Boise Creek, Enumclaw WA
Just another overcast day full of beautiful streams and forest
Note: this is the first post in a new section, Photography Instruction. I will put longer-winded, more technical posts in this section from now on.
I did a test of the new 32mm lens to see how sharp it is. I took the above photograph over and over with different apertures. Generally speaking, you expect a good quality lens to get better from wide open to about f/8 or f/11, and then to slowly get software due to diffraction effects.
That is not what happened with this lens. That could have gone two ways: so bad that it never got good, or so good that it never got bad. It was at its best wide open, a rare quality in photographic lenses.
Even so, I used the f/8 image in the series—it has more depth of field than the f/4 image. (I’ll do a post on depth of field at some future point.) It has the best balance of depth of field, sharpness, color, and contrast. See my comments on the last three qualities below.
I used what is becoming my favorite landscape setup for this photo: the 32mm Rodenstock Digaron lens on the Cambo WRS-5000 camera with the IQ3 100 Phase One digital back. (I guess I should point out that it’s not that unusual in the land of medium format camera to mix and match like this.)
I have taken photos along this creek before, but I took another path this time and found this small, delicate cascade. I use a long exposure to smooth out the white water and draw more attention to it. To do this I needed to close the lens down to at least f/8, which gave me a 1/10th second exposure. Smoothing of flowing water starts to happen at about that shutter speed. (But you can do 20 minutes if you like, with a proper neutral-density filter. I forgot to bring the neutral density filter (left it in the car), so I had to improvise with a series of smaller and smaller apertures to allow longer exposure times.)
It’s just an ordinary stream, but the camera turns it into a subject that your eye can wander around in to find fascinating little details.
Technical info follows.
Sharpness: a lens has good sharpness when it can separate very small details from one another. Bad sharpness looks like the details got smooshed together; good sharpness resolves tiny details, like blades of grassy or veins in a leaf, clearly.
Color: a lens has good color when the colors are not washed out, but are rich and vibrant.
Contrast: a lens has good contrast when small details with slight differences in brightness can be clearly seen as different.
Note: evaluation of sharpness requires a sensor with high enough resolution to see the sharpness that the lens provides.
Let’s look at a series of images taken at various apertures (f/4 to f/32) to see what effect aperture has on these three qualities. These are crops of one small area in a series of images with approximately the same framing as the image at the top of the post.
At f/4, the sharpness of the image is obvious. For most lenses, the sharpness will improve at f/5.6 and f/8, then start to soften. For this lens, which seems to be exceptionally sharp, it is at its best wide open at f/4.
Note: this image is slightly lighter overall. I tried to do the same processing on each image, but it wasn’t perfect.
At f/5.6, we see that the sharpness is still very good, and color is also about the same (but it’s hard to tell with the change in brightness; my bad). Contrast is down a very small amount; it may not be obvious if you do not regularly pore over images tweaking the settings. I had to use a slower shutter speed to compensate for less light through the aperture, so the water now looks smeared instead of frozen in action. This is normal. For me, it is also desirable; in many situations, I like to see the water flow made smoother.
At f/8, we are still in good shape, but some of the superb sharpness we saw at f/4 is gone. It’s a tiny bit harder to differentiate the individual blades of grass. Contrast is also, again, slightly down, which adds to that problem. The image still looks quite good, however. The water is a little more smeared and smoothed.
At f/11, we can now see that contrast is noticeably reduced. The image is still good and usable, but not at full size. Shrinking image size will make the reduced contrast and sharpness much less of a problem. Color is also starting to be affected; saturation is not as good. Again, this is a slightly lighter image, so the effect is not obvious, but contrast is beginning to be compromised at f/11.
At f/16, the image begins to look a little soft due to loss of sharpness (itself due to diffraction effects with small apertures) and reduced contrast/loss of saturation, but it’s not dramatically different from f/11. This is still a very useable aperture. The water is now very smoothed (thought not completely), and the depth of field is very large—objects close to the camera are in focus, and objects at infinity are also in focus. If depth of field is your requirement for the shot, then either f/11 or f/16 is likely to work for you. Again, I would not use the image full size, but it should be excellent at 50% or smaller.
At f/22, the can really see the changes. Contrast is reduced, color saturation is still pretty good but declining. Sharpness is poor, for two reasons: diffraction effects, and the length of the exposure. The exposure duration has been doubling since f/4, when it was 1/160th of a second, to 0.8 second at f/22. So not only is the water very smoothed out, but some of the grass has been stirred by a light wind and looks fuzzy as a result.
At f/32, the smallest aperture available on the 32mm lens, we see less wind stirring, more water smearing, and reduction in sharpness, contrast, and color saturation (although of the three, color saturation is holding up better). The image is probably still useable at 33% or 25%, and will have huge depth of field, but it is not going to have any of the ‘zing’ of a really good, sharp image.
What are the diffraction effects I keep mentioning? And why does it apply to smaller apertures?
First: when light passes an edge, it is a wave so it get disturbed—just like waves get mixed up at the edge of a body of water. It’s not fully accurate, but think of the light waves bouncing in various ways off of the edge and making mud out of that part of the image.
If you have a large aperture, these edge effects are a minor contribution—they make up a minor part of the area of the aperture.
As the aperture becomes smaller, these edge effects become a larger and larger percentage of the aperture. So the edge effects start to dominate, and at some point (f/32 in this case) the whole image suffers because it’s all too close to the edge as it comes through the aperture.
You need a good reason to limit image quality by stopping the aperture down below f/16 (and in some cases, even f/16 will be too much).
The better the lens quality, the earlier in the process the edge effects will begin to dominate. This is an exceptionally lens because the best image quality occurs wide open. (If the lens quality were not as good, small imperfections in the surface and/or curves of the lenses inside would degrade the image more than the diffraction effects take over at f/8 or f/11—up to that point, stopping down the lens means less glass is being used, and the image will improve in an average lens up to about f/8.)