Books are not movies; books are books, and therefore it is not as easy to put an image into a reader's mind. How do writers combat this?

Think of all your senses: touch, taste, smell, hearing and sight.

To help me describe scenes in my novel, sometimes I practise describing things that are not in my novel, but in my immediate vacinity.

"I grasp the pen, and find it is very cool to the touch. The white imprint near the end reads 'Faber Castell', and is accompanied by a tiny image of two jousting knights. As I roll the pen between my fingers, I notice that an immovable beam of light expands along the length of it. Six rows of tiny bumps line the plastic; a simple, yet unique design choice. I pull at the lid, and hear a hollow sound as it lifts off. It is a soft sound. A pleasing sound."

Did you notice I didn't mention that the pen is black? Or that it is a felt-tip pen? Or that it is a rounded-triangular shape? When readers aren't given every detail, they either consciously or sub-consciously make up the rest of the image in their mind. It is not necessary to give every single detail, but you must decide which details are worth keeping.

When you are writing a scene, either:
1) Make sure it gives the reader a clear image of the scene so they know where the characters are, or
2) Give the most important details that are relevant to the plot, background, setting etc.

As I said before, see how many of your senses you can use to describe. So, if I want to make a point about the architecture in the story, you could mention the cathedral. Talk about the colour of the building, but also talk about the cracks where it has aged, and the places where it has been spoilt by bird-droppings. Mention the nest at the corner of the roof. Is the roof flat? Triangular? Domed? You can hear the birds screeching, and their chicks tweeting. Where is the sound of that bell coming from?

That's if you want a clear picure of the scene. Now, what if you want to describe the things that are important to the plot here? The architecture and the birds become less important. Perhaps the most important thing here is the bell. Once the bell tolls, our main character knows that demons will rise from below and he will have to fight for his life. At this point, the character doesn't really care about the bird's nest on the roof, and neither should the reader. The reader should be in the moment with the character, worrying about the impending event. Descibe the character's heart racing.

Books are not movies; books are books. Thus, it is important that we choose exactly how we are going to describe a scene, because it will have a great impact on how the reader reacts to it.