100-300 with Kenko extension rings - focus range

Started Jan 11, 2013 | Discussions thread
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Tony Rogers
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100-300 with Kenko extension rings - focus range
Jan 11, 2013

This is a post for those wondering whether the extension rings from Kenko are suitable. Hopefully it will add info to the couple of threads that I have seen previously.

Up until now, I have been using a Canon 500D for close-ups on either the 45-175 or 100-300 lens. It works very well but sometimes it does bug me. In particular, I hate screwing the lens on and off the front and sometimes I drop it. I even managed to drop it followed immediately by standing on it one time in my rush to get the shot!

With the 500D the focus range is between 300 and 500mm on any lens you put it on. This is great for using the zoom to change magnification but gets limiting when the thing you want to get a picture of is say 1m away. Thus the screwing on and off all the time.

So I have been interested in whether the Kenko extension rings would behave in the same way or differently. Well I got a set (10mm + 16mm) for Christmas and now I know. The answer is no, they don't!

With extension rings, closest focus distance varies with the focal length of the original lens and the amount of extension. With both rings on and the focal length at 300mm, closest focus is about 1200mm, which provides quite a significant increase in magnification over the lens alone at 1500mm. More interesting though is that the maximum focus distance is somewhere up around 3.5-4m which provides a lot of extra flexibility.

As the focal length reduces as you zoom out, the focus range reduces until at 100mm the range is between about 500 to 650mm. At closest focus of 500mm, the magnification is about the same as with the lens at 300mm and 1200 distance. So, maximum magnification stays about the same throughout the zoom range. Didn't expect that!

Anyway, the result of this is that I can use the 100-300 with focus distances between about 500mm and 4m by adjusting the zoom. If the subject doesn't move and you can get in close, you can use a shorter focal length where the lens is sharper. If it's a critter that will be scared away if you get too close, you can stand back. Reducing the extension, reduces the magnification but expands the focus range. Quite flexible really.

The only real downside as far I can see to using extension rings is the loss of light which can be quite significant with shorter focal lengths and big extensions. I haven't got a lot of practical experience yet since the weather has been so crap since Christmas but so far they seem very promising.

Here's a sample shot from this morning at 100mm at about 500mm focus.



 Tony Rogers's gear list:Tony Rogers's gear list
Olympus E-M1
Canon EOS 500D (EOS Rebel T1i / EOS Kiss X3)
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