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tele converters, DOF, and fstops
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PostPosted: Mon Aug 03, 2015 3:15 pm    Post subject: tele converters, DOF, and fstops Reply with quote
so here is a pretty basic question: i put a 2x teleconverter on a 50/2.0 lens. that makes it a 100mm lens with a DOF of 4.0, but light gathering ability of its original 2.0 maximum aperture. correct, partially correct or entirely incorrect?
tony


PostPosted: Mon Aug 03, 2015 3:20 pm    Post subject: Reply with quote
My understanding would be that your lens will become the equivalent of a 100mm f4 as you will lose 2 stops of light using the converter.


PostPosted: Mon Aug 03, 2015 5:23 pm    Post subject: Re: tele converters, DOF, and fstops Reply with quote
rbelyell wrote:
so here is a pretty basic question: i put a 2x teleconverter on a 50/2.0 lens. that makes it a 100mm lens with a DOF of 4.0, but light gathering ability of its original 2.0 maximum aperture. correct, partially correct or entirely incorrect?
tony


DigiChromeEd is correct.

The light gathering ability is from its 50mm/f2=25mm diameter aperture. With a 100mm lens which the combination now is a 25mm apperture gives f/4.
It has the same light gathering power but spreads the light over a larger area giving longer exposures.

Image quality will typically be worse than a dedicated 100mm f4 of the same quality. A quality teleconverter can still be useful, but modern digital cameras can usually crop to give an effective image more successfully than any but the best converters. These days teleconverters only really become worthwhile on seriously long/fast lenses.


PostPosted: Mon Aug 03, 2015 6:07 pm    Post subject: Reply with quote
thank you both.

so it seems that with a 2x crop sensor, like m4/3, the light gathering ability of this 50/2 will stay at 2.0, whilst the fl changes to 100mm and the DOF changes to 4.0. but with a teleconverter, both the DOF and the maximum aperture for light gathering purposes becomes f4.
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