Duffy makes use of contrast here, comparing a peaceful ceremony such as a mass to this man laying out photos of horrible, violent images of war
Shows how much care he takes when developing the images, he treats his job with respect
Could be parallelism between him as a war photographer seeing a lot of pain and death, and a priest seeing the same regularly
"Belfast. Beirut. Phnom Penh"
Duffy creates a list of places destroyed by violent wars, and this emphasises the sheer amount of violence taking place across the world, and the sheer variety of people affected
Creates a depressing mood as it could be used to show that suffering has no bias, anywhere can be affected by war
"which did not tremble then
though seem to now"
Contrast between the man's life before and after having this job for a long time (did not/then...though/now)
Shows the impact this work has had on him, he is physically shaking with anxiety because of all the horrors he has been subjected to
"Home again
to ordinary pain which simple weather can dispel"
More contrast between the life of the photographer and those whom he photographs
He is able to go home and be at peace, in a place where the troubles seem to superficial that even just a nice day can lift people's moods
'ordinary' is word choice which emphasises this contrast - war is the opposite of ordinary, normal, calm, etc.
"from which his editor will pick out five or six"
Contrast between the hundreds of photos he has taken over his career, and how only the smallest portion of these will ever be seen
Creates a sense of sadness of the many photos and people who will not be acknowledged or remembered
"at where
he earns his living and they do not care"
'they' in this line could refer to refer to either the people being photographed or the people seeing the photos
The people suffering in the war don't have time to worry about this man taking photos, they have much bigger problems in their life
Those who get to sit at home and peacefully browse the catalogue of photos don't care because they don't stop to acknowledge the real pain contained behind them
'do not care' is quite blunt/informal word choice to emphasise the harsh reality of ignorant people
"to do what someone must"
This line fits into the analogy of the photographer being like a priest, suggesting that this job is more like a vocation than a choice - the man continues his work despite it bringing him great pain because he feels he has a responsibility of justice toward these people
Shows him as a kind person as he treats his job with respect and sensitivity as a priest would