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Century Precision Optics - 10mm -> 6mm Focus

Focus Adaptor

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#1 Dustan Lewis McBain

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Posted 16 January 2013 - 04:49 PM

Hello Ladies and Gents,

When using an adaptor, 10mm to 6mm, is there a specific method or chart to follow when attempting focus. Is there any difference in depth or light loss to factor in. Although the lens is refocused to a different size, im worries that the focus will be slightly off if i simply follow the 6mm charting ratio's and such. Best,
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#2 Bernie O'Doherty

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Posted 16 January 2013 - 05:28 PM

If the adaptor is set to specs, everything should focus as it should. I would assume there would be a light-loss factor. In a pinch, you could measure the difference between the adaptor being on and off with your light meter behind the lens, and that would approximate the stop difference. I would guess a stop.
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#3 Chris Millar

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Posted 16 January 2013 - 06:37 PM

I have this lens for my Bolex Switars.

Funny how topics come in little clusters - in one of the black-lagic threads we were discussing how with adaptors when you go wider you actually get more light - as in a light-gain factor - it makes sense to me thinking that you're sucking more 'image' into the same area, so the density per area, per time is greater = more exposure.

Mind you, I don't know what I don't know about optics so I'm looking forward to learning.
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#4 Bernie O'Doherty

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Posted 16 January 2013 - 08:23 PM

Interesting point Chris. I'd like to meter both and see. Theoretically the wider lens would bring in more light, but practically that light has to pass through more glass to do the job ! Hopefully, a lens designer might be looking on....,and enlighten us !!
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#5 Dom Jaeger

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Posted 16 January 2013 - 08:37 PM

Chris, a wide-angle adapter is different to the Metabones Speed Booster we were discussing in the Black Magic thread. Wide angle adapters go in front of a lens, gather a wider view yes, but not more light. The system is still limited by the original lens T-stop. The additional glass actually adds more reflective surfaces and refraction so that you lose some light.

The Speed Booster thing goes at the back of a lens and condenses the image circle, so that the same amount of light fills a smaller area, hence increasing the speed. It doesn't create a wider angle of view than the original lens had, it just allows the same angle of view on a smaller imaging area, effectively reducing the lens focal length.
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#6 Bernie O'Doherty

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Posted 16 January 2013 - 08:51 PM

Makes perfect sense Dom. I am a stop brighter !
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#7 Chris Millar

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Posted 16 January 2013 - 09:02 PM

The additional glass actually adds more reflective surfaces and refraction so that you lose some light.


Yep that's a given - but besides the point (but I know you know that I know that we know that ^_^)

I'm hitting a conceptual block - too many variables for my cache.

When you say 'they gather a wider view yes' I see a big evenly illuminated erm, 'void' (infinitely even and 'white' in all aspects for the sake of discussion) - ok, so standard optic gathers 1 'light unit' and presents it across the imaging sensor which has a unit area - the ratio between light and area is '1' (flux if you will). Now double your angle of view, the sensor now 'sees' 4 times the original viewable area of that infinitely even white ether - where does all this light go ? I see it hitting the same sensor - what I'm not understanding is why it isn't 4 times denser when it gets there - same sensor area, but it's 'collecting' area is 4 times bigger. The thing I'm not factoring in is the effective aperture - need to draw some diagrams but I'm stuck here with no dang pen.
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#8 Chris Millar

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Posted 16 January 2013 - 09:11 PM

Its all pun and games until someone loses an eye :rolleyes:
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#9 Dustan Lewis McBain

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Posted 21 January 2013 - 08:11 PM

Interesting comments. These lenses are used on a bolex 16mm. I'll try out the light meter trick, however, this camera doesn't offer that much light out the viewfinder.

I've also just completed a test, and the result with only the 10 mm lens, and then the 6 mm lens did not have much difference in light loss (at least not by a stop, anyway). So, it would appear that focus would now be my main question - and if you factor in Chris' comment of 'sucking more into a lens', perhaps the lens remains at it's base lens focus (in this case 10 mm) and as an addition, the frame is provided a wider angle. So, given that the 10 mm lens contains the preferred depth of field, adding a 6mm adaptor will offer more viewing to the scene. Seems like a bonus to me...
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