s and teleconverter
s are two tools of a photographer that act to change the behavior of a lens
. These often take the shape of an tube that sits between the lens and the camera. The teleconverter and extension tube both act to extend the range of the lens though at opposite ends of the capability.
The extension tube is simply a hollow tube that links between the camera
and the lens used in macro photography
. Often these come in different sizes and can be coupled together to provide a range of lengths.
Using the extension tube, the photographer sacrifices the ability to focus at infinity and gains the ability to focus closer along with an effective magnification.
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A B C
- Film Plane
- Original Film Plane
- The focal length of the lens is "B-C"
- The extension length is "A-B"
In the above, diagram
the image projected on the original film area shown
as '?' now extends to the entire film plane on A. This is the
magnification ability of extension tubes. However, this comes at a cost:
less light. By moving the lens away from the film some of the light that
originally would be cast upon the film is lost. The amount of light that
is lost is a function of the focal length of the lens - the shorter the
lens, the more the light is spread out, the more the light is lost.
The extension tubes also decrease the minimum focusing distance (often
abbreviated MFD). With short lenses this can be rather dramatic - my
experiences with photographing a flower
with a 50mm lens and extension
tubes occasionally had me stick the lens an inch or less away from the
With extension tubes, the effective aperture changes. While this is
not of much concern to cameras with through the lens (TTL)
if using a manual camera or manual mode on an automatic camera this
is of some concern.
f = a * (1 + m)
The effective aperture (f) equals aperture on lens (a) times one
plus the image magnification ratio (m).
m = e / f
Magnification ratio (m) is equal to the amount of added extension (e)
divided by the focal length of the lens (f).
Closely related to the extension tube is the bellows which can be
thought of as a variable length extension tube. Realize, however, that
the bellows system is not as portable as the extension tube as it
often requires bulky stabilization while the extension tubes are very
is in essence an extension tube with some optics in it.
It does the same thing - magnify the center of the image. However,
a teleconverter trades minimum focal distance
for a change to the
effective length of the lens and keeping the ability to focus at distant objects.
Teleconverters most often come in two sizes: 1.4x and 2x. The
teleconverter also loses light for the same reason as an extension
tube. Teleconverters often lose between one stop and two stops
depending upon the amount of magnification (the 1.4x loses one stop,
the 2x loses two stops, and occasionally off brand teleconverters
are not exactly 1.4x or 2x).
The 'beauty' of the teleconverter is to take a 300mm f/4 (costs
about $1000) and for only another $600 get a 600mm f/8. Realize that
the 600mm f/4 costs over $7,000 making it far beyond the reach of all
but the most serious of wildlife photographers.
A bit of both
Sometimes, photographers will use a combination of a long lens
(sometimes with a teleconverter too) and a
small extension tube to change the minimum focal distance and
depth of field. This will change a photograph of a bird on a branch
with distracting leaves around it to just the bird and nothing else.