An image sensor may include an array of image sensor pixels that are read out using analog-to-digital converters (ADCs). The ADC may be shot-noise-matched to reduce the number of decision cycles required. A ramp with limited resolution spanning only a small portion of the full scale voltage range may be used. For small analog input voltages, this limited ramp range is sufficient. For large analog input voltages, less resolution is needed due to the increasing shot noise in the photo signal. The larger input voltages may be successively divided by a selected attenuation factor until the analog input signal is within the range of the reduced ramp. The ADC keeps track of the number of divisions being performed to determine an exponent value for a floating-point output value and then convert the residual signal with the smaller ramp to determine a mantissa value for the floating-point output value.