Why Extension Tubes with a Long Lens?

Thread starter #1

rip18

Senior Member
I got a question this morning about why I was using an extension tube with my 600 mm lens when extension tubes are typically thought of as being used for close-up work. I thought I would share my response here...

First, what are extension tubes? Extension tubes are simply hollow tubes that have male and female fittings appropriate for the camera body & lenses you are using. You put them between the lens and the body. This puts the lens further away from the film or digital sensor and decreases the minimum focus distance - you can focus on things closer than you are "supposed" to be able to with any given lens. It also messes up your long-distance focusing, so when you have it on, you can't take long-distance pictures. Extension tubes typically come in a set of 3 with a short (around 10 mm) medium (around 15 mm) and long (around 25 to 35 mm). Extension tubes can be "stacked" to make longer lengths.

Extension tubes effectively increase the focal length of the lens. So when used with a short lens (say using a 12 mm extension tube with a 50 mm lens, you increase the effective lens length to 62 mm while also allowing you to get closer.) Extension tubes also decrease the depth of field, especially with shorter lenses.

Extension tubes are NOT teleconverters or tele-extenders. These are a different kind of device that goes between the body and lens to make objects bigger. They are typically available in 1.4x, 1.7x, and 2x factors. They do contain glass lenses and are used to make things bigger. So a 2x teleconverter on a 50 mm lens would turn it into a 100 mm lens - but at the cost of 2 "stops" of light. You have to have good light to use a teleconverter because the glass in it does reduce light transmission significantly.

Okay, so why was I using a 27.5 mm extension tube with my 600 mm lens - because I had birds near my minimal focus distance. When I am shooting birds near a feeder or other place where I can get them near full frame, I know that I am close to minimal focus distance, so I put on the extension tube - because it really makes me frustrated when the bird is TOO close to focus on (and there it is FULL FRAME!!! AAARRRGGHHH!!!!!). (It also turns my 600 mm - 12x magnification - into a 627.5 mm - 12.55x magnification). In fact, in the marsh hen shot I had my 27.5 mm extension tube on the 600 mm and THEN had my 2x teleconverter between that assembly and the body - meaning that I had 1255 mm of effective lens out there - 25.1x magnification!

So, I use the extension tube on my long lenses for things that are going to be too close to focus on otherwise - birds, frogs, flowers, etc. And when I use my 2x teleconverter AND my extension tube, I put my extension tube in front of my teleconverter so that it's length is doubled as well...

So far, I haven't stacked a 1.4 and 2.0 teleconverter AND one or more extension tubes, but I would if the situation called for it...

Note: Many lens/body combinations will lose their ability to autofocus (and some lose their ability to meter light) when you use an extension tube - so manual focus is very commonly used with extension tubes.
 

Hoss

Moderator
Great info Rip. I think we have one that needs to be linking at the top. Thanks for sharing the information. I've been considering getting extension tubes, just haven't gotten around to doing it (or taking many photos for that matter).

Hoss
 

FERAL ONE

Shutter Mushin' Mod
rip, i have not used it on my long lens yet, but i sure like it on my macro. it allows me to get even closer to the subject . i can't wait to see what it will do with bugs !!! i think i have some neat moss and lichen shots from devils tower that i shot this morning. time will tell !!! thanks for the very good explanation !!!
 

leo

Retired Woody's Mod 7/01-12/09
Thanks rip,

that makes sense:biggrin3:
 

jason308

Senior Member
Great post Rip.....A lot for me to learn on this subject....
 
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