A backlit photo of a dead fly trapped inside the inner elements of a Canon 70-200mm F2.8 lens, photos kindly provided by Lensrentals.

Thanks to ever-increasing reliance on firmware, the latest lenses can sometimes have bugs, but the bug Roger Cicala and his team at Lensrentals recently came across inside a 70-200mm F2.8L IS II lens wasn't about to be fixed with the latest firmware update from Canon. That's because it wasn't a bug in the programming they encountered, it was a fly that managed to trap itself inside the optical elements of the lens.

Time for its close-up!

Somehow, a sneaky fly found its way into the deepest parts of a 70-200mm F2.8 lens, where it remained until its untimely demise. Naturally, Roger and his team wanted to remove the little critter, but not before they used it as an opportunity to see just how much a fly in the lens affected the performance of a lens and share the results (and process) with the world. In Roger's own words, the blog post was written up for two reasons:

'1) [To shot that] this big-ass fly had almost no effect on image quality, so that little dust speck you’re frantic about sure doesn’t, and 2) getting flies (or dust) out of a lens is a lengthy and difficult process that takes a long time.'

Just how well did the lens perform with the fly stuck inside of it? 'Pretty well, actually,' according to Roger. The Lensrentals team took the time to run a complete set of MTF tests and stopped down the lens across all focal lengths.

What the fly looked like stopped down to F13.

As noted in the blog post, there was no sign of the fly until the lens was stopped down to F13, but even then all that was visible was a shadow in the image. Stopped down to F22 and zoomed out to 70mm showed a much more prominent black spot, but even then it only took up a fraction of the frame.

What the fly looked like when stopped down to F22 at 70mm.

The remainder of the post details the teardown process and shows just how much effort goes into thoroughly cleaning out the inner-elements of a lens, especially a zoom lens as intricate as a 70-200mm IS lens. For all of the nitty-gritty details of the teardown process and more photos, head over to Lensrentals' blog post.

Finally, the fly is discovered.

Roger's takeaway message from the image tests with the fly-laden lens is 'if your dust spec is smaller than a fly (about 4mm by 1.5mm) it’s not showing up in your images.' He goes on to say in the 'Final Thoughts' section:

I think this does serve as a good demonstration of how little effect even a very large object inside the lens has. It would have a more dramatic effect, probably, if it had been located closer to the rear element. It’s also a really good demonstration on why we (nor anyone else who’s qualified) won’t clean the dust out of your lens for $50. Sure, a lot of lenses are easier to do than this one, but it’s still time-consuming and often technically difficult.

As for how the fly got into the lens, Roger deduces that it likely made its way into the lens through the small gap that's made in the rear of the lens that can open up during the zooming process. The lens had been in Lensrentals' rotation for 15 months, so he believes it found its way inside the lens after it left the factory. However, Roger admits he's not a bug expert, so he's not against hearing other possibilities.