04-19-2014 04:56 PM
I can not manually or automatically focus my T3i Rebel on distant objects when using the Canon EF 25 II extension tube on Canon EFS 18-55mm and EFS 55-250mm lens. Close up objects will focus fine.
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04-19-2014 07:08 PM
Yes - this is as it should be... nothing is wrong.
When you use an extension tube, you move the lens farther away from the sensor. The result is that the ENTIRE focus range is shifted to physically closer distances.
This means your closest focusing distance is closer than it used to be.... and it also means your FARTHEST focusing distance is ALSO closer than it used to be (you can no longer focus to infinity.)
Extension tubes are added specifically to allow the camera to focus closer. If you don't need to focus closer, don't use an extension tube.
04-20-2014 11:57 AM
Thank you Tim for the reply. I had read a number of forum reviews with similar situations with an extension tube, including your feedbacks, and I kinda thought that was going to be my situation also. You confirmed it is. I do still want the close up shots the tube will give me thus I may just keep the extension tube because it does a wonderful job. And I guess I will live with the ranges the other two lenses give me. I may be getting to greedy for distance shots because we will be in Alaska for a couple weeks this summer.
Thank you again,
04-20-2014 12:15 PM
Extension tubes allow the camera to focus on subjects which are closer than the lens would otherwise focus (e.g. think "macro photography".
Teleconverters (sometimes called tele-extenders) alter the focal length (but also the focal ratio) of a lens and this may be what you were thinking of. E.g. if you put a 2x teleconverter on a 100mm f/2.8 lens, then it effectively becomes a 200mm f/5.6 lens.
BEFORE you rush out to the nearest store to buy a teleconverter, there are some caveats that you will want to know about.
Firstly... a given teleconverter has to be checked for compatibility with the specific lens you want to use with it. Not every teleconverter can be used with just any lens.
Second... what many people forget is that while a teleconverter is increasing the focal length... it's ALSO increasing the focal ratio and this can have some very bad side-effects which make it unusable with some lenses. For example... you mentioned that use have the EF-S 55-250mm lens... that lens has a VARIABLE focal ratio (very important) of f/4-5.6. With THAT lens, a teleconverter (which wont work, btw... but I'll use it anyway as an example) would become a 110mm-500mm f/8-f/11 lens.
This is a big problem... your camera cannot focus when using an f/8 lens (and to be f/8 you'd have to be on the "short" 110mm focal length range where you dont plan to use it anyway because you could get 110mm without putting a teleconverter on so what would be the point). You'd want to use it out at the 500mm end where it's f/11. No DSLR camera on the market can auto-focus at f/11.
The lens/teleconverter combination should not have a focal ratio above f/5.6
This means 1.4x teleconverters should typically not be used on lenses with focal ratios higher than f/4 and 2x teleconverters shoudl typically not be used on lenses with focal ratios above f/2.8 (there is a tiny bit of wiggle room here... but not much.)
Third... there's also the problem that EF-S lenses have a rear-most lens element that protrudes beyond the back of the lens slightly and may not even mount to most teleconverters. But given the focal ratio constraints, there's not much point in making teleconverters for EF-S lenses since most of them have either variable focal ratios that are too high to be useful OR have fairly short focal length ranges (or are prime lenses) and there are much more suitable lenses that don't require teleconverters.
Lastly... you get a degradation in optical quality when you use a lens with a teleconverter.
You can rent lenses... so if you just need a lens for a special trip and you don't want to buy an expensive long focal length lens... you could always rent one.
04-20-2014 08:56 PM