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Contrast 'real' steps when using GratingStim

I’m preparing an experiment where participants have to detect grating patches at threshold visibility. To adjust difficulty I’m using the contrast parameter and I was wondering how much I need to increase or decrease contrast to actually produce a change on the screen.

For example, is there a physical change if increase the contrast from 0 to .05, or even to .005, I’m actually changing the contrast or are these values being rounded to 1 decimal point? I guess that depends on the screen calibration, and if so, is there a way for me to check on the particular screen I’m using whether a .05 change in contrast actually has an effect?