Originally Posted by
Steaphany
Traditionally, the ISO has always been set, by selecting the film to be used, and then the shutter and aperture combinations determined by the measured exposure. Contemporary digital cameras still follow this model - no camera exists where you lock down either the shutter or aperture leaving the other to be set as the ISO is adjusted. You still need to set the ISO and work with the others.
The philosophy of the lens baby sounds very similar to how one photographs terrestrial scenes with an astronomical telescope serving as a long focal length lens. The aperture is fixed, you select the ISO, and then just work with the shutter speed to get the exposure. If scene element movement requires a specific shutter range limit, you select the ISO accordingly and go back to finding what shutter gets you the image you're after.
You may be able to use your camera's meter to select the exposure. Put your lensbaby on, install the aperture disk that you want to use, set your camera to aperture priority mode with the aperture set to the numerical value of the disk, set your ISO, and when your camera meters, it should be selecting only the shutter leaving everything else fixed. If your camera biases the exposure expecting the lens aperture to be under it's control, just adjust the exposure compensation for the difference. ( two stops too dark, select two stops brighter )
I do not know about Nikon, but my Sigma SD14 meters fine with my Orion 1250mm fixed aperture f/13.9 ( 90mm Ø ) Maksutov Cassegrain.