On the Olympus E-M1, I shot with the Olympus 12-100mm F4 zoom and the Olympus 12-40mm F2.8 zoom at 12, 14, 18, 25, 34 and 40mm, using F4 and F8. That’s twelve images with each lens. I then compared the four corners and the center of the comparable 12-100 and 12-40 images.
Because this was a brick wall experiment at about 50 feet, many might characterize it as a worthless experiment as it was not a "real world" situation. I understand this argument. After all, how often in real life is one interested in just one two-dimensional plane within a three dimensional image? Nevertheless, primarily to be sure it is working properly, I do this little experiment with any new lens during the 30-day return period when purchased from Adorama or BH Photo.
So you can visualize the experiment, here is the brick wall at 12mm:
I downloaded the twenty-four photos to Lightroom and compared them side-by-side using the X:Y feature. Let’s make this real simple: At 1:1 magnification I could see no difference whatsoever!
I am so pleased with that result. I had been ready for a small compromise in image quality in exchange for an 8X zoom range; but, it doesn’t look like there will be any loss of quality, at least when used in the F4 to F8 range.
I then compared the images at 2:1 magnification. The first time viewing it seemed the 12-100 was sharper than the 12-40 at 40mm and that otherwise they were identical. But the second time viewing, due perhaps to somewhat different lighting in my computer room, or perhaps flip flopping the side-by-side images from the left to the right side of the computer monitor, the images this time seemed identical. Go figure! I think I’m starting to split hairs, and my eyes are beginning to cross. I think this is another case of too close to call.
I think if I were starting from scratch with mFT I’d buy the 12-100 and not the 12-40. The faster aperture of the 12-40 is appealing, but the 12-100 has a bigger zoom range. Perhaps I’d supplement the 12-100 with a couple of the Olympus primes for those situations where a bigger aperture is desired. If I were to pick two primes they would be the 25mm F1.8 first and the 45mm F1.8 second. On the other hand, to keep within a tighter budget, I’d buy either an Olympus 14-150 or Panasonic 14-140 travel zoom as my first zoom lens. Then, if there is any money left, I'd make the 25mm F1.8 my first prime lens.