Canon 17–85 mm IS USM lens: EF-S to EF mount conversion

Why bother?

The Canon EF-S 17–85 mm IS lens has an EF-S mount, and will not fit traditional EF mount cameras. This is for the very good reason that the mirror in older Canon SLRs will hit the rear of the lens when the shutter is released.

However, I’d heard rumours that the rear lens element of this lens will not hit the mirror of 1.6 crop factor EF-mount cameras such as the Canon 10D, owing to the smaller mirror, and that the lens could be modified to fit. Apparently, the black plastic baffle in the centre of the metal mount of the lens needs to be removed or changed to one from an EF lens. So, I compared the rear of this EF-S lens to EF lenses of mine (35 mm f/2, 50 mm f/1.8 Mk I), and the rear element protrudes by very similar amounts for each lens - it will definitely clear my 10D’s mirror.

With the clearance checked, I removed the plastic baffle from the 17–85 mm lens, and tried it out on my 10D: it worked like a charm. Note that removing the baffle doesn’t harm or permanently alter the lens in any way, and the original EF-S baffle can easily be replaced at any time.

The lens will work without a baffle – but I decided to see if one from an EF lens would fit. Looking at my lens collection, it seems that any baffle from an EF metal lens mount would do so long as the central hole clears the EF-S rear lens element. I ended up using the baffle from my Canon 24–85 mm USM lens, and ordered a spare EF baffle for £12 from a Canon repair centre. Figure 1 shows the lens before and after the conversion.

Figure 1. Canon EF-S 17-85 mm IS lens. Left: lens with original EF-S baffle. Right: lens with EF baffle.

I‘m pretty sure the modified lens will clear the mirror on all full-frame EOS cameras (try at your peril!). However, I‘d expect full-frame cameras to show some vignetting with this lens, which is designed for crop factor cameras.

Procedure

If anyone wants to try this, here’s how to remove the baffle (it’s very straightforward – but is of course at your own risk!):

1. Buy a small jeweller’s crosspoint screwdriver (most hardware or electronic stores will sell these).

2. There’s a raised screw on the side of the metal lens mount about 15 mm to the left of the electrical contacts that retains the baffle. Undo it. (Note: do not undo the two inset screws that retain the electrical contacts.)

3. Undo the four larger screws on the rim of the lens mount that fix the lens mount to the lens body. (Note: the lens barrel casing is now loose, so don‘t hold the lens by just its barrel once you’ve removed these four screws.)

4. Gently lift one side of the lens mount upwards at the point furthest from the electrical contacts, enough to be able to get your fingers underneath. (Note: the lens mount won't move much near the electrical contact owing to connecting wires – so don't try pulling the lens mount off.)

5. Push the baffle out of the lens mount from underneath whilst holding the lens mount firmly, taking care not to pull out the wires (note: the wires are firmly attached, so this would take some force). The baffle’s held in (not very firmly) by four plastic clips, and will ping out when pushed from behind.

6. Reassemble. (Note: if you’re replacing the baffle with an EF one, this can be added after reassembling the lens).