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Many professional and amateur photographers have abandoned Canon lenses in favour of Olympus, Leica, Zeiss (and even Nikon!) lenses for their Canon digital cameras. Given the fact that a (frequently expensive) adaptor is required, and that the user is robbed of autofocus and required to deploy stop-down metering, some have rightly wondered: is it worth it? Is it just a fad; or is it plain snobbery?
In a nutshell: it depends. It depends on whether your subject matter stays still long enough to compose a shot wide open, then stop down to the required aperture and shoot. If you cut your teeth on view cameras, this discipline is second nature; if not, the procedure may seem hellishly protracted.
It also depends on how you feel about autofocus: for sports photographers, life without AF is unthinkable. For architectural photographers and f1.2 portrait shooters, AF is a potentially hazardous irrelevance.
And again it depends on your preferred focal length: Canon's 85mm f1.2, 135mm f2 and 200mm f1.8 L series lenses are as close to perfection as matters. On the other hand, until the launch of the 16-35mm Mark II, you wouldn't find a Canon lens below 35mm worth using as anything but a paperweight (unless you stumbled across a rare nominal 24mm f1.4 L).
Furthermore, it also depends on the demands made by your camera. A 300D couldn't care less about the edge performance of your Zeiss 35-70mm f3.4; ditto a 20D – despite its much more demanding 9.9ü pixel pitch. For Canon's full frame sensors, however, it very much matters. Your 1Ds Mark I, 5D, 1Ds Mark II or even III (in increasing order of fussiness) want lots of lpmm – they want it across the frame, and they're not going to get in sub 35mm focal lengths from Canon lenses. Adapted enlarger lenses, for instance, offer a unique range of tilt and shift movements combined with perfectly flat-field rendition and extremely high resolution that makes them peerlessly useful in many fields of technical and creative image-making.
There's also the question of brand loyalty: many have a legacy of superb lenses left over from the demise of support for the Contax and Olympus OM systems. It's cheaper to use them than to invest in new Canon glass.
Finally, there are aesthetic considerations: there is no doubt that certain lenses just have a bit of unaccountable magic and sparkle that suits our personal style of shooting. There are Olympus 90/2, Nikon 24 AIS and Zeiss 28/2 users, for instance, who have become inseparably attached to their glass over the years – and there's a certain glow produced by Summicrons and fast Takumars that no Canon lens quite duplicates . . . . That metal ring grants acces to a world of optics with unique characteristics: from oscilloscope lenses like the charming Helios 85mm f1.5, to the largely uncharted delights of the Pentacon Six range: the Sonnar 180mm f2.8, for instance, is a budget-minded astrophotographer's dream.
Fortunately – and I'm sure Canon didn't design it purposefully to boost used Distagon sales – the EF mount has a larger throat than almost any other 35mm system, and has a focal-plane-to lens-mount (register) distance that is very short, which handily means that many other manufacturer's lenses can be mounted with an appropriate adaptor that adapts down to the third party lens, and pushes it away from the focal plane by just the right amount. This tends to be in the 0.5-2.5mm range.
It cannot be stressed too strongly that all the arguments in favour of adapted lenses are nullified by using a badly matched adaptor (or adapter). Symptoms of bad matching include blurring in one half of the image circle, atypical colour fringing, failure to acquire infinity focus, and overall resolution loss. The jury is still out on whether adaptors of different thicknesses should be used for lenses in the same system with different focal lengths – for instance, demanding a thinner Contax-mount adaptor for the 15mm Distagon than the 35mm Distagon. Sample variation affects adaptors, too. If you're suffering any of the above symptoms, try a different lens on the same adaptor – then a different adaptor on the same lens. When you find a match that works, stick with it!