*Originally posted by sweat100 *

**So the formula does not apply here?**

For example if a 50mm lens that is at F1.4. If i add on a 2X TC, its effective focal length will be 100mm ( with the camera still set to F1.4) from the formula, it will be F2.8. Isnt that 1 stop? (or i got the definition of "Stop" mixed up?)

All along i thought, if u add TC infront, you will use the effect focal length and divide by the diameter of the opening. Since the focal length is increased and the diameter remains the same, so by right it will change the F number?

Wouldnt it be better if i am using a film SLR and use a digital camera TC so i woundnt get a reduction in the amount of light? :dunno:

In your example, your 50/1.4 with a 2x TC becomes a 100/2.8, that's 2, not one stop from 1.4. Every stop has a 1.4 factor, so the f-stop scale goes like 1, 1.4, 2, 2.8, 4, 5.6, 8, 11, 16, 22, 32 and so forth.

For those TC's in front, the formula of aperture = focal length/f-number no longer applies.

For film cameras, if you try to use a digital camera TC/WC, you might see some softness, especially at the edges.

Regards

CK