Extension Tube or Close-up Lens?

The full frame (APS-C size)
Konica-Minolta Dynax 7D
To focus closer than a lens’ minimum focus limit usually permits, one can use close-up lenses or extension tubes. It’s a common misconception to believe that close-up lenses degrade image quality while extension tubes don’t, as they don’t contain any glass that could affect the rays of light. In fact, extension tubes do degrade image quality substantially because they force the lens to work at a image distance it is not designed for. The performance loss caused by high-quality close-up lenses however is mostly neglectible.

To prove this, I’ve taken a few shots of the address sticker on a parcel that came my way via UPS Standard (see left). The magnification was 1:3 (i. e. an image field of approx. 47 70 mm) which is sort of half-way between close-up photography and real macro photography — a typical range where non-macro lenses plus tubes or close-up lenses are employed. I compared three 50 mm lenses on a Konica-Minolta Dynax 7D which is an APS-C-format digital SLR camera:
  • Minolta AF Macro 50 mm 1:3.5
  • Minolta AF 50 mm 1:1.4 with two (!) achromatic close-up lenses
  • Minolta MC Rokkor-PG 50 mm 1:1.4 on a 17 mm extension tube
f/11
Minolta AF Macro 50 mm 1:3.5
50 % view, corner
f/11
Minolta AF Macro 50 mm 1:3.5
50 % view, center
The image quality of the Minolta AF Macro 50 mm 1:3.5 lens, at magnification 1:3, is impeccable from center to corner at f/5.6 and f/11. When looking carefully, you can even see some minor image degradation at f/11 due to diffraction.

Regular images are at f/5.6; rollover images are at f/11.
Minolta AF 50 mm 1:1.4
Minolta Close-up Lenses No. 1 & No. 2
50 % view, corner
Minolta AF 50 mm 1:1.4
Minolta Close-up Lenses No. 1 & No. 2
50 % view, center
To arrive at a magnification of 1:3, it took two close-up lenses on the Minolta AF 50 mm 1:1.4 lens. The Minolta Close-up Lens No. 1 is +2 dpt; the No. 2 is +3.8 dpt. Still, image quality is virtually as good as with the macro lens.

However, note the purple hot-spot at the image’s center which become smaller and brighter at smaller apertures. This comes from the lens not being designed for use on digital cameras.

Regular images are at f/5.6; rollover images are at f/11.
Minolta MC Rokkor-PG 50 mm 1:1.4
17 mm extension tube
50 % view, corner
Minolta MC Rokkor-PG 50 mm 1:1.4
17 mm extension tube
50 % view, center
For the image taken with extension tubes, I had to resort to the Minolta MC Rokkor-PG 50 mm 1:1.4 because I don’t have any tubes for the Alpha mount. This older lens is one of the finest standard lenses ever made, it’s in no way inferior to the AF 50 mm 1:1.4 used in the images above. On extension tubes, the AF lens would give basically the same poor results.

At the frame’s center, image quality is just fine. However on the periphery it degrades considerably, even at f/11 — and that’s on APS-C format! On the full 35-mm format, loss of image quality would be even worse. Furthermore, these images suffer from significant barrel-shaped distortion (not visible here).

Regular images are at f/5.6; rollover images are at f/11.
Lesson learned from this comparion: Non-macro lenses on extension tubes — even on fairly short ones — yield poor image quality. Close-up lenses — even when stacking two of them — on non-macro lenses yield an image quality which is almost on par with a dedicated macro lens.

The shorter the master lens’ focal length and the longer the extension tube, the worse the results will be. Still, extension tubes do have their place. They work well when combined with lenses designed for use on long extensions (e. g. macrophoto lens heads), with enlarger lenses, or with reversed wide-angle lenses at magnifications greater than 1:1. Long telephoto lenses on short extension tubes also often work pretty well, for two reasons: first, the tube is very short in relation to the lens’ focal length; second, the angle of view is narrow so the lens isn’t as sensitive to additional extension as shorter lenses are.