An image sensor may include an array of image sensor pixels arranged in rows and columns. Each image pixel arranged along a given column may be coupled to analog-to-digital converter (ADC) circuitry that is capable of converting analog pixel signals into a digital floating point equivalent representation. The ADC circuitry may be configured to obtain an illumination value during an auto exposure period. The illumination value, which serves as an exponent value, can be stored as tile data in a tile column memory circuit. During actual readout, the ADC circuitry may be configured to perform mantissa conversion. During mantissa conversion, the ADC circuitry may use a reference voltage value that is adjusted based on the tile data. A mantissa value that is obtained during the mantissa conversion may then be combined with the exponent value for that tile to yield a final floating number point for that image pixel.