There is a common mis-concept among students:
[i]Blocking part of the lens surface would block the corresponding part of the image.[/i]
The intensity of the image for the object is reduced is some light path were blocked (the whole image is still there).
Click "[b]block part of light path[/b]" button or drag lower end of the black line to lower position to block part of light paths.
The intensity is not the same if part of the light paths were blocked.
You can change the slit position and its width with mouse drag and drop function.