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metering display vs. actual aperture value (e.g. Nizo Professional)

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#1 Simon Lucas

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Posted 31 March 2013 - 08:02 AM

It's been discussed here before : The internal meter on a Super 8 camera (typically) compensates for the loss of light split off by the prism for the meter and the viewfinder and changes due to focal length. I have read as many threads on this topic as I can but there seems to be one question that has not been asked.

 

When the viewfinder shows an f-stop - is that the value before or after compensation for light loss?

 

 

I'm using a Nizo Professional but think this a general question.

 

 


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#2 Zac Fettig

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Posted 02 April 2013 - 12:37 PM

It should be the value after compensation. Otherwise, what's the point? It would always be underexposing the film 8-20% (depending on the prism).


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#3 Dom Jaeger

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Posted 02 April 2013 - 10:18 PM

Given that the internal meter would have been calibrated at the factory to match the light measured at the film plane, the f-stop displayed would be a compensated value, showing the geometric aperture required to properly expose the film.

 

It could conceivably be up to 1/2 stop more open than an external meter reading, since with big S8 zooms there would be 1/4 to 1/3 stop light lost through the lens itself, on top of the loss to the viewfinder prism.

 

Zac's figure of 8-20% sounds about right for a S8 prism, the light lost through the lens depends on the coatings and number of elements. The only S8 zoom I'm aware of that had T stop markings is the Angenieux 6-80 which lost about 1/3 stop (f/1.2 to T1.4 at 6mm).


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#4 Simon Lucas

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Posted 04 April 2013 - 10:30 AM

Thank-you for your replies. Sounds like 1/2 - 3/4 stop light loss. I cant actually see any change in the metering take place between wide and zoom, though.

 

So, if the display shows the actual f-stop after compensation, then would it also follow that when setting aperture manually (from a hand-held meter reading) the exposure would be correct. (This pre-supposes using correct ASA and shutterspeed values and correct meter usage.) Effectively - we would be ignoring any light-loss issues. 

 

You can see where I'm going with this, but I find the way that the metering works slightly opaque in the Super 8 camera.  As I'm also trying to pin down my film development times I need to know what the actual exposure was when I shoot a test strip.


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#5 Dom Jaeger

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Posted 05 April 2013 - 01:15 AM

So, if the display shows the actual f-stop after compensation, then would it also follow that when setting aperture manually (from a hand-held meter reading) the exposure would be correct. 

 

No, the internal meter is giving you a reading that is measured through the lens and calibrated for correct exposure at the film plane. An external meter will give you a pure reading that needs to be adjusted for light loss within the camera/lens. Typically I'd say you would need to open up the lens 1/3 to 1/2 stop more when using an external meter.

 

That's pre-supposing the factors you mention, on top of the accuracy of both meters and how they might differ in response to different lighting conditions. On top of how accurate the manual aperture control of the camera is (ie whether setting the iris to f/4 actually opens the iris to f/4), which might be  less consistent than auto-exposure. I've no idea if that's an issue with Nizos.

 

So really, it's somewhat theoretical until you shoot a test. 


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#6 Simon Lucas

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Posted 06 April 2013 - 04:58 AM

No, the internal meter is giving you a reading that is measured through the lens and calibrated for correct exposure at the film plane. An external meter will give you a pure reading that needs to be adjusted for light loss within the camera/lens. Typically I'd say you would need to open up the lens 1/3 to 1/2 stop more when using an external meter.

 

That's pre-supposing the factors you mention, on top of the accuracy of both meters and how they might differ in response to different lighting conditions. On top of how accurate the manual aperture control of the camera is (ie whether setting the iris to f/4 actually opens the iris to f/4), which might be  less consistent than auto-exposure. I've no idea if that's an issue with Nizos.

 

So really, it's somewhat theoretical until you shoot a test. 

 

Dom, thank-you. I realise that I got my interpretation arse-about-face, now.

 

I've tested my camera meter and my Weston V with a 18% grey card so i can see approx. 1/3 stop difference in reflective readings. A very minor loss of light does occur when zoomed in. So i think I can judge quite well how to adjust the reading for the manual camera setting. But I know the final results will be what matters.


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#7 Mark Sperry

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Posted 28 June 2013 - 07:09 PM

I can't see how this is true.  What would be the point of manual exposure?  

 

 

No, the internal meter is giving you a reading that is measured through the lens and calibrated for correct exposure at the film plane. An external meter will give you a pure reading that needs to be adjusted for light loss within the camera/lens. Typically I'd say you would need to open up the lens 1/3 to 1/2 stop more when using an external meter.

 

That's pre-supposing the factors you mention, on top of the accuracy of both meters and how they might differ in response to different lighting conditions. On top of how accurate the manual aperture control of the camera is (ie whether setting the iris to f/4 actually opens the iris to f/4), which might be  less consistent than auto-exposure. I've no idea if that's an issue with Nizos.

 

So really, it's somewhat theoretical until you shoot a test. 

 

What would be the point of having a manual mode then?  It doesn't say anywhere in the instructions that exposure needs to be adjusted when hand metering.  I can't see how this is true.  Also, why is my film coming out so well when I have been hand metering Tri-X, and absolutely not compensating for light loss?  If anything I tend to over expose when doing it this way.


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#8 Dom Jaeger

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Posted 30 June 2013 - 09:56 PM

If the method you use with your particular camera and lightmeter works then stick to it. I'm just explaining the physics. Super 8 was an amateur format, manufacturers virtually never marked T stops on the lenses or explained about viewfinder light loss probably because it complicated things. Manual mode was most likely intended to be used in conjunction with the built-in meter needle, to overcome certain lighting conditions that auto-exposure doesn't handle very well, like strong back light, snow scenes etc.

 

With your own findings, there are variables that can affect your results when using an external meter, as mentioned, which is why testing is the only way to really be sure. 

 

Once you move up to 16mm, lenses (particularly zooms) were often marked with T stops and if a reflex system used a prism that ate up some light, it was mentioned in the manual. The Bolex "compensated" shutter speeds confuse the hell out of a lot of people, but it's just trying to include the prism light loss in the exposure table. The Canon Scoopic is probably the closest thing mechanically to a Super 8 camera, its manual mentions that the lens is marked in T stops which accommodate light lost to the CdS cell and the viewfinder, taking it from an f/1.8 lens to T2.5.


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