With luck, this post will answer some of those questions (while no doubt prompting others
).Background is that over the years I've accumulated a variety of lenses that I've found useful for macro work, and I finally decided to set up a head-to-head test on a common subject and see what happened.
Test subject was the snout beetle face that I had previously posted.
Lighting was fiber optic halogen, with a pingpong ball diffuser.
I tested seven different lenses:
- El Nikkor 50mm f/4 enlarging lens, reversed
- Mamiya Sekor 55mm f/1.8 lens from 35mm film SLR, reversed
- Olympus 38mm f/2.8 bellows macro lens
- Olympus 20mm f/2 bellows macro lens
- Zeiss Luminar 16mm f/2.5 macro lens (looks just like a microscope objective with an aperture ring)
- aus JENA 10X NA 0.25 microscope objective (effectively 14.5mm f/2)
- Edmund 20X NA 0.40 microscope objective (effectively 7.6mm f/1.25)
For lenses with variable apertures, I shot a series of single frames stepping aperture over each lens's full range. From those, I picked the aperture giving highest resolution, to use for stacking.
Along the way, I noticed that a couple of my lenses seemed to have a lot of flare when used without a macro lens shade, so I made one of those. It helped a lot.
Here are some pictures that summarize the results.
First, full frames illustrating the field size for each lens test. You can figure out the magnifications yourself, from the scale bar in earlier post. This image also shows the macro lens shade and its effect. You can see from the Photoshop histograms that without the shade, all the dark tones suffer pretty badly from stray light
Second, here are some illustrations of what happens as various lenses get stopped down.
All of the lenses have lousy resolution when stopped down too far, mostly due to diffraction. But they vary in what happens at wider apertures.
The Mamiya lens in the middle is typical of what I find with "ordinary" lenses -- performance peaks around f/5.6 and falls off for wider apertures. There's also a significant shift in focus point over the first couple of f/stops, so it's a bad idea with this lens to focus wide open and then stop down to shoot. (I've seen the same effect with other lenses, including my Sigma 105mm macro.)
In contrast, the Olympus 38mm macro lens actually has highest resolution wide open at f/2.8, and starts to degrade noticeably at f/5.6.
By the way, it would be a mistake to compare across lenses from these pictures. These are actual pixels, but the magnifications are different.
Finally, here is a direct comparison of center resolution from finely spaced stacks for all the lenses. What I've done here is to resize the result images so that they're all the same magnification. The Olympus 38mm image is just about actual pixels, and the others are scaled up or down to match. (There's a bit of detail lost in the 20X image, but I think the others keep everything that matters.)
So what's the bottom line?
Well, it depends...
At one extreme, we've got the standard microscope objectives. They give great resolution and they're pretty cheap, but they give very short working distance and the DOF per frame is so shallow that you have to shoot really deep stacks, which costs time and wears out your camera. At the other extreme, the Mamiya and El Nikkor lenses give great working distance and they're cheap too, but the resolution isn't quite up to the dedicated macro lenses and flare may be more of a problem. In the middle there are those dedicated macro lenses, which are real nice if you've got 'em, but they're fairly expensive and can be hard to find.
Note that center resolution doesn't tell the whole story either. Go very far off center, and some lenses start to lose resolution and pick up color fringes. The dedicated macros seem to shine in this regard, showing essentially no change in image quality across the whole frame (as shown by other tests, not these).
I'll stop here, 'cuz I really don't know what might be of interest to anybody reading this.
Questions, comments? George?
--Rik
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