Flash is the Only Option”
Images by John Bebbington FRPS
use flash in the first place?
I personally prefer to use available light for
close up photography with both film and digital cameras, especially for
flower photography (it's only my opinion, but to me something of the
subtle beauty of many flowers is lost when flash is used). Many
invertebrates will, if approached carefully, allow the photographer to
set up a tripod and long exposures are possible if there is no wind.
I used Kodachrome 25 almost exclusively until its
sad demise and often gave 2-3 minutes at f32. My longest successful
available light exposure is 11Ω minutes, of a Mottled Beauty moth on a
tree trunk, taken just before a violent thunderstorm. It is sharp and
the colours are true!
However when the subject is moving -
whether it is a mobile insect or a flower blowing in the wind - and
reasonable depth of field is required, available light may not be
enough (figure 1).
Electronic flash is a convenient way of providing
sufficient extra light to enable the use of smaller apertures and/or
shorter exposure times.
Flash can be used either as the only effective
light source or balanced with available light. This can be done with
the camera and flashguns set manually, or by using automatic (TTL or
non-TTL) flash metering. For manual and non-TTL auto work, a basic
knowledge of close-up flash technique is essential; for TTL flash this
basic knowledge is still very useful.
light output from flashguns
A flashgun on manual setting will provide a short
burst of light - usually of about 1/1000 second duration. A flashgun on
auto setting - whether it is TTL or non-TTL - will vary light output by
changing the duration of the flash, so very short effective exposures
Whatever the power and duration of the flash, the
light output attenuates with distance according to the inverse
square law. For example, an object 20cm from the flashgun
receives 1 unit of light; the same object 40cm from the flash only
receives º the amount of light, as it has spread over an area 4x as
large (figure 2); thus two stops more exposure is needed if flash is
the only light source.
The flash power for a gun on manual setting is
normally expressed as a guide number (GN). This
will be the maximum output for a totally automatic
GN = flash-to-subject distance (m) x f no. needed
for a correct mid-tone exposure at ISO 100.
Thus if f11 gives the correct exposure with the
flashgun 2m from the subject, the GN will be (2 x 11) = 22.
The guide number should be stated in the
manufacturer's literature but on older models can be calculated from
the dial or table on the flash by matching up distance and f number.
Most flashguns are calibrated by the manufacturer
in lab or studio conditions where there are white reflecting surfaces
close by and these will contribute to the amount of light reaching the
film or sensor - in field conditions the nearest white reflecting
surfaces my be clouds at several thousand metres so they are no help!
For manual flashguns a close-up guide number (CGN)
should be calculated. This is usually about 2/3 the manufacturer's GN
which means that 1 stop more exposure is needed.
For the example above f8 rather than f11 would
give the correct exposure at 2m so now the CGN is (2 x 8) = 16.
With non-TTL auto flashguns the flashgun sensor
reads reflected light and controls flash duration. With TTL auto
flashguns the camera itself meters exposure and controls flash duration.
lenses of different focal lengths
Longer focal length lenses have the disadvantage
of weight but are especially useful for insect photography because they
allow a greater working distance, with less chance of disturbing the
Where flash is the sole light source, there is
often a problem with dark or black backgrounds. The shorter the focal
length of the lens used, the greater the problem is likely to be.
For example, let's suppose we are photographing an
insect at half life-size with a background 10cm behind it. Using first
my 90mm then my 180mm macro lenses I made the measurements below:
With a 90 mm lens, flash-to-subject distance =
0.15m and flash-to-background distance = 0.25m. The background is about
1.67x as far away as the subject and is about 1Ω stops underexposed.
With a 180 mm lens, flash-to-subject distance =
0.5m and flash-to-background distance = 0.6m. The background is about
1.2x as far away as the subject and is only about Ω stop underexposed.
In any case, if the background is too far away, it
will be black and this will probably look unnatural, as in this image
of a parasitized Eyed hawkmoth larva, taken deliberately against a
distant background to show the effect.
the effect of macro lenses, extension tubes or bellows and
teleconverters on the amount of light reaching the film or digital
The lens iris diaphragm is effectively the light
source for the film or digital sensor. If this light source is moved
further from the film/sensor plane so the light intensity decreases
according to the inverse square law and either the aperture has to be
widened or the exposure time increased to give enough light.
With all the different macro lenses which I have
tried, 2 stops more exposure is needed when the lens is focused at
life-size (1:1) than at infinity.
Extension tubes or bellows
increase the distance between the lens and the film/sensor plane and so
reduce the amount of light reaching it.
Teleconverters reduce the
amount of light reaching the film/sensor plane by 1 stop (x1.4
converter) or 2 stops (x2 converter)
When setting up manual or non-TTL auto outfits it
is necessary to take these effects into account; with TTL metering the
camera will compensate but the flashgun(s) may not have enough power to
allow the use of small apertures.
manually controlled flashguns
Positioning and mounting the flashgun
Ideally the flashgun should be mounted on a
bracket with a tilt-and-swivel head, which means that it can be set at
the appropriate angle for the lens and magnification ratio being used.
Hot-shoe mounting is less satisfactory as the lens may prevent light
reaching the subject. The flash may be positioned directly above the
lens as in figure 4 or slightly to one side and 'toed in' as shown in
With the flashgun bracket-mounted as above the
simplest calculation is to work out the appropriate aperture (f no.).
Set up the kit on a table top and preset the magnification at 1:2 (half
life-size). Look through the lens and move a suitable object (say a
film box or similar) into focus. Measure the distance (d) (in metres)
from the flash tube to the object.
Work out the relative extension (RE):
RE = 1+ magnification ratio, or
1+ (length of extension tube or bellows/focal length of lens)
The correct aperture (f) for a mid-tone subject
can now be calculated:
aperture = close-up guide no/(flash-to-subject distance x relative
extension) or f = CGN/(d x RE).
Repeat the exercise with the magnification preset at 1:1. The two
calculated f numbers are starting points and it is now necessary to
calibrate the setup.
For film this should be done
outdoors with slide film because, when negative film is printed, the
processing lab will try to produce a decent print whatever the
exposure. Take careful measurements and notes and be logical
and methodical - then you will be able to repeat successful results!
Select a mid-tone subject and photograph it
firstly at 1:2 and then 1:1, starting with the aperture 2 stops wider
open than the calculated value: for example, if f16 was the calculated
value, start at f8. Stop down half a stop at a time until the aperture
is 2 stops smaller than the calculated value (here f32). Remember
to allow the flashgun to recharge fully between exposures and include a
label with the f number in the frame!
Repeat the process with a white textured subject
and a black textured subject . The processed slides should be compared
side-by-side on a light table and the 'best' results projected for
final choice. If the flash is not powerful enough, then it can be moved
closer; or if it is too powerful a diffuser can be fitted to it.
Positioning and mounting the flashguns
Again the flashguns should be mounted on
brackets with tilt-and-swivel heads. My preferred configuration is
shown in figures 6 & 7.
The 'main' flash is positioned above and to the left of the lens axis
and the 'subsidiary' flash close to and just below the lens axis.
These are slightly more complex than for single
flash work. It is best to mount the flashguns more or less as in
figures 8 & 9 and then choose the aperture (say f22) in order
to calculate flash-to-subject distances.
The main flash should give about twice as much
light as the subsidiary flash. This the subsidiary flash should be less
powerful - or, if the same model, a diffuser is needed.
For the main flash, flash-to-subject distance
dmain is calculated by
dmain = CGN/((f + Ω stop) x RE)
Here f + Ω stop = f19.
For the subsidiary flash, flash-to-subject distance dsubsid is
dsubsid = CGN/((f + 1Ω stops) x RE)
Here f + 1Ω stops = f13.
The end result is that the main flash is 1 stop
more powerful than the subsidiary flash, giving a 2:1 balance which is
an ideal starting point, giving soft shadows behind the subject.
Follow the same procedure as for a single flash,
keeping careful notes.
For manual flash, stop down about 1 stop for
white or very bright subjects and open up about 1 stop for black or
very dark subjects.
non-TTL auto flash with film SLRs
or twin flash
Positioning of flashgun(s) can be as for manual
flash. If the flashgun has a detachable sensor and sensor lead, this is
best mounted on the end of the lens; if not, make sure that the sensor
is not obscured and can read light reflected from the subject.
According to instruction books, most non-TTL
auto flashguns will not work closer than 1m - this is because the flash
duration will not be short enough at closer range and overexposure will
My first non-TTL auto flashgun was a National
PE-3057 and for a distance of 1m, f11 was given as the aperture. I
checked it out and all was OK. However I needed to work at 0.5m; so I
stopped down to f22 (remember the inverse square law - twice as close,
4x as much light - stop down 2 stops). It worked well and flash
duration was short enough to enable me to capture a wild specimen of
Broad-bordered Bee Hawk in flight (figure 8).
Calibration is necessary to be sure that the
sensor can work adequately. For non-TTL auto flash, open up about 1
stop for white or very bright subjects and stop down about 1 stop for
black or very dark subjects.
TTL flash with film SLRs
the sole light source
With my film SLRs I use twin TTL flash,
configured as in figures 5 & 6. Flash synchronisation speed is
1/125 second. When using ISO 100, the flash duration is short enough
(probably about 1/10,000 second) to enable me to photograph insects
such as Honeybees in flight at f22 (figure 9). The exposure is
effectively 1/125 sec at f22, which is so far below available light
exposure as to render ambient light irrelevant. There may be problems
with black backgrounds if they are too far away.
with available light
If a much slower shutter speed (say 1/15 sec) is
used then ambient light will affect the exposure and there will be less
problems with dark backgrounds. However unless the camera is tripod
mounted there may be ghost images resulting from camera movement during
exposure, as in figure 10.
old manual flashguns with digital SLRs
Most older manual flashguns have a very high
discharge voltage which will damage the electronics of a DSLR if they
are connected! However it is possible to use them on a DSLR without
All that is needed is a slave unit mounted in front of the DSLR's
pop-up flash and connected to the flashgun(s). A black hood is placed
over the pop-up/slave combination - this prevents the camera meter
'seeing' the light from it (figures 11 & 12).
The same calibrations as for film SLRs are used,
with the advantage that results are instantaneous. Shoot in
JPEG format for calibration, as most software will 'correct'
exposure when RAW files are opened.
TTL flash with digital SLRs
the sole light source
Follow the same procedure as for film SLRs
above. Figure 13 shows a typical result with dark background.
with available light
The same remarks apply as for film SLRs. Figure
14 shows a typical result with available light giving background
Trailing curtain slow sync
Sometimes the flash during a longish (e.g. 1/15
sec) exposure may disturb the subject and it will move before the
shutter closes. When using trailing curtain (aka second curtain) slow
sync the flashgun does not fire until the end of the exposure and
subject movement becomes irrelevant.
And finally - a checklist!
All this detail may seem a lot to digest but in the end it all becomes
instinctive and you can concentrate on the image in the viewfinder.
- Always keep careful notes when setting up and
calibrating flash kit. It is then easier to repeat your successes and
avoid repeating your failures!
- When you have set up a good piece of kit,
unless you can leave it permanently assembled, photograph it and put
the baseplate on a piece of paper and draw round it, showing where the
flashguns are angled
- Before starting to take images ensure that all
batteries are fully charged, all the connections properly made and all
camera settings are correct.
- Use manual focusing - autofocus probably won't
work with this type of subject. The easiest method is to preselect your
magnification ratio and move gently towards the subject, pressing the
shutter release at the appropriate moment.
- Remember that the welfare of the subject is
paramount - always observe the Nature
Photographers' code of practice
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