Extension Tube or Close-up Lens?
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The full frame (APS-C size) Konica-Minolta Dynax 7D |
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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
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Minolta AF Macro 50 mm 1:3.5
50 % view, corner |
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Minolta AF Macro 50 mm 1:3.5
50 % view, center |
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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.
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Minolta AF 50 mm 1:1.4
Minolta Close-up Lenses No. 1 & No. 2
50 % view, corner |
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Minolta AF 50 mm 1:1.4
Minolta Close-up Lenses No. 1 & No. 2
50 % view, center |
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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.
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Minolta MC Rokkor-PG 50 mm 1:1.4
17 mm extension tube
50 % view, corner |
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Minolta MC Rokkor-PG 50 mm 1:1.4
17 mm extension tube
50 % view, center |
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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.
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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.
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