Kenko 36mm and Canon 400mm F5.6

I have been searching for a bit for information regarding the maximum obtainable magnification with the combination of Kenko 36mm Extension Tube and Canon 400mm F5.6.

The closest I've come is a blog stating that with the Kenko 36mm, that the 400mm focuses to about 6 feet, see URL:

http://blog.theeyegame.com/2010/05/0...forgotten-400/

I have a Canon 25 mm extension tube which achieves 0.21x magnification at 7 feet, but would like to potentially use the Kenko 36 mm, but for it to be worthwhile I think I need to get around 0.34x or more at 6 feet.

Has anyone here obtained the magnification at the closest focus for this pairing? (Unfortunately Kenko's own website does not provide the nice tabulation like Canon does for their extenders)

Chris,

It is a simple computation. The relevant formulae are:

1/F = 1/a + 1/b, and M = b/a

where F = focal length, a= subject to lens distance, b = lens to image distance, and M = magnification.

In your case, F = 400 mm. If the subject distance, a, is 6 feet = 6*12*25.4 =1828.8 mm. Then
1/a = 1/400 - 1/1828.8 which means b= 512 mm, so M = 512/1828.8 = 0.28.

If you stack a 24 mm and a 36 mm extension tube, then the 512 mm becomes 512+24 = 536 mm, and a= 1576 mm = 5.2 feet, and M = 0.34

Roger

Hi Roger,

Thank you for the posting with the formula. I just tried to derive the "tabulated" value for 400mm with the 25mm extension tube which states that at a distance of 7 feet, the magnification should be 0.21x, however the calculated value is 0.23x. I suspect there is some alteration of the focal length at short focus distance. That's why I was wondering if someone had actually obtained a "field" reading for the magnification because I was concerned a calculation based upon theory might be off, but in this case the discrepancy appears small and is unfortunately insufficient magnification.

Not sure on the magnifications Chris but here is a chart I made a few years ago with the approx min.max focusing distances with Extension tubes for the 400/5.6 if that helps.

http://i130.photobucket.com/albums/p.../Tubetable.jpg

Thank you Roy for the tabulation.