Cokin p series filter system

[odd jim]

I bought a Cokin p series filter system and an graduated nd filter mainly for landscapes. It works perfectly with my canon 10-22 albeit with Vignetting below 13mm which I knew it would do...

As I had the 77mm adaptor ring I thought I'd try it on my sigma 70-200 EX. Now I know you wouldn't normally use it on such a lens but I've though of the odd occasion where I might use it to prevent blown skies. However, when I use it with this lens the camera massively over meters, resulting in a totally blown out image even when the meter tells me it's fine. I have under expose (on the meter) about 3 stops to expose the image properly.

I put it back on 10-22 and again it worked fine and I couldn't replicate the issue.

Anyone know why it might do this?

 

[AJQS]

Probably because your grad now covers, to the camera's eye, a lrger area. On your wideangle, it is seeing light from a much greater area, on the Tele it's seeing a very small area, and you've darkened half of it. Using an ND8 Grad on my GH1 requires around 1-2stops negative exposure compensation. Your meter is only a guide anyway.

 

[odd jim]

This is what I thought so I moved the grad upwards, leaving the clear glass in front of the image while zoomed in and it still over metered. I'm thinking maybe it's light refraction from the back of the filter?

Anyway it's not a massive issue, just means I have to go back to trial and error rather than using the meter! I was planning on getting a 67mm adaptor ring to use with my 18-135 but I wouldn't want the metering issue with this lens...