First, let me give you a little background. I was a hardcore Canon guy. Even so, I really was always open to good systems regardless of name.
For really long wildlife shots, I used a Tamron 150-600mm Zoom which, to be honest, was nothing short of amazing. Super sharp photographs, super fast autofocusing, and very well built. The price was really reasonable for the level of excellence it provided. However; it was huge, heavy, and most often I would not take the lens into the field on one of my two harnessed cameras.
Why? Because it was huge, heavy, absolutely requires a tripod or monopod at the least, and that all is simply too much to wrangle... No other reason than that.
It was simply easier to leave the huge heavy lens in my lens bags and use a lesser smaller and lighter zoom lens, and miss the potential shots of a lifetime... I see and hear this a lot among other photographers, "I don't carry my huge heavy lens because it is a huge heavy lens."
I switched to MFT and decided upon Olympus as the system I would switch to. Olympus glass is really really sharp.
I have always found that Olympus MFT glass is extraordinary, light by comparison, and VERY easy to operate quickly in the field when the shot is fleeting, take a second too long and you miss the shot of a lifetime.
The 300MM PRO 4.0 is equivalent to 600mm and I have the Olympus 1.4 teleconverter mounted on my OM-D M1 Mii. It mates perfectly and the quality is very complimentary.
The 300mm PRO 4.0 is REALLY well built, and it feels like it in your hand... It is super solid, meticulously detailed, and actually really nice to look at... I know, not THAT big a deal, but sometimes lenses take a beating in the brush and tumble of the daily working photographer. Being strong and beautiful is a good thing...
I can't pretend to be very technically knowledgeable so I will skip all that mumbo jumbo and I will tell you this.... Daaaaaaang this glass is super sharp!! Because this is a prime lens and not a zoom, it is much more difficult to acquire and lock on flying birds, but when you do, WOW!
With the 300mmPRO4.0 it is really imperative that you learn how to do custom sets on your C1 C2 C3 settings on the M1... Why? Because if you are shooting flying birds you need to actuate the tracking feature on the M1, which is awesome. (There are lessons here on this site to help you with these things)
If something is still I use manual focus almost all the time. I just find it easier, especially if the background is complex. So you could set up a C1 setting for flying birds, or moving subjects that require tracking, and you could set up C2 to settings which capitalize on the high functionality of this system and lens for incredibly sharp photographs.
This lens is comparatively light and the OM-D M1's stabilization is so extraordinary, and the lens stabilization is sickeningly good, I confess I handhold a lot more than I could do with other systems.
I should use one of my two tripods or my monopod, but I end up handheld and this camera and lens is so extraordinary, it comes through almost perfectly. I write 'almost' because a tripod or monopod at the very least most often provides that additional mind-bending level of clarity and sharpness that this camera and lens is known for.
I have only shot approximately 4,000 images in the few days I've owned this lens and I would say I am still just learning the lens and camera system. That said, what in the world will this system do once I actually learn to use it expertly!?
For some the price is an objection... I get that, no one GIVES me lenses or camera systems and I save and save... and save for equipment just like everyone else, hoping I am not wasting my money, and that I get lots and lots of good use out of it... That said, I, and probably all other photographers have this objective to eliminate unnecessary dead weight in their go bags in order to go farther and faster into the field...
In order to do this, we always hope we select the JUST RIGHT lenses so we can consolidate our glass to cover all of our bases photographically, and generate great work without carrying the kitchen sink with us.
As I am sure you can imagine, that is the lure of zoom lenses, multiple focal lengths in one lens.
So you might ask, "Why then do you advocate for the 300mm PRO 4.0 since it is a PRIME lens?" Well, THIS lens is entirely different than any lens you may have ever used. It is TACK Sharp edge to edge and even with the teleconverter it is simply amazing.
It is the biggest (size wise) lens in the MFT subset, but the more you use this lens, its impeccable construction and design make it a joy to use.... If feels perfect in your hands, at least it does in mine. Set the focus to C-AF-TR for moving subjects and set the fire rate to whatever rate you choose.
I use as small a focal point as I can because I am an addict for bird or animal eyes... I am addicted to super sharp feathers or fur on the animals I am photographing... and, although I am not a moon photographer, with the Olympus 300mm 4.0 PRO any putz can be a good moon photographer, and if a person is NOT a putz, and actually uses a tripod and cable or wireless release, you will capture amazing moon shots... truly so sharp you can actually discern what type of cheese is in each part of the moon and even more, you can determine if it is organic cheese or cheese product... Yes, it is THAT sharp...
I love this lens.. I love this system... Imagine when I can actually use it to it's fullest capability! You might ask, why four, and not five stars...? Simply because the sliding buttons that actuate the Lens IS are too easy to click off without knowing it... Also, the hood is cool that it is retractible, but I haven't figured it out totally yet.
I am excited to keep learning this system... Olympus is an extraordinary company with an extraordinary system...
Might I add one brief note not related to the lens or Olympus specifically? Photography is a precious thing to me... and those photographs you shoot, at some point, whether accidentally or not, will capture something amazing and that image might happen only once in your life.... Revere it... Photography captures moments to remember. Once in a lifetime... Might as well use the best system available to do it.