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07/08/2005 07:26:43 AM · #1 |
I have been reading about the extension tubes and converters for lenses. Pardon my beginners knowledge on these but I will be glad if someone can explain the following questions:
-- what is the difference between the both and where/why are they used?
(I understand that one is used for macro reasons and one for enhansing the zoom.... Please feel free to crrect me.)
-- check my profile for the lenses that I have. I want to increase the zoom on my 70 - 300 lens. What should I buy. Any links in UK will be helpful.
-- In lack of a proper macro lens I have taken some fairly nice macros (for my purpose atleast) using the macro feature on my lenses but want to enhance them. Do I necessarily need a macro lense or can I use an extension tube / something else to do this.
Will really appreciate some answers on this.
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07/08/2005 07:34:20 AM · #2 |
One thing to watch for when using a TC to extend the reach of your zoom:
it reduces the max aperture available. Therefore, if your 300mm has f/5.6, stacking it with a TC would close it beyond the point your camera is available to auto-focus on.
Also, the pic quality degrades with the TCs, but you do get the extra zoom. It depends on what you do with it, sometimes you can get the same result by cropping the picture.
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07/08/2005 08:21:29 AM · #3 |
An extension tube reduces the minimum focal distance. For example if your 70 - 300 has a minimun focal length of 3 ft the extension tube can reduce this to 1.5 ft. Works well for macro shots if the subject is not timid.
The Teleconveter (TC) will extend your focal length. This is good for shots where you can't get any closer. However, like srdanz stated you can loose image quality. With a 2X TC you loose 2 f stops. They are recomended for lens that have an aperature of 2.8 of better. And many times you loose the AF capabiltiy when using them.
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07/08/2005 08:41:35 AM · #4 |
Thanks for the insight guys
... so to summarize for my macro needs I guess I will go ahead and use (some) extension tubes. I know the lense is capable of taking good macros (of not too timid objects). See an example here.
Well that brings to another question that can you use multiple extension tubes with a a lense? In theory if one extension tube reduces the distance to 1.5 ft from 3 will two reduce it even further? I hope so.
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07/08/2005 08:53:18 AM · #5 |
Yes, you cna stack multiple tubes. The tubes are often sold in sets of three, in lengths of 12mm, 25mm, and 37mm or thereabouts. You can stack them in any combination. Note that because you're spreading the light out more as it travels toward the sensor, you'll need to increase exposure somewhat.
The range of focus of a zoom lens (from nearest to farthest focus) can be quite narrow with extension tubes. Still, a great way to get close in on those skittish bugs.
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07/08/2005 09:11:32 AM · #6 |
Yes, you can use multiple extension tubes, but they will futher decrease the available light and depth of field. Extension tube sets come in different lengths (like 12mm, 20mm and 30mm) so you can vary the focusing distance and stack them in different combinations for more flexibility.
Extension tubes contain no glass, so image quality remains the same as your original lens. Note that any extension tube will drastically cut your depth of field and focusing distance. Typically, you will only be able to focus from a few inches to a few feet, and DOF will range from a few inches to a fraction of an inch. The only extension tubes that will autofocus (within the diminished focus range) are the two individually-sold Kenko tubes (not the set) and Canon's own model. Since the tube will reduce your aperture, a fast lens (f/2.8 or less) will make focusing easier.
Teleconverters DO contain glass which can degrade the image quality of your lens, so it's important to get a good model. The Rebel will not autofocus if the maximum aperture of your lens is smaller than f/5.6, and that might limit your choices. For example, a 1.4x Canon teleconverter will reduce an f/4 lens to f/5.6 and autofocus will still work, but a 2X TC would take it to f/6.3 (I think), and autofocus will no longer work. For a 2X TC, you'd need an f/2.8 lens or faster for autofocus to work. |
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07/08/2005 09:27:12 AM · #7 |
Originally posted by scalvert:
Teleconverters DO contain glass which can degrade the image quality of your lens, so it's important to get a good model. The Rebel will not autofocus if the maximum aperture of your lens is smaller than f/5.6, and that might limit your choices. For example, a 1.4x Canon teleconverter will reduce an f/4 lens to f/5.6 and autofocus will still work, but a 2X TC would take it to f/6.3 (I think), and autofocus will no longer work. For a 2X TC, you'd need an f/2.8 lens or faster for autofocus to work. |
I had a really cheap Cosina 100-300 5.6-6.7 lens that autofocussed on my 300D in good light. I know that the Canons (300/350/20/10) are rated to autofocus up to 5.6, but I don't believe that they will rigidly refuse to focus above that if light is good enough, up to a point, of course.
Message edited by author 2005-07-08 09:27:42.
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07/08/2005 09:34:16 AM · #8 |
Originally posted by AJAger:
I had a really cheap Cosina 100-300 5.6-6.7 lens that autofocussed on my 300D in good light. I know that the Canons (300/350/20/10) are rated to autofocus up to 5.6, but I don't believe that they will rigidly refuse to focus above that if light is good enough, up to a point, of course. |
Others have reported similar experinces, even with TCs. To believe canon's literature, the lens will report the max F-stop to the camera and if it sees more than 5.6 is will stop.
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07/08/2005 09:53:57 AM · #9 |
The reason that there's confusion on whether a Canon non-1-series DSLR will try to AF when the minimum aperture is smaller than 5.6 is because of third-party teleconverters.
These teleconverters don't tell the camera they are there, so with a Tamron 1.4x converter, for instance, the camera still sees f/5.6 as f/5.6, not f/8, adn will still try to focus. Use a Canon converter, however, and you get the correct f-stop reported, and camera iwll not try to focus.
Add to that, some lenses like the Sigma 50-500 lie to the camera, telling it they are f/5.6 when they are really something like f/6.3. The effect is that the camera will not stop trying to AF.
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07/08/2005 10:16:07 AM · #10 |
Originally posted by kirbic:
The reason that there's confusion on whether a Canon non-1-series DSLR will try to AF when the minimum aperture is smaller than 5.6 is because of third-party teleconverters.
These teleconverters don't tell the camera they are there, so with a Tamron 1.4x converter, for instance, the camera still sees f/5.6 as f/5.6, not f/8, adn will still try to focus. Use a Canon converter, however, and you get the correct f-stop reported, and camera iwll not try to focus.
Add to that, some lenses like the Sigma 50-500 lie to the camera, telling it they are f/5.6 when they are really something like f/6.3. The effect is that the camera will not stop trying to AF. |
My Third party Kenko Teleplus Pro 300 DG 1.4x reports to the camera and adjusts f/stops accordingly. My 70-200mm 4L becomes a 5.6 and when for fun (It's soft @ 300 to begin with) I put my Tamron AF 28-300mm 3.5-6.3 on there; the second I hit the zoom ring it goes to 7.1 and hunts like a mad dog. Don't know about the other Third Party TCs.
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07/08/2005 10:35:26 AM · #11 |
Yep, forgot that at least one of the "pro" converters, the Kenko, do indeed report aperture correctly.
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