Tele-converter math?

Question: Tele-converter math!?

i am a little confused!. how do u get the focal length of a lens if u got a tele-converter on it!? cuz i got a 600mm n i put the 1!.4x and 2x converter on it with the 1!.6x crop factor on my 30D n i got 2 different answers with two different equations!. is it: 600x1!.4x2x1!.6=2688mm!? or is it: 1!.4+1!.6+2= 5( all the crop factors) then do 5x600 which equals to 3000mm!? this is confusing me a littleWww@QuestionHome@Com

Best Answer - Chosen by Asker:

You multiply the whole bunch!.
600mm lens * 1!.6 focal length multiplier of the 30D * 1!.4TC * 1!.6TC = 2688mm effective focal length (and one batshit crazy contraption!.)
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just checking back!.
Let's think about this for a moment:

Put a 600mm lens on a 30D and what have you got!?
600mm * 1!.6FOV= 960mm

Add a 1!.4x TC!?
960mm * 1!.4= 1344mm

And stack a 2x TC on top of that!?
1344mm * 2 = 2688mm!.

As for the aperture, again, take it one step at a time to calculate the cumulative effect:
If the 600mm lens is f/8 (as Edwin proposes), a 1!.4TC will generally chew up a full stop (down to f/11) and a 2x TC will normally detract 2 stops, giving you f/22 for the whole setup!.
So in this case you add instead of multiply!.Www@QuestionHome@Com

You simply multiply the focal length of the lens by the converter power!.

The crop factor doesn't increase the focal length so it doesn't belong in the equation!.

To see how the crop factor actually works, go here:

http://www!.dpreview!.com/learn/!?/Glossary!.!.!.

VWww@QuestionHome@Com

1!.4 + 2 = 3!.4

3!.4 X 600 = 2,040

You also have to multiply the f-stop by 3!.4 so if the maximum aperture on your 600mm lens is f8 it becomes equivalent to an f27!.2 - so you'll need a good tripod and a whole lotta light!. A really clear day is also needed since the tiniest amount of particulate matter in the air will be magnified!. Early morning would be the best time to use this combination!. If its the least bit windy - even the slightest breeze - you may as well forget about using this set-up unless you're completely sheltered out of the wind!.Www@QuestionHome@Com