Like many things in this world, our understanding of what characterizes straight photography has evolved.
Our first definition of straight photography emerged as a result of George Eastman's invention of the Kodax box camera in 1888 that made snapshot photography possible. As millions of snapshots became produced, it became necessary to define how photography could be seen as art. In 1890, English photographer Peter Henry Emerson first developed the concept of straight photography in his book titled Naturalistic Photography for Students of the Art. In his book, he proposed the idea that photography as art should create sharp images of scenes exactly as they appear without manipulation or staging. He termed what we call straight photography naturalism and related naturalistic photography to naturalistic art. Using a tree as an example, Emerson explained that the naturalistic painter "would endeavour to render the impression of the tree as it appeared(it) to him" (p. 25). So, too, would a naturalistic photographer strive to capture a scene exactly as it appears to the photographer in reality, and this can only be achieved through sharp, focused images because only such images portray a scene exactly as we see it in reality.
In contrast, in 1869, photographer Henry Peach Robinson published Pictorial Effect in Photography: Being Hints on Composition and Chiaroscuro for Photographers in which he presented the idea that photography could be allegorical and symbolical if images were manipulated to create special effects. We call this pictorialism, and naturalism, or straight photography, and pictorialism, have waged a war that has lasted even to this day, a war that eventually influenced even our understanding of straight photography.
In 1932, a group of photographers who called themselves F.64 opened an exhibit at San Francisco's M.H. de Young Memorial Museum, Ansel Adams being one of the photographers. Their exhibit protested against pictorialism and promoted straight photography using a new modernist aesthetic. Their aesthetic combined the desire to capture a scene as it appears in reality with the desire to also capture the emotion expressed by the scene. To do so, aside from using the sharp focus espoused by naturalists, they also developed what they called previsualization in which they used framing techniques to capture the scene as they saw it. Therefore, while they rejected manipulation techniques espoused by pictorialists, such as staging and image tweaking, they did use framing to capture their own interpretations. Since framing stands in contrast to the naturalism espoused by Emerson, we can see how straight photography has evolved over time to encompass the idea of interpretation espoused by pictorialism without using greater manipulation techniques than framing.
Hence, straight photography as we understand it today captures an image of a scene exactly as we see it in reality, using sharp focus and refraining from image and scene manipulation. But it also makes use of framing and high contrast to capture the photographer's interpretation of the emotions expressed in the scene.