Un­der­stand­ing im­age res­o­lu­tion

Hans We­ich­sel­baum dis­cusses the var­i­ous im­age-res­o­lu­tion op­tions and di­men­sions, and clar­i­fies which op­tions to use for a va­ri­ety of im­age-use sit­u­a­tions

The Shed - - Tutorial - Hans We­ich­sel­baum

The topic may sound triv­ial, but you’d be sur­prised at how many pro­fes­sion­als, pho­tog­ra­phers, desk­top pub­lish­ers, and print­ers are con­fused by the con­cept of im­age size and res­o­lu­tion. No mat­ter what you’ve whipped up in Adobe Photoshop, sooner or later you’ll need to ei­ther up­size the file for a su­per-large print or scale it down for the web — and that’s when the pix­els and inches, the ppi, dpi, and lpi, will come back to haunt you.

Size verses res­o­lu­tion; mon­i­tor verses prin­ter

The im­age size is de­ter­mined by noth­ing else other than its pixel di­men­sions. A 12-megapixel cam­era will give you 4000x3000 pix­els to work with. If the im­age is go­ing to be dis­played elec­tron­i­cally (mon­i­tor, web­site, pro­jected on a screen), you only need to look at the pixel di­men­sions; there’s no need to worry about the im­age res­o­lu­tion. The res­o­lu­tion of a mon­i­tor or pro­jec­tor is de­ter­mined by the hard­ware. For ex­am­ple, I am look­ing at a screen 52x32.5cm that dis­plays 1920x1200 pix­els. This works out to be 36.9 pix­els per cen­time­tre or 93.8 pix­els per inch (ppi). If an im­age is dis­played on my mon­i­tor at full screen, and it has fewer pix­els than my mon­i­tor’s na­tive 1920x1200 pix­els, it’s go­ing to be up-sam­pled by the dis­play driver. If the file has more pix­els, it will be down-sam­pled. That’s all done by the dis­play driver, and there is no need to worry about im­age res­o­lu­tion — though you might no­tice the poor qual­ity of an up-sam­pled im­age.

Al­ter­na­tively, if the im­age is head­ing to a prin­ter, the im­age res­o­lu­tion be­comes im­por­tant, and you need to look at the ppi set­tings. Your photo is made up of im­age pix­els — tiny blocks of colour that have no pre­de­ter­mined size. ‘Res­o­lu­tion’ is the num­ber that de­ter­mines how many pix­els get packed into a given space, which, in turn, con­trols how big or small those pix­els are. Com­monly, res­o­lu­tion is re­ferred to as ‘dpi’, which stands for dots per inch. This term isn’t strictly cor­rect — it should be pix­els per inch, but it means the same thing.

The Im­age Size di­a­logue box

If the mon­i­tor doesn’t show you the true res­o­lu­tion of an im­age, what can you trust? Well, you can rely on the Im­age Size di­a­logue box. In Photoshop, you’ll find it un­der Im­age > Im­age Size (see Im­age 1). It doesn’t just show you the size and res­o­lu­tion — from this screen, you can also con­trol both of them.

A lot has changed since the early ver­sions of Photoshop. The new im­age pre­view shows you the re­sults of the op­tions you en­ter be­fore you com­mit to them. Noth­ing hap­pens to the file size if you leave the Re­sam­ple box unchecked. You can change the di­men­sions (in cen­time­tres or inches) or the res­o­lu­tion (in ppi or pix­els per cen­time­tre), and the over­all file size will not change — you can’t do any dam­age to your im­age.

How­ever, if you tick that Re­sam­ple box, the pro­gram gets ready to change the res­o­lu­tion and the over­all pixel di­men­sion of your im­age. In other words, you can up-sam­ple your im­age if you need a big­ger file for a large print or down-sam­ple to get a small web im­age. The chal­lenge is to man­age this with­out send­ing the im­age qual­ity down the tube.

Re­mem­ber that ‘up-sam­pling’ means that your edit­ing pro­gram adds new pix­els that weren’t orig­i­nally there. This is called ‘in­ter­po­la­tion’ and is done by guess­ing the colours of the new pix­els by look­ing at ex­ist­ing ones. Need­less to say, this won’t give you any im­age de­tail that wasn’t there to start with.

Photoshop gives you a whole pile of op­tions in the drop-down menu next to the Re­sam­ple check box. In the early Photoshop days, there were just three choices: Bicu­bic (the one to use for pho­to­graphs), Near­est Neigh­bour (for line art with hard edges), and Bi­lin­ear (which is re­ally use­less). To­day, Adobe makes it a lot eas­ier by help­ing you to se­lect the right al­go­rithm for en­large­ment or for re­duc­tion (see Im­age 1).

Note that there are two op­tions for en­large­ment — in my next ar­ti­cle we’ll com­pare the two, plus some third-party spe­cial­ized up­siz­ing soft­ware. In short, the Pre­serve De­tails (en­large­ment) op­tion sharp­ens ar­eas of fine de­tail, which tend to get softer by up-sam­pling. The prob­lem here is that im­age noise also gets en­hanced, and, there­fore, you’ll get a Noise slider when you choose this op­tion. The other al­ter­na­tive, Bicu­bic Smoother (en­large­ment) works sim­i­larly to the old Bicu­bic set­ting but blurs the in­ter­po­lated pix­els slightly to get a smoother and more nat­u­ral tran­si­tion. Bicu­bic Sharper (re­duc­tion) will give you bet­ter re­sults when you need to down-sam­ple your im­ages to make them smaller. Of course, you can for­get about all those sub­tleties and sim­ply choose Au­to­matic to let Photoshop pick the best method, whether you’re mak­ing your im­age larger or smaller. In­ter­est­ingly, the Crop tool and the Free Trans­form com­mand use this al­go­rithm as well.

Need­less to say, the Photoshop guru will pre­fer to use the stan­dard Bicu­bic set­ting, and ap­ply his or her own sharp­en­ing/soft­en­ing/nois­ere­duc­tion magic set­ting for the per­fect out­come.

It’s im­por­tant to re­mem­ber that you pro­tect the im­age’s qual­ity when the Re­sam­ple op­tion is turned off. If you plan to print the im­age, you can fid­dle with the res­o­lu­tion with­out do­ing any harm to the im­age. You’re just chang­ing the pixel size not the num­ber of pix­els. If the Re­sam­ple check box is ticked, this means that you are chang­ing the num­ber of pix­els, ei­ther forc­ing the pro­gram to in­ter­po­late new pix­els or to throw pix­els away.

But there’s more: the lat­est Photoshop ver­sion also sports a Fit To drop­down menu (as seen in Im­age 2). Here, you get a handy list of com­monly used sizes, and you can add your own pre­sets too. Note that the given print set­tings all come in 300dpi — I’ll ex­plain more about that later. Click­ing the gear icon in the di­a­logue box’s top-right cor­ner dis­plays just one menu item: Scale Styles. When se­lected, this op­tion tells Photoshop to re­size any layer styles as well. This is im­por­tant, be­cause oth­er­wise that pretty drop shadow that you added might end up big­ger or smaller than your re­sized im­age.

Re­siz­ing im­ages for print

Com­mer­cial print­ers rou­tinely ask for 300dpi files (re­mem­ber: this should ac­tu­ally read ‘ppi’). This fig­ure goes back to the res­o­lu­tion of the first LaserWrit­ers in the mid 1980s, and 300dpi has be­come some­thing like a magic num­ber for print­ing (like the 72dpi for mon­i­tor dis­play). Strictly speak­ing, laser and off­set print­ers work with lines per inch (lpi), and that’s the unit we should use in­stead of ppi.

In off­set print­ing, the print res­o­lu­tion is de­ter­mined by the halftone screen fre­quency. The file res­o­lu­tion should be be­tween 1.2x and 2x the screen fre­quency. News­pa­pers are printed with a coarse 80-line screen. A 2x fac­tor will give us a file res­o­lu­tion of 160lpi. Glossy mag­a­zines typ­i­cally use a 133-to-150–line screen, and high-qual­ity art is printed with a 200-line screen. At this high level of screen fre­quen­cies, a fac­tor of 1.5 is con­sid­ered suf­fi­cient. This gives us a max­i­mum file res­o­lu­tion of 225 for glossy mag­a­zines and 300lpi for the best fine-art print.

The Fit To drop-down menu (seen in Im­age 2) gives you an Auto Res­o­lu­tion op­tion (see Im­age 3). If you know the lpi of the prin­ter, you can en­ter it in the Screen field, and Photoshop cal­cu­lates the res­o­lu­tion (in ppi) for you. You’ve got the choice of three qual­ity set­tings: Draft gives you

a res­o­lu­tion of 72ppi, Good mul­ti­plies the lpi by 1.5, and Best uses a fac­tor of 2

hen you send your files to a pro­fes­sional prin­ter, it’s al­ways a good idea to ask what res­o­lu­tion they want and what colour pro­file they pre­fer. If they don’t know what you’re talk­ing about, find an­other prin­ter — fast!

To­day’s high-res­o­lu­tion inkjet print­ers work by putting 1440 to 5760 drops of ink per inch onto pa­per. Now, here we are talk­ing dots or drops per inch, so dpi is the cor­rect unit. Ev­ery pixel of your file needs to be dithered with many ink drops to sim­u­late thou­sands of colours. You cer­tainly don’t need to (and you shouldn’t) feed your prin­ter with 1440 to 5760ppi files.

First, a print res­o­lu­tion of 1440dpi (dots or drops of ink per inch) is plenty for matte and fine-art pa­per, and you will be hard-pressed to tell the dif­fer­ence be­tween 1440 and 2880dpi, even on the best photo-gloss pa­per. Se­cond, the res­o­lu­tion of the file should be an in­te­gral di­vi­sor of the prin­ter res­o­lu­tion. Most ex­perts agree that a file res­o­lu­tion of 240ppi is suf­fi­cient for pho­to­graphic im­ages, which hap­pens to be 1/ of 1440. If you are a

6 per­fec­tion­ist, I sug­gest you make a se­ries of prints of one of your su­per­sharp pho­tos. Re­sam­ple the file (prefer­ably down-sam­ple) to res­o­lu­tions of 180, 240, 300, 360, and per­haps 480ppi, and print to iden­ti­cal phys­i­cal size. Use the high­est-qual­ity set­ting on your prin­ter. You will prob­a­bly need a loupe to dis­tin­guish the 240ppi print from any­thing higher.

Due to the view­ing dis­tances, poster prints can be done at fairly low file res­o­lu­tions, typ­i­cally 150ppi down to 75ppi. If the file res­o­lu­tion is too low, we will start to see in­di­vid­ual pix­els, most ev­i­dent in the ‘jag­gies’ on di­ag­o­nal sharp lines. If you print a generic-size im­age (for ex­am­ple, 5x7 or 8x10 inches) straight from Photoshop, you can turn on Scale to Fit Me­dia in the Print di­a­logue box (see Im­age 4). The pro­gram will cal­cu­late the res­o­lu­tion for you, but you won’t have any con­trol over how the im­age is be­ing cropped to fit the page.

An­other clever way of re­siz­ing im­ages is to use Rus­sel Brown’s Im­age Pro­ces­sor, which can be found via File > Scripts > Im­age Pro­ces­sor (see Im­age 5).

This script comes in par­tic­u­larly handy when you want to re­size a whole folder full of im­ages. It also al­lows you to re­size land­scape and por­trait shots to a max­i­mum di­men­sion by en­ter­ing the same num­ber for both height and width (don’t worry, you won’t get square im­ages).

A list of ways to re­size im­ages would not be com­plete with­out men­tion­ing the Save for Web com­mand. It gives you su­perb con­trol when down­siz­ing im­ages for the web. We’ll dis­cuss it in an­other in­stal­ment when we fo­cus on web im­ages.

In the next is­sue, we’ll look at var­i­ous ways of up-sam­pling an im­age us­ing Photoshop’s op­tions and com­pare the im­age qual­ity we ob­tain with that ob­tained us­ing some third-party pro­grams.

Im­age 2 — the Fit To op­tions of the Im­age Size di­a­logue box

Im­age 1 — Im­age Size di­a­logue box in Photoshop CC 2015

Im­age 4 — scal­ing print size in the Print Setup in­ter­face

Im­age 5 — the Im­age Pro­ces­sor script

Im­age 3 — the Auto Res­o­lu­tion op­tion for off­set print­ing

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