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Teleconverters on a crop sensor

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Thread started 22 Aug 2009 (Saturday) 14:51   
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MikeMcGowan
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I'm going to be picking up a Canon 70-200mm f/4 L(non IS)this week, and I have a 50D. I was planning on getting either a 1.4x or 2x teleconverter later on for extra reach. Since I already have 1.6x crop factor, that makes the lens 112-320 already right? Now if I add either one of the teleconverters does it add onto the already stretched focal length? And what brand of teleconverters do you suggest? I would just go Canon, but I'm sure there's one that's just as good for a smaller amount of money.

Also, if I used the 1.4x converter on this lens, it would make me one stop down to f/6.3. Now does that mean I can't shoot at f/4, or that I can but the amount of light coming in will be equivalent to what it is at f/6.3?

Post #1, Aug 22, 2009 14:51:45


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joedlh
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The crop factor would be assigned to the lens-teleconverter combination. A 1.4 would make it a 280. Multiply that by 1.6.

Be aware that you will lose two stops with the 2x converter.

Can't help you with other brands.

Post #2, Aug 22, 2009 14:57:43


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SCOTTinNJ
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If you use a 2x on the 70-200 f/4 it will be a minimum of f/8 and you will lose autofocus on the 50D. Better to go with the 1.4x. Reportedly the Tamron/Kenko Pro are just as good as the Canon, and will work with lenses that the Canon version will not.

Post #3, Aug 22, 2009 15:10:14


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District_History_Fan
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a 1.4 teleconverter would net you a 35mm eqivalent of 448mm, f5.6.

Post #4, Aug 22, 2009 15:23:33


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DreDaze
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if you've never used a FF camera, then you don't really need to do the whole 1.6X to figure anything out...it's just going to cause people to come in here and tell you that your lens really isn't a 112-320mm...it's just the equivalent FOV...and all that...

the kenko 1.4 is a good TC and can be used on other lenses, while the canon can only be used on a few L's i believe...also you do lose 1 stop which as mentioned above is actually 5.6...using a 2x loses two stops so it becomes f/8, and no autofocus...

you don't even have the lens yet right?...i'd wait to see if you even do need the extra reach...

Post #5, Aug 22, 2009 15:29:02


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MikeMcGowan
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So me losing one stop means the biggest aperture I can shoot with is f/5.6? As in the camera won't let me go lower, or that I can shoot at f/4 but it's as dark as f/5.6. I'm not too sure, and I am going to wait until after I get the lens to see if I need the extra reach.

Post #6, Aug 22, 2009 16:38:48


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pwm2
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MikeMcGowan wrote in post #8505423external link
So me losing one stop means the biggest aperture I can shoot with is f/5.6? As in the camera won't let me go lower, or that I can shoot at f/4 but it's as dark as f/5.6. I'm not too sure, and I am going to wait until after I get the lens to see if I need the extra reach.

Note the expression f/4.

There is a mathematical relation between the focal length and the aperture.

When you use a 1.4x extender, you will both multiply the focal length from 200 to 280 and at the same time multiply the possible aperture from 4 to 5.6.

With the 1.4x extender, the camera will not allow you to select any aperture past 5.6. If you tape the extra connectors on the extender, you may fool the camera. It will show a focal length of 200 and a aperture of 4, but in reality you will still have a focal length of 280 and an aperture of 5.6.

Taping the extra contacts on a 2x extender would allow you to autofocus even at f/8, but it will not always work. In low-light situations, then it may hunt which will result in extra wear on the lens. Letting the lens hunt for for a longer time could potentially overheat it.

Post #7, Aug 22, 2009 17:02:40


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SkipD
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MikeMcGowan wrote in post #8505021external link
I'm going to be picking up a Canon 70-200mm f/4 L(non IS)this week, and I have a 50D. I was planning on getting either a 1.4x or 2x teleconverter later on for extra reach. Since I already have 1.6x crop factor, that makes the lens 112-320 already right? Now if I add either one of the teleconverters does it add onto the already stretched focal length? And what brand of teleconverters do you suggest? I would just go Canon, but I'm sure there's one that's just as good for a smaller amount of money.

Also, if I used the 1.4x converter on this lens, it would make me one stop down to f/6.3. Now does that mean I can't shoot at f/4, or that I can but the amount of light coming in will be equivalent to what it is at f/6.3?

Mike, the part I put in bold in the quote above is NOT correct. A 70-200mm lens is, and always will be, a 70-200mm lens. Nothing can change that.

If you add a 1.4x extender to the 70-200 f/4 lens, then the combination becomes a 98-280mm lens. The maximum aperture becomes one stop smaller (higher f-stop number) with the 1.4x extender. Thus, the maximum aperture of the 70-200 f/4 with a 1.4x extender mounted would be f/5.6.

To the "crop factor" issue... Please read this "sticky" covering the "crop factor". The one thing to understand is that the focal length of a lens DOES NOT CHANGE when putting it on different format camera bodies. Note that "Format" refers to the size of the film frame or digital sensor in a camera. The "crop factor" essentially has one purpose in life. That is to compare the field (angle) of view provided by a particular focal length on two different format cameras, one of which is considered the reference format for the comparison.

The camera manufacturers have decided to use the 35mm film format as the reference format for DSLRs. However, the 35mm film format is not, never has been, and never will be the "master" format against which all other formats are measured. Thus, the current Canon marketing lingo for a 50D reads thusly: "35mm-equivalent focal length is approx. 1.6x the lens focal length". This language is far better than the old "magnification factor" that Canon used to use or even the very common "crop factor" term that you see/hear a lot today.

Post #8, Aug 22, 2009 17:27:04


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MikeMcGowan
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That explains everything, thanks a lot.

Post #9, Aug 23, 2009 00:05:06


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yogestee
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MikeMcGowan wrote in post #8505021external link
I'm going to be picking up a Canon 70-200mm f/4 L(non IS)this week, and I have a 50D. I was planning on getting either a 1.4x or 2x teleconverter later on for extra reach. Since I already have 1.6x crop factor, that makes the lens 112-320 already right? Now if I add either one of the teleconverters does it add onto the already stretched focal length? And what brand of teleconverters do you suggest? I would just go Canon, but I'm sure there's one that's just as good for a smaller amount of money.

Also, if I used the 1.4x converter on this lens, it would make me one stop down to f/6.3. Now does that mean I can't shoot at f/4, or that I can but the amount of light coming in will be equivalent to what it is at f/6.3?

Mike,,your Canon 70-200mm F/4 will become a 98-280mm F/5.6 with a 1.4x TC

and a 140-400mm F/8 with a 2x TC.. Skip has explained so eloquently what happens with Field of View when comparing APS-C sensors and 35mm..

With either TC your auto focus will suffer and so will some image quality especially with a 2x TC.. With a 1.4x TC it's not too bad..

I have a Tamron SP AF 1.4x TC (the white model) which I team up with my Sigma EX 50-150mm F/2.8 APO with good results if I need that extra reach.. There is a little image degradation but nothing too severe and because the Sigma is a F/2.8 lens auto focus isn't affected too much..

Post #10, Aug 23, 2009 00:25:23


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