Eliot Porter, a pioneer color photographer best known for his large-format books published by the Sierra Club, died Friday in Santa Fe, N.M. He was 88 and had been hospitalized since Oct. 27 for treatment of Lou Gehrig's disease. Mr. Porter earned a medical degree from Harvard and conducted bacterial research there before turning his interest in photography into a career. He had lived in Tesuque, N.M., since the mid-1940s. His work was exhibited at the Metropolitan Museum of Art in New York City, and he donated hundreds of prints to the University of New Mexico and the Fine Arts Museum in Santa Fe. Mr. Porter took his last photograph in 1986, the same year he was diagnosed with Lou Gehrig's disease.