This is a new one on me: I had careful focus locked into manual on Nikon D5500, tripod, steady subject, etc. Frame 1 great ... but over 900 frames focus went away like a smooth ramp to blurry ... no one frame jump or anything. Baffled! Camera was mounted on tripod 'sideways' to get framing I wanted; that's the only different thing.

Hi, could it be that some how you experienced some cripping on the lense due to the positioning of the camera/lens because of the framing?

Regards,

Rodrigo

Thanks.  You mean the focus mechanism actually can slip slowly, from having the camera sideways? If that's even a possibility I'll have to avoid that on my next attempt.

Yes, it can do, it depend on the lens, specially if it is pointing upwards. What lens we talking about?. Point you camera and lens upwards and see if slowly start cripping back, it could be ever so slowly. Some lens have a lock button, to stop that from happening.

Regards, 

Rodrigo

That's it ... It happened again ... slight pullback of focus ring enough to mess up by the end of another long sequence.  See attachment.  Corner white space is larger on the last shot. This time camera was pointed down for 5 hours ... Maybe tape or a rubber band to fix? 

Thanks for your help ...

Hi, I am glad you sorted it out, yes a bit of gaffer tape placed over the junction between the focus ring and the body will stop the lens from cripping.

Regards,

Rodrigo