A new Canon 50mm f/1.2 L?

Started Dec 20, 2012 | Discussions thread
Rick Knepper
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You've got this mostly wrong but then you are not the first.
In reply to Moogles, Dec 23, 2012

Moogles wrote:

I am wondering if now's a good time to get the current 50mm f/1.2 L famous for back-focusing and some said it's not, or should I wait.

The following is not a technical discussion but rather it is more of a historical perspective of this form.

The 50L does not have the same corrective measures for spherical aberration that other Ls employ from the 100L to the 24L. Within ten feet AND stopped down up to f4-f5.6ish, the lens will shift focus, one of the symptoms of spherical aberration. Beyond f5.6, DoF will cover any shifts. At super wide apertures other than wide open, even a shift in focus onto a tiny closer or further surface of say the face will create the resemblence of a back focus. Not back focus as we common think of it, but a shift of focus. Shooting wide open should not produce the focus shift phenonmenom.

By the way, all lenses have spherical aberrations because their elements are rounded.

Having said this, there was a batch of 50Ls that indeed back focused wide open (as we commonly think of back focus) and if we had had MA back in the day, this probably would not have been a matter of concern. The hysteria of this issue was exaserbated by the focus shift issue and eventually melded into one issue by folks truly ignorant of lens design (and with reading comprehension problems because the following link was posted dozens of times by myself and others).

Common hardware corrections for focusing at close distances are aspherical lenses and floating elements. Every L lens from 24mm to 100mm has a floating element (including the original 50L) except for the current 50L. In fact, it is rare for 50mm lenses from any manufacturer to have floating elements.

Common non-hardware corrections for lenses without floating elements and the need to focus on subject matter that is closer than eight to ten feet are 1.) don't use it like this, 2.) learn how your lens shifts and correct for the shift before the shot or 3.) stop down past the DoF needed to bring the area into focus including the surrounding humps and bumps.

So what do you think the conclusion should be for someone considering the purchase of this lens and knowing that it does not have a strong correction for focus shift when close focusing? If you want to buy this lens to take a head shot of your newborn from 3 feet away, what would be your conclusion about the 50L. Not the right tool maybe? Better choice: the 85L maybe and step back, or the 35L and crop.

Here's the technical description of spherical aberration and focus shift that helped end my ignorance of this particular lens design and allowed me to see that my 50L wasn't defective, I was just using it incorrectly or not as the manufacturer intended.

http://toothwalker.org/optics/spherical.html

Do I wish Canon had included a floating element in this lens (which we take for granted in the other Ls from 24mm to 100mm)? Yes, I do but that's the design choice Canon made which has produced the seemingly endless controversy fueled by ignorance of course.This forum can't even discuss solutions because there aren't enough folks that understand the issue and that being, would a floating element detract from other elements of the lens IQ and would one pay the extra cost? It's similar to the controversy of IS in the 24-70 II (though much better understood by users). But if we were to simulate the same kind of ignorant discourse on the 24-70 II that we've seen on the 50L, we'd see complaining that 24-70 II is too soft and prone to shake. At least here, most folks know why they think this. Now you know the why fors of the 50L.

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Rick Knepper, photographer, non-professional, shooting for pleasure, check my profile for gear list and philosophy.

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