I tried several times to get an alpha channel from a render I've set up in Poser 7. When I open the TIFF-File in Photoshop CS2 the alpha channel is there but it's all white...
Any ideas?

Try using PNG instead. I've found that it works better anyway, since you don't need to derive a mask from the alpha channel - it comes in (to PSP at least) with the background already transparent.

It's possible that the appropriate image handling plug-in has become corrupted. A reinstall may fix it if you want to try it.

Also, if at any time you have loaded a background picture it doesn't seem to work.

Thank you guys for your answers. Strange enough - Photoshop allways gives an error message when I try to load a PNG or BMP-File which was generated with Poser 7.

yeah, png files exported from P7X import to APS with no alpha channel, but transparent. 
tiff files exported from P7X import to APS with an alpha channel, usually mostly black.
a solid white alpha channel may mean there's no empty space behind the objects, but
ISTR there was a bug in earlier versions, where some users had an inverted channel.
in a tiff file exported from poser, an alpha channel  that's mostly white, with some grey,
may be the result of activating the depth cue.

Why don't you just export as .psd? Works perfectly, at least, it does with Photoshop 6.01. Including the alpha channel.

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Quote - I tried several times to get an alpha channel from a render I've set up in Poser 7. When I open the TIFF-File in Photoshop CS2 the alpha channel is there but it's all white...
Any ideas?

EF changed the way P7 handles the alpha channel in TIFF.  In previous versions the alpha channel would load in the channels pallet if you saved to TIFF.  There's a very long involved thread in the CP forums from February of last year regarding this.  According to Uli K, the way it is now is the way the alpha channel is supposed to be handled.  All previous versions of Poser were wrong.

Here's a link to the thread if you're interested

http://www.contentparadise.com/forums/showthread.php?t=4131&highlight=igohigh

I tried it with the psd-format as well but no alpha channel.

Thanks for the link - I will check it out.

My guess is that you are rendering with a wall or some object in the background. To get an alpha you have to have some area that is 'empty' of objects. If you only have a couple small areas of 'empty space' then your alpha will be mostly white. You can render over a background image and still get an alpha, as the image isn't an object. I prefer to use tiffs with alphas as I like to be able to easily edit the mask. I usually at the least apply a little gaussian blur to soften the edges. To use the alpha as a mask, go into your channels pallette and ctrl click on the alpha thumbnail. This will load it as a selection (or you can use the selection menu).

There is no wall or anything else in the background. I tried now another Poser-file and it worked. So I guess the one file was kinda a damaged.