SE­LEC­TIVE SHARP­EN­ING

HANS WEICHSELBAUM RUNS US THROUGH THE VAR­I­OUS ADAP­TA­TIONS POS­SI­BLE BY UTI­LIZ­ING THE HIGH PASS FIL­TER

The Photographer's Mail - - Tutorial - HANS WEICHSELBAUM

Even af­ter many years of work­ing with Adobe Pho­to­shop, you’ll stum­ble across tools and fil­ters you’ve never seen or heard of be­fore. Many of these are spe­cial­ized gadgets that you’ll rarely use, but there are also use­ful fil­ters — some of which you may not even know that you need.

The High Pass fil­ter is one of the more fa­mil­iar in the group of ob­scure fil­ters and is widely used to mit­i­gate the soft­en­ing ef­fects of the cam­era’s an­tialias­ing fil­ter. While not tech­ni­cally a sharp­en­ing fil­ter, it can pro­duce re­sults that are as good as, or even bet­ter than, Pho­to­shop’s ac­tual sharp­en­ing fil­ters — while, at the same time, be­ing just as easy to use. But, there’s more! As con­tra­dic­tory as it sounds, you can also use this fil­ter to soften por­traits and to en­hance mid-tone con­trast, too.

The High Pass fil­ter is not the type of fil­ter that you sim­ply ap­ply to your images like most other fil­ters. To see what it does, open an im­age and run it through the fil­ter (Fil­ter > Other > High Pass). You’ll be greeted with an im­age that has a sil­ver-grey em­boss over it. In most cases, the High Pass fil­ter is not ap­plied di­rectly on the im­age but in­stead to a du­pli­cate layer sit­ting on top of the orig­i­nal. Then, the blend­ing mode is changed to one mem­ber of the Over­lay fam­ily of blend­ing modes (Over­lay, Soft Light or Hard Light). The ef­fect you get will de­pend on your com­bi­na­tion of Ra­dius set­ting, blend­ing mode, and the top layer’s opac­ity set­ting.

HIGH PASS SHARP­EN­ING

Man­ual sharp­en­ing is an es­sen­tial part of pro­cess­ing any im­age, though it dif­fers greatly ac­cord­ing to the im­age. Images with lots of fine de­tail will need a dif­fer­ent kind of sharp­en­ing than will images with no sharp edges (think of the soft, moody qual­ity of a foggy land­scape, as op­posed to the strik­ing lines of ur­ban ar­chi­tec­ture, for in­stance). Some­times, you need se­lec­tive sharp­en­ing — for ex­am­ple, a por­trait should have sharp eyes, but you prob­a­bly won’t want to high­light wrin­kles and ev­ery skin im­per­fec­tion. Again, the op­ti­mum set­tings will de­pend on the size of the im­age and its fi­nal out­put — images for mon­i­tor dis­play de­mand a very dif­fer­ent amount of sharp­en­ing than do large prints. Sharp­en­ing, there­fore, should al­ways be the last step in the edit­ing process.

Ideally, a small amount of sharp­en­ing should be ap­plied dur­ing RAW con­ver­sion to com­pen­sate for the ef­fect of the Low Pass fil­ter, also called the ‘an­tialias­ing’ (AA) fil­ter.

The stan­dard tool for sharp­en­ing was (and still is) the Un­sharp Mask fil­ter, while High Pass sharp­en­ing is some­thing to­tally dif­fer­ent. This fil­ter high­lights edge de­tail and has a lot in com­mon with the Find Edges fil­ter (Fil­ter > Styl­ize > Find Edges).

High Pass sharp­en­ing is par­tic­u­larly help­ful if an im­age doesn’t con­tain well-de­fined sharp edges that the Un­sharp Mask fil­ter can latch onto. An­i­mal pho­tog­ra­phy can of­ten fall into this cat­e­gory, es­pe­cially shots taken un­der­wa­ter. Com­pare Im­age 2 with the High Pass sharp­ened ver­sion (Im­age 3).

The Un­sharp Mask fil­ter would have re­quired a very high Ra­dius set­ting to pro­duce a sim­i­lar sharp­en­ing ef­fect; the re­sult show­ing very no­tice­able ha­los around the edges.

Sharpen us­ing the High Pass fil­ter by mak­ing a du­pli­cate of the cur­rent im­age layer, then ap­ply­ing the High Pass fil­ter to the du­pli­cated top layer (Fil­ter > Other > High Pass). I used a Ra­dius set­ting of 10 for this par­tic­u­lar im­age (Im­age 4).

You can see that all the flat ar­eas come up as mid

grey, whereas the edges are ac­cen­tu­ated. Now, let’s think about blend­ing modes. Re­mem­ber that the Screen blend­ing mode light­ens the lighter ar­eas, while Mul­ti­ply dark­ens the darker ar­eas of the un­der­ly­ing im­age. Over­lay is a com­bi­na­tion of both; there­fore, it light­ens light ar­eas and dark­ens darker ar­eas. Mid-grey re­gions are not af­fected. If you change the blend­ing mode of the up­per layer to Over­lay you will get the ef­fect shown in Im­age 3. If the re­sult is too strong, you can re­duce the opac­ity of this layer. An­other way of chang­ing the amount of the sharp­en­ing is through chang­ing the blend­ing mode to Soft Light for a softer ef­fect or to Hard Light for a more pro­nounced re­sult.

A fur­ther vari­able is the Ra­dius set­ting of the High Pass fil­ter. For proper sharp­en­ing, it should be in the range of 0.5 to 3.0. As men­tioned, I used a set­ting of 10 pix­els for this im­age, which is not re­ally a text­book sharp­en­ing but, rather, an in­crease in mid-tone con­trast. A good way of see­ing the ef­fect of the Ra­dius set­ting is to change the blend­ing mode of the du­pli­cate top layer to Over­lay be­fore ap­ply­ing the High Pass fil­ter. This will al­low you to see ex­actly what the Ra­dius set­ting will do to the sharp­en­ing process.

The next im­age (Im­age 4) has lots of fine de­tail. Here I used a small Ra­dius set­ting of around 1. The re­sults are sim­i­lar but not quite the same as with the Un­sharp Mask fil­ter. In the case of highly coloured images, it is also a good idea to set the sat­u­ra­tion of the top layer to zero (Im­age > Ad­just­ments > Hue Sat­u­ra­tion). This elim­i­nates any colour fring­ing around the sharp­ened zones.

SOFT­EN­ING AN IM­AGE

The High Pass fil­ter can also be used for soft­en­ing an im­age and is es­pe­cially ef­fec­tive for skin tones. The ef­fect is sim­i­lar to the old dark­room tech­nique of hold­ing a dif­fu­sion fil­ter un­der the en­larger dur­ing the ex­po­sure.

Again, start by du­pli­cat­ing the back­ground layer. Run the High Pass fil­ter over the top (du­pli­cate) layer, but set the Ra­dius to a fairly high value, around 15. Next, go to Im­age > Ad­just­ments > In­vert to get a neg­a­tive of the top layer, then change the blend­ing mode to Soft Light. This will cre­ate a beau­ti­ful, ethe­real soft-fo­cus ef­fect.

You can stop right there, but you might want to keep parts of the im­age sharp, such as the eyes and the mouth in a por­trait. Im­age 5 was soft­ened with the High Pass fil­ter at a fairly high Ra­dius set­ting of 20, with the blend­ing mode set to Lin­ear Light and the Opac­ity re­duced to 30 per cent.

Here is an ex­am­ple of a por­trait — the orig­i­nal (Im­age 6) com­pared with the soft­ened ver­sion (Im­age 7). The High Pass fil­ter with a large Ra­dius of 30 was used, then the layer was in­verted with a Soft Light blend set­ting, and set to 88-per-cent opac­ity.

The sharp eyes and lips and part in the hair were re­cov­ered from the un­der­ly­ing orig­i­nal with a layer mask. To do this, sim­ply paint with a black brush onto the white layer mask. For more con­trol over the ef­fect, use a soft-edged brush and re­duce the brush opac­ity. Here, for in­stance, I brushed a few times over the eyes to get the full orig­i­nal sharp­ness back, while the hair was only painted over once or twice in some places.

IN­CREAS­ING MID-TONE CON­TRAST

This is a third way of us­ing the High Pass fil­ter and is very sim­i­lar to the sharp­en­ing tech­nique, but it uses a fairly large Ra­dius of more than 10. This now in­creases the con­trast not along the edges but be­tween larger ar­eas. (See Images 8 and 9.)

If there is no dis­tinct dif­fer­ence when us­ing the High Pass fil­ter at a medium or a large set­ting, it’s best to ap­ply the blend­ing mode first, be­fore the High Pass fil­ter. Then, you can fol­low the ef­fect di­rectly by chang­ing the Ra­dius set­ting.

The in­crease in mid-tone con­trast can also be dra­mat­i­cally in­creased by us­ing the Hard Light blend­ing mode in­stead of Over­lay. In con­trast, the Soft Light blend­ing mode will give you a more sub­dued ef­fect.

Im­age 4 — The High Pass fil­ter

Im­age 3 — Af­ter High Pass sharp­en­ing (Ra­dius 10)

Im­age 2 — Be­fore High Pass sharp­en­ing

Im­age 9 — High Pass fil­ter with Ra­dius 40

Im­age 8 — Land­scape by An­dris Apse prior to us­ing High Pass fil­ter

Im­age 5 — Im­age soft­en­ing

Im­age 4 — High Pass sharp­en­ing with small Ra­dius

Im­age 6 — Orig­i­nal por­trait

Im­age 7 — Soft­ened por­trait

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