What stop

[slat]

If you had a lens with a 6.3 aperture and used it with a 1.4 teleconverter you would be at 9.0 aperture with the converter correct?

 

[Dave442]

Just using the general guide taking the TC magnification and consider that and f-stop and then see how many stops the f-stop is from f/1. So f/1 to f/1.4 is one stop so the answer to your question is "yes".

The real math uses the TC magnification applied to the focal length of the lens while the lens entrance pupil remains the same size as the f-stop is the focal length divided by the entrance pupil, however you did not give a focal length of the lens so I did not calculate using this formula. But could also estimate using the formula E=2*log2*(TC magnification)=2*log2*(1.4)=0.971 (basically one stop).

What I usually do is calculate the entrance pupil based on f/2. So 50mm lens at f/2 = 25mm entrance pupil. Now multiply 50mm * 1.4 TC multiplier = 70mm. Now 70mm/25mm = 2.8, so: f/2 to f/2.8 is one stop of light.

If I calculate the entrance pupil for a 50mm lens and apply that using f/6.3 (sqrt2^5.33333) I get an entrance pupil of 7.87451mm and then use the new focal length of 70mm/7.87451mm I get a new f-stop of f/8.89 (basically one stop from f/6.3496 to f/8.89 - f/9 is actually the sqrt2^6.333333=8.98) The numbers are not as easy to see as it is using f/2.

 

[480sparky39 38]

The short answer: Yes.

 

[TCampbell]

Yes -- and the reason that 1.4x is the most popular teleconverter power level is because that number ... 1.4 ... is extremely close to the actual value for the square root of 2.

In photography, that number (1.414) is significant. Each full "f-stop" of aperture is actually the previous stop... multiple by that value. It also works for distances when using a flash. Each time the distance increases by that multiplier, the amount of light is exactly halved.

This is because if you increase the radius of a circle by that value, then the area of the circle is exactly doubled.

So using a 1.4x tele-converter keeps the math really easy... you just change the aperture by 1 full stop.

 

[Braineack]

If you had a lens with a 6.3 aperture and used it with a 1.4 teleconverter you would be at 9.0 aperture with the converter correct?

going to have a lot of trouble auto-focusing it with most cameras on the market...