Putting the ‘post’ in pro­cess­ing

Bob New­man on know­ing the im­por­tance of pro­cess­ing as a key stage in the pho­to­graphic method

Amateur Photographer - - Tech Talk -

One of the ter­mi­no­log­i­cal slip-ups that I can get aer­ated about is mis­use of the term ‘post-pro­cess­ing’. This is a term bor­rowed from the film and video in­dus­try and, ac­cord­ing to Wikipedia, it ‘is the process of chang­ing the per­ceived qual­ity of a video on play­back (done af­ter the de­cod­ing process)’. I think that the clause in paren­the­ses is key – post-pro­cess­ing is clearly what hap­pens af­ter pro­cess­ing.

Why is this dis­tinc­tion im­por­tant? It is be­cause ‘pro­cess­ing’ is a key stage in the pho­to­graphic method, and learn­ing to get what you want from it is im­por­tant. The pro­cess­ing phase is when the la­tent im­age is trans­formed into a vis­i­ble im­age, and in­evitably in­volves the loss of some in­for­ma­tion that was con­tained in that la­tent im­age. In film days, you could vary the chem­i­cal com­po­si­tion of the de­vel­oper, its strength, tem­per­a­ture and the length of de­vel­op­ment, as well as many other sub­tle tech­niques that skilled dark­room tech­ni­cians mas­tered. Vary­ing th­ese many dif­fer­ent pa­ram­e­ters changed the out­come and dic­tated what you could achieve from that im­age.

Dig­i­tal has a great ad­van­tage in that the de­vel­op­ment process is non- de­struc­tive, so if you don’t like the out­come of what we would now call a ‘raw con­ver­sion’ you can try again, but the aim is the same, to use the de­vel­op­ment process to pro­vide for the range of out­comes that you want. Once you have dis­carded in­for­ma­tion dur­ing pro­cess­ing, be it shadow or high­light de­tail, no amount of post-pro­cess­ing will bring it back.

This is made more dif­fi­cult be­cause many dig­i­tal im­age ma­nip­u­la­tion tools blur the dif­fer­ences be­tween pro­cess­ing and post-pro­cess­ing. Mostly, the ad­vanced com­mer­cial tools do a bit of both. Ap­par­ent raw pro­cess­ing tools such as Light­room per­form some func­tions af­ter they have pro­cessed the im­age, while im­age ed­i­tors such as Pho­to­shop in­clude pro­cess­ing abil­ity. None­the­less, keep­ing the dis­tinc­tion clear does help the un­der­stand­ing that al­lows fine-tun­ing of tech­nique.

One of the oft- quoted ad­van­tages of cam­eras with a large dy­namic range is that they al­low the pho­tog­ra­pher to ‘lift the shad­ows in post’. Un­for­tu­nately, ‘lift­ing’ shad­ows dur­ing post­pro­cess­ing is likely to achieve poor re­sults un­less the im­age was pro­cessed so as to pre­serve the de­tail in those shad­ows in the first place. If it was pro­cessed ‘cor­rectly’ ac­cord­ing to the cho­sen ISO (of which more later) then it would be likely that most of the shadow in­for­ma­tion would have been dis­carded, and no longer be there to ‘lift’. More­over, the de­sired tone curve, that re­sulted in lighter shad­ows, could have been ap­plied in pro­cess­ing; so no post­pro­cess­ing would be re­quired.

ISO and pro­cess­ing

Es­sen­tially, the ISO con­trol does two things (it may also do other things, such as chang­ing volt­age gain some­where in the read chain, but that is a mat­ter of man­u­fac­turer im­ple­men­ta­tion, rather than the ISO stan­dard). It de­fines a tar­get ex­po­sure, set­ting what the me­ter de­fines as ‘cor­rect’ ex­po­sure, and it de­fines a pro­cess­ing regime that will re­sult in that ex­po­sure be­ing ren­dered with the light­ness re­quired by the ISO stan­dard. Thus, any use of ex­tended dy­namic range means a de­par­ture from the pro­cess­ing dic­tated by the ISO set­ting.

This im­age was un­der­ex­posed with re­spect to the ISO set­ting to avoid high­light clip­ping, then pro­cessed to the de­sired light­ness range. No ‘post-pro­cess­ing’ in­volved

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