Macrophotography in a digital age - extension tubes
Extension tubes usage in macrophotography:
- extension tubes are just spacers between your camera and lens and thus do
not add extra glass that might introduce aberrations or lens flare.
- perhaps unexpectedly for the uninitiated, they do however decrease the
amount of light hitting your film or sensor such that a 25mm extension tube
on a 50mm lens will require exposure to be increased by about 1 stop.
- in fact racking your 50mm OM macro lens to its closest focus (by its own
25mm "in built" extension tube) will in itself require exposure to
be increased by about 1 f stop and this will be additive if a 25mm extension
tube is also used.
- to put this in perspective though, you could use a 1.4x teleconverter
instead of an extension tube which will provide a further 1.4x magnification
of the image at a cost of 1 f stop exposure while maintaining the same
working distance to the subject AND still allowing you to focus up to
- a 25mm extension tube will cost 1 f stop exposure but give a further 2x
image magnification BUT require a shorter working distance AND restrict you
to a smaller range of working distances eg. 6-12cm with an OM 50mm f/3.5
- an extension tube may also mean that autofocus capability is no longer
possible and with some lenses focus is not possible (such as fisheye lenses)
- thus in some situations the teleconverter option may be more suitable and
- rather than a fixed extension you can alternatively use either:
- variable extension tube:
- these are telescopic tubes which you can alter the amount of
- eg. Olympus
OM Telescopic Auto Tube 65-116
- more restrictive than a bellows but easier to use allowing
extension range of 65mm to 116mm
- matches nicely with the Olympus OM Zuiko 1:1 80mm f/4 macro
lens or 135mm f/4.5 macro lens
- allows a wide range of extension - eg. the Olympus
OM bellows allows 36mm to 198mm extension range
- magnification = image size on film / actual subject size
- magnification = (distance from optical centre to film - focal length) / focal length
- when a lens is focussed to infinity, distance from optical centre to film
= focal length
- when a lens is focussed to infinity and an extension tube is added:
- extension tube = (distance from optical centre to film - focal length),
thus, magnification = extension tube / focal length.
- subject distance in mm = 1 / [ (1/f) - (1/(f+extension)) ]
- thus maximum distance you can focus a 200mm lens with the following
extension tubes are:
- 7mm extension => ~6m
- 14mm extension => ~3m
- 25mm extension => ~1.8m
- (hope my maths is correct!)
- when a lens is focussed to its closest focus without an extension tube:
- amount of internal extension in mm = (1 / [ (1/f) - (1/(closest focus
in mm)) ] ) - f
- thus a 200mm lens which can focus to 2.5m has internal extension of
- thus, this same lens when used with a 25mm extension tube (max.
combined extension of 42mm) will have a focus range of ~1.2m to 1.8m and
a magnification range of 0.085x to 0.21x.
- if M = magnification & f = focal length in mm:
- distance from subject to film = (1+M)(1+M)f/M
- distance from optical centre to film = Mf[1 + (1/M) ]
- thus if magnification = 1, then distance from subject to film = 4 x f
and optical centre is in the middle, as distance from optical centre to
film = 2 x f.
- thus, for a 50mm lens on a 35mm camera, to get 1:1 macro you need 100mm
total extension including any in-built extension within the lens.
- exposure compensation for extension:
- exposure factor in f-stops to compensate for lens extension = log[(focal length + extension length)/focal length]/log(sqrt(2)