100-400mm f5.6 Focus?

Hi Guys

Sorry to seek comments on such a possibly well explored problem. But: When I am using my 100-400 f4.5-5.6 L IS at its 400mm end, the images seem to have a slight double image effect. This is particularly noticable when photographing a ships rigging for instance at say 0.5 - 1mile. The secondary/shadow rigging is only just disceranable as it is not as bright as the main image. Maybe I'm expecting too much at 1 mile - dunno! I've tried using a tripod and all the usual stabilising tricks with mirror lock-up etc. I get the same effect taking moon shots - noticable as a very faint and narrow displaced edge to the bright moon. The effect is on the left of the main images.
I will not be posting sample shots.
There are no problems at all between 100 and 300/350mm.

Does anyone else out there have similar problems.

In anticipation of your comments and thoughts - Many Thanks.

Do you have a filter on it? If so, what brand and type? Cheap filters may cause problems. I use B+W MRC or Hoya HMC (or better) on my lenses and don't have such a problem with any of them, including the 100-400.

Hi Jon

Thanks - I have tried without the filter. I usually take it off for the more fussy distance shots and where detail is absolutely essential - but I will try again before I start shouting too loud. I must admit to using a cheap filter for protection. The filter is fine for less detailed and closer subjects.

Do you think 0.5-1 mile is too far to expect good definition?

Not with a good filter or no filter. However it may be too far to expect to see much detail in the rigging. At a mile, each pixel will be covering about an inch (20D); if you're talking about normal sailboats, that's much wider than the normal standing rigging. If you're shooting tall ships, you may actually be seeing paired lines. But for protection, get a good filter.

OK Jon

I'm talking mainly general shipping on the Thames. I did at first think of paired lines - but no it wasn't.

Examples you can post? Maybe a couple of 100% crops of problem areas?