120 B&W film question about Negative corners

[akfreak]

Hello, I just had a question about my 120 b&w negatives, The corners have some weird hour glass thing going on, I have to assume it is either the pressure plate not pressing the film flat, the masking area inside the light box is is misshapen. A combo of both, the is FILM very curly (lucky SHD 100 in 120). I am shooting a pentax 645 with a 120 back. I am making sure the film is taught when I load it to the start line.

Lastly I have the same issue with Delta 100 is I know it isn't the film. I dont think this is normal, however I could be wrong.

here is a pic of what is happening

HourglassNegs by Thomas Shue Photography, on Flickr

 

[chioque]

That's normal

 

[Ian Grant]

Quite normal but more noticeable with some cameras/backs than others.

Ian

 

[Ian Cooper]

If you take the back off the camera and have a look at the gate/frame which the film rests against, you'll notice these notches in the corners. The Hasselblad backs famously have two small 'V' notches in the middle on the left hand side. My Bronica has corner 'serifs', like your example. Some people who have multiple backs will file additional notches of their own so if a back starts leaking light etc. they can readily identify which one it is.