Old 28-04-2008, 08:42 PM
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Mod for a Canon EF-S lens

For various reasons, it seems that probably the most common DSLR for astrophotographers are the Canons and many would know that there are two types of Canon lens mounts – the EF and the EF-S lenses. The EF lenses fit on all Canon EOS SLR cameras – film, full frame digital, 1.6x crop digital such as the 300D/350D/400D/450D and 20D/30D/40D, and the in-betweens like the 5D. However the EF-S lenses only fit the 1.6x crop digitals (and I believe a few of the APS film cameras).

The reason for this, we are told, is that with the EF-S – the ‘S’ stands for ‘Short Back Focus’ - the rear lens element is closer to the focal plane than for the EF lenses and with the non 1.6x crop cameras, the larger reflex mirror hits the rear of the lens. To protect the lens and the camera, the EF-S lenses have a rear protrusion that prevents the EF-S lenses being mounted on anything other than the 1.6x crop cameras which have a recess to allow the protrusion to fit in.

Now I’ve been thinking of getting my 40D modded with a clear filter, but haven’t done so for a couple of reasons, the main one being the high cost of the screw-on filters that go over the lens to convert the camera back for normal photography. With my modded 30D (since stolen), I had a 67 mm B+W 489 filter on the EF-S 17-85 mm IS f4/5.6 and a 58 mm for my 28 and 50 mm lenses. For my 400 mm, I didn’t have a filter as I usually only used that for astro. As well as the insurance supplied replacement 17-85, 28, 50, and 400, I have since also bought an EF 70-200 mm f2.8 and I’m considering a few others (if my bank account recovers…) Now if I were to get my camera converted (about $400), I would also need to buy 5 new 489 filters, and that would set me back about another $1200.

My other option would be to get an insert filter such as the Hutech or Astronomik for $200 - $300, but then I can’t use any EF-S filters with them, so I would still need a screw-on or two and every time I changed from EF to EF-S I would also have to remove or replace the insert filter.

Now I’ve also got an EOS3 film camera and I would maybe like sometimes to stick the EF-S lens on that – but it won’t fit.

Or will it? And can I get the insert filter to work with the EF-S?

So over the weekend I pulled apart my 17-85, all the while thinking that if I don’t get it back together again, I will owe my camera repair man a bottle of whisky as I promised I would stop bring in cameras and lenses that I have pulled apart and not been able to get back together again…

Anyway, after removing four screws on the mount ring and the two on the electrical contact and having a closer look, I discovered that all I needed to do was to remove the rubber ring from the back protuberance and then with a small screwdriver or something, lever of the plastic bit and – hey! - it fits on my EOS3 and by the looks there is enough clearance for the insert filter! The plastic bit is simply held on with three crappy plastic lugs. If you remove the plastic bit, be careful that the lugs don’t break off and fall inside the lens… Also with the plastic bit off, there is a greater chance of dirt or dust getting inside the lens.

Have a look at the photo – there is an EF 28 mm f1.8 next to the EF-S 17-85 with the plastic bit. The rear lens element of the 17-85 does stick out further than that of the 28 mm, but only by about 0.5 mm and I don’t think this will be a problem with the insert filters.

Obviously there may be some EF-S lenses where the rear lens element DOES stick out far enough to be a problem. Also some of the lenses – possible the 10-22 and the 18-55 f2.8 - may have a better quality protuberance that needs the bayonet ring removed before the plastic bit comes off, but not sure on this.

Also with the 17-85 on the EOS3, there is vignetting like you ain’t seen before!! The image is circular and covers about ¾ of the film area – rather like some of the fish-eye lenses. But then who really cares? I don’t. I just crop the film – I usually scan the film anyway in a 20 MP scanner, so I’ll just crop it down to about 15 MP or so.
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Old 28-04-2008, 11:00 PM
Dazzled by the Cosmos.

Dennis is offline
Join Date: May 2005
Location: Brisbane
Posts: 11,452
Hi Susan

Wow - nothing like a bit of adventure and experimentation...sounds like you've found a solution for your particular configuration.

I have a 40D and EF-S 60mm F2.8 Macro but I don’t need to bung the macro on anything other than the 40D so I have been spared the task following in your footsteps.


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Old 29-04-2008, 11:48 AM
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EzyStyles (Eric)

EzyStyles is offline
Join Date: Jan 2006
Location: Melbourne, Victoria
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Hi Susan thanks for posting it. nice mod you have done on the lense.
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