Flat Object

[Tony]

Last night I was testing some film and used a very flat plaster of paris slab.
Every result looked as though it had moved.
The were fairly large rings. The image was only visible a odd angles (not the angle I shot).
So to better eliminate movement I set the plate of top of the plaster, same result.

I was wondering if I'm seeing something else, some wave front or something to do with the laser.
Has anyone seen this before? Is it bad to shoot a flat object?

Seeing my over whemming response from my last post, I look forward to this one as well
Who says the forum is dead

[kaveh1000]

Hard to say without more detail. Any chance of a photo showing set up and also the image and fringes?

[kyodai]

From the description it does sound like classic movement. Especially soft objects can be a bitch to shoot. If you wanna make sure its not the setup then place a penny near the plaster. If both show movement it might be your setup (Table) if only the plaster is moving then maybe the material is hard to shoot (let it settle much longer?)

[Paulos]

Last night I was testing some film and used a very flat plaster of paris slab.
Every result looked as though it had moved.
The were fairly large rings. The image was only visible a odd angles (not the angle I shot).
.......

It looks like your object expanded or contracted during exposure.
How long did you let it settle? You should probably wait one day ...or even more !!!
Plaster of Paris easily interacts with air humidity, so changing in its dimensions.
Also, in order to avoid transfer of moisture from your hands to the plaster, it is advisable not to handle such objects with bare hands, always use gloves.
Water absorption happens fast, while drying may last many days, depending on environment conditions.

[Tony]

Many thanks all.
Plaster is normally pretty stable for me.
But.... there was a hitch. the plaster had cracked when I was installing some nails as a film holder. I am thinking the epoxy I use to stick it back together was not as dry as it could be. I had waited an hour but maybe it was not long enough.

I will try this again and see what happens.

[Johnfp]

My experience with plaster is that it has to cure for days before it can be shot.