In this second round of legacy 28mm prime lens testing, we continue to evaluate the impact of field curvature, at the optimal aperture for landscape shooting. Since two of the lenses being tested are slow 28mm f/3.5 lenses, we’ll also measure light falloff in the corners, using Imatest. The source files for those measurements were set to equal exposures in Photoshop, which made the numbers relevant across both lenses. Here are the corner measurements:
SMC Pentax-M 28mm f/3.5 at f/3.5: -3.34 (f-stops)
Vivitar(Kiron) 28mm f/2.0 at f/4: -2.69 (f-stops)
At ~f/10, both lenses were nearly equal, at -1.99 (f-stops) and -1.86 (f-stops), respectively. To sum it up, you do pay a corner darkness penalty with slow glass on a full-frame sensor, but it’s largely gone after the lens is stopped down hard.
The other SLR lenses that were also evaluated for flat fields on the Sony a7R were the Olympus G-Zuiko Auto-W 28mm f/3.5, the Minolta MD Celtic 28mm f/2.8, and the Canon FDn 28mm f/2.8 lens.
Ease of focus at long distances: The center focus point was on the “5610″ street address, on the back of the building. The worst of the bunch for focusing was probably the Canon FDn 28mm lens; it always seemed like either one side or the other was slightly blurry. The Olympus 28mm lens doesn’t have a half-stop mark between f/8 and f/11, which isn’t really focus-related, but it makes the lens feel cheap. The Pentax 28mm lens probably gets the nod here, 1 point.
Lens contrast: Center contrast appears adequate on all of these lenses; this is a subjective call, and it’s influenced by things like changing light conditions, and unequal exposures.
Sides of the images: The Pentax 28mm lens(2 points) takes a narrow win over the Vivitar 28mm lens(1 point). Look at the “5620″ street number, towards the left side of the picture, is it readable? The Olympus 28mm lens has a problem with the left side, which seems to be par for the course for that brand. The Minolta MD Celtic 28mm lens is weak on both sides, with the left side being slightly worse. Don’t look at Minolta for a clean 28mm legacy prime, for full-frame sensor use.
Center of the images: The 36mp Sony a7R seems to favor the centers of every lens that it sees. Would there be more substantial differences between the lens center areas of these photos, if a higher density crop sensor camera was used for this testing? The biggest center failure was with the Oly, but it’s not by much, possibly even in the realm of a slight mis-focus. You have to look hard to see it.
Summary: SMC Pentax-M 28mm f/3.5, all the way. In addition to everything else, it could have the smallest amount of CA. The Vivitar 28mm f/2.0 lens is the winner of the consolation round, at least so far. Both copies of that lens showed very slight out-of-focus areas at wider apertures, at the same spots. We’ll take a look at more apertures than just f/10, with both of those lenses, at a later date. Brands to avoid in the legacy 28mm focal length are the losers here; Canon FDn 28mm f/2.8, Olympus due to unreliability(3 out of 3 build quality failures for this author), and all of the legacy Minolta 28mm prime lenses, except possibly the f/2.5 version, which has a problem with yellow glass. That radioactive yellow tint should be fixable with UV light, similar to what you would do with the old Pentax 50mm Takumars.
What is the next step? Locate and test a Pentax 28mm f/2.8 lens; they are reputed to be slightly better than the slower f/3.5 Pentax lens that was tested here. The unfortunate problem with some of these legacy 28mm lenses is the fact that multiple brands only offered five aperture blades in their camera lenses, which can make for some funky bokeh at certain times. In general, though, 28mm is a big step up over 24mm lenses, on the Sony a7R.
Here is the test image, it’s about 21mb in size, **be sure and view it at 100% size**: