Extension Tube Question EF 70-300 IS

[rixtix]

I was reviewing the information provided regarding the working distance of lenses when used with an extension tube.  I couldn't figure out if there is a specific formula for determining the changes.   I have a 70--300 IS it closest focus distance is about 5 feet.  I need to know if extension tubes could be used to reduce that by 50% or more.   

[Bob Atkins]

Probably not at 300mm, but possibly at 70mm.

Extension tubes have maximum effect on focus distance with short focal length lenses. It is possible to calculate the approximate change in magnification and focus distance, but it's a rather complex process. I should probably write an article on how to calculate it, or at least how to get some sort of rough approximation of it. For a simple one element lens it would be easy. For a complex zoom lens it isn't because it depends on knowing the effective focal length and positions of the principle points

Focal length isn't as easy as reading it off the lens scale because almost all zoom lenses actually change focal length when close focused, so a 70-300 set to 300mm and close focused probably isn't acting as a 300mm lens anymore. It might be as short as 200mm.

If you just look at data for some older 100-300 zooms (see http://www.bobatkins.com/photography/eosfaq/closeup.htm) you can see that the minimum focus distance with a 25mm extension tube was around 1.2m at the 300mm setting and 0.6m at the 70mm settings (focus distance is defined as the distnce between the sensor in the camera and the subject). Without the tube the minimum focus distance of those lenses (see http://www.bobatkins.com/photography/eflenses/lens_list.php) was around 1.5m.

[rixtix]

My goal is to get the working distance down to about a foot at 120mm.  Do you think that may be possible? 

[Bob Atkins]

Sure, you can get the working distance down to almost anything with enough extension. Quality suffers with a lot of extension and  image brightness drops, so using the minumum extension you can get away with is desirable.

Your best bet is to go with a set of tubes like this one:

Pro Optic Auto Extension Tube Set for Canon EOS

That way you get 31mm,21mm & 13mm tubes for $75 that you can stack until you get the extension you need for any particular job.

[rixtix]

I understand the compromises. This will be used for product photography in a very controlled shooting environment with most shots being taken at f8 or f11 and long exposures.  A 120mm or 180mm macro lens is a big investment so if I can use an existing zoom lens the cost is significantly lower and the results may not differ enough to warrant the expense.