This is for those who are considering changing formats. I've shot Canon full frame for years and love the line. It was getting heavy though and I'm now trying the MFT format. Using the OM-D M-1 MKIII body, this lens is very good. I posted a couple of test shots against my Canon EF 500mm L IS lens which is awesome. All shot at f/8 ISO 100 without additional post processing. They are crops of the 13X19 test target that's included so you have a sense of scale. As you can see, it is not quite as good, but considering I can hand-hold an 800mm lens and walk around with it all day, the difference in sharpness isn't quite as bad. Sure was less expensive too. Very fast to focus and light enough to take on hikes, it will be a lens that I can use frequently. Very pleased.

Olympus hit it out of the park with this one.
Imagine quality is great and this lens is a joy to use.

One pleasant surprise is the build quality is better than I had expected.

I like this lens much better than the Panasonic 100-400, which suffered from quality control issues and had a bad zoom.

The Oly feels great and works great, plus it takes a teleconverter!

Since its bigger brother is forever out of stock, I took this to my trip to antarctica. That turned out to be a great decision. It is light and it is long enough. I brought with me the 2x extension and never used it. I took crazy 1000-2000 shots a day whenever we had a landing. The first two were taken on a very shaky zodiac and the third was taken on the moving National Geographic Explorer cruise ship. This together with the E-M1 Mark III made my trip perfect.

I use this lens on my Olympus M-1 MarkIII camera for bird and nature photography. The 100-400 zoom is a good magnification range for most situations and the image stabilization is good enough for hand held shooting for the majority of my needs. The image's are very sharp and detailed even with significant cropping. Focus is fast an accurate. I'm very happy with this lens.

I purchased this lens primarily for wildlife - and especially birds. There have been reviews that indicate this lens gets soft at the 400mm end, but my experience is that this lens is very sharp throughout the total range. Now, having said that, using the current Olympus OMD cameras, the real limitation is the sensor. Because this lens has an aperture range of 5-6.3, you will need to shoot in plenty of light in order to keep the ISO of the OMD camera low. I've had great success with this lens shooting up to ISO 400 and even 800, buy anything higher really degrades the images - but that is the sensor, not this lens. So, as long as you understand the limitations of the cameras this lens is designed to work with, you'll get great images!

I first tried out a Panasonic/Leica 100-400 hoping it would work out due to its diminutive size. Unfortunately, it was not consistent performer. I ordered a Olympus 100-400 to take it's place hoping for the best. That is what I got. Not only is its build very reassuring as it's solid as can be and the focal range ring and focus ring are smooth as butter (unlike the rough Panny). From 100mm right on out to the 400mm end it's crystal clear sharp wide open. However, you may want to know that when I was using an OM-1 body, if you have the lens IS on it is NOT as stable a result as with it in the OFF position allowing the body to do the work of the IS. I have NO complaints about this lens as it's as close to perfection as you can get optically. Focus is immediate and accurate every time. Highly recommended. It can be used as a semi Macro lens as well since it has excellent close focus capabilities as well with great bokeh.

Wildlife photography use - excellent lens. Much more convenient that FF heavy lenses for adventure photography for us older folks. Image quality and build great. Weather resistance a must.

Focuses quickly compared to their other telephoto zoom lenses

I had my doubts, not as to this lens's quality, as it is a Zuiko, but as to justifying it's price and weight considering it offers only a 200 mm extra reach compared to my excellent Zuiko 75-300 (150-600)mm zoom which costs less than 1/3 and weighs much less than its 3.5 pounds and which has given me many in my opinion very good photos over the last year of COVID19 backyard photography; UNTIL you factor in the Zuiko 2X converter and then you have a whopping 400-1600 mm reach with absolutely no loss of resolution or sharpness even if handheld (no tripod) due to the excellent IS system of both the lens and my OM1 MKIII combined; That makes all the difference and I would recommend to anyone considering buying this lens that the 2X converter be considered PART of the lens ; I leave mine on the lens all the time and use my 75-300 when I don't need the extra reach or wand to give my left arm a break; It is a superb piece of equipment for a bird- or wildlife photographer.

The Olympus 100-400 F/5-6.3 is a very sharp lens. Autofocus is quick and it feels more like a pro lens. It is a little heavy but definitely manageable. Close focus works very well, great for nature, sports, and event photography. Image stabilization works great, I was able to hand hold in slow shutter speed at 400mm and get crisp images. I also used this lens hand holding in High Res mode stationary subjects and the results were superb. For outdoor photography I take the 12-100 F4 Pro and the 100-400 to get an incredible focal range. I highly recommend this lens.