does polarising filter decreasing focus?

[williamchew85]

d90 user, i bought a "pirated" 77mm filter tru online which cost me around 15-20 bucks ... i bought it for testing purpose..

when i use it with my 70-200mm f2.8 lens.. it seems can't focus well... photo taken was kinda slight double image.. is polarising filter suppose work in this way? or something wrong with the fake filter?

[catchlights]

My guess is you bought a linear polarizing filter, which is cheaper.

for auto focusing camera are recommended to use circular polarizing filter, for overcome metering and auto focusing issue which only happen on linear polarizing filter.

[boyboy]

my guess is that a proper polariser for 77mm lens will cost waaaaaay more than "15-20". i think i paid something like 70-80 for my Keep 52mm CPL. dunno whether kena chop or not but i was super green n noob tt time

[Blur Shadow]

Is it a linear or circular polarizer?

[Snoweagle]

Originally Posted by williamchew85 d90 user, i bought a "pirated" 77mm filter tru online which cost me around 15-20 bucks ... i bought it for testing purpose.. when i use it with my 70-200mm f2.8 lens.. it seems can't focus well... photo taken was kinda slight double image.. is polarising filter suppose work in this way? or something wrong with the fake filter?

A linear polariser will somehow affect Auto Exposure (AE) but focusing will not. To combat this, get a circular one which definitely cost a lot more than your indicated price.

[Octarine]

Originally Posted by williamchew85 d90 user, i bought a "pirated" 77mm filter tru online which cost me around 15-20 bucks ... i bought it for testing purpose.. when i use it with my 70-200mm f2.8 lens.. it seems can't focus well... photo taken was kinda slight double image.. is polarising filter suppose work in this way? or something wrong with the fake filter?

Very nice example of cause and effect. You paid only peanuts... If you have a good lens please get a proper CPL.
Beside this: focus cannot be increased or decreased... only spot on or not with the effect of a sharp or soft/blur picture.

[williamchew85]

Originally Posted by Octarine Very nice example of cause and effect. You paid only peanuts... If you have a good lens please get a proper CPL. Beside this: focus cannot be increased or decreased... only spot on or not with the effect of a sharp or soft/blur picture.

yeah its kinda soft and blur...and its a CPL filter...

think cheapo filter is like tt 1 lar hahaha

[agws1970]

Originally Posted by williamchew85 d90 user, i bought a "pirated" 77mm filter tru online which cost me around 15-20 bucks ... i bought it for testing purpose.. when i use it with my 70-200mm f2.8 lens.. it seems can't focus well... photo taken was kinda slight double image.. is polarising filter suppose work in this way? or something wrong with the fake filter?

You remind me of a friend who bought a $100,000 Car but uses $65.00 tyres. When you spend so much on a pristine piece of glass then muck it up with a Cheapo filter, results will be what you describe.

Please Please Please Please Please Pretty Pretty Please get yourself a decent filter. Do justice to your lens.