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View Full Version : 2 use of teleconverter ??


jann1033
10-13-2007, 10:48 AM
i asked this on the other thread but think it got buried..sooo any help would be appreciated

says the f number="total f number of prime and the converter"so i am guessing that means if it says 5.6 in the viewfinder,it really is 5.6, it isn't really 6.3( deducting the stop of light i lose).

and since it's really making my lens more magnified i assume i need a faster shutter to be sure not to show shakes? (ie at 200mm, 1/500 ish with the crop and teleconverter taken into account)

MarkBarbieri
10-13-2007, 11:58 AM
says the f number="total f number of prime and the converter"so i am guessing that means if it says 5.6 in the viewfinder
With the Canon teleconverters, the f-stop you see is the f-stop you get. I assume that the Tamron works the same way, but I don't know that for a fact.

One way to test would be to point the camera at an evenly lit scene. Meter it with the converter off and meter it again with the converter on. If the exposure is the same and correct for both, then the f-stop reported is correct.

it really is 5.6, it isn't really 6.3( deducting the stop of light i lose).
Just to be picky, one stop slower than f/5.6 is f/8.0. F/6.3 is one-third stop slower. A full stop on aperture is the equivalent to doubling the shutter speed or the ISO.

The f-stop numbers progress in a weird way. The number is the diameter of the entrance pupil divided by the focal length. So f/4 means that the entrance pupil is 1/4 the size of the focal length. The weird progression is because the entrance pupil measurement that matters to the amount of light entering is the area of the entrance pupil, not the diameter. If you do the math, you see that each full f-stop is the prior f-stop times the square root of two.

and since it's really making my lens more magnified i assume i need a faster shutter to be sure not to show shakes? (ie at 200mm, 1/500 ish with the crop and teleconverter taken into account)
Yes. The greater magnification means that shake is also magnified. We have had considerable debates as to whether the crop factor matters when determining safe hand-hold speed (I think it does, others disagree), but I think that everyone agrees that using a teleconverter is no different than using a longer lens and thus justifies a higher shutter speed.

jann1033
10-13-2007, 04:43 PM
thanks...it must since i put it on p with and with out the converter and it read 4 and 5.6..i do see the drawback now though since getting it at f8 and 500 is going to take some light. i hope to use it at mid-late afternoon for birds so i'm going to have to go out and work with it to see what i have to do to use it like that