Zone Focus

Many articles discussed sharpness. Yes it's important and many may will disagree with me. 

But do you need to track the eye and spend money to purchase new cameras, "gun site" accessories? IMHO No.

Consider zone focus. 

For me with moving subjects I use Photopills DoF pill to decide will the subject be in focus.

When I'm using my 75-300mm glass wide open at f/6.7 and estimated distance of 100ft my DoF is over than 6 feet. That's enough for me to capture many subjects with acceptable sharpness. At f/8.0 DoF is 8 ft. That works for me. YMMV.

For those not M43 shooters need to consider the sensor size. Using a full frame at 600mm it would cut the DoF about half.

 DoF Simulator

PhotoPills DoF Pill

Choose your aperture

My DoF Rant

I'm new to (fill in the blank) I just ...

It's not mine to reason ...

(Ours not to reason why, ours but to do and die.

Alfred Lord Tennyson)

But why we, well some, get some shiny item then purchase accessories that they don't know to use or fit.

For example, "Hi, I’m fairly new to photography and have an Olympus E-M10 Mark II camera, I just purchased a secondhand lens ..." That doesn't work likely the wrong mount.

The message is understand your equipment before you accessory. For cameras learn its capabilities and photography. Better photos is not paved by gear. Learn the craft then you will learn the gear you can actually use or need.