After trying macro lenses for the last five years, I decided it was time to try extension tubes. Typically, one uses extension tubes for higher magnification than is possible with a dedicated macro lens. However, at present, I’m more interested in experimentation with decreased minimum focus distance for regular lenses. In particular, I have the SMC 150mm f/3.5 lens, which is quite sharp (seems sharper than the 120mm macro), but has a minimum focus distance of about 55in (1.4m). I want to use it to shoot flowers, but I need more magnification than is possible at the MFD — ergo, the extension tubes.
Like many things, obtaining Pentax 645 extension tubes was easier desired than accomplished. Complete sets rarely appear for sale here in the US. The few sets I’ve seen over the years were too pricey for something that might get little use, so it took until two months ago to find a complete set at a reasonable price, in decent condition.
The standard set comes in three independently usable sections: 13.3mm, 26.6 mm, and 39.9mm. These lengths allow one to adjust the desired amount of magnification with ease. Of course, greater magnification is accompanied by a decrease in the depth of field. In addition, the greater working focal length makes hand-held shots nearly impossible, so out came with the tripod.
Since this was my first time using extension tubes, I wanted to get a feel for how much the DoF was affected. I grabbed a vase of flowers and shot a series of images using the 13.3mm extension tube segment. I kept the same f-stop but moved the camera closer to the subject for a series of shots. I used both the Pentax 645 A 150mm f/3.5 and the 645 A 120mm f/4 macro lenses for the tests. At this point, the goal was to see how well each performed over its base ability.
I used Kentmere 200 for all images. Unfortunately, I developed the film before realizing my thermometer was off by 18°F, so that film is overdeveloped (accidentally pushed two stops). The first two studio images use the 150mm f/3.5. The first image is at the MFD of 55in, and the second image is at 46in. Forty-six inches gives an image that’s the size I want with this lens. Next, I tried using the 120 macro lens (MFD 15.5in) at 27in, then at 19in to see its behavior without the tube attached, and for comparison to the 150mm image’s magnification and DoF. Finally, I took the 120mm macro outside to get pictures (with the 13.3mm tube attached) for reference purposes.
Despite the development error, these shots don’t look horribly bad. I had to scan some frames multiple times because the Kentmere 200 was curled even after being pressed between heavy pads for a week. After seeing these initial images, I decided to shoot a roll of Portra 400 using the 120mm macro lens and the 13.3mm tube, as well as one using Lomo 400 CN and the 150mm lens along with the 13.3mm tube. The color results will appear in future posts.
Overall, this was a worthwhile experiment. DoF shrinks quickly, and balancing magnification with DoF is going to be tricky. Were I using a digital camera, I would do focus-stacking, but I want to see what I can achieve with film. I haven’t developed the Lomo 400 yet, but the Portra 400 looks great. Now, if this heat would go away, I could go outside again.
