lightmeter doesn't recognize contrast filter!

[wallace]

I was surprised today when I recognized that the light meter of my Olympus 35RC doesn't read a difference between using a yellow (2x) filter or not. I then made a test with my Weston hand held meter, it shows only one half stop difference. Can somebody please explain?


Thomas (wallace)

 

[Sparrow]

Comparing meter readings without a known light source and reflective surface, and without allowing the FOV, frequency sensitivity and K factor of each meter would, I suspect, prove a thankless task I'm afraid

and as Richard said it's only half a stop

 

[wallace]

Comparing meter readings without a known light source and reflective surface, and without allowing the FOV, frequency sensitivity and K factor of each meter would, I suspect, prove a thankless task I'm afraid

and as Richard said it's only half a stop

well, in case of the Oly it makes one full stop. I even pointed the camera against the blue sky. thought that would make a difference, but it did not. still strange to me...

 

[philipp.leser]

It says -1.5 stops on my Heliopan Yellow 8 filter, but I guess it depends on the spectral sensitivity of the film and the spectra makeup of the scene how much light is actually transmitted or blocked. Now, if the meter has a different spectral sensitivity than the film (and I think it usually has), the meter readings won't be reliable if measuring through a filter.

Filters generally confuse me, because it's very hard to actually measure correctly because of what I wrote in the last paragraph... I generally increase my exposure and try not to worry too much about it 😉

Regards,
Philipp

 

[wallace]

It says -1.5 stops on my Heliopan Yellow 8 filter, but I guess it depends on the spectral sensitivity of the film and the spectra makeup of the scene how much light is actually transmitted or blocked. Now, if the meter has a different spectral sensitivity than the film (and I think it usually has), the meter readings won't be reliable if measuring through a filter.

Filters generally confuse me, because it's very hard to actually measure correctly because of what I wrote in the last paragraph... I generally increase my exposure and try not to worry too much about it 😉

Regards,
Philipp

It sounds logical that light meters may have a different spectral sensivitity than panchromatic film, good thought!
So I guess I better adjust the asa dial at about 250 next time I use the filter with Tri-X....

 

[Sparrow]

well, in case of the Oly it makes one full stop. I even pointed the camera against the blue sky. thought that would make a difference, but it did not. still strange to me...

What Richard said x2 is twice the light, one stop, so your just half a stop out. 🙂

 

[Sparrow]

????

Light meter manufacturers make their meters with the same spectral response as film, that's what they're designed for.

I suspect they try ... and fail

 

[philipp.leser]

Be careful how you interpret those numbers guys!

They are not "f stops".

They are "filter factors".
...

It depends on what numbers you mean by "those numbers". The number I was talking about was f-stops. Literally, it says "3x / -1.5" on the filter. Heliopan at least always indicates the filter factor by that "x" and also gives the stops.

Regards,
Philipp

 

[wallace]

????

Light meter manufacturers make their meters with the same spectral response as film, that's what they're designed for.

Are you sure??? Which film? Black&white or color?

 

[sevo]

????

Light meter manufacturers make their meters with the same spectral response as film, that's what they're designed for.

I'd like to have a EPT, Velvia 50 and T-Max 400 meter each, please...

Film does not have a linear spectral response - and different film types differ in their aberrations. Most meter manufacturers merely added filters to linearize their sensors where necessary (silicone photo transistors in particular have a increased red/IR response and need a cyan filter - hence the "silicone blue cell").

Zone VI modified Pentax Spotmeters to spectrally match some then popular film type (IIRC Plus-X or Tri-X) - but these meters may be more off for some other film than a regular one, so the practical value of that is a bit questionable.

Sevo

 

[Sonnar Brian]

The Olympus uses a CDS meter, and it is generally sensitive a bit more into the IR than is film. Take a white sheet of paper, put it in direct sunlight, then test with and without the yellow filter and see what you get. Then, try the same test under artificial lighting.

CDS cells deteriorate, usually lose sensitivity at the extremes of the scale. I'm not sure if the spectral response changes as well.