Cokin ND filter usage

I am new with ND filter. I bought two Cokin ND filter P154 (ND8) and P153 (ND4) recently. When I use them seperately the result is fine. However if put them together on the holder the result is quite different with red tone. I am not sure if I use them incorrectly or something else. I attached the resulted photos here. Any advice is highly appreciated.

Yi

https://www.cambridgeincolour.com/fo...achmentid=5562
#1 ND filter
https://www.cambridgeincolour.com/fo...achmentid=5559
#2 ND 4
https://www.cambridgeincolour.com/fo...achmentid=5560
#3 ND 8
https://www.cambridgeincolour.com/fo...achmentid=5561
#4 ND4+ND8

Hi Yi,

It's a known problem with resin filters; Singh-Ray think it has something to do with infra-red light.

Best way around it if you need bigger attenuations is to use a Singh-Ray Vari-ND filter; they give attenuation of between 2 & 8 stops, with no colour shifts.

Hi Colin;

It makes sense now. Thank you for this information!

Regards!