Whay are teleconvertors 1.4X ?

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John
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I was looking throught he range of teleconvertors for Canon, Nikon and Sigma and I noticed they all make 1.4x and 2x teleconvertors. It struck me that 1.4x is an odd number to choose so there must be a scientific reason for it :thinking: Can anyone enlighten me on why 1.4x was chosen rather than say 1.5x?
 
All to do with f stops. Each f stop is a 1.4 multiplication or division of 1.4 from the previous stop. Eg f 2,2.8,4,5.6,8,11 etc.

This is because each stop halves or doubles the amount of light entering through the lens and the square root of 2 is 1.4 (or thereabouts) I.e. I increase the diameter of the aperture by 1.4x, then the area is doubled therefore twice as much light therefore half the shutter speed for same exposure.
 
Good question. And crazy quick reply! I've learnt my thing for the day now, may as well leave work for the day...
 
Very interesting. I had wondered briefly before, but never thought to ask the question as I didn't think there would be such an easy, logical answer. :thumbs:
 
It also conveniently lines up with F4 lenses to make them F5.6 which is the minimum aperture required by a lot of central focusing points (in AF cameras) to work. 2x converters line up with f2.8 lenses for the same reason, used on F4 lenses and you lose AF.

Matt
 
Then you start looking at the Nikon 1.5x and 1.7x converters which are available - no logic to their conversion factor in the mathmatical sense.
 
MatBin said:
It also conveniently lines up with F4 lenses to make them F5.6 which is the minimum aperture required by a lot of central focusing points (in AF cameras) to work. 2x converters line up with f2.8 lenses for the same reason, used on F4 lenses and you lose AF.

Matt
Click to expand...

The 1.4x and one stop metering difference actually existed way back in the seventies, thus predating AF by a good few years.
 
Remarkable what I've picked up from these forums. problem is I am now one of the sad people who now can expound the reasons behind the conundrum. I doubt it'll make me more attractive to the opposite sex ;);)
 
PsiFox said:
The 1.4x and one stop metering difference actually existed way back in the seventies, thus predating AF by a good few years.
Click to expand...

I bow to your better memory, I only recall 2x converters then, some had removable central elements thus doubling up as extension tubes.
Matt
 
ive just read this thread, and spent the last 20 minutes looking at mathematical stuff i aint got a clue about, thank you John, now my head hurts lol
 
RichardtheSane said:
Then you start looking at the Nikon 1.5x and 1.7x converters which are available - no logic to their conversion factor in the mathmatical sense.
Click to expand...

Nikons crop factor is different to canon's, so the 1.5x works well on smaller crop camera's and the 1.7x on the 1.3x crop bodies
 
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