Testing gravity theory in the strong field region becomes a reality due to the observations of gravitational waves and black hole shadows .
In this paper , we discuss how to constrain the possible deviations of the classical general relativity with the image of M87* observed by the Event Horizon Telescope .
More precisely , we want to know where is the event horizon for a non-rotating black hole .
General relativity predicts the horizon is located at the Schwarzschild radius r _ { \textrm { s } } , while other gravity theories may give different predictions .
We propose a parameterized Schwarzschild metric ( PSM ) in which the horizon is located at r = nr _ { \textrm { s } } , where n is a real free parameter , and prove general relativity with nonlinear electrodynamics allows n \neq 1 .
In the weak field region , the PSM is equivalent to the Schwarzschild metric regardless of the value of n .
In the strong field region , the difference between the PSM and Schwarzschild metric would leave an imprint on the shadow image .
We present detailed calculations and discussions on the black hole shadows with large background light source and accretion disk in the PSM framework .
More importantly , we point out that n \approx 2 can be used to explain why the black hole mass measured by the shadow is a factor of about two larger than the previous gas dynamics measurements .
If this explanation is confirmed to be right , then this phenomenon , together with the late-time cosmological acceleration , will be very important to test gravity theories .