First, the optics.
On the left, a Nikon CFI60 Plan 10X NA 0.25 objective, mounted on a Sigma 105 mm f/2.8 macro lens focused at infinity.
On the right, A Nikon CF BD Plan 5X NA 0.13 objective, mounted on a old Vivitar 200 mm f/3.5 telephoto lens.
Now, ignoring potential differences between the 200 mm telephoto and a real Nikon tube lens, it's apparent that the 5X BD Plan is being used "as designed" at 5X, while the CFI60 10X is being dragged way outside its design space, operating at measured 5.15X on a nominal 105 mm "telephoto" that isn't even really a telephoto.
Comparing these two lenses, the results should be a foregone conclusion. The 5X lens will probably have less central resolution because of its smaller aperture, but should be pretty uniform, while the unfortunate 10X lens that's being forced to cover twice its usual field may have a really nice center, but its corners are going to be junk.
And indeed, when we look at the images, the corners for one lens are notably deficient, while the corners for the other lens look quite good. The following are actual-pixel crops from a Canon T1i, 15.1 megapixels on a 22.3 x 14.9 mm sensor.
But here's the surprise: the good corners come from the CFI60 10X dragged down to 5.15X on the 105 mm macro.
Here are the corners at actual-pixels with proper order and labeling. Those are followed by the centers at actual-pixels, then a reduced full frame showing the crop locations. Zerene Stacker PMax, no post-processing except laying out for posting.
As mentioned, this is a surprising result.
The obvious answer, of course, must be that the 200 mm Vivitar is just a duff lens.
Well, that's hard to test rigorously, especially since it's after dark right now and I can't just point the thing at some target a long ways off. Come to think of it, that wouldn't necessarily be a good test anyway, because maybe the thing would work OK by itself but not play nicely with an objective.
So how about we just put the CFI60 10X on the 200 mm Vivitar and see how that looks?
Here's the answer, corner and center, comparing CFI60 10X with BD 5X, both on the 200 mm Vivitar. In this set, the CFI60 is with a different part of the wing (different day, after some other tests). But I think the result is clear enough. The centers have similar sharpness, but with the CFI60 10X there's not much difference from center to corner, while with the BD Plan 5X the corner is notably deficient.
Conclusion: the problem is in the BD objective, not the telephoto. I suppose I could nail this down even farther by putting the BD on a different telephoto too, but really, enough is enough!
No doubt different people will see different things in this exercise, but here's my set of take-aways:
1. The CFI60 Plan 10X NA 0.25 objective also makes a very good 5.15X objective, when paired with a 105 mm rear lens on a 1.6 crop-factor sensor. There's detail at the pixel level, corner to corner at 15 megapixels.
2. At least this particular CF BD Plan 5X NA 0.13 will make a fine addition to my collection of unused lenses. Is it a duff sample? I can't tell, except to say that I can't see evidence of abuse or disassembly. But it's pretty safe to say that this model of lens is not going to appear on my personal short list of lenses to recommend to other people. This is annoying because I bought the 5X objective expecting it to work well.
3. There's a big difference between these two objectives, despite that they're both Nikon CF infinity's. The newer design CFI60 objective beats the socks off the older BD Plan. Would this hold up with more samples and other lenses from the same lines? Can't tell, more testing required.
4. Tests beat preconceptions, sometimes quite badly.
Hope you find this interesting.
--Rik
, objectives that perform well at image circles larger than was ever needed for their intended use. Some seem to be capable of doing so, others do not. Other than trying each individually, there does not seem to be a universal answer, even in the same vintage and line of objectives.
