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Focal Reducers and Light.
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PostPosted: Mon Jan 08, 2018 8:52 pm    Post subject: Focal Reducers and Light. Reply with quote

I understand the reasoning behind getting an extra stop or so's worth of light using a Focal reducer,such as the Zhongyi LT2 etc. What does puzzle me is the fact I see no mention of Losing light when using FF lenses on M4/3 sensors with a 'Dumb' adapter,due to the fact the Sensor is only seeing Half of the Light/Image being 'seen' by the Lens??
Am I barking up the wrong Tripod with my thinking?


PostPosted: Tue Jan 09, 2018 3:26 am    Post subject: Reply with quote

I suspect the answer might have to do with light intensity rather than area of coverage. If the intensity is the same, regardless of the area of coverage, then it seems to me the exposure would be the same. Does that make sense?


PostPosted: Tue Jan 09, 2018 7:45 am    Post subject: area v\s intensity Reply with quote

Absolutely yes to the answer.

Any lens on a purely mechanical adapter will behave as usual, sending light on to the film plane. If you shade out half of the film, cut up the sensor in smaller pieces or use a ready made, smaller sensor, it receives the same amount of light.

A focal reducer will concentrate the same light from the motive on a smaller area, so each area unit will get more light. Not absolutely exactly twice as much due to a tiny loss in the intermediate optics, but close enough -probably well within the margin of error of the rest of the equipment. In exchange, the same setup used on a full-frame sensor (if the mount had accepted it so as to focus) would have shown a correspondingly smaller circle of light.

p.


PostPosted: Tue Jan 09, 2018 7:52 am    Post subject: Reply with quote

As others referred to, the f# indicates light intensity per unit area -- no matter the area, the intensity of light falling on each unit is the same.


PostPosted: Tue Jan 09, 2018 9:22 am    Post subject: Reply with quote

Some of the full frame brigade make a big fuss over total light seen by the sensor. As far as exposures go it's irrelevant & it's light /area that's important - which does not change with a simple adapter.

Total light will effect the signal/noise ratio but so do many other factors based on the design of the camera. Newer models are generally less noisy than older ones as these factors are improved.

BTW I would consider a dumb adapter to be one without electronic connections. My focal reducer is dumb (despite the carefully designed optical elements) while some glass less adapters translate AF/aperture signals & are thus not dumb IMO.


PostPosted: Tue Jan 09, 2018 12:28 pm    Post subject: Re: Focal Reducers and Light. Reply with quote

Eddie46 wrote:
Am I barking up the wrong Tripod with my thinking?


Yes, but your reason for questioning it is a reasonable one.

You should forget about the total amount of light the full frame lens is projecting into the camera, and think about how it attempts to spread that light evenly. That even level of light will fall across an M4/3 sensor just as it would a full frame one. It kind of works like this when using a full frame lens on a cropped sensor - put a sheet of paper out within a puddle of light, then place a smaller sheet on top of it centrally. The smaller sheet doesn't darken, as if receiving less light across it. The smaller sheet is your M4/3 sensor.

I say attempt, above, as lenses can suffer from light fall off to the edges (and produce vignette). This is less of a problem if a sensor only uses a centrally based portion of the lens, like the M4/3 sensor does when using a full frame lens.


PostPosted: Tue Jan 09, 2018 8:56 pm    Post subject: Re: Focal Reducers and Light. Reply with quote

Sciolist wrote:
Eddie46 wrote:
Am I barking up the wrong Tripod with my thinking?


Yes, but your reason for questioning it is a reasonable one.

You should forget about the total amount of light the full frame lens is projecting into the camera, and think about how it attempts to spread that light evenly. That even level of light will fall across an M4/3 sensor just as it would a full frame one. It kind of works like this when using a full frame lens on a cropped sensor - put a sheet of paper out within a puddle of light, then place a smaller sheet on top of it centrally. The smaller sheet doesn't darken, as if receiving less light across it. The smaller sheet is your M4/3 sensor.

I say attempt, above, as lenses can suffer from light fall off to the edges (and produce vignette). This is less of a problem if a sensor only uses a centrally based portion of the lens, like the M4/3 sensor does when using a full frame lens.


Brilliant description 'Sciolist'.explained it beautifully for me.


PostPosted: Tue Jan 09, 2018 9:07 pm    Post subject: Reply with quote

I did also think "brilliant" on seeing his reply, but for a different reason, being the kindness and sensitivity exhibited in the answer.