From: john@nmt.edu (John Shipman)
Subject: Flash projection for bird photography (long)
Organization: Zoological Data Processing
Date: Tue, 29 Sep 1992 01:34:46 GMT

1. THE PROBLEM WITH WARBLER PHOTOGRAPHY

In a previous posting, I talked about the style of bird
photography I prefer, emphasizing portable equipment and
stalking wild birds for action shots.  For many situations,
like shorebirds, raptors, and waterfowl, you will usually
have plenty of light on your subject.

But when I went into the woods to try to document rare
warblers and other denizens of deep woods and thickets, I
got frustrated in a hurry.  It's really dark where the
MacGillivray's Warblers and Ovenbirds prefer to lurk!

I bought a strobe to help in this situation, but my strobe
was not much good past about 10' or so with the film and
aperture I was using (Kodachrome 64 at f/5.6). I almost
never get *that* close to a wild bird.  Furthermore,
long telephoto lenses don't usually focus that close.

2. FLASH PROJECTION

In a book called "Field Photography," by Alfred Blaker
(published by Freeman), I learned about a technique called
flash projection.  This technique is great for deep-woods
photography, at the cost of a slight increase in the size
and weight of your rig.

The basic idea is to use a Fresnel-type lens to focus most
of a normal strobe's light into a relatively narrow beam.
The results were stunning: I found that I could shoot
Kodachrome 64 at f/5.6 out to almost FIFTY FEET, an
effective increase of roughly FIVE STOPS in exposure.

Assuming that you have some kind of strobe (almost any kind
will do), the first step is to get an optics catalog from
Edmund Scientific in Barrington, NJ (?---anybody have the
address handy?).  Edmund sells a variety of inexpensive
plastic Fresnel lenses.  For example, they sell a lens that
is 4" x 5" with a focal length of 8.5".  I found by
experimentation that placing this lens 9.5" (slightly more
than the focal length) in front of my strobe worked best
for me, and easily covered the field of view of a 300mm
lens.

3. POSITIONING THE FRESNEL LENS

One way to determine the optimum spacing between the strobe
and the Fresnel lens is through experimentation.  Mount on
your camera the *shortest* telephoto you are likely to use,
say a 400mm, and put this rig on a tripod.  At some
reasonable distance like 25-30' or so, put up a target
(such as a big piece of butcher paper) and mark clearly on
this piece of paper the rectangle where the field of view
of your telephoto falls.  Then switch to a shorter lens
(say, a 50mm) and take pictures of the target using the
strobe (mounted on the camera's hot shoe or whatever
bracket you like) with the Fresnel lens at various
distances in front of the strobe.

Start by placing the Fresnel at its focal length and try
moving it away by 1/4" increments until the spacing is 2-3"
beyond the focal length.  For each spacing you try, write
the distance on a card and attach the card to your target
before shooting that frame, so you can pick the best
distance by looking at the resulting film.

Develop the film and examine the patterns produced by each
flash-to-Fresnel spacing.  Pick a pattern that concentrates
as much of the strobe's light as possible in the target
rectangle, while still covering the rectangle completely
and evenly.  At some flash-to-Fresnel spacings you may see
a grid or bar pattern; avoid these.

Once you have found the optimal flash-to-Fresnel distance,
then design a bracket to hold the Fresnel lens in the
correct orientation relative to the strobe.  My friend Pete
LaTourrette made his out of two pieces of coat-hanger wire
bent into V shapes, with the points of the V attached to
the sides of the strobe and the arms of the V attached to
the corners of the Fresnel lens.  His design is light,
rugged, and compact.

4. CORRECT EXPOSURE

Getting the exposure right with this system is no different
than with any strobe.  Assuming that the available light is
negligible, the exposure depends on the film speed and on
the distance from the strobe to the subject.  For every
film speed, a given strobe has a GUIDE NUMBER, which is the
product of the aperture and the subject distance (in feet)
for optimum exposure.

To find the guide number, experiment by shooting a standard
target (an 18% gray card or a Macbeth Color Checker) at a
known distance and a wide range of apertures.  Pick the
best exposure and multiply the target distance by the
f-stop to compute the guide number for that film speed (EI
or exposure index, formerly called ASA number).

For example, suppose that for Kodachrome 64 at 30 feet the
best exposure is the one for f/8.  The guide number for all
EI 64 films would be 240.  You must figure out the guide
number separately for each film speed you use.

I made up some tables showing the aperture as a function of
distance and pasted them on my strobe.  Here is a table for
the preceding example:

   KODACHROME 64: GN240

   Distance  Aperture
   44'       f/5.6
   30'       f/8
   22'       f/11
   15'       f/16
   11'       f/22

The maximum distance for your exposure tables will be
determined by the aperture of the lens.  The minimum
distance is determined by the close-focusing limit of the
lens.

When shooting, estimate the subject distance and set the
aperture accordingly.  As always, bracket your exposures
for best results.  For example, if with the above table you
figure the bird is about 25' away, you should probably take
two exposures, one at f/11 and one at f/8 (assuming you get
a second chance).

If your strobe gives you the ability to deliver partial
flashes, you can also adjust the exposure by this method.
For example, I have a Vivitar strobe with an attachment
that allows you to dial partial flashes down to 1/64 of the
normal intensity.  This method has the advantage that your
flash will recycle faster after a partial flash.  However,
you may prefer to use a smaller aperture rather than a
partial flash to compensate for closer subject distances,
since this gives more depth of field.

5. SOME COMMON PROBLEMS

This technique yields the best results when the light from
your strobe is much brighter than the available light.
When the available light is significant, you will have
problems with "ghosting."  This produces slides with a
sharp image where the strobe went off and smears from the
bird's other motions when the shutter is open.

There are two ways to reduce ghosting problems.  Raw power
is one antidote.  Small, light "speedlight" strobes don't
put out very much light.  The big "potato masher" strobes,
in combination with flash projection, can really deliver a
lot more light than the ambient light.  I found, though,
that the in-between models, small enough to fit on a hot
shoe but brighter than speedlights, were a good compromise,
delivering a reasonable amount of light without seriously
compromising weight and bulk.

Another way to reduce ghosting is to use a camera body
whose flash sync speed is as short as possible.  A sync
speed of 1/200 second or better is much better than the
common 1/60 or 1/90 flash sync speed for this application.
The beauty of this solution is that it does not cost you
weight or bulk.

If the axis of the strobe/Fresnel system is close to the
axis of the camera/lens system, you will probably have some
problems with red-eye (reflection from the subject's eye).
If you offset the strobe axis some distance from the camera
axis, you may instead have parallax problems, as well as
making the whole rig bulkier.  Since my aim is more towards
documentation than art, I prefer to keep the strobe axis
close to the camera axis.  I don't mind a little red-eye,
since strobes render plumage colors so well.

If you find that pictures at short distances are not
illuminated along one edge of the frame, this is probably a
parallax effect.  Such problems can be minimized by two
techniques.  You can try to adjust the flash-to-Fresnel
distance so that the light covers an area well outside the
area seen by your lens.  You can also change the axis of
the flash system so that instead of being parallel to the
camera system's axis, it is angled slightly toward the
camera axis, so that there is full coverage at the middle
distance and parallax problems occur only at extremes of
range.

One question that comes up a lot: doesn't the strobe scare
the birds?  Sometimes, yes, it does, so it helps to assume
that you will get only one shot.  In my experience, though,
the majority of birds seem to ignore the strobe.  Perhaps
they think it's lightning.
--
John Shipman (john@nmt.edu)
-- 
John Shipman/Zoological Data Processing/Socorro, NM/john@jupiter.nmt.edu
``Let's go outside and commiserate with nature.''  --Dave Farber
Newsgroups: rec.birds