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35mm SLR, 300mm f-2.8 APO lens, 2X teleconverter, Fuji Velvia 50 pushed 1 stop, Bogen Tripod and ball head. Multi-segment metering on aperture priority, 1/60 sec at f-5.6. Autofocus.

Pushing film means rating the film speed or ISO higher than the film is designed for. An ISO 100 film for instance, can be rated at ISO 200 or even 400. By setting ISO 100 to 200 is pushing the film one stop. Going to ISO 400 is two stop push. Pushing films let you get one or two additional stops of light which means faster shutter speeds. Lets say you are photographing wildlife with ISO 100 film and your 400mm f-5.6 lens wide-open gives 1/60 sec shutter speed in low afternoon light. You know 1/60 sec is too slow to freeze moving wildlife. By pushing the film to ISO 200, you get 1/125 sec shutter speed or 1/250 sec if you push the same film two stops to ISO 400. You have to shoot the whole roll at the same ISO and tell the lab how much you pushed the film. You can't change ISO in the middle of the roll. What you are really doing by pushing film is underexposing each frame by one or two stops. If your film is processed normally, all images will be underexposed. You must tell your lab how many stops each film was pushed. What the lab will do is push process the film for additional cost to correct exposure. Pushing film has its draw backs. Contrast is increased and grain will be more visible. My advice is to push films one stop with ISO 100 or faster films, except Fujichrome Velvia ISO 50 which can also be pushed with good results. Pushing two stops should be used as a last resort. Experiment with different films to see if you like the results. I push Fujichrome Velvia one stop with very good results and have pushed Velvia two stops with acceptable results. I see very little difference between Velvia pushed to ISO 100 than Fujichrome Provia ISO 100.
Opposite of pushing is pulling. Pulling film means rating the film slower than its normal speed. Why would anyone want to do this? two reasons: 1- To correct a film that was accidentally shot at a faster speed. 2- To reduce contrast when duplicating slides. Contrast is increased when you make duplicate slides. Films that are designed for normal shooting produce very contrasty images if used for duplication. Fuji Velvia or Ektachrome Elite films for instance, produce excellent results for outdoor shooting. If you use these films to make duplicate slides, results will be very contrasty. You can use these or similar films if you need to make a few duplicates by overexposing them one stop and under develop the films by one stop to reduce contrast. Simply tell the lab the film needs one stop pull process.