Help me understand...

My sigma 50mm (sigmalux) was focusing randomly in the past few months until I decided to bring it at sigma center. What the tech did first was calibration. He made me tried the lens again but performed worst than it was before. I showed him the results and so he took the lens again and brought it inside their working area. After an hour, he showed up and told me that the rear element got loose a bit and some grease soaked a sensor (pointing at the focusing ring and don't know what he was talking about ) which caused the lens to focus randomly. So he made me test the lens again and I was happy with the results. I tested it with subjects ranging a distance from 10 feet to infinity. The results were spot on, mfa disabled. The downside is, I didn't clicked on a subject closer than 10 feet due to my overwhelmed feelings. When I got home, I took some shots (this time ranging from 3 feet to at least 8ft.) and found it front focusing consistently on the same focal plane. Mfa'd it and got +15, for near subjects and no mfa for far objects. The lens is consistent this time and its easier to use but its kind of annoying of getting inside the menu to change mfas all the time.

Did the tech did something wrong? I am currently thinking if the lens could be adjusted again by sigma to make it more accurate at all distances, is this possible? What I dont understand is, why was he able to make it accurate for long distances and not in short distances?

Just to make it clear guys, I am not in despair with the result of the repair. Just wanted answers to my questions. I can get around with the mfa annoyance but it would be better working with one mfa value only. Regards!!!

Canon says that MFA should be re-applied for every new shooting situation. New light? Redo MFA. Different distance? Redo MFA. I'd seriously consider replacing the lens.

The fact that it's different for near and far distances either means that the lens is not working properly, or needs further adjustment. I wonder if the Sigma dock works with this lens.

The sensor the tech was talking about is probably the one which detects the distance range of the focused subject; Canon lenses have somewhere between 4 and 32 distance ranges they can detect, depending on model. This sensor is very fragile, so it's possible that if the Sigma has one it was ruined by the grease.

This lens is not compatible with the dock. According to the tech, the sensor only needed cleaning and was not ruined or damaged. A big improvement after the repair actually, focused plane is now consistent and mfa can manage it but as mentioned, different values at near and far subjects. I am basically thinking that it could still be adjusted properly. The question is how would I explain it technically to the tech cause I do not know anything on how the lens works internally.

Take it back one more time, and get a receipt of the work. Be upfront about the reason for selling it with potential buyers. Place it for sale and move on. If the buyer has the same issue with the lens on their body as well, they have justification for Sigma to fix or replace it.

I have had no issues with my 50L personally and I find the 'focal shift' to be very real, but manageable.
I also hear that it is much softer than the 50mm Sigma Art lens, but I find now reason to replace it.
The new 50 1.8 STM is better build quality than the previous models, just like to 40 STM.

Best of luck to you. I hope you find a 50mm that you like.

I think selling it would be a great idea.., I have tried the new 50mm stm and found it great... However, selling this piece would be a big challenge for me...

I think the unfortunate thing about this is you'd be trying to palm off a rubbish lens to someone knowing full well its no good. Perhaps you'll adjust the price to reflect the state of it

what apertures are you having trouble with at close focus distance? I wonder if this is just focus shift.

Before you head out and sell it though, do you plan on shooting at less than 10ft? If most of your shooting is going to be medium to long distance maybe don't worry about it and keep in mind that if you are planning on shooting close distances with it to use some MFA.

Adding the 50mm STM isn't a bad idea either and probably so cheap that it may not be worth the hassle of trying to sell a lens that you know has reasonably pronounced differences in focus calibration at different distances

No, I don't intend to sell it to someone at a high price, if ever I decide to. Bad carma is not my deal.

I frequently use the lens at 6 to 8ft distances. Seldom in long distances. Its consistently front focused at f2.8 to max aperture. In a cloudy day, mfa is +9; 0 at infinity. Low light indoors mfa +15; 0 at 10 ft onwards.

The good thing is I am now able to control the lens with mfa compared before the calibration. Oh, one more thing... The photo of the scale (the tech used to calibrate) is still in my memory card. To my estimate, the distance between the camera and scale target is most likely at 3ft. He set the aperture at 1.4, ss at 4000th and iso at 1600. Did he do the right procedure?