(Photographer unknown. If you know who took this get in touch so we can give credit where it's due!)
This photograph of two men standing at the bottom of a very large ship in a dry dock is a really stunning example of scale in photography. Using scale in photography is very important because as humans our minds figure out the size of our surrounding world in various ways. Of course your brain knows the size of a human and in this case that means that is one HUGE ship!
Scale gives the person looking at your photograph a frame of reference. Scale is often an under-utilized technique in photography. In fact many beginners in photography will even go so far as to remove people from a landscape photo thinking people in the image detract from the view. When in fact having people in your landscape photos can often be great! For example look at how much better this image is because there is a person in the foreground:
photo by steved
Be aware your brain can and will play tricks on you:
In fact this caused problems for astronauts on the moon! Here on Earth, distant objects are obscured by the atmosphere, and we use that to mentally estimate distances. That makes things a little tricky on the moon because there is no air! An object can be very far away on the Moon and still razor sharp to the eye. You can't tell if a boulder is a meter across and 50 meters away, or 10 meters across and 10 miles away!
To give you an example this watch the rock the start of this video. Try to guess the size of the rock at the start of the video. Then watch until the end and see if your original guess was indeed correct.
Here is another fantastic example of scale in photography:
Scale is a great technique to improve your images and shouldn't be overlooked. You can even have fun with it like this. As humans we need scaled to know how large and small the surround world is. In photography it helps us to enjoy the image instead of adding mental work to figure out the size of various elements.
Here is another video about scale (and polarizing filters)
Now go forth and take some great photographs with scale!
Here is a video that uses scale nicely: