These days manual focus can work in a multitude of ways:
The second case is somewhat rare, but some lenses have a "switch" where one setting is autofocus and the other is manual. Sometimes the switch is not an electrical switch, but a mechanical coupling that either connects the autofocus motor to the lens element that needs to move, or it connects a focus ring you can grab to the lens element. More common is a "focus by wire" scheme that acts the same way, but in reality the focus ring is sending electrical information to some kind of controller in the lens that then commands the autofocus motor. The camera body is not involved at all in such an arrangement. But you don't care, you just move the switch on the lens and somehow the lens makes the ring perform its focusing duties.
The last case involves telling the camera body that you want to be focusing manually. So a ring on the lens is sending commands to the camera body and it is sending commands to the autofocus motor in the lens. The important difference here between this and the second case described just above is that you must change a setting on the camera body, not the lens for this to work. One lens I have (the RP 24-105 STM lens) requires you to both move a switch on the lens and to tell the camera body you want to do manual focusing.
With my RP 24-105 STM, I must flip the switch on the lens to tell it that the control ring should serve as a focus control. Some lenses have a dedicated focus ring and for those lenses, this won't be needed. Then you have to tell the camera body you want to use manual focus. Press the menu button. Select AF submenu 1, and at the bottom you will see "Focus mode". Press set and choose either AF or MF. Press set again and you are in business. Most lenses are not this awkward, but that is the price you pay for this lens not having a dedicated focus ring. This lens does not support combined use of autofocus and manual focus like the lenses I will discuss next.
Next I mount my EF 24-70 f/2.8 lens using the adapter with control ring. This gives me 3 rings (the control ring, the zoom ring, and the focus ring). I no longer have the menu entry with the AF or MF choice. But life is even better. The lens will do autofocus, and I can use the focus ring on the lens to make manual adjustments. Exactly how this all plays out depends on how you have your autofocus buttons set up. In my case each time when I half press the shutter button, autofocus takes place, but does not continually track if I move the camera. The only way to make adjustments with manual focus is to hold the shutter button half down, tweak the focus, and shoot.
This can be changed in at least 2 ways. You can set your camera up for back button focus (as is widely recommended). Then the shutter button won't initiate autofocus. You can ignore the buttons on the back and just do manual focus, or you can use the back button to autofocus and then make adjustments with the focus ring.
My EF 100 macro lens works in exactly the same way. It has a dedicated focus ring. Probably any lens with a dedicated focus ring would work nicely like this. It is my RP 24-105 STM that is the "odd man out" and cannot mix autofocus and manual adjustment for whatever reason.
The magnifier is quite useful. Note that there are two levels (6x and 15x). If you have a focus point set up, it will zoom in centered on that point, but it generally zooms in on the image center. You can use the 4 way "joystick" thing to move around, or put your finger on the viewfinder and drag the image.
Focus peaking must be turned on if you want it. On my camera this is in the AF-2 menu. You can set up a button to turn it on/off. You also get to choose between high and low sensitivity. It is claimed (somewhat surprisingly), that selecting "low" allows you to make more precise focus settings. I am not convinced. I find that I need to set it "high" to be able to reliably see it. My previous Sony A6000 was much bolder in the way it rendered focus peaking. You can also choose the color. You get 3 choices: red, yellow, or blue. I am used to red from my other camera where I had no choice, but experimenting is recommended. Your chosen "picture style" can have a big effect on focus peaking. I shoot raw and tend to ignore this, but remember that a conversion to JPG is done to produce the image on the viewfinder or back screen. Additionally, the focus peaking display itself is produced from the converted image data.
The "focus guide" must also be enabled if you want to use it (also in AF-2). It is cute! It uses the autofocus sensors to give you information as you manipulate the lens in manual focus. You can also assign a button to enable/disable this. You get green when in focus and grey indicates it just can't cope with things.
The focus guide works great with my EF 100 macro (when it is in MF mode). It also works well with my RF 24-105 STM in MF mode. Sadly, it does not work with my fully manual Samyang EF 14 lens
When can you use manual focus when in AF mode? It is complex, but note that you get to do this in certain cases when back button focus is enabled that are disallowed when you are using front button focus. It is complex and confusing. Here are some factors:
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