With free weights and most exercise equipment resistance is constant throughout the range of motion. You are limited to lifting whatever amount you can lift through the sticking point, that point where you are at a mechanical disadvantage. That will be the hardest part of the movement; the reminder of the repetition will be relatively easier.
Most leg press machines provide movement in a straight line. With straight line movement there is more shearing force at the end point of the movement, and if one locks out at the end point there is greater potential for hyper-extending the knee. As an example see here (warning graphic). At Austin Fitness Trainers and at New Orleans Fitness Trainers we use MedX equipment. The MedX leg press provides an arcing range of motion similar to the rotation of the knee, and it can be set so that there is no locking out at the end point.