Using an old FF a-mount lens instead of the hypothetical FE24-105...
...and comparing it to its 18MP e-mount APS-C counterpart on the a7RII.
In another thread I was whining… again… about the lack of (for me) a suitable walk-around lens for our FF aX cameras. The OP of that thread was saying that the FF FE24-240 was surprisingly good and might make a suitable 24-105 substitute, given those focal lengths are in its sweet spot optically. I won’t, and didn’t, argue the imaging merits of this lens but it is my contention that makes for a really large walk-around 24-105.
Heresy that it is, for this purpose I use the compact e-mount PZ18-105G f4.0 in 18MP crop mode. I know I’m giving up a lot to do this. In the thread I stated that I could use my ancient 35 year old a-mount Minolta 28-135 via the LA-EA adapter but it's also bulky and heavy-ish. Right from the beginning when I went mirrorless I refused to do this. Another poster apparently considered using this lens also , but pointed out some known faults and stated the PZ18-105 was optically far superior. For the purposes that I actually use a lens and where my threshold of pain lies this has not been my opinion but I had no proof other than what had been coming out of my mouth. That's a dubious source at best.
I knew going in that testing the two and comparing them is an exercise in frustration and in compromises. What I thought was going to happen did…pretty much. Overall the old glass shows it age appropriate defects and is also still hanging in there…much like myself .
Full disclosure, I placed my camera beside my radial arm saw so I had a flat spot beside me. That alone determined the distance to my 30”x48” test chart setup. I used a cable release, big Sirui ball head and a heavy tripod capable of pulling engines. In the lighting I had, I used 1/10 sec, ISO 100 and f6.3 because that’s a long-known good walk-around aperture with both lenses.
In this case the APS-C e-mount was shot at 55mm and showed a similar FOV to the FF a-mount at what registered as 90mm. While they both framed the test target similarly, at 100% the FF lens file was much larger at 42MP vs. the 18MP crop. I’ve provided one example below at both 100% full size and also down sampled to emulate the (also shown) center of the APS-C 18MP crop-shot... unfair but real-world practical.
Obviously the focal length was arbitrary and as with most zoom lenses quality varies focal length to focal length but this gives an idea of what generally happens. I shot at other focal lengths also and (to my surprise) the 28-135 hit a low spot at 110mm for some reason but came back strong at 135mm. In all my tens of thousands of images with this lens I never suspected this.
In the 18-105G's favor is a substantial list. It has significantly better micro-contrast, optical in-lens image stabilization, supports some native functions like eye AF, a much shorter MFD, a more compact form factor and the transmittance of the newer lens is much better also. I also know from practice that it is very tolerant of most all lighting situations ...all non-surprises. On the other hand, the 35 year old FF lens acquitted itself well, I think. It has the benefit of a mechanical zoom, which I like…a lot. But most importantly… overwhelmingly importantly… it is obviously providing more usable image information that I can use. It's still too big and bulky however. So I still think the 18-105 is fine (FWIW it actually gets better at 105mm) within the framework of a compromise on size. Everything is always a compromise. The takeaway of my test however is that if I want best image quality I think I can generally extract more out of that old FF a-mount lens. I’m glad I felt pressured into actually proving this to myself.
Something for you to look at:
All from the center of the capture. APS-C lens shown on left (100% from 18MP shot), similar FOV FF lens in center (100% from 42MP shot) and on the right, that same center section down sampled to match APS-C lens on left.
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