TTL metering confused by yellow filter..?

Prarry · Jan 6, 2008

I have recently added a Heliopan Light Yellow Filter (5) to my standard lens on a Nikon F90x and it appears to "confuse" the metering by something close to the filter factor (1.5 to 2.0), so around one stop. I had assumed the TTL meter would automatically adjust, i.e. no adjustment for the presence of the filter would be needed. Is this is recognised effect or am I looking at getting the camera serviced or replaced?

AgX · Jan 6, 2008

What do you mean by `confuse´?

Depending of the colour of the measured subject the meter will give a different response in relation to a corresponding measurement without that filter.

cowanw · Jan 6, 2008

In addition, some manufacturers' metres react differently to different colours of light, I am told. For example I believe Contax metres do not see yellow very well.
Of course youu should see a difference in reading of the expected filter factor as AGX says
Regards
Bill

Ralf · Jan 6, 2008

A light yellow filter should not affect TTL metering. Extreme filtering - yes, but not a light yellow one.

pentaxuser · Jan 6, 2008

Simple experiment. Set camera on tripod or at least ensure it doesn't move. Fix aperture. Take a shutter reading with and without the filter. I'd expect the reading to be about 1 stop different with a light yellow filter. Check with the filter manufacturer's instructions to see what it says. The camera meter should agree with the filter factor. If it doesn't then it suggests that that TTL meter may be faulty or at least if you know the filter factor, say 1 stop, and the TTL meter adjustment( less or more than one stop) you can then make a further adjustment to the camera shutter speed.

pentaxuser

AgX · Jan 6, 2008

The filter factor is related to a colourless subject.
To distinguish any faults in the system, one has to state the subject the measurement is taken on.

Chuck_P · Jan 6, 2008

AgX said:
What do you mean by `confuse´?

Depending of the colour of the measured subject the meter will give a different response in relation to a corresponding measurement without that filter.

I may be mistaken, but I don't believe that meters are affected by the spectral wavelengths of light. A meter, IMO, only measures the intensity of the light to provide an EV with associated ranges of apertures and shutter speeds. The only response to the individual wavelengths of color is from the film you are using and the filter one is using affects those wavelengths that actually make it to the film. But I'm sure a more experienced user of filters may chime in and set the record straight on that, IDK.

AgX · Jan 6, 2008

Chuck,

In principle there are three main sensitivity curves for selen-, photoresistor- and photodiode cells. In addition there is sometimes optical filtering done within the meter (blue chip e.g.).

Further there is the issue to what extend a meter corresponds to a given film sensitivity curve. In theory a meter would have to be calibrated to the very film in use...

However these are minor issues, and this was NOT what I tried to say.

I rather tried to hint at the fact that when you measure a colourless subject with and without a coloured filter there will be a difference in measured luminance of the subject, equal to the given filter factor.
However if you measure a yellow subject through a yellow filter there won't be a difference in the meter reading.

Prarry · Jan 7, 2008

Dear All,
Thanks for the comments and advice.
I will wait until next week-end to check things in some daylight, but a quick try out in artificial light shows the filter has no effect on the metered value. I will try it against different coloured backgrounds and run a film through with a range of positive compensation "dialled in".

John Koehrer · Jan 8, 2008

In this wonderful world of technique & opinion you're going to find that there are as many ways of doing things as there are bodily orifices.
Some folk stick the filter on the lens & shoot away, others will meter & compensate for the filter factor. Whatever floats your boat is the way to go.
Different light sensing cells do have different spectral sensitivity AND to make life even more interesting they are different in Daylight or tungsten situations.

Nick Zentena · Jan 8, 2008

A #5 is so light I'm suprised the filter factor is supposedly 2.

I wonder if what you're seeing is more rounding then anything else. The filter blocks so little light the change isn't obvious to the metered F/stop and shutter speed. I don't know the 90x but how fine does it set things?

herb · Jan 8, 2008

TTl and filter factors

It is interesting that Gordon Hutchins, author of The Book of Pyro, has a set of filter factors that are added on after one takes a measurement thru the filter with a spot meter, i.e. with a dark red, it can be two stops on top of what a pentax spot meter sees thru the dark red by itself.

That should indicate the filter sees differently than the meter.

AgX · Jan 8, 2008

Which would indicate the film sees differently than the meter.

TTL metering confused by yellow filter..?

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