It seems that Tiffen have just released a new set of ND filters to resolve this issue.

Tiffen XLE Filters

From article/press relase

The affect you are getting is similar to the affect I got when trying link. I have used the Camidox filter redbusa mentions above on my K30 without any problem.

It seems that from a quick google search you are not alone in experiencing problem with the tiffen filter link

I thought i would do something unusual (for me) and post some pictures! All taken at the climate demo a few weeks back
















This one is blureed and over exposed and recoved but i still like it.

Interesting photos, thanks for posting.

I have used cheap variable ND filters of the ebay, although there was a color shift it was manageable. I cant seem to find any photos I am sure were taken with one.

What is worth noting is that when you increase the density to higher levels you get uneven exposures, from what I have read this can also be true of more expensive variable filters. I would do some research before spending alot of money on one.

Also found the markings on the side bear no relation to the affect you get.

The Reading London Camera Exchange store is having an in store day on 28th August, chance to play with 645Z if anyone local is interseted

link

I have been experimenting with back buck focusing a and have found it allot easier. Had a few instances when i first started where I forgot i had changed settings and tried focusing with shutter button and couldn't understand why it did not work and work. Other than that it was easy to get the hang of

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I like the shot of the bird walking into the shoe shop, works well with poster

I don't think there is a big difference between the two cameras, what are you looking to gain from the upgrade?