Polarising filters and Non-metered MF cameras

[dean_waters]

I tend not to use too many filters, just an orange for Black and White scenics

and a polariser for transparency sometimes, plus the occasional ND grad, but

coming from a 35mm SLR with TTL metering I never had to worry about exposure

compensation. The orange filter is easy, I just figure in the filter factor

onto my Pentax spotmeter and away I go, but what about the polariser as

exposure would change depending on the angle of rotation, and what about the

grads? Any sugestions?

[david_henderson]

I'll comment on the grads first. The answer is that it depends what you're using them to achieve. If what you're doing is the darken a sky relative to the land/foreground then you wouldn't really change your exposure vs what you measured without the grad fitted. What you might well do is measure the difference between the foreground/land and the sky so as to choose the grad strength that gives you the right difference between the two- which will generally mean that the sky should mostly be somewhat lighter than the "land" and about a stop lighter than any reflection of itself in water.

 

There are more complex scenarios- for example I quite often use a grad on the sky to enable me to lighten a too-dark foreground. So I may use a two stop grad to darken the sky one stop and open up the foreground one stop. Clearly to achieve this I'd need to give a stop more exposure than I would need to render the foreground as it seems to the eye.

 

But the simple answer is that there are no easy filter factors to plug into a spotmeter. Each time you use a grad to have to know what you want to achieve with it and expose accordingly.

 

A polariser is simpler most of the time. I use a filter factor with a polariser, and I've found that a two stop factor has worked best with all the (several) polarisers I've used since I started spot metering some years ago. Yes, it means that a sky at 90 degrees to the sun is going to be a little darker than at say 45/135 degrees to the sun, though of course you can alter the degree of polarisation by rotating the ring if its looking heavy. I wouldn't worry about differential exposure on "non-sky" polarisations either.

 

As you get more used to a polariser there are a couple of issues that may emerge that you might notice, and which might mean you want to go away from the "two stop filter factor" temporarily. First it does seem like different colours respond to polarisation in different ways. I've often found that reddish foliage or reddish landcsapes darken more with polarisation, for example. Equally the extent that the sky darkens is not the same in all conditions/locations. The more intense the blue of the sky the more it seems to darken with a polariser.

 

The more I photograph, the less I polarise and the more I use grads.

[q.g._de_bakker]

Dean,<br><br>That's right: you use a fixed filter factor for polarizers just as you would for other filters.<br>Yes, depending on the degree of polarization of the light, and the orientation of the filter, the amount of darkening varies. But you do not need to compensate for that: it (selectively making polarized bits darker) is why you use a polarizer to begin with. (Think of it this way: when using an orange filter, you wouldn't compensate exposure to the degree the filter makes blue skies darker, end then end up with a sky on your negative with density as if you had not used a filter at all, but all the rest terribly overexposed/much too dense, would you? ;-))<br>So do just like you do when using that orange filter: dial in the fixed filter factor of the polarizer.

[dean_waters]

Good points, I hadn't thought of it in that way. Thanks a lot!

[helenbach]

There are times when a fixed filter factor is inappropriate for a polarizer - such as when it is used to cut reflections from water or glass. In those cases there is usually no alternative than to measure through the filter in the correct orientation, because you are measuring the brightness of whatever is behind the reflecting surface, and you can't do that when the reflections are present.

 

Best, Helen

[q.g._de_bakker]

That's right. Sort of...<br><br>What is inappropriate then is not the fixed filter factor, but the metering method.<br>Use incident metering, and there's no problem at all.<br>;-)

[helenbach]

...unless the subject is behind tinted glass, or the water is not perfectly clear or...

[john_hancock]

I rotate the polariser in front of my eye to get the desired effect, then hold it in front of a handheld meter for exposure.

 

The only problem I've ever had -- and I've had it with both my Canon F1 and my 'blad -- is getting the lens hood on without changing the polariser rotation. Any ideas?

 

John Hancock / Sydney Australia