Rear Gelatin Filter?

I'm looking at buying a Sigma 12-24mm lens for a Canon EOS350D but

cannot find a link that explains how a polarising rear gelatin

filter works. Could someone please explain how this? Also, it would

be great if someone could comment on this lens...is it the right buy

if I want a wider angle?

A rear polarizer works into one fixed direction. You'll have to cut several ones and try them out, allways removing the lens and collecting more dust on your sensor...

 

I'd desperately try using something in Front of the lens, what should be possible according to the digital cropfactor. Maybe You'll have to mount a pro bellows-lensshade (either in the hotshoe or at the tripod mount) to use it if you have no filter ring in front of the lens but the convenience should be worth the effort.

Just out of interest, who makes polarising gelatin or polyester filters that are thin enough and of good enough quality to use behind a lens?

 

Thanks,

Helen

In my 35+ years of photo experience in pro equipment retail and operated my own studio I have never came across gel polarizers, but I could be wrong.

 

There is also the issue of direction orientation if these are mounted internally - How do you see the effect you want?

 

I have heard and seen polarizer used in wide angle landscapes with large area of sky or water and the result of uneveness might not be what you want.

Probably the Edmund Scientific polarizing sheets that can be cut to size and shape. Not very convenient for this purpose.

Apart from optical quality issues, isn't that polarising film too thick for use behind the lens - especially a wide angle lens?

 

Best, Helen

The addition of a gel filter will shift the focus slightly, so the lens must be focussed, AF or MF, after the filter is added. (The filter will shift the focus towards the rear by about the equivalent of 1/3 the filter thickness, so the correction will be moving the lens forward by that amount. Don't adjust the focus after you have manually or automatically focussed.)

Remember that the focus shift is not truly uniform over the range of ray angles. With gelatin filters this is not important because the filter is very thin and gelatin has a low refractive index. The thicker the filter and the higher the refractive index, the more the image is defocussed. There's also an issue of how much degradation the polarising film would cause because of lack of parallelism.

 

Of course it may well be that there's no problem. I have some polarising film somewhere and if I can find it I'll try it on the back of an ultra-wide, though it's not something I could see a lot of practical use for.

 

Best, Helen

You are correct in that the focus shift does depend slightly on angle and wavelength. Fortunately, in most applications, the effect is small.