Calculation of focusing distance by adding ext tube

philipcs · 11th May 2006

anyone know how to calculate the minimum focusing distance for a lens after attach a 25mm ext tube?

let take tamron 28-75mm as example because i am planning to use this lens with the tube.

Here is the manufacturer info:

Minimum Focus 0.33m (entire zoom range, 28-75mm)
Macro Mag. Ratio 1:3.9 (at 75mm)

After adding the 25mm tube, how is the figure looks like?

Minimum Focus ??m (entire zoom range, 28-75mm)
Macro Mag. Ratio 1:?? (at 75mm)

sweat100 · 12th May 2006

As for the Macro magnification, it is the tube length divided by the focal length and set the focus at infinity. So for 75mm, it is 1:3. If you change to ur minimum focusing distance, u magnification will increase. To calculate that, it depends whther ur lens focus my barrell extension or internal focusing. If it is by barrell extension, jus add on the additional length. If it is internal focusing, i dont think it is possible to calculate. As for the working distance, i am not that sure.

11th May 2006	#1
philipcs Member Join Date: Jun 2005 Location: JB & KL Posts: 368	Calculation of focusing distance by adding ext tube anyone know how to calculate the minimum focusing distance for a lens after attach a 25mm ext tube? let take tamron 28-75mm as example because i am planning to use this lens with the tube. Here is the manufacturer info: Minimum Focus 0.33m (entire zoom range, 28-75mm) Macro Mag. Ratio 1:3.9 (at 75mm) After adding the 25mm tube, how is the figure looks like? Minimum Focus ??m (entire zoom range, 28-75mm) Macro Mag. Ratio 1:?? (at 75mm) __________________ pixelraves.com \| lessthanten.net \| philipcs.com

12th May 2006	#2
sweat100 Member Join Date: Jul 2002 Location: Singapore Posts: 711	Re: Calculation of focusing distance by adding ext tube As for the Macro magnification, it is the tube length divided by the focal length and set the focus at infinity. So for 75mm, it is 1:3. If you change to ur minimum focusing distance, u magnification will increase. To calculate that, it depends whther ur lens focus my barrell extension or internal focusing. If it is by barrell extension, jus add on the additional length. If it is internal focusing, i dont think it is possible to calculate. As for the working distance, i am not that sure.