How does SLR measure light when using B-gate or T-gate?

What you said belongs to low-light photography (the light is quite weak). You can use the camera's exposure meter to measure light, set the ISO to the highest (for example, 1600), and get the exposure reference value (for example, F2, 1 second). In order to shoot the star orbit, when using ISO 100, it is necessary to calculate and use a small aperture (such as F 10). According to the reciprocal rate, it should be 1024 seconds (that is, 17 minutes). In fact, when calculated in weak light, the reciprocity rate has failed (that is, the reciprocity rate curve has entered the nonlinear region). If the exposure is carried out according to this exposure value, it is necessary to increase the exposure. The longer the exposure time is calculated, the more compensation is needed, such as+1 ~ 2EV.

In extremely low light photography, exposure value mostly relies on experience. In the part of the reciprocal rate curve far from the linear region, there is little difference between exposure for 2 hours and exposure for 4 hours.