Nifty fifty STM - focus issues, or human error?

It is not s front back micro adjust issue, however to see what is going on you can diagnose and understand in the same way you remedy a front/back focus issue. I explain...:

-take a tape measure or ruler at a 90 degree angle to use as a subject and put camera on tripod or somewhere stable.

-set aperature to 1.8 focse at say 8 inch marker from whatever distance you chose (something like 5 feet should be okay)

-take a shot, un focus and re-focus take another...repate a couple of times unfocusing and refocusing between shots. a good way to do this is by putting your hand in front of the lens to throw focus off before refocusing.

-Check the tick markers on your measuring tape, you will see exactly where to focus landed and the depth of field

-Repeat at F 2.8, F5.6, F8

What you will notice is

1) if you need to micro adjust (not that important in your case)

2) You will notice that the focus is consistent when the aperature is above 2-2.2Why is that ?

Well, the lens is simply not sharp at 1.8 nor F2.0 really and truly. It is pretty good but not sharp to the degree that the focus system is fully settled. at around F2.8 you should see more focus consistency as the lens itself is more sharp and able to decide on a point to focus on better than F1.8. It is still not perfect as there is some play in the teeth of the focus mechanism but it is much better at those higher apertures.

I have had about 3 or 4 of the nifty fifties they pretty much all exhibited what I mention above. however, I have only tested on a full frame, I am not sure if on an APSC it gets "shape" and has consistency at 1.8 or at 2.8...have a look I am interested to hear your results if you decide to do the geeky test outlined above.

I should also mention that I love this lens, really one of my favorites. It is phenomenal for shooting Panos at F8.