Modifying the Canon EF-S 18-55/3.5-5.6 lens for use on a Canon EOS 10D

The Canon EF-S lens has been the subject of significant interest since it was introduced along with the EOS Digital Rebel last fall. It is only sold as part of a kit with the Digital Rebel body, it's not available as a separate item from Canon.

There's a full review of the EF-S 18-55/3.5-5.6 in the reviews section of this website. Canon EF-S 18-55/3.5-5.6 review

The "S" in EFS stands for "Short back focus", which means the distance between the rearmost optical surface of the lens and the film (or in this case the digital sensor) is shorter than that of a normal Canon EF series lens. A shorter back focus does have some advantages when designing wide-angle lenses, but there's a limit to how short it can be in an SLR lens since it can't be so short that the SLR mirror hits it when it flips up. The lower limit on the size of the mirror depends on the size of the film (or sensor). Medium format SLRs need a big mirror, 35mm SLRs need a smaller mirror and SLRs with a digital sensor smaller than a full frame 35mm frame can use an even smaller mirror.

Since the sensor in the Digital Rebel is only 22.7mm x 15.1mm, smaller than the 24mm x 36mm full frame 35mm frame size, the SLR mirror can be (and is) smaller, so it can use a lens with a shorter back focus than full frame cameras, hence the EF-S lens.

Since the lens can have a shorter back focus distance, and it can also have a smaller image circle then a lens designed for full frame 35mm use, it should be possible to make a better, smaller, lighter lens for a lower cost than if it had a standard back focus and had to cover a full 43mm image circle.

Now Canon aren't dumb, and they know their users. They could have made the EF-S lens fit on a standard EOS body, but told purchasers NOT to mount it on anything except for the Digital Rebel. Then, when someone mounted it on their EOS-3 or 1Ds and the mirror smashed into the back of the lens (damaging the mirror, the lens - or both) they could have said "Sorry, we told you you could only use it on a Digital Rebel". This would not have made them many friends, so they took the smart route. They put an EF-S lens mount on the Digital Rebel, which could take all regular EF series lenses AND the specially modified EF-S lens. The EF-S lens has a protrusion at the back which prevents it being attached to anything but a Digital Rebel. Unlike all other EOS cameras, the Digital Rebel has space inside the body for this protrusion.

The observant Canon user might well say "wait a minute, doesn't the 10D also have the same small sensor as the Digital Rebel, hence a small mirror, hence the ability to use an EF-S lens if only you could get it attached to the 10D body?". Well, the answer appears to be yes, it does and it could. I made measurements on the position of the rearmost surface of the EF-S lens and the position of the 10D mirror when it flipped up and according to my calculations, they wouldn't hit. They'd come close, but there'd be a gap.

So the rest of the story is obvious. Take a EF-S 18-55 lens and a hacksaw, saw off the bits that don't need to be there, mount it on a 10D, zoom to 18mm, press the shutter and see if there's a noise of breaking glass. There wasn't when I did it!

Below is a shot of the original lens and the modified version of the lens. You can see the part that needs removing - it's the stuff above the red line. Now you could just take the lens, the hacksaw and get to work, but the better way to do this is to remove the lens mount from the lens first. You will need two precision Phillips head screwdrivers. One small one and one very small one. If you don't have the right size or type of screwdriver you may damage the screw heads and make life miserable for yourself. Be warned. The mount is attached by 4 obvious screws. However there are two more which hold the electrical contact block in place. Look carefully and you'll see them. A magnifying glass may help. They are indeed small, but obvious. Having the right sized screwdriver will help a lot. I don't know if the screws are put in with any sort of locking compound, but I would not be surprised if they were. It takes a little bit of force to break them free, which is where the right size screwdriver will work and the wrong size screwdriver round off the heads.

Once you've removed the mount, sawing off the bit you don't want is pretty simple. Just take it slow and careful. It's a plastic mount, not all that strong. Clamping it really firmly in a vice may not be the best idea. When you've removed the part you don't want, clean up all the bits of plastic dust, smooth off any jagged bits with sandpaper or a small file and attach it back to the body of the lens. Let's hope you haven't lost those really tiny Phillips head screws.

At this point what you will have is shown below:

Now it's up to you to mount it on your 10D. On mine, there's no problem. It works fine at all zoom and focus settings and there is no collision between the mirror and the lens. I have no control over what you do. If you don't remove enough plastic from the back of the lens, or if for some reason your lens or your 10D is different from mine, you may not be so lucky. What I'm basically saying is that if you have problems, it's not my fault. I'm not suggesting that you do this, all I'm doing is telling you what I did. You're on your own at this point.

Here's a bigger warning: 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. If you ask me if the lens will work OK on a D30 or D60, the answer is I have no idea. I have not tried the lens on either one. You might guess that since they use the same size sensor as the 300D and 10D, it would be OK, but you might be wrong. I don't know what size mirror the D30 and D60 use, or how close the mirror on those cameras gets to the lens mount. I''m certainly not going to assume they are the same as the 10D, and you shouldn't either.

I did try this modified lens on an EOS IX lite - an APS film camera. It seemed to work just fine, even at 18mm. There was no sound of breaking glass and the shutter operated normally, so it seems, as you might expect, that the reduced film and mirror size on the Canon EOS APS bodies enables a modified EF-S lens to be used without causing damage.

Why would you do this anyway - it's just a cheap lens

Yup, it's a cheap lens, just $100 when bought with a Digital Rebel body - and in fact that's the ONLY way you can buy one from Canon, as part of the Digital Rebel Kit. If you want one without buying the kit, I suggest looking on eBAY. There seem to be a few for sale each week for around $100 each.

But why would you want one? Well, it's the cheapest way to get a "wide-angle" lens for the 300D or 10D. In 35mm equivalent terms its more like a 29-90 zoom, so it's not all that wide, but your alternatives (EF 17-40/4L for example) are a lot more expensive and cover a smaller zoom range.  It's actually not a bad lens. I've done extensive tests and it holds it's own against the more expensive non-"L" consumer zooms. It's a bit weak at full aperture in the corners at the wide and tele ends of the range, but it's decent when stopped down, and in the center of the frame it's pretty good at all apertures and focal lengths. It has good flare resistance and uses 58mm filters. It's a pretty good "walking around" lens. Normally I use a 28-135 IS (plus 72mm filters) as my "walking around" lens, and it's great, but 28mm isn't very wide on a 10d (35mm full frame equivalent 45mm). That means I also carry a 20-35/3.5-4.5 (plus 77mm filters). The 18-55 can cover a lot of situations without having to switch lenses (or filters), and it's smaller and lighter than the 28-135 + 20-35, in fact it's smaller and lighter than either one of them!

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