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Defocus Smoothing on Canon’s 85mm Costs You 1.5 Stops of Light at f/1.2

Does that photo of the new Canon 85mm f/1.2L DS IS lens look a bit dark to you? Well, that’s because the new Defocus Smoothing coating Canon applied to its latest prime lens isn’t without its tradeoffs. Though the Defocus Smoothing that helps improve the look of bokeh in photos, it’s going to cost you a stop and a half of light wide open.

If you read how it works, Canon states that the “coating allows off-axis flux to pass through the periphery of the lens that gradually decrease the transmittance of light, which results in images with beautifully blurred backgrounds.” That sounds great, but did you catch that part about decreasing the transmittance of light?

Canon confirmed with PetaPixel that the lens does actually cost you a stop and a half of light at f/1.2, instead acting like a T2.5 lens when wide open. If you’re wondering how a lens can be f/1.2 but not actually be f/1.2 in terms of the light it lets in, there is a reason for that.

“The f-stop is a physical measurement of the aperture. It’s not just a light transmittance measurement — that’s a T-stop,” Drew MacCallum, the Senior Technical Specialist at Canon, tells us. “The transmission is one and a third stops darker than f/1.2, but that does change as you stop down the lens.”

Defocus Smoothing becomes minimized as you stop down the lens. Canon has stated both a 1.3 stop loss in light in a recent video (below) and that was echoed by MacCallum in the statement to PetaPixel, but we’ve also seen 1.5 stops referenced in other Canon materials, such as this blog post. For all intents and purposes, you should expect somewhere between 1.3 and 1.5 stops of light lost when shooting wide open with this lens.



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