Dum-Dum Polarising Filter Question

TheBigYin

TheBigYin

Moderator
Messages
16,673
Name
Mark
Edit My Images
No
Time for my dumb question of the month...

I've just received a nice second hand 62mm Hoya Linear Polarising filter for my Bronica. Amazingly enough it actually came in the original case, with the original box and leaflet, as out of the shop!

Anyway - the leaflet states that you should apply a filter factor of 3-4 in general use. Would I be correct in my assumption that this equates to 1 2/3 to 2 stops?

At the back of my mind, I vaguely remember that Filter-factor to stops was calculated by

Code: 
stops = log(filter factor) / log(2)
which gave me 1.59 to 2 stops - so give or take a smidge, 1+2/3 to 2 stops.

So - have I remembered correctly, or am I talking through my harris?



 
erm couldn't you place the filter over the lens of your digital camera to see what the stops are....or is that a dumb answer.
 
excalibur2 said:
erm couldn't you place the filter over the lens of your digital camera to see what the stops are....are is that a dumb answer.
Click to expand...

I think that some linear polarisers (yep - I went pikey for once, as the 62mm is no use whatsoever on any other lens systems I have) cause metering problems with modern digital cameras, so I've not bothered as yet - I did stick the Pola. filter over the Sekonic L358/Lumigrid, and came up with between 1 and 2 stops, so its a bit of a belt-and-braces check, but I just thought someone would know off the top of their heads...
 
Does it not depend upon how it's oriented? I only use a polariser on my SLRs and when you turn it through 90 degrees, the indicated exposure varies a lot. If you have a flat screen monitor on your PC, look at it through the filter as you turn it - you can black out the image totally at one angle.
 
For negative film, 1/3 stop is nothing. The film's latitude will take that up easy.
 
NickT said:
Does it not depend upon how it's oriented? I only use a polariser on my SLRs and when you turn it through 90 degrees, the indicated exposure varies a lot. If you have a flat screen monitor on your PC, look at it through the filter as you turn it - you can black out the image totally at one angle.
Click to expand...

I think you're confusing circular polarisers and linear polarisers Nick. Quite different animals.
 
The only difference between a circular and linear polariser is that a circular one has a quarter wave plate behind the polarising layer, turning it into circularly polarised light, which does not confuse the beam splitter used for autofocus and metering in modern cameras as they linearly polarise the light. Both types do the same effect although linear ones apparently are slightly better for minimising reflections.

Putting my polariser on my Dynax 5, and measuring the exposure at different polriser angles makes a tiny to no difference. All thats happening is the camera is measuring the brightness of each individual component (especially when using matrix metering), so it will automatically want to expose more if the sky for example if darker. Its better to meter with the polariser rotated to where you want it first.

The reason that an LCD computer screen blacks out at a certain angle is because the light is polarised, the screen wouldn't work if it wasn't. The camera will show a different exposure for the simple reason that if it see's something black in the majority of the metering area its going to automatically want to increase exposure, its nothing to do with the polariser, just use exposure lock with the screen visible and then rotate it until its black.

All the polariser does is have a small filter factor for doing these effects, its near impossible to make something have no filter factor if its actively doing something to the image like a polariser does.
 
Just tested on my Canon A1 with a linear and a circular one. Both give a variation of half a stop as the filter's turned looking at a view which is 2/3 sky. From memory I think it could was up to a stop and a half when I was at 6,000ft altitude on a sunny day. Both types will block an LCD screen out completely at a certain angle.
 
NickT said:
Just tested on my Canon A1 with a linear and a circular one. Both give a variation of half a stop as the filter's turned looking at a view which is 2/3 sky. From memory I think it could was up to a stop and a half when I was at 6,000ft altitude on a sunny day. Both types will block an LCD screen out completely at a certain angle.
Click to expand...

as explained above - thats purely because the light from a LCD screen is already polarised.

Back in the day of "foster grant" polarised sunglsses, they used to come with a test card to prove that they were real polarised lenses and not knock offs! The Test card was a small scrap of polarising film in a card. You held the shades up to the light, and turned the test card. When the 2 polarising films (one in the card, one in the sunglasses were at 90 degrees to each other, the test card was blacked out.

If I ever actually manage to get out and shoot, I'll probably just use half a roll of slide stock to check my exposures - shoot a control frame sans polariser, then 1 to 3 stops in 1/2 stops, and see which frame comes out nearest the control on the lightbox. I'm betting it's the 1.5 or 2 stop frame though :)
 
Last edited:
You must log in or register to reply here.
Back
Top