As I mentioned recently, I’m working on some in-depth tutorials for Shutter Muse about autofocus microadjustment, which is a feature of some mid-to-high end cameras that allows you to fine-tune the autofocus performance of particular lens and camera combinations.
Perhaps microadjustment is something you already do, and maybe you even own a LensAlign II or a SpyderLENSCAL? Well, during my research, a bit of lateral thinking lead me to discover something which many people are getting wrong when they are performing microadjustments.
These days all cameras have a live view function, but what many people aren’t aware of is that the focus system in live view is totally different to that which is used if you are looking through a viewfinder (on a DSLR). When live view is being used, the mirror of the camera is flipped up, and the camera is using a focus method called contrast detection. When the viewfinder is being used, and the mirror is therefore down, the camera uses phase detection autofocus.
I’m not going to delve into the technical differences of how the two work right now, but what you need to remember is that the two systems are vastly different, and they use totally different systems in the camera o calculate the focus point.
AF microadjustment is a feature that lets us fine tune the AF system so that we can nail a consistent, accurate focus when using the viewfinder with phase detection AF. It is therefore entirely wrong to perform AF microadjustment using live view, because when you’re using live view, the default behaviour is for the camera to use contrast detection AF. In other words, the AF results you see from live view, will not match what you are seeing when you go to use the viewfinder in regular shooting.
With tools like the LensAlign, where the idea is to analyze the position of the plane of focus on a special ruler, it is very tempting to use live view to make that easier. Do not do it! It completely invalidates your test results.
Now with Canon cameras there is an option in the menu system to switch live view autofocus to a mode called “Quick Mode”. What quick mode does is temporarily flip down the mirror and use the phase detection circuity to perform the focusing. You’ll see the screen go black for a split second while it does this, and you’ll hear the flap of the mirror. At first I thought that this would be a good way to get around this problem. In fact one of the most popular tutorials on this topic, by bird photographer Art Morris, says that you should do this.
However, whilst doing the research for my own tutorials (yes, I like to be thorough) I actually shared a few emails with the creator of the LensAlign tool, Michael Tapes, and I asked him about this. He told me that you should not use Quick Mode, because whilst it does use the phase detection AF circuitry, the order in which all the processes are carried out is different to how things work when you use the viewfinder for regular shooting. He said that whilst you can use live view to confirm focus position once it has been focussed, the actual act of focussing MUST be done with live view turned off entirely if you want to get the most accurate results. He also confirmed that the popular (and now quite old) tutorial by Art Morris is incorrect.
So there you have it. Do not focus in live view if you are performing autofocus microadjustments!
NOTE: If you didn’t read the in-depth guide to L-plates that I mentioned in yesterday’s post, I also condensed all that info into a smaller tutorial that is a bit more bite-sized for all you busy people. You can find that one here: 3 Reasons You Should Be Using An L-Plate.