Holiday Happy Snap: teach me how to use a polarising filter!

[mattyjacobs]

My wife and I recently went on a holiday up to Nelson Bay, just North of Newcastle. Beautiful place ... lots of mansions!

Anyway, with lots of sun and some really scenic beaches, our year-old canon eos 350d got a bit of a work-out.

Earlier in the year we bought a circular polarising filter, without fully understanding how they work and how to use one properly.

Here's a photo that I took (that's my wife on the left), that I thought turned out alright, but I'm concerned that the blackness in the top right corner shouldn't be there, and that I could have gotten rid of it by adjusting the filter ... what do you think?

Teach me!

[losfp]

What you're seeing is perfectly normal with a CPL and an ultra wide angle lens. I forget the exact technical explaination, but the colour depends on the ANGLE from the sun, and an UWA lens covers such a wide angle that you can't get even colour in the shy etc. Best to rotate so that it is even colouring (despite losing the "deepness" of the colour")

[mattyjacobs]

Cool, thanks for the quick reply!

So if I moved, so the sun was behind me, I'd get less blackness?

Also, when composing the shot, what should I be looking for as I adjust the CPL? At some points, the image goes really dark (like wearing dark sunnies), and at other points, it seems to be light and glary ...

[MHD]

The blue light we see is scattered light from the sun off the atmosphere and is polarised (The process is known as Reighley scattering, and scatters light pre-dominantly in the blue)

The So light you see which is 90 degrees away from the sun is strongly scattered and is strongly polarised... so is attenuated the most through the polariser on the lens...

[mattyjacobs]

umm ... I'm not that smart.

"the So light" I see? Can you please explain?

and by '90 degrees away ... strongly scattered ... strongly polarised', you're talking about the black bit right?

sorry...

[johnd]

Matty, look towards the direction of the sun. Not straight at it because we don't want you to go blind. The polariser will have least effect on the sky in the direction you are looking and in the direction behind you. It will have most effect directly to your right and left. By most effect, I mean deepest blue colouring. As was stated above, wide angle lenses have such a wide angle of view that they often take in parts of the sky near the sun (light blue) and at 90 degrees to the left or right of the sun (dark blue), resulting in the uneven colouring of the sky.

Polarisers are a great filter, but they do have limitations on wide angled lenses (as you have experienced). Another place where they cause more trouble than they're worth is when you are trying to stitch panoramas together (where you are effectively using multiple exposures to make an ultra, ultra wide angled lens).

Hope this helps,
Cheers
John

[Yi-P]

Make your hand pointing as you're playing children's gunfight (index out, thumb up, fist closed). Now, point your index finger straight at the sun, but dont look with your eyes (close enough will do okay).

Now, twist your wrist around in motion where your index finger does not point away (I guess you get what i try to say here).

Wherever your thumb is now pointing is your '90 deg' angle from the sun, point your lens towards the direction of your thumb, turn the CPL filter until you see things go dark or reflections go away, and there you go, a simple rule of thumb for CPL. Wasnt that hard is it?

[Justin]

Also note that the strength of the effect of the polariser can also depend on your aperture - the more wide open, the stronger the effect - at 1.8 you will get skies that are nearly black (and thus quite effective in B&W)

These are at 3.5 you can get skies that are deep blue, as per this photo -



and this one -

[mattyjacobs]

wow, cool ... learning so much already!

thanks for the rule of thumb, and the other examples of CPL pictures. I think my pic was shot at f/3.5 ... I'm tending to shoot in AV, and letting the camera set the shutter speed for me.

still got so much to learn! and lusting after the 50mm f/1.8 ...