5D, 85mm, front focus and sigma 150mm macro.

Started Oct 15, 2012 | Questions thread
David Franklin
Contributing MemberPosts: 951
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Re: 5D, 85mm, front focus and sigma 150mm macro.
In reply to winwin, Oct 16, 2012

winwin wrote:

Hey guys, I'm new in the forum but I need to ask a question, I bought an 85mm f1.8 a few months ago, when it arrived, I noticed it had back focus problems, so after testing, I concluded that it really was, by around 1", so I sent it back, got another 1 and same problem, back focus by 1" (must be same batch production), so I sent it to canon, had it calibrated and came back, not it has around .5" back focus, sent it back, and it came back okay.

A few weeks ago, I was shooting our school intramurals, I noticed things were a bit blurry even with flash so I tried focusing and manually adjusting after, and notice it was front focusing by 5feet at 30ft distance. But up close, it seems to be okay.

So, my question is, is it possible the lens is okay up close but is front focusing at far distances or is it really front focusing but just a lot more obvious when it's far away?

2nd question is, would it be better to have 50mm F1.8 and sigma 150mm instead of 50mm F1.8 and 85mm F1.8? Thanks a lot and sorry for the long story...
(By the way I'm from the Philippines, so return policy and Canon service might be different.)

You've asked a lot of questions for which there are nothing but complex answers.

First of all, yes, a lens can focus properly at close distance and still have front or back focus problems at distance. However, if the lens has autofoocus accuracy issues, it would be more likely  to have them in the opposite way, because the tolerances for missing focus up close are so much smaller and more sensitive than for greater distance focusing. Second, your problem could also be a function of the camera's focusing syystem, independendent of the lens.

The best way to test these propositions, to get a good idea of what is actually happening, is to adjust your lens for micro focusing with the camera's built-in MFA program. Adjust the lens three times, once for a close-up target, once at a mid-distance, and, once again, with a target at far distance, and see whether the peformance with any of the three adjustment sets works well at both the prescribed distance and the other two. If one of the three adjustments gives a fairly good result for every other distance, then your choice is clear; if no setting gives acceptable results for the other two distances, both the camera and lens should be sent back to Canon.

However, if you have an original model 5D, it doesn't have MFA built-in. If so, you are stuck with only one choice - sending the camera and lens back to Canon.

As to buying the 50 f/1.8 and Sigma 150 f/2.8 macro, well, of course, that depends on what you will be shooting. If your "close-ups" are what you do most often, and if close-up means mostly at repproduction ratios of between, say, 20:1 and 1:1, and people head-and-shoulder portraits are not what you most like to do, then the Canon/Sigma combo you mentioned is a better choice than the Canon 85mm f/1.8 lens alone. Think about your goals and decide for yourself. Since the Sigma would also function as a "long-ish" portrait lens, the 50mm f/1.8 can function as a very cheap "bonus' lens that can also work as a nice choice for portraits showing more of the subjects body - like a "knees-up" vertical, for which it is near perfect - and for other general purpose situations.

One more thing. If you decide to get the Sigma, save some money, and still get a much better performing lens by buying the one generation older Sigma 150 f/2.8 macro without the built-in vibration reduction system; the slightly older (and I think still available as new) lens is a much better performer, probably the best performing Sigma lens of any focal length. I have one, and it is the only non-Canon L lens I've bought since shooting Canon. It is amazingly sharp across the frame at f/2.8 and just about as perfect as a production lens gets at f/4.0. Simply amazing. The autofocus is good, not great, but it is a macro, and slow autofocus, or even manual focusing, is not a real handicap in most real macro focusing siiituations. Unfortunately, when Sigma changed ther lens construction in its latest model to accomodate its version of Canon's IS system, it resulted in a lens quite a bit less sharp and contrasty. Too bad.

Regards,

David

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