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I own a crop sensor body and naturally have EF-S lenses because of my budget. I am extremely happy with my 55-250 EF-S lens, but it isn't that great for nature photography.

Now, I know that EF-S won't work on full frame, but full frame lenses work on a crop sensor body. Could I put a 2x extender on my 55-250mm and use that to get to 500mm on my camera? (I could technically afford a 70-300, but it doesn't have IS and I don't want a mostly-useless lens in my bag.)

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The Canon extenders will not physically mount to an EF-S lens. Even if you defeat the keying the front part of the EF 2X (any of the three successive versions) would extend into the rear of the EF-S 55-250mm and almost surely contact and damage the rear lens element or the front element of the extender or both, at least at certain focal length and focus distance combinations.

In 2015 Kenko introduced Teleplus HD DGX 1.4X (and its 2.0X counterpart). These teleconverters officially support some EF-S and EF lenses. There is an exhaustive compatibility list on their website.

Some third party teleconverters, such as the older Kenko AF 2X Teleplus PRO 300 DGX Teleconverter for Canon, are not officially supported. They might or might not fit your EF-S lenses but then you have other issues to worry about.

  • Your 55-250mm f/4-5.6 lens becomes, effectively, a 110-500mm f/8-11 lens. This means your camera would be unable to use autofocus and the viewfinder would be very dim when trying to manually focus. You might be able to use magnified Live View to manually focus.

  • The EF-S 55-250mm f/4-5.6, while a pretty good lens for the price, probably isn't of a high enough optical quality to accept a 2X converter and give you results you would be happy with. All of the flaws of the lens are magnified along with the image it projects. Sharpness would suffer. You also would give up a little contrast. Chromatic aberration (purple fringing) would likely be increased.

  • Just out of curiosity, what are your thoughts on using a 12mm extension tube between the extender and the lens? Assuming the fit works - you'd have some changes in focusing distances...but the combo would otherwise work, no? – Hueco Nov 16 '17 at 16:24
  • @Corey, that all depends on how you define works. – Michael C Nov 16 '17 at 16:31
  • @Corey I don't think there are any general rules in that area. There are far too many variables with each specific lens, extender/TC, extension tube, and camera body. You can only really answer that by trying various specific combinations and comparing the results. – Michael C Nov 16 '17 at 16:33
  • I figured. I'm lacking in an extender at the moment, but maybe I'll rent one to test it out. Cheers, – Hueco Nov 16 '17 at 16:52
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If the extender in question has a white square alignment mark on the lens-side bayonet, then it will accept the EF-S lens. If it has only the red circle, then it won't (but if there is physical clearance it ought to work optically if you defeat the keying on the mount).

But see this answer, as not all EF lenses have clearance, so EF-S would be worse.

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