Hallo, what I describe below is something I noticed yesterday that I suppose many here already know, but I didn't and I would have liked I had known it before, so here is my sad case.

I had noticed some kind of a dark "curl" in the sky of several frames of a couple of rolls taken during the last month. My notes told me that the rolls were taken with the same camera, my beloved Minolta X-700.

It was evident that there was some kind of small particle of dust, hair-shaped, in the mirror box of the camera, but, as it often happens, the camera had a roll of slides in it.

Well, "no problem" I thought. I'll sacrifice a frame, take a shot in Bulb mode, and observe the mirror box carefully. That I did near a window, and found nothing wrong.

Next I inspected my most used lens and I actually found a minuscule hair-shaped fragment of fine dust trapped near the inner side of the bayonet mount. I eliminated it exercising as much violence on the little bastard as I could (was thinking about torturing it)

When I developed the roll yesterday I found that three frames were ruined, not one. The central one was totally blank, as one would expect. The two frames on both sides of it were for about half of the length also entirely totally white.

Aaarrgghhh. How could that have happened?

After some brain squeezing I suppose that the indoor but bright light was so "strong" that it "travelled" along the film base and the emulsion. The film was a Fujichrome Sensia. Had it been a film with a polyester base things would have been probably even worse.

So I learned two lessons:

1) If you suspect there is some small particle of fine dust somewhere between the rear lens and the focal plane, check the back of the lens first!

2) If you don't have mirror lock-up and have to inspect the mirror box, which I suppose you will do in daylight or indoor, before triggering the shutter in B pose advance the film twice, and then twice again before taking the next picture. That will spoil three frames but it will not ruin pictures that you actually thought you had taken. It goes without saying that the frames around the "B" one are to be taken with a lens cap (or with a normal shutter speed).

That teaches two more lessons:

When instructions recommend loading film in subdued light, they do it for a reason;
If using film with a polyester base, be careful about how you handle film and how your "roll your own".