A post on DPReview got me thinking about the performance of the Olympus 45mm f/1.8 versus the old legacy fast 50’s out there. There were many claims of, to exaggerate only slightly, “Oh, wide open they might not be quite as good, but stop it down to f/2.8 and it is basically as sharp”.
Just to do a small amount of “disproving” of this. Here is a center resolution test of my Olympus Zuiko 50mm f/1.8 Made In Japan lens, reputedly the sharpest of the old 50mm Zuiko lenses. Keep in mind, I have gone through a total of 7 50mm f/1.8 lenses over the years as well as 3 different 50mm f/1.4 lenses and I can vouch that the 50mm f/1.8 MIJ that I have kept is the sharpest of all of the 50mm OM mount Zuiko lenses I have come across.
First, Olympus m.Zuiko 45mm f/1.8 center performance.
Click on the image for full size, this is a 100% crop at apertures of f/1.8, f/2, f/2.8, f/4, f/5.6 and f/8 in descending order.
Same for the above, except it is from the Olympus Zuiko 50mm f/1.8 MIJ OM mount lens on my OM-D E-M5 and at apertures of f/1.8, f/2.8 and f/5.6.
Just about equal stopped down to f/2.8??? I don’t think so. The 50mm f/1.8 MIJ shows much lower contrast at all f-stops and resolution is noticeably lower at f/1.8 and f/2.8 compared to any f-stop of the m.Zuiko 45mm. At f/5.6 the resolution is roughly close enough to be considered “the same ballpark”, but contrast is still a little lower.
Needless to say the corners are even worse than the center.
For the m.Zuiko 45mm f/1.8 it is a single series, S-AF autofocused and then switched to manual focus to keep the same focus for all apertures. For the 50mm f/1.8 I refocused between each shot, took 3 at each aperture (f/1.8, 2.8 and 5.6) using magnified view to confirm focus and took the best shot from each of the 3 to comparisons sake. All tripod mounted, 2s shutter release.
Several things to consider. Not only is the 45mm f/1.8 a much newer design (and significantly more complicated optical formula), but it is also designed both as a short telephoto from the get go, instead of a normal lens that just happens to be a short telephoto focal length when mounted to m4/3, but it was also designed to cover the image circle of m4/3. The Zuiko 50mm f/1.8 MIJ was designed to cover a much larger image circle, which is now cropped down to 25% of the original (or the inner 50% diameter), which is going to magnify any flaws, but it was also designed to resolve well on 35mm film, which is not exactly as demanding as a 16MP CMOS sensor, let alone one using only 1/4th of the imaging circle and 16mp.
The lens resolves just fine on 35mm film, but just wasn’t meant to be pushed as much as it is mounting on an OM-D E-M5. It doesn’t make the 50 a bad lens, just not the best choice for m4/3 use, especially 16MP m4/3 use (IE the sensor significantly out resolves the lens, at least until stopped down to roughly f/5.6 and then probably still outresolves the lens a little bit).
As a cheap portrait/short telephoto or a lens where you are looking for some “character” to it and by character I mean can render the subject softened and lower contrast (an ideal some do look for in a portrait lens) when shot anything close to wide open, then it is a great lens for m4/3 use. It however is not in the same league as the m.Zuiko 45mm f/1.8.