There’s a game you can play at this lively exhibition, which is enjoying a stretch in Edinburgh after a run at London’s National Portrait Gallery. Of the 50 paintings on display — whittled down from 1,081 entries — which three would you choose as the best and would your selection match that of the judges? It’s unlikely they would. Not because they are weak pictures, but because the competition is so fierce.

Of the three, it’s easiest to see why Gregory Cumins’s third prizewinner is there. Technically, it’s the most original work in the room. At certain angles, you might not realise it is a portrait at all, just a large black square.

But stand back and you’ll see a face emerge from the gloom, a black-on-black study of a balding, middle-aged man, Richard Deacon, staring out, contemplative, perhaps a little troubled, and almost not there at all, as the light just — and only just — picks out his features from the inky background.

You could spend a long time figuring out how Cumins created this meticulous, ghostly memory. Even if he leaves you guessing, you won’t deny the sober mood his painting brings to the exuberance that surrounds it.

In second place came Saul Robertson’s The Universe, a full-length portrait of a young artist in a paint-spattered studio. His universe is demarcated by the radio, book and mug of coffee at his feet — and the picture postcard propped up on a chair behind — all suggestions of a distant world outside.

The Glasgow-born artist, who trained in Dundee, shows a mastery of colour and shade, but you can only guess why the judges thought this picture superior to other competent and arguably more inspired works here.

No doubt you could raise similar questions about the winner, Dean Marsh’s Giulietta Coates, although there is unquestionably something arresting about this vibrant study. This is one of those portraits whose eyes follow you around the room. If nothing else, you are in awe of Marsh’s tenacity in picking out each tiny metal ring in the scarf around his subject’s neck, a precision that’s reflected in his subtle command of skin tone.

So what aspects of the work would you reward? If it were creative ambition, you’d want to praise Cain McLoughlin’s self-portrait, a monochrome impression that layers on the oil paint almost sufficiently to be sculpture; Brendan Kelly’s My Brother (Paul), a large, bloated and distorted image full of wayward character; and Roy Eastland’s Person 2004/5, an exquisite echo of a portrait all but obliterated by scratched numerals and letters on the surface.

You might prefer to acclaim photographic realism. In this case, you’d go for the brilliantly executed work of Stephen Earl Rogers in The Oswestry Connection, three fashionable young men in relaxed conversation; Andrew Tift’s Daniel, every hair of a voluminous grey beard delineated; and Stefan Towler’s Arrival, highlighting every contour of a cotton hoodie.

More instinctively, you might opt for those that leap out at you: Domanda e Risposta, the striking double- headed image by Davide Castronovo; the sickly child in Annemarie Busschers’s Chicken Pox; and the soft tones and hard-edged profile of Craig Wylie’s gorgeous KR.

If your taste is none of the above, it’s only because you’ll have favourites of your own in this engaging collection. True, nothing strays far from convention — on the contrary, many pay deliberate and rather conservative homage to past styles — but there’s more than enough variety, skill and humanity to compensate.

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