Originally posted by Papa_Joe But why do all the people tell you, you need more focal length for shooting portrait?
Working distance.
Subject isolation using background blur is a happy side-effect, but not the main reason. If the lens is close to the model, the laws of perspective dictate an unflattering exaggeration of facial features. Want a big nose, ear, mouth, and/or chin? Shoot close! If a group setting, the nearer subjects will appear much larger than those next to them, depending on how the group is arranged. This effect may be worked to advantage if one wishes to place emphasis on an individual, but is usually not desirable.
Of course, the other advantage to greater working space is the comfort of the subject. A head shot with a 24mm lens on 24x36mm FF calls for the subject to be 40 - 50 cm from the lens. A professional model might not mind, but most of the rest of us prefer not to feel the photographer's breath on our face while sitting for a portrait.
Originally posted by Papa_Joe Larger format = less DOF.
This is so very true, despite being endlessly disputed in online discussions. A few minutes shooting with a medium or large format camera is usually all it takes bring home why there are no "fast" medium format lenses.
Steve