LensTurbo II samples

Started May 31, 2014 | Discussions thread
OP forpetessake Veteran Member • Posts: 4,892
Re: LensTurbo II samples

Mark Q wrote:

Pentax 28-105mm/3.2-4.5 lens, which becomes approx. 20-76mm/2.3-3.3 with focal reducer.

The above numbers are lens' physical parameters. Focal reducer modifies the optical formula resulting in a lens with all those parameters scaled by approx. 0.72.

Thanks for the post regarding the new lens turbo. I have been considering investing in one of these, especially for the extra spot of light in dark conditions.

Just to clarify the effective focal length with using lens turbos:

for the combination of APS-C sensor (crop of 1.5) and this lens turbo (reduced to 0.72) the effective focal range of this lens would be (28mm x1.5x0.72) - (105mm x1.5x0.72) which is 30-113mm ??

Now you are talking about an FF equivalent parameters. When calculating the FF equivalent you have to multiply those physical parameters by 1.5 (crop factor). So the FF equivalent becomes 1.5*0.72 = 1.08x of the markings on the lens, i.e 30-113mm f/3.5-4.9. Because the 1.08 crop factor is close to 1, many people prefer thinking of FR as a simply eliminating crop factor from an APS-C camera. If you look at the sensor through the focal reducer it would look almost FF size when looking from the same distance.

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