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Making a 40/160 camera into a 64/160


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#1 Tom Doolittle

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Posted 29 November 2006 - 07:08 PM

I've been kicking this idea around for some time, and I've decided to give it a try. I mentioned it once before inside another thread but nobody seems to have picked up on it...

A compact Nizo I own "reads" the cartridge film speed notch with a simple pin that can only has two possible positions: in or out. With 64T, it gets pushed in. With the side of the camera off, you can see that pushing this pin in moves a tiny blade of ND material in front of the photocell. Because the camera is calibrated for only two film speeds (40 or 160), I suspect this ND filter represents the difference between those two speeds. When the pin is pushed in, presumably by 40ASA film, the filter blocks some of the light reaching the photocell, causing it to think less light is available, and therefore open the aperture more. When a higher speed film is loaded, the pin remains out. So the little ND filter optically calibrates the meter to 40, and removing it resets it to 160.

The difference between 40 and 160 is two stops, correct? So, if one were to replace that little blade of filter material with a piece that represented the difference between 64 and 160, the camera would then read 64T correctly, right? We need to let more light though to the photocell, making the system close by 2/3 stop.

Another idea is to simply use a very small drill, like a jewelers hand drill, and make a tiny hole in the center of the existing filter to let just a wee bit more light reach the photocell. Because all this takes place after the beamsplitter, there would be no impact at all on the image. The only change would be how much light the photocell sees. The question is, what size should the hole be? I guess it would be a function of the diameter of the photocell, but how would I account for the remaining ND surface area?

I'd appreciate hearing your thoughts, before I get out the screwdrivers...
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#2 Clive Tobin

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Posted 29 November 2006 - 09:36 PM

So, if one were to replace that little blade of filter material with a piece that represented the difference between 64 and 160, the camera would then read 64T correctly, right? ...


Assuming that there is no light meter filter for 160 ASA, the filter for 40 ASA would be 0.6 ND.

To increase the meter sensitivity by 2/3 of a stop (to 64) would mean reducing this filter to 0.4 ND.

To do this by making a single hole in the filter would have to be done by trial and error since we don't really know what sort of angle or area the photocell is seeing. If you could instead make an array of small holes in the entire ND filter these should total almost exactly 20% of the total area to convert the 0.6ND to 0.4ND.

This is a far more economical approach than buying a 0.4ND filter to cut out a replacement, if you have priced ND filters lately.
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#3 Tom Doolittle

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Posted 30 November 2006 - 02:49 AM

Assuming that there is no light meter filter for 160 ASA, the filter for 40 ASA would be 0.6 ND.

To increase the meter sensitivity by 2/3 of a stop (to 64) would mean reducing this filter to 0.4 ND.

To do this by making a single hole in the filter would have to be done by trial and error since we don't really know what sort of angle or area the photocell is seeing. If you could instead make an array of small holes in the entire ND filter these should total almost exactly 20% of the total area to convert the 0.6ND to 0.4ND.

This is a far more economical approach than buying a 0.4ND filter to cut out a replacement, if you have priced ND filters lately.


Clive,

Thanks for your input. You are correct to assume no filter for the 160 setting.

It would be difficult to put an array of tiny holes in this thing. It is VERY small. I plan on trying the single hole, trial and error approach. I'll shoot for a hole area of 20%, but it will be hard to get any good measurement on the size or orientation of the photocell. Worst case scenario: I'm left with a camera that STILL can't read 64T. If it works, the lightmeter can stay in the bag a little more often.
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#4 David W Scott

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Posted 30 November 2006 - 11:01 AM

This is a far more economical approach than buying a 0.4ND filter to cut out a replacement, if you have priced ND filters lately.


You can get all the ND that you would need for free. (I am assuming that the ND on this photocell assembly isn't huge.)

Go to a decent photography/video store and ask for a free LEE Filters swatch book. You will get your ND, and a few hundred other interesting bits. Just cut out the piece you want.
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#5 Mark Dunn

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Posted 30 November 2006 - 12:17 PM

I'm not sure that ND comes in anything other than .3ND, 1 stop, increments, but any combination of filter material adding up to one and one third stops ought to do. Like two layers of 85A.
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