Invisible light leaks

[Mark Sampson]

The other day I made two photographs. I was excited by their potential, and went home and processed the film. To my dismay I found a large (20% of the image) amount of fog on all four negs. I quickly located a light leak in the bag bellows and repaired it, mumbling under my breath... I can go back to the location and reshoot, that's no problem. But I wonder why this was invisible on the groundglass yet was so obvious on the neg? Wide open or stopped down, there was nothing unusual visible. If I'd seen it, I could have thrown the focusing cloth over the camera and saved two nice photos and two hours of work. Ideas anyone?

[David A. Goldfarb]

A number of possibilities. One is that you may have been covering the leak with the focusing cloth when focusing, but not during the exposure. Another is that if you were using a bag bellows, you were probably using a wide lens, so it can be hard to see what's going on at the edges of the frame anyway. If you've got a fresnel, then the angle that's best for seeing the image might not be so good for catching a light leak. If the leak was projecting toward the top of the glass and the camera was positioned high, it might have been difficult to see.

I'm sure there are more. I'm always surprised by the things I miss on the groundglass.

[Donald Qualls]

It's also very likely that the leak had a lot longer to impress on the film than the lens image -- a leak gets to work on the film from the moment you pull the dark slide until you reinsert it, while the lens image only has the shutter time. With my typical methods, that would give a leak about a 3 to about 10 stop advantange on exposure time (how long do you spend waiting, dark slide out, for a lull in the breeze before actually making a 1/4 second exposure?) -- the leak could easily be a couple stops dimmer than the image and still overwhelm the image on the film.

[Mark Sampson]

Two very sensible answers. I've glued the bag bellows back together, and now I'm waiting for evening light similar to last week's. The recent thread about not being able to "go back" notwithstanding, I'm quietly hopeful about a re-shoot. Thanks!