1. An Architect has hired you to photograph the facade of a new ten floor building he has just completed, constructed of concrete. There are five story buildings on the street opposite and he wants an accurate rendition of his work.
You want the 28mm shift lens here. The shift is for distortion correction and will prevent the building from looking like it is falling over backwards
2. An architect has hired you to photograph the facade of a new twenty floor building he has just completed. The design is very modern and he is looking for a creative photo to enhance his design. There are ten story buildings on the street opposite. The building is made of stainless steel and glass.
Go wide with the 20mm. You can use the ditortion of the lens for some creative control and caputure some reflections in the glass. May be fluffy clouds on a clear day.
3. A magazine has hired you to shoot a cover shot of a model's head. There will be hair and makeup artists and they want an outdoor shot on an overcast day.
Probably the 135mm (although you could get away with the 85 or 100) It will allow you frame tightly on the models head and have a nice silky out of focus background. You may want a reflector to bounce some light into the models face or an off camera flash/studio light to compensate for the lack of light on an overcast day.
4. A magazine wants you to photograph a fashion story on active sportswear using male and female models. They are looking for high energy shots with models interacting physically with each other.
Something fast that will allow you to freeze action but capute all of the scene. Either the 50 or the 85 (or maybe 35 if you want to get in closer)
5. An advertising agency wants you to photograph models with long legs for a panty hose ad. They want the look to be arty and the length of the legs exaggerated. The girls are to be photographed standing and walking.
You could use the 20mm or 35mm. This will allow you to get in close to...