Why AF Fine Tune on the Nikon Z cameras? As I said above, it’s generally assumed that cameras with the autofocus sensor built into the image sensor (i.e. mirrorless cameras) do not need any AF calibration. There’s no chance of the AF sensor being misaligned or the pre-programmed internal calibration between the AF sensor and image sensor to be incorrect like there is in a DSLR (which has a separate AF sensor in it’s base). However, Nikon has included AF Fine-Tune in their Z6 and Z7 cameras – so why is that, and is it really needed?
為何在Nikon Z系列上要有AF Fine Tune功能? 如我上述所說,這功能普遍在使用CMOS上的AF模組時是不太需要的 因為無反相機的特性使的AF模組不太有偏差或是組裝偏移問題像在DSLR上發生的(因為DSLR的AF感測器是分離式置放於機身底部的) 然而在新一代的Z6/Z7相機上,Nikon卻仍舊將AF Fine Tune功能置入,為何如此並且需求在哪?
If the camera uses purely contrast-detect AF (CDAF), you wouldn’t need any sort of calibration. CDAF works in the same way as a human focusing a lens – you look at how sharp the image is, and if you’re not satisfied with the sharpness you move the focus point a bit in a random direction and see if it gets sharper or not. You repeat this process until you get a sharp image. This is a closed-loop, iterative process – closed loop means you adjust and then check your result, and iterative means you repeat the process until you’re happy with the outcome. The end focus position is probably about as good as you’re going to get (although you can never be quite sure as you have to adjust the lens to check, which might then mess up the focus!). The down sides are that CDAF is quite slow and requires good light.
Phase-detect AF (PDAF) looks at two paths of light coming to the sensor and with a single measurement can determine both the magnitude (how much you need to adjust the focus) and the direction (towards the camera or towards infinity). So one quick measurement can give you everything you need to know – then you drive the lens the right amount in the right direction and voila, perfect focus…
… not quite. This is an open-loop system. It’s fast, but it relies on your measurement being accurate and the lens doing exactly what you tell it to. When the camera says move x amount in this direction, it assumes that x is the perfect amount, and the lens will correctly move to this point.
So what happens if your lens motor/gearing is a bit worn and doesn’t behave quite the same as it did when it was new? What if the lens mount or lens elements are fractionally misaligned due to manufacturing tolerances or an accidental knock 6 months ago, and the focus isn’t measured quite correctly or the movement of the motor doesn’t quite move the lens element where is should be? These are the sorts of things can result in a fractional offset of the focus position, and an out-of-focus image.
But as long as the focus error is fairly consistent, then AF Fine Tune is your knight in shining armour to get that perfect focus back!
但只要對焦錯誤是相當穩定的,AF Fine Tune功能就可以成為你的救命稻草讓你再次對焦精準
Other mirrorless cameras offering AF Calibration There are a few other mirrorless cameras which offer AF tuning. Cameras like the Olympus OM-D E-M1 even have the capability to adjust calibration for each individual focus point!
I’ve consistently seen reference to Sony mirrorless support AF Fine-Tune but this isn’t technically correct and I wanted to clarify here. There are a few lens mount adapter available for Sony cameras, and at least 2 of them (LA-EA2 and LA-EA4) use Sony’s “Translucent Mirror” technology and incorporate a PDAF sensor in the base of the adapter. When you use AF Fine Tune on a Sony mirrorless, this adapter PDAF sensor is what you are actually calibrating, so it’s only of use if you’re using one of these specific adapters.
過去曾持續地看見一些資料說明Sony無反支持AF微調功能,但這並不正確我必須在此澄清 Sony的AF微調功能只針對部分接環可使用,例如LA-EA2或是LA-EA4(使用Sony的半透明反光鏡科技)此種接環利用置放於轉接環底部的PDAF感測器一同工作 當你在Sony機身上使用AF Fine Tune時,你其實在調整的是該轉接環上的PDAF系統,所以Sony的AF微調只局限於該種特殊轉接環