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tele-converters for canon eos xti?

i have recently bought a canon eos xti dig camera. i enjoy taking action pics, but only as fun. i also got the ef 75-300mm f/4-5.6 III lens with it. my main goal for the camera with that lens was to be able to take action/ sports pics with that combo. i don't want to drop alot of money on lens. i was looking into tele-converters for it, because i will be taking pics of my kids playing sports and of surfing pics (which i can't get too close to). so i was looking at these converters: quantaray 2x af tele-converter, sigma 2x tele-converter ex apo, and the canon extender ef 2x II. in that order because of price, i would rather $80 than $300, if they are pretty equal is the pics i will get. any advice?

Mon, 31 Dec 2007 18:21:08 +0000

Before you get your hopes up, dc, let me ask you a question that you may not have considered. Are you willing to sacrifice the ability to auto-focus (AF). If you are not willing to sacrifice AF, your only choice is to buy a higher-end lens. You see, with a 2x teleconverter, you lose 2-stops of light. That means your f/5.6 lens becomes f/11, meaning your XTi won't be able to auto-focus with that lens.

In addition, with an aperture of f/11, you'll have to be shooting in bright sunlight. I don't know if your kids plays indoor or outdoor sports. But the small aperture won't cut it indoors or at night.

The image quality will also be somewhat disappointing, especially when you start with a lens that doesn't have spectacular resolving power in the first place. Teleconverters will get you the best bang for the buck when you use them on excellent lenses (professional-grade). Some of the higher-end teleconverters are made specifically for one or two high-end lenses. Look toward Sigma's high-end teleconverters; they are generally compatible with upward of ten or so high-end lenses.

Ok, having said all that, it's still fun to shoot with 2x teleconverters at f/11 without AF. See the "Stacking Teleconverters" article. If you still want to use a teleconverter, I would look for a 6 or 7-element 2x teleconverter. If you settle for a 4-element 2x teleconverter, go for the one with the lowest price (they are all pretty similar to me).

Chieh Cheng
Mon, 31 Dec 2007 19:23:23 +0000

thanks for the info, i'm fine with the manual focus. on my last camera the canon t70 i had a 200mm manual focus, so thats not a problem. also my kids play outdoor sports for now, and all of their games are still during the day. i will now look at those elements of the conerters and go from there. thanks for the help.

Tue, 01 Jan 2008 15:51:04 +0000

ok, i don't know much about aperture, so that's confusing to me (i guess will have to research more about aperture). the 6 element convert was $200 while the 4 element was $79. my lense that i will use it for is not a high end lense (canon ef 75-300mm), so will i really notice a big difference between the 6 and 4 element?

Tue, 01 Jan 2008 16:05:35 +0000

Yes. Because the teleconverter will degrade your image quality. The 6-element teleconverter degrades the image quality less than the 4-element teleconverter.

You can play with aperture on your XTi. It has a stop-down preview button, right? That is the button next to the lens, below the lens release button. Set the camera to aperture-priority (Av mode). Turn the dial so that your camera says f/11. Hold that button down. Now, go outdoors and try shooting at f/11 with manual focus. That's what will happen to your lens at f/5.6 when you mount a 2x teleconverter.

Chieh Cheng
Wed, 02 Jan 2008 00:31:04 +0000

yes i do have that, didn't notice it before. we went out today and i took pics in the av mode messing with the aperture, to get pics of the kids, with the city skyline in the background. with the higher f number the more focused the background, right? like i said i didn't notice that feature before so some came out pretty bad, but now i know. next i will try the f/11 doing it manual over the auto focus. i decided not to buy the cheap one just yet, i figure i will mess with it some more, and try it out at the next soccer game or next time were surfing and see how it does without the converter and maybe take some with the f/11 setting and manual focusing. thanks again for all the advice and help. i'm so glad i found this site.

Wed, 02 Jan 2008 01:05:45 +0000

ok, one more thing. i saw that the canon 2x extender II on the canon site says its a 7 element and that the auto focus will work with lenses of f/2.8 or faster. well how does the f/stop speed go? is lower faster or is higher faster? i did a google search but kept finding info about the f/stop diameter and not how the speed goes.

Wed, 02 Jan 2008 01:35:50 +0000

Yah, most 2x teleconverters will allow AF on a f/2.8 lens. That is because a 2-stop light lost changes the f/2.8 lens to f/5.6 (that what you start with on your lens), which still provides enough light to the AF sensor.

Lower f-number means bigger aperture hole, more light, and faster shutter speed. Higher f-number means smaller aperture hole, less light, and slower shutter speed.

Be careful with you look at the Canon teleconverters. Take a look at the photo attached below. Notice that there is a black piece of plastic to protrude from the front of the teleconverter? You have to make sure that doesn't come in contact with your zoom lens. Move your lens's zoom ring and MF ring back and forth. Does the rear lens element comes all the way out? If so, then it probably won't work with the Canon teleconverters. The Canon teleconverters are specially designed to only work with two or three professional level Canon telephoto lenses. So Canon was willing to make this design trade-off for optimal image quality.

Attached Image:

Canon Extender EF 2x II.jpg

Chieh Cheng
Wed, 02 Jan 2008 22:00:13 +0000

ok, i took my lens and switched it to manual focus and moved it back and forth nothing on the back (where it connects to the camera) seemed to move at all. so it would most likely work with this lens, but if i buy another lens (if i move up to 500mm or so lens) the extender may not work with it. so it looks like i'm back to the sigma for around the $200, since it would work with my lens and has the 6 elements which will give me a better pic than the 4 element. now i went to play with the camera with the ap setting like you said, and you told me to go to f/11, but there were a lot of other steps between it and 11. 6.3 then 7.1 was second 8,9,10 were the next then 11, so with all those steps before 11, would adding the extender move it up to 7.1 (two steps) or is that different than the lens? do lens have their own step, and how do you know what two steps will be from each lens? i also have a 18-55mm lens with a f/stop of 3.5-5.6, so if i added the extender to that how would i find out the two steps? thank you again for the help.

Wed, 02 Jan 2008 22:40:06 +0000

Yah, you are confusing "steps" with stops. A stop is an actual mathematical definition of how much light can pass through an aperture. Each stop less is half the light gathering area of the previous stop. So when I say a 2x teleconverter causes 2-stop light loss, it means the imager sensor gets 1/4th the amount of light with the teleconverter than without.

Based on the f-numbers you've given me, your XTi is set-up to report aperture in 1/3-stop "steps". Take a look at the "F-Stop Quick Reference" article and let me know if that clarified the stops and "steps" for you.

Chieh Cheng
Wed, 02 Jan 2008 23:50:51 +0000

still not sure how the steps go. you said the lens with lose two stops of light, making it a f/11. i see the f/11 on the chart, just not sure how it happens to go from f/5.6 to f/11 is there a chart for that? i guess i need to go get one of those photo for dumbass books, and do make research about this topic. thanks again, i can't thank you enough for everything.

Thu, 03 Jan 2008 21:15:16 +0000

Glad I can help. If you take a look at the top chart again, there are 5 columns. Note that the five columns are under two column headings. The left two column falls under the 1/2-stop heading. The right three column falls under the 1/3-stop heading.

So if you look under the aperture column under the 1/3-stop heading, you will see that to move 1-stop down from f/5.6, you have to go from f/5.6 -> f/6.3 -> f/7.1 -> f/8.0. Remember, each number is 1/3 of 1-stop. To go another stop down from f/8.0, you have to go from f/8.0 -> f/9.0 -> f/10.0 -> f/11.

Hope that helps.

Chieh Cheng
Thu, 03 Jan 2008 21:24:15 +0000

wow, got it. didn't really get the 1/3 stops making three of them one. thanks.

Fri, 04 Jan 2008 18:57:49 +0000

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