My Learning Curve - Part 2
1. Order of Extension Tube and Extender Combinations
I have been reading and experimenting recently with the use of an Extension tube (Canon Extension Tube EF25 II, 25mm) and a 1.4x Canon Extender on my trusty Canon 300mm F4 IS lens all mounted to a Canon 50D. What follows are my findings on the order in which these accessories can be combined.
The details below are applicable to Canon equipment and I have ignored the differing effects of extension tubes and extenders on light transmission, metering etc. as most cameras these days meter through the lens and should exposure compensation be required it can easily be made via the histogram. I am therefore only interested in what I will call the ‘magnification’ and ‘macro’ effect of these combined accessories.
Southern Hawker dragonfly taken using above combination.
By way of background:
An Extender is a piece of optical equipment that increases the focal length of a lens by the factor stated, e.g. a 1.4x extender on a 300mm F4 lens changes the 300mm lens into a 420mm lens (300mm *1.4) but you loose one f stop, so this lens now has a minimum f stop of 5.6. Extenders above 1.4x tend to degrade image quality markedly, especially if they are of poor optical quality. For this reason I rarely use my Canon 2x Extender, even though it is a high quality instrument, unless I really have to and prefer to try at all costs to get closer to the subject. A 2x Extender also reduces the minimum f stop of your lens by 2 stops, so an f4 lens becomes an f8 lens and so I would then loose autofocus capabilities on my 50D (though on a Canon 1D the central autofocus point would still function). The image quality of a lens and extender combination can be improved by using them above their minimum f stop i.e. even though the 420mm combination above has a minimum f stop of 5.6 I will always try to use them at f6.3 and ideally f7.1 to f8. By increasing the focal length you appear to get closer to your subject from a given fixed distance. Because Extenders multiply the focal length, their ‘magnification’ effect is greatest with longer lenses i.e. with a 50mm lens and 1.4 x Extender the focal length becomes 70mm, a gain of 20mm, whereas a 1.4 Extender on a 500mm lens gives a focal length of 700mm, a 200mm increase (though admittedly the percentage increases are identical).
On the other hand an Extension tube is simply a spacer with no optical elements (usually in 12mm, 20mm, 25mm, 36mm or 50mm depending on the make) that moves the lens away from the camera body thereby reducing a lenses minimum focus distance, allowing you to take close up/macro images without any image degrade. However you will loose the ability to focus at infinity with your lens. In other words your range of available focus distances is reduced but you get the upside of being able to gain focus when you are a lot closer to your subject. This ability to get closer is greater with shorter focal length lenses than longer lenses i.e. you can get significantly closer (percentage wise) to your subject with a 50mm lens and 25mm extension tube than with a 300mm lens and 25mm extension tube. The longer the extension tube the greater the effect, extension tubes can be stacked to good effect e.g 37mm can be attained by stacking 25mm and 12 mm tubes, however the greater the extension the greater the loss of light which could become an issue.
When purchasing either Extenders or Extension tubes be careful to check how they will affect the autofocus capabilities of your gear. With Canon Extenders and Extension tubes they preserve autofocus on Canon bodies other makes may not, also image quality can suffer hugely with inferior quality Extenders.
Now getting back to the main reason for this article, the order of these accessories - Until recently I had always used this collection of gear in the following order, based on what seemed a logical order to me and what I thought I had remembered reading:
1. Camera Body (50D)
2. Extension Tube
I put them in this order because to me it seemed logical to move the entire optical elements (lens and Extender) further from the camera body and therefore reduce the minimum focus distance. I was sure I had read somewhere that this was also the correct order but now cannot find it anywhere!
I had found that in this order the minimum focus distance had need reduced thereby enabling me to get closer to the subject and so fill the frame but also for a given fixed distance there was a ‘magnification’ effect making the image appear bigger in the frame. Also in this order the auto focus and metering of the camera work flawlessly, though I had lost the ability to focus at infinity.
However I now realise the above order is not technically correct, if you really want to get as close as you can to the subject and so make the most of what I will term the ‘macro’ benefits of these accessories (though you still loose infinity focus) the order should be:
1. Camera Body
3. Extension Tube
I have taken the following images which may help to explain what I mean. The first 3 are from a fixed subject distance of 2.0m (which show the ‘magnification’ effect). The second 3 are all taken at the minimum focus distance of each combination (which show the ‘macro’ effect).
Canon 50D, 1.4 Extender then 300mm lens (2.0m fixed distance from subject)
Canon 50D, 25mm Extension Tube, 1.4 Extender then 300mm lens (2.0m fixed distance from subject)
With this combination the image appears about 10 to 15 % bigger to my eye than the standard camera and 1.4 extender combination image above, the auto focus and through the lens metering work perfectly.
Canon 50D, 1.4 Extender, 25mm Extension Tube then 300mm lens (2.0m fixed distance from subject)
With this combination the autofocus does not work reliably but you still get the ‘magnification’ effect though it may be a fraction less than above.
Canon 50D, 1.4 Extender, 25mm Extension Tube then 300mm lens. Minimum focus distance 1.2m (subject to camera body) hence improved ‘macro/close-up’ ability of combination. Autofocus seemed to work at this close distance, though not well.
Canon 50D, 25mm Extension Tube, 1.4 Extender, then 300mm lens. Minimum focus distance now 1.35m, improved ‘macro/close-up’ performance but not as good as above. Autofocus works perfectly.
Canon 50D, 1.4 Extender then 300mm lens. Minimum focus distance now 1.53m. Autofocus works perfectly.
Therefore in conclusion for my work, primarily non macro, I am usually less interested in the ‘macro’ effect of this combination (particularly if I loose reliable autofocus capabilities) but very interested in the benefits of the ‘magnification’ effect. Often I am unable to get any closer to my subject and so to draw them a little nearer via optical means is a real advantage.
So for normal bird, butterfly and dragonfly images I will continue to use the theoretically ‘incorrect’ combination (Extension tube on the camera body, then Extender, then lens) due to the benefits of ‘magnification’ (10 to 15% with a 25mm tube), no loss of autofocus reliability and I still get a reduced minimum focus distance (from 1.53m to 1.35m). But, if I need to maximise the possible ‘macro’ benefits I will change the combination to the theoretically correct order (Extender on Camera body then Extension tube, then lens) but will suffer from reduced autofocus reliability.
The following link is also a useful illustration of the ‘magnification’ effect of Extension Tubes.
Link To – Further Extension Tube Magnification Images
Common Blue - The "closeness" helped by extender and extension combination, yet still maintianing a good working distance when mounted on Canon 300mm F4 L IS.