Marco Nero wrote:
CESA wrote:
"Be wary of copy-variation from both EF lenses because they were known to be problematic occasionally. When I bought my EF 85mmL lens it needed to be replaced the following day due to massive back-focus problems. For example, it would focus two inches beyond a closeup subject and three feet from a subject 40 feet away. This was adjustable via the MFA setting on my DSLR camera but I was right at the very limit of my MFA adjustment settings with no room to move. I then managed to select a new model from three other new lenses and was extremely satisfied with the results. In theory, backfocus ought not to be a problem with mirrorless lenses. Yet some folks complain that their EF lens with back-focus issues can occasionally struggle to nail focus on a mirrorless camera. Usually, this always/only applies to f/1.2 lenses."
Marco can you elaborate more here? I have the.EF 8m f/1.2L II and I feel like that some times I miss the focus. I haven't fully calibrated the lens using FoCal but once I do that I will do some experiments because I feel like I have the issue you have just described. Because I haven't calibrated the lens can't conclude anything. However in my case it looks like it is not front focusing.
Most lenses tend to back-focus although some may front focus. Since you're talking about calibrating for the lens, I can only assume you are using a DSLR. Mirrorless cameras won't have a means to calibrate (eg MFA setting) but some entry-level DSLRs won't either. Using an EF 85mm f/1.2L lens on an APS-C DSLR camera (including the EOS M cameras) will generally produce fairly average looking pictures. We discussed this a couple of years ago on another thread once and other members here noted the same. The pictures were mundane looking and we now assume it's because APS-C crops out the middle of the scene and the lens character is then lacking. If you haven't calibrated your camera for the lens via an MFA, then you will probably need to if using a DSLR.
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I'm not sure what camera you are using. If the lens required the FoCal for lens calibration, I assume this means you are using a DSLR. Higher models of DSLR have a microfocus adjustment setting (MFA) in the menu but I'm not aware of Mirrorless cameras that possess this feature. Brighter lenses usually allow for more accurate Auto Focus than ones with smaller apertures. But if there's an issue with a lens, the only way you can tell is with locking the camera down on a tripod and using FLAT targets at different distances from one another. A technique that dealers like to use is to stack a number of batteries upright on a table with the text *(usually on the rear of each battery) facing the camera lens. Then they place their hand over the lens before each shot to force the lens to "infinity" before then focusing on each target. This forced the lens to shift from "infinity" before focusing on the battery (target). If the lens is unable to lock target accurately, or if it back-focuses (which is common enough), then you have a problem.
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When cameras are made, there's extremely small differences in where the sensor sits in relation to the rear of a mounted lens. If the sensor mount is off by a millimeter, you get focus problems unless a mirrorless camera is used. If the lens construction have a variation of just a few millimeters, you get problems too. But add any camera to any lens and you have the potential for focus issues, most notably with fast aperture lenses with a thin DOF.
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You will usually only notice focus problems with wide aperture lenses (eg f/1.2). Those lenses with narrower apertures (eg f/4) will probably never display a problem that you can see. This difficulty is why so few third party lens companies are able to make an f/1.2 lens with Auto Focus. Instead, they make it Manual Focus Only or try their hand at f/1.4 instead.
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DSLR + LENS focus test: Take two shots with the camera locked down on a tripod. One with AF and another using the Live View (which employs the sensor to nail the the focus). In theory, both shots should be in focus and look the same. But if they don't. the shot taken with Live View will be sharp and the one taken with EVF and Auto Focus will be out of focus slightly. This means you need to use the MFA adjustment feature on your DSLR to match the lens to the body more more accurately. (This method won't be useful for mirrorless cameras). * This is an example from my own good-copy of the EF 85mm f/1.2L II USM lens. it was the shot that showed me I needed to make an MFA adjustment on my camera.
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People using very bright lenses with very shallow DOF will sometimes find their shots are out of focus slightly, even on a Mirrorless camera. In almost all cases, this is user error and it happens for the following reasons:
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* Using the "Focus & Recompose Method" is disastrous with f/1.2 lenses.
* Keep the delay between AF lock and taking the picture to a minimum.
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If you attempt to 'focus & recompose' a shot, even with something static and up close, you will throw out the focus almost every time when using f/1.2 lenses. You just can't use that technique with these lenses because the moment you shift the direction of the camera, you are shifting the plane that the sensor is attached to. Mere millimeters are enough to ruin a shot completely.
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If you are a slow-poke who locks focus and then pauses before taking the shot, your body is going to shift slightly. All it takes is a couple of millimeters or more at f/1.2 and you've already lost the focus. Be more assertive and snap the shot the moment you hear that camera beep to confirm AF-lock.
What did you do to the copy that you were experiencing this?
My first copy was seriously backfockusing. I returned it to the store and selected another one. They, in turn, returned the problematic lens to Canon.
How do you go around this problem without having the luxury of selecting from 3 different lenses the one that performs best?
If you're spending serious money on a lens, don't buy it online unless you know the seller. Buy it from a local dealer so you can check the copy in the store. Most dealers will let you mount and test the lens before you leave with it. Sure, you might need to pay for it first but if you rest the camera on the counter in the store and then focus on something that is facing the camera with contrast (eg text on a box), you should know instantly if there's an issue. In the case of my EF 85mm lens, I had to take a 3 hour journey into the city and back again to replace the defective lens the day after I bought it. Canon have pretty decent quality control so a lens that is problematic usually never gets past the testing phase at the factory. But it can happen and when it does, the problem is usually a unique one.
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When I bought the RF 85mm f/1.2L USM lens, I made sure to take a quick shot (regardless of using ideal settings) just to check the focus. I just wanted to ensure that the lens was focusing properly. This is the shot here:
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My in-store test with the RF 85mm f/1.2L USM lens the other day.
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Can this be sent to canon to get fixed? What causes this? The lens still in the warranty and it was bought from a used shop.
Conduct a focus test and be sure to lock the camera down on a tripod to prevent errors in your testing. If you still experience errors in the AF accuracy or reliability, then you might need to look at resolving by returning the lens if this was conducted with a Mirrorless camera with no MFA feature. If you are using a DSLR then you'll need to calibrate it for the lens using the MFA feature in the menu. Don't be tempted to conduct a focus test without using a tripod.
Thanks
Thanks for your comprehensive response. I have a canon 6D. Will do as you suggest in here. Will setup different targets at different distances and will also do the trick with the batteries.
I think that you should be able to focus at any distance right? From minimum or say 1m up to whatever. I normally shoot at 1.5m up to 7m but sometimes like to do some at 1m.
I will take the time to do some assessment and follow your advice. Will come back with more.
