All because 1.4142 is the square root of 2. It turns out that 2 and its
square root are very important numbers in photography because of the
relationship with measurements of areas. If you use a 1.4X
teleconverter you change the aperture by precisely one stop because the
area of the aperture opening relative to the new focal length is exactly
one half that without the converter. In addition, the area covered by
the photograph is reduced by exactly one half as well because the length
of each side is reduced by 1.4. Change to a 2X converter and you drop 2
stops, lengths are reduced by 2 and areas are reduced by a factor of 4.
2 and its square root are also vitally important in determining (direct)
flash exposure because the area covered by the flash varies by the
square of the distance from the flash. For example; assume a 50mm lens
on a 35mm camera focused at 10 feet. The area covered by the image is
7.2 x 4.8 feet or roughly 35 square feet. If the lens was instead
focused at 10/1.4 = approximately 7 feet the area covered by the image
would be only half as much at 17.5 square feet and the intensity of the
light over the smaller area would be twice as much. If the correct
flash exposure at 10 feet was f/8 the correct exposure at 7 feet would
require closing down to f/11. Conversely, if the focus distance was 10
x 1.4 = 14 feet the area covered would be 70 square feet, the amount of
light over double the surface area would be reduced by half and you'd
need to open up to f/5.6 for correct flash exposure. Focusing at 5 feet
vs 10 would require a 2 stop change from f/8 to f/16. You should also
notice that correct flash exposure varies quite dramatically at close
distances. To avoid estimation errors it's best to shoot at longer
distances. However, the greater the distance from the flash the harsher
the shadows.
Dr. Flash
Andrew Fildes wrote:
> 1.4x costs you exactly one stop, 2x cost two stops. That's my
> understanding. Some sort of geometric, logarithmic scale thingy. I
> suppose a 3x costs you 3.141..... stops :-)
> Andrew Fildes
> afildes@xxxxxxxxxxxxx
>
>
>
> On 30/09/2009, at 9:35 AM, Clay Nichols wrote:
>
>> Why are the Olympus teleconverters rated at 1.4x mag instead of
>> 1.5x which is half of the 2x teleconverter?
>
--
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