Hi Al,
If you are using autofocus with a linear polarising filter, depending on the angle of the filter, it may not allow enough light though for the AF sensors to detect when the subject is in focus.
You can use this filter, but bear in mind that the AF may not work, you almost certainly will have to switch to manual focusing, especially as the 70-300 will probably have a smaller max aperture at full zoom.
Why not get a step-up/step-down filter ring for a couple of quid and try using the existing filter you have with the bigger lens? This will let you see if it's worth getting the circular polariser. It all depends on how important AF is to you.
Hope this helps,
Duncan.
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