the Digital Rebel Announcement on Rob Galbraith's site (discussuing the 18-55mm lens):

To accommodate this lens, Canon designed the Digital Rebel's mirror to flip up and back during operation, to prevent it from slapping the rear of the new lens.

Skip Douglas , Mar 14, 2005; 09:23 a.m.

Yakim, the rear element of the EF-S zoom lenses can project beyond the mount (into the camera body) at some focal lengths - something that does not happen with conventional EF lenses. I believe that's the part of the puzzle that you are missing.

Paul - , Mar 14, 2005; 09:43 a.m.

The 10D is the only non-EF-S body with a slightly smaller mirror. The mirrors on other non-EF-S bodies would hit even the modified 18-55.

The Digital Rebel's mirror moves back when it flips up, but the 10D's mirror doesn't do that.

Bob Atkins , Mar 14, 2005; 11:06 a.m.

I'd guess the D30 and D60 have the smaller mirror too.

As pointed out, the rear element of the 18-55 lens sticks further into the camera body than that of EF series lenses. It only does this at the wide end of the zoom range though. My estimate is that you could use the modified lens on ANY canon EOS SLR or DSLR as long as you didn't zoom into the 18-22mm range.

Your mileage may vary. Try this at your own risk.

Yakim Peled , Mar 15, 2005; 12:11 a.m.

I'm relieved. It's not that I am inherently stupid :-) I just didn't know all that.

So, do all EF-S cameras have back flip mirrors?

Happy shooting, Yakim.

Stephen Crary , Mar 15, 2005; 03:18 a.m.

I've tried it on a EOS film camera. Since I manually focus 98% of the time, to prevent damaging the mirror on the film body, after the modification to the lens I set the lens flush on a table, adjusted the lens until it "bottomed out" onto the table, and etched and colored in red the mark to let me know the "safe" limit for the film body. BTW it's about 21mm for those that are interested. But another thing to remember is the smaller glass for the 1.6x factor.

Stephen Crary , Mar 15, 2005; 03:19 a.m.

I guess I should have said image circle.

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