Reflections are a very popular subject to photograph- our eye is attracted to the odd nature of seeing part of the world upside down!
While it seems very straight forward to take a picture of a reflection it is more difficult than you think if you want to get everything you see in focus. this is because, optically, the reflection doesn’t sit on the reflecting surface as it seems. Instead the subject that is being reflected is actually as far away from you as it would be without the reflection. Let me explain.
Let’s say you are looking down into a puddle covered with red leaves and a reflection of a tree. Optically, the distance to the red leaves is 5 feet but the distance to the branches of the tree you see in the reflection is actually 5 feet to the puddle PLUS the distance back up to the tree. If the branches are 40 feet above your head, optically the distance to the branches is 45 feet.
This is important if you want all of your photo to be in focus. You have to stop way down, to f16 or so, to get the leaves on the water and the branches to all be in focus. In other words, focus is determined by the distance the light travels from your camera- the depth of field you will need is from 5 feet to 45 feet.