My two cents' worth, just having returned from two weeks in Colombia with Glenn Bartley. First of all, using DXO PureRaw, issues of image noise up through ISO 6400 largely disappear, and if pictures taken with my M43 rig were weaker in this respect than those taken by others using Canon and Sony gear (no Nikon users in this group), the differences were very, very small. Others were routinely going to ISO 12,800, however, and at that level the Olympus images clearly were less correctable for image noise. But come on, ISO 6400 solves a lot of low light problems.
To invoke my second point, I must recount the painful fact that in a moment of inattention, I failed to prevent my tripod-supported rig comprising the OM1 plus Olympus 150-400mm zoom from toppling to the ground (actually a gravel road) in the Colombian Andes, ouch. The viewfinder was apparently damaged. The "good news" was that otherwise, the camera still operated, so in some situations I could take photographs using the rear screen. For fast-moving birds in confusing vegetation this did not work so well, but at (say) feeder setups it was do-able.
Fortunately, I followed the practice of bringing along a backup body. To save weight and space, I had brought a "low-level" Olympus OM-D-M5 mark iii as my spare. To my surprise, this camera proved quite capable. I gave up bird/eye focus. Otherwise, what was the big difference? The sensor size is pretty much identical, I could use Pro Capture with effectiveness, and if the image quality was inferior, the difference is very, very tiny. I went back to the "old ways" of dealing with autofocus, including using the joystick to move the focus spot to the best location, of using the much cruder but still usually effective "subject target mode," Of using single autofocus and recomposing, and of resorting to manual focus adjustments in some critical situations. That there was no "focus peaking" was a small loss, definitely.
I will add that it was apparent that compared to the Sony and Canon systems, low-light autofocus with Olympus/OM1 cameras is not as good. I lost reliable autofocus while the rest of the group was still having no difficulties. Having to resort to manual focus in these situations was not "suffering," but the difference was definitive. Also, with some bird photo situations, including dealing with the gorgeous but exposure problem-prone Multicolored tanager (attached, I hope) with its oh-so-pale pale yellow back patch, it was apparent that the dynamic range of the M43 sensors is not as extensive as with the full-frame cameras. I had to underexpose remarkably drastically to avoid blowing out that back patch. The good news was that even at -1.3 EF, shooting RAW and using DXO PureRaw as a first postprocessing step rendered the issue fairly moot. After a first session that produced blown-out tanager back patches, I rallied during the second go-round and got some great photos of this amazing bird.
My damaged OM1 is now off to New Jersey for repairs. The generic estimate was a curious $417, and OM Systems told me the turnaround was under two weeks. We'll see. I am off to Patagonia in early March, and I am still hoping to take the OM1. Because this is not a bird photography workshop (it is a Road Scholar trip, a "general tourism" type trip with my wife), I will not bring the 150-400mm zoom. Instead, I will be bringing the 100-400mm zoom, lighter and much less conspicuous; the rig can be placed in a holster-type hip pack to protect it from the lousy weather of Tierra del Fuego.
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