Resolution
I normally don't do a lot of testing of my camera lenses, because I don't own a lot of lenses that could possibly substitute for one another. I don't know whether my 24mm f2.8 is sharper than my 200mm f4, and frankly, I don't care, because that kind of knowledge couldn't possibly make me choose one over the other.
For several years, I've owned a 105mm f2.8 AIS micro, and I've been very pleased with it. But on my last trip to the camera store, I saw a beautiful, pristine, 105mm f2.5 AIS Nikkor, the classic portrait lens with perhaps the best reputation of all portrait lenses for the 35mm format. It was priced well, and I bought it.
Now, with two lenses of identical focal length, it's time for a shootout. Prepare for the "Battle of the 105mm Nikkors"
I loaded a roll of Kodak Royal Gold 100 into my FM2n. Perhaps not the best film, but the best film I had around at the time, with reasonably fine grain. I put the camera on a tripod, and used the self-timer for all photos, to minimize vibration. I scanned all negatives at 2400 DPI using my HP PhotoSmart S20 film scanner. I used no exposure or color corrections, and I didn't sharpen any of the photos in PhotoShop.
 105mm f2.5 at f8 |
 105mm f2.8 Micro at f8 |
Both look very sharp to me, and I'd have a hard time choosing one over the other based on this test.
OK, let's look at them at f2.8 (I didn't test the f2.5 lens completely wide open, figuring it's fairer to test both at the identical aperture of f2.8).
 105mm f2.5 at f2.8 |
 105mm f2.8 Micro at f2.8 |
Here, you can clearly see that the 105mm f2.5 is sharper. There's a hint of color fringing at the bottom edge, but it's still a better image than the 105mm f2.8 micro. In fact, it compares fairly well with the lenses at f8.
One of the prized qualities of a good portrait lens is the way it renders out-of-focus highlights. To test this, I took a series of pictures of my backyard deck. I focused on a patio chair about 10 feet away (perhaps a bit farther than a typical tight portrait). I made sure to include some foliage in the background, and took the photos late in the day, when the sun shines on that foliage, making lots of bright specular highlights. I tried to take the series of photos as quickly as possible, but I wasn't instantaneous, so the sun moved a bit, changing the details of those specular highlights slightly from one photo to the next.
Let's look at both lenses at f8:
 105mm f2.5 at f8 |
 105mm f2.8 Micro at f8 |
There's definitely a difference in the character of the Bokeh, but it's subtle. I'm not sure the difference is enough to choose one lens over the other. I notice that the dark portions of the background look darker on the f2.5 lens. Perhaps this is due to less flare in this lens, with fewer elements? But those darker shadows do tend to make the background less smooth.
Let's look at them both at f2.8:
 105mm f2.5 at f2.8 |
 105mm f2.8 Micro at f2.8 |
The difference is now less subtle. The f2.8 micro shows a definite ring around the outside edge of each specular highlight in the background. A hint of those rings is visible at f8, but it shows up much more clearly at f2.8.
I'll keep both lenses. The micro is great for its intended purpose -- extreme close-ups. And it's not bad as a general purpose 105mm lens. But the 105mm f2.5 is smaller, lighter, more convenient to use (built-in slide-out hood vs. reversible snap-on hood for the Micro), a tad faster, sharper, and with smoother bokeh as a bonus. It's also easier to focus, since a given movement of the focusing ring results in a smaller change in focused distance on the 105mm f2.5.
