The „Composing“ originally comes from  „to compose“ in the musical way of meaning.
And that is exact waht we do here in the visual area. We compose a new picture.
A lot of photographers, especially the „Out of Cam“ people are very critical with this part of photography, saying that it is no real photo any more.
But if you go back to the times when photography did not yet exist, then just think of the following example: There were painteres who painted landscapes to show them to people who had not been there, so they could see them too. they painted realistic with all details. Then later came the  Impressionists, who wanted to show the influence of the surrounding on the people, then the Expressionists, who wanted to present the world solely from a subjective perspective, distorting it radically for emotional effect in order to evoke moods or ideas and the Surrealists, who painted unreal dreamlike surroundings that allowed the unconscious to express itself.
Wasn´t theat painting any more????
We have similar examples in literature. A journalist who tells what has happened. He uses a perhaps a polished language bit he keeps to the things that took place. A professional writer.
But what about the poets, the essay-writers, the phantasy authors? Are they not writers?
In my oppinion this is similar in the photography. We take photos of things and landscapes and build new images out of theis stuff. Things which do not exist or which are too difficult to take photos of.
For me it is a part of the photography which goes its own artistic ways.
I do not like every artform, which does not denies them their existence authority.
Advertising uses this part of the photographyto show products that not yet exist, or situations, which would be too expensive to take photos of.

I think it is the part of magic we can find in the composings. Ir looks so credible that it seems like reality.
J.R.R. Tolkien wrote an essay in 1939 called  „fairy-Stories“. Darin beschreibt er folgendes:
“What really happens, […] is that the story-maker proves a successful ‘sub-creator’. He makes a Secondary World which your mind can enter. Inside it, what he relates is “true”: it accords with the laws of that world. You therefore believe it, while you are, as it were, inside. The moment disbelief arises, the spell is broken; the magic, or rather art, has failed. You are then out in the Primary World again, looking at the little abortive Secondary World from outside.”
The sub-creation shows the dependancy of the creation on the real world, as the author has to fall back on known things to describe the unknown things. (https://de.wikipedia.org/wiki/On_Fairy-Stories).
That is the same with photocomposings. As sub-creators we have to  stick to the rules of the real world to make people believe in our work. As there are perspektive, shadows, light, iris-blur, all the natural laws have to be correct, otherwise it remains a unbelievalble absurdity.
This only works with a lot of practising. So even if we do not work „out of Cam“ we have to be good photographers. We have to know all about the things that appear in photography. For our creation we have to make the right photos with the needed perspective, the needed light and the right focus.
If I see an object that I might use for one of my composings, I always take photos from different angles. One never knows in what surrounding it will bei used, so I keep several views in stock.

In the world wide web there are thousands of tutorials concerning things you could need for composings. Pavel Kaplun and Miho Birimisa have made lots of helpful videos. Matthias Schwaighofer is well-known and he sells several CDs with tutorials. One nice tutorial I found is on on reflections in water from „Kulturbanause“ https://www.youtube.com/watch?v=jd9nr1EZssM
If a problem appears, just go to Google and put in "photocomposing" and the name of the problem and you will get several results to find tutorials, videos and information.
And look at pictures. That is one of the best exercises, look at photos, paintings and illustrations. Pavel told in one of his videos to go to a museum and have a look how the well-known painters build up their paintings. Try to copy that with photoshop just to practise.
At first it might be frustrating but with the time you will get better. Your reward will be the amazement of the viewers, a "Wow" at Instagram oder "Likes" in Facebook and, who knows perhaps some money of a customer.

Please have a look also at "Tipps and Tricks", there is more intersting stuff to be found.

There are lots of reasons to make photocomposings.

• Because we want to tell astory
• Because we want to illustrate something which does not exists in reality
• Because it is fun (this should be listed three times)
• Because it is a "clean" solution of paintng (there is no dripping colour out of computers)
• Because we want to invent new worlds
• Because a customer wants us to do it
• and even more......

A long time ago I made Airbrush illustrations. I was sick of dirty clothes, a dirty office and the frustration that came when you made a mistake when your work nearly was finished. And then came the first Mac with Photoshop. It was like a christmas miracle.