For an expert to give opinion testimony, the opinion must be based upon facts within the knowledge of the expert, or the expert may give an opinion based upon a hypothetical question which itself is based upon facts already in evidence. State Farm Fire Cas. Co. v. Sawyer, 522 So.2d 248, 251 (Ala. 1988); see also C. Gamble, McElroy's Alabama Evidence, § 130.01 (3d ed. 1977). In either event, the facts known to the expert or hypothesized must be facts in evidence.

Furthermore, an expert is not required to possess direct personal knowledge of a matter in order to express an opinion. State Farm Fire Cas. Co. v. Sawyer, 522 So.2d 248, 250-52 (Ala. 1988) (expert witness allowed to give an opinion even though hypothetical question was based on photographs and expert had not personally inspected scene at time of fire); Bastian v. Laffin, 54 Md. App. 703, 712-14, 460 A.2d 623 (1983) (expert's appraisal based solely on a description of items was admissible as evidence). Just as a medical expert may base an opinion on an X-ray or as any expert may render an opinion based on a hypothetical question, so also may an expert give an opinion based on photographs.