its tripod when you press the shutter release – but the
benefit is somewhat reduced because you’re going to have
to touch your camera between shots, to adjust the exposure.
Actually, bracketed sets of photos can sometimes be taken
automatically with a single press of the shutter release button,
but this isn’t available on all cameras – and even when
available, it might not give you enough control for HDR work.
Click click click
The process involves fixing your camera on a tripod,
framing the scene, taking a photo at one exposure,
altering the exposure setting using your camera’s exposure
compensation feature, taking another photo, and continuing
until you’ve got all the exposures you need. You’ll need one
correctly exposed shot and, as a very minimum, shots that
are underexposed by one and two stops (-1EV, -2EV) and
overexposed by one and two stops (+1EV, +2EV). For some
scenes you’ll need to go further than -2EV and +2EV, but you’ll
learn from experience, and it’s always better to get more shots
than too few.
Try to work as quickly as possible to eliminate any problems
of clouds moving in the sky between shots – which,
depending on the method you use for combining the shots,
could cause problems. Don’t worry if you notice, when you
review your set of photos, that some are almost all black while
others are almost all white. While such shots would be
virtually useless as standalone photos, they could still contain
small areas that are correctly exposed and, therefore, will
makeusefulcontributionstothefinalHDRphoto.
Also,byallmeanscaptureasmanyexposuresasyouwant,
but bear in mind that, when you come to process them, you
may not be able to use them all. In particular, shots over-
exposed by more than about 3EV could well suffer from
effects that will manifest themselves either as haze in parts of
the image or even a prismatic rainbow appearance. The
eagle-eyed might just notice this effect in a couple of the
images presented here.
With a bit of time and effort it might be possible to minimise
these artefacts in post-processing, but it’s easier to avoid it
from the start. The effect is known as lens flare and is a fairly
common phenomenon when shooting into the light.
Overexposing a shot increases the chances of it being visible.
Using a lens hood is a good way of reducing flare.
Finally, we need to provide some guidance on how to
underexpose and overexpose your shots. Generally speaking,
there are two methods. You can set the camera to Aperture
Priority mode and pick an aperture, so that when you use your
camera’s exposure compensation feature it alters the shutter
speed. Or you can set the camera to Shutter Priority mode and
pick the shutter speed, so when you use your camera’s
exposure compensation feature it alters the aperture. Both
have their pros and cons for general use but, in most cases,
you’ll get better results for HDR work if you use Aperture
Priority. This is because the aperture affects the depth of field,
which is something you’ll want to be the same between
individual shots.
The manual method
A common experience is to take a photo which ends up
having an almost featureless white or pale grey sky, even
though it was a pleasing blue colour with contrasting white
clouds to your eye. This happens when the camera exposes
theshotsothattheforegroundiscorrect,withtheresult
that the sky is overexposed.Tostartourinvestigationof
how to process a setofphotostogenerateanHDRresult,
we’ll take exactly thiscaseasanexampleofhowto
increase dynamic rangemanually,usingjusta photo
manipulation package.
This isn’t the only scenarioinwhichthismethodis
suitable, but it’s probablythemostcommon.Theeffect
is different from thatachievablewithdedicatedHDR
software that we’ll lookatlater,buttheresultoften
appears more natural.We’regoingtouseGIMP(GNU
Image Manipulation Program,www.gimp.org),butyou’ll
be able to get the sameresultsusinganyphotoeditor
that supports layers andmasks.
First, you need to selectjusttwophotosfromyourset
of bracketed shots, oneinwhichtheforegroundis
perfectly exposed, andexactlyasyou’dlikeit toappearin
the final photo, and oneinwhichtheskyhasallthedetail
you want. Before openingtheminGIMP,it mightmake
things easier if you firstrenamethemwith‘foreground’
and ‘sky’ in their filenames.Nowopenbothofthephotos
in GIMP– they’llappearinseparatewindows.OurnextjobBy using GIMP to combinea photowiththeforegroundexposedcorrectly
and one with a properly exposedsky,theresultantimageincludesa dramatic
sunset and a fully visible foreground.Creatinga properHDRimagerequiresseveralexposures
but, if you only havetheone,youmightbeabletogo
some way to replicatingtheeffect.Thisis possiblewhen
some tonal differencesarepresentin areasthatlook
RVWHQVLEO\XQLIRUP7KH PRVW LQWXLWLYHZD\ RI ÀQGLQJRXW LI
this is the case in GIMPis toselectColors> Brightness-
Contrast and try adjustingtheBrightnessslidertotheleft.
If the result is thata uniformwhiteskysimplybecomesa
uniform grey, thenthereis nodetailin thesky,soit’s
game over. Conversely,movingtheBrightnessslidertothe
right will reveal if there’sanyhiddendetailin apparently
uniform dark areas.
If you discover hiddendetailin thelightanddarkareas,
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seen in the light areasandsavetheresultwitha different
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now have two photographsofthescene,onewithdetail
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now use these twoimagestogenerateanHDRimage
using GIMP.SINGLE-IMAGE HDR
“It’s common to take a
photo with a featureless
sky, even though it was a
pleasing blue to your eye”
April 2020 | |^35
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Photo editing in GIMP