1. ## Extension tubes ?

How do I calculate how much does the extension tube affects the minimum focus distance of a lens ? If I have a 70-200mm Lens with min focus distance of 1.2m, what extension tubes are available and how much do they reduce the min focus distance ? Does anyone knows how much roughly does an extension tube cost ? Thanks

2. The first question I will leave to the experts to answer. I believe this topic was discussed before, you might want to do a search on it.

There's the Teleplus 25mm extension tube which is retailing for abt \$85 at AP. You can also purchase a set of three tubes (made by Kenko I believe) of varying lengths but which cost more....perhaps ard \$200+.

3. Originally Posted by Garion
The first question I will leave to the experts to answer. I believe this topic was discussed before, you might want to do a search on it.

There's the Teleplus 25mm extension tube which is retailing for abt \$85 at AP. You can also purchase a set of three tubes (made by Kenko I believe) of varying lengths but which cost more....perhaps ard \$200+.
oic.. thanks.. I just saw a thread which says that it needs extension tube of the whole focal length of the lens to half the min focus distance. So I will need 200mm extension tube to get to 60 cm MFD..
a 25cm extension tube will only reduce the MFD of the 200mm lens by 7.5 cm..

4. Originally Posted by majere2sg
oic.. thanks.. I just saw a thread which says that it needs extension tube of the whole focal length of the lens to half the min focus distance. So I will need 200mm extension tube to get to 60 cm MFD..
a 25cm extension tube will only reduce the MFD of the 200mm lens by 7.5 cm..
Having an extension tube that has the same length as the focal length of the lens would mean a 1:1 magnification, not much on the min focusing distance. If you want to increase your min focusing distance, you have to put at least 1.4X TC between the cam and the extension tube, this will multiply your extension tube length as well. (your 25mm ext tube will become 35mm with 1.4X TC attached)

Well, I haven’t seen any write-up that discuss on the min focusing distance using an extension tubes (even J. Shaw didn’t mention this topic in his book). They normally discuss the magnification, which is apparently ext tube length divided by the lens focal length.

Using your 70-200mm, the min focusing distance will vary as you change the focal length (by zooming the lens). Assuming using a 25mm extension tube, when you zoom the lens to 100mm, it will give you the min focusing distance that is enough to reach the 1/4 life-size. Changing the focal length would mean changing the min focusing distance.