Based on the mathematical model of modulation transfer function in atmospheric turbulence,the combined effect of atmospheric coherence length and optical pupil diameter on integral resolution and spot radius were investigated.Approximate algebraic solutions of resolutions and spot radii are derived for long-term exposure and short-term exposure by use of interpolation method.The result shows that the resolution is mainly limited by the pupil diameter while the pupil diameter is smaller than atmospheric coherence length,and can be enhanced by increasing the value of the pupil diameter.However,the resolution will not be significantly improved with further increase of the pupil diameter,since the imaging performance is gradually limited by turbulence while the pupil diameter is larger than atmospheric coherence length.There are limiting values for the resolution and spot radius restricted by atmospheric coherence length while the ratio of the diameter to atmospheric coherence length tends to infinite.For short-term exposure,since the tilt effect due to large-scale eddies is eliminated,the integral resolution and the spot radius are better than that of long-term exposure.Especially,for the near field of short-term exposure,the maximum value of integral resolution is approximately increased by a factor of 3.5 and the minimum value of spot radius is approximately decreased by a factor of 1/2,as compared with the case of long-term exposure.