Post by USM on Dec 14, 2006 9:03:11 GMT 8
There is a difference between using an extension tube and a teleconverter for macro photography.
Beside what others had mentioned that extension tube is basically a hollow tube without any glass element while the TC has elements in it, which may will degrade the image quality (can't hardly notice it).
The differences are :
1) Extension Tube
- loss of light is inversely proportional to the focal length of the lens. For e.g., a 25mm tube results in a 1 stop loss on a 50mm lens, but only a half stop loss on a 100mm lens, or a quarter stop loss on a 200mm lens.
- no degradation of iamge quality since there is no glass element
- focal length of the lens used remains the unchanged
- Unable to focus at infinity (not an issue since you are shooting macro)
- reduces the minimum focusing distance of the lens, and thus increases the magnification ratio of the lens
- usable for most lenses
- formula to calculate magnification ratio of using a 25mm extension tube :
Magnification = Extension Tube (mm) / Focal Length of Lens
E.g. using a 25mm extension tube on a 50mm f1.8 II lens
Magnification = 0.68x
E.g. using a 25mm extension tube on a 200mm f2.8 II lens
Magnification = 0.32x
It follows that the shorter the focal length of the lens, the greater the magnification produced by a given extension tube.
2) Teleconverter
- increases the focal length of the main lens
- loss of f-stop, 1 stop on a 1.4x TC and 2 stops for a 2x TC
- image quality may be affected as TC affects it especially 2x TC
- magnification or working distance or both increases due to increased focal length of the lens. For e.g. a 1.4x TC will give you 40% more magnification at the same distance or 40% more working distance at the smae magnification or a combination in between
- minimum focusing distance of the lens remains unchanged
- able to focus at infinity
- not usable for many lenses due to protruding element
Given the above differences between using an extension tube and a TC, how does it affect a macro lens? For e.g. EF 100mm f2.8 Macro USM lens.
Using the lens alone, it will give you the magnification ratio of 1:1, i.e. magnification of 1x.
With a 25mm extension tube, the magnification ratio becomes 1.39:1, i.e. the magnification is 1.39x. This means that you are getting larger than life size.
With a 1.4x TC, the magnification ratio becomes 1.4:1, i.e. the magnification is 1.4x. However, only the 3rd party TC like the Kenko can mount onto the lens. The original Canon TC can't use with the lens due to the protruding element.
Therefore, it seems that there is no much of a difference in using an extension tube or a TC on a 100mm macro lens
Beside what others had mentioned that extension tube is basically a hollow tube without any glass element while the TC has elements in it, which may will degrade the image quality (can't hardly notice it).
The differences are :
1) Extension Tube
- loss of light is inversely proportional to the focal length of the lens. For e.g., a 25mm tube results in a 1 stop loss on a 50mm lens, but only a half stop loss on a 100mm lens, or a quarter stop loss on a 200mm lens.
- no degradation of iamge quality since there is no glass element
- focal length of the lens used remains the unchanged
- Unable to focus at infinity (not an issue since you are shooting macro)
- reduces the minimum focusing distance of the lens, and thus increases the magnification ratio of the lens
- usable for most lenses
- formula to calculate magnification ratio of using a 25mm extension tube :
Magnification = Extension Tube (mm) / Focal Length of Lens
E.g. using a 25mm extension tube on a 50mm f1.8 II lens
Magnification = 0.68x
E.g. using a 25mm extension tube on a 200mm f2.8 II lens
Magnification = 0.32x
It follows that the shorter the focal length of the lens, the greater the magnification produced by a given extension tube.
2) Teleconverter
- increases the focal length of the main lens
- loss of f-stop, 1 stop on a 1.4x TC and 2 stops for a 2x TC
- image quality may be affected as TC affects it especially 2x TC
- magnification or working distance or both increases due to increased focal length of the lens. For e.g. a 1.4x TC will give you 40% more magnification at the same distance or 40% more working distance at the smae magnification or a combination in between
- minimum focusing distance of the lens remains unchanged
- able to focus at infinity
- not usable for many lenses due to protruding element
Given the above differences between using an extension tube and a TC, how does it affect a macro lens? For e.g. EF 100mm f2.8 Macro USM lens.
Using the lens alone, it will give you the magnification ratio of 1:1, i.e. magnification of 1x.
With a 25mm extension tube, the magnification ratio becomes 1.39:1, i.e. the magnification is 1.39x. This means that you are getting larger than life size.
With a 1.4x TC, the magnification ratio becomes 1.4:1, i.e. the magnification is 1.4x. However, only the 3rd party TC like the Kenko can mount onto the lens. The original Canon TC can't use with the lens due to the protruding element.
Therefore, it seems that there is no much of a difference in using an extension tube or a TC on a 100mm macro lens
hahaha 
