Equipments for Visual Effects Photography
The tools used in Visual Effects photography are much the same as those
employed for the main action camerawork; it is what they are used for that is different.
Although an important part of modern film language, and one which
does not unduly tax normal production, a moving camera adds significantly to
the complexity and expense of effects sequences.
The photographer’s main resource is the lens and this is no less the case in
VFX. Very careful thought must be given to the choice of lens, not just for its
conventional variables such as f/stop and speed (maximum aperture) but also
to its quality and the effects of the choice further down the line.
Lenses are analogue devices and none of them are perfect.They suffer from
a number of distortions and, in particular, the high-quality short-run hand-built
lenses crafted for movie cameras are very inconsistent.
All lenses suffer to some degree from the four principal defects of: chromatic
distortion, shading, flare and geometric distortion. In normal photography these
are usually not a problem and will not be noticed by the casual observer.However,
in effects work, images are often combined, making comparison possible and
perhaps showing up the differences between lenses.
Chromatic aberrations in most lenses are kept to some specified minimum
which will not be intrusive to anyone viewing the image created, however in effects
the criteria have to be much tighter. Imagine for instance a lens which focuses light
of different colours in slightly different planes. Let us say that our lens causes soft
red and blue edges on sharply defined subjects.
For example the subject is an
actor standing against a blue screen. On one side there will be blue on the inner
edge and on the other side there will be red on the outer edge.
The result of this is that the key detection will ‘eat’ into the subject on one
side and will see imperfect blue down the other. The outcome will be that extra
work has to be done to make the blue screen elements composite properly and
indeed may culminate in an imperfect result. This is just one example, but it
shows how lenses of a much tighter specification are required for effect work.
This means careful selection and testing are essential.
Shading can create the same sort of problem with blue, green or any colour
of chroma matting screen, since it will cause the colour to be uneven across the
background. This can be corrected, but with a cost in time and money. Where a
tonal matte is being used shading will create even worse problems.
The biggest headache of all is where the components of a shot are being
made on different lenses with different degrees of shading. Let’s imagine two
elements being combined, where one is shot with a lens which has shading,
and the other with a lens which is perfect.When you look at a normal shot made
with a lens which has shading and a character walks across it, he will get darker
as he moves into the shaded area – but this will not be noticed because the
background in that part of frame is itself equally darker. However, if the shot
described is created from two images, one with shading which has the actor
walk across it, and a background shot with a distortion-free lens, then he will
appear to get darker for no apparent reason. An even worse scenario is where
a split screen is being made between two shots with these different lenses – the
matte line will become visible.
And this does not just apply to ‘real’ lenses; in computer-generated backgrounds
the virtual lenses are perfect and so a shaded image will show up there
too. To deal with this an evenly lit white card should be shot for use by digital
post to create an identically shaded ‘virtual’ lens for the CG image.
Flare is where light bounces around inside the lens and so produces lighter
patches – effectively the converse of shading where dark areas are created. All
of the same problems occur but additionally flares are often coloured so they
can create patches of unwanted colour such as blue in the middle of the subject
matter in a blue screen shot. Flare can also cause loss of contrast and detail
which again can draw attention to added elements. On the other hand, a romanticized
idea of flare is one of the most popular effects to be added over a composite
in post, so as to make it appear realistic – e.g., virtual circular flares from
a camera looking at an added CG sunset.
Geometric distortion is where straight lines stretching across the frame look
curved. This tends to increase with nearness to the edge of frame.
Filters and Lens Attachments
In normal production practice it is common to use additional elements on the
front or back of the lens. These are most commonly methods of controlling the
color or resolution. Since VFX are often color- and/or resolution-dependent
for their success, extreme care has to be taken in this area.
Accurate color is essential in all VFX whether immediately dependent on
it or not. For example, blue screen shots must have accurate color to work
at all efficiently, since the quality and saturation of the blue determine how
well the key is made, and unbiased non-blue color on the subject is essential
if it is going to sit invisibly into the background image. Processes which are not
color-dependent, such as split screens, also need very pure and precise
color to avoid pinpointing the effect itself (e.g., revealing the existence of the split)
or of drawing attention to the effect shot (e.g., not exactly matching the look of the
shots on either side of it).
This has a two-pronged effect on the choice of filtration. It demands that film
stocks are correctly balanced for the illumination type so that the color reproduction
is as accurate as possible. But, it also insists that filters which deliberately
distort color reproduction should not be used. Thus for instance the use
of an ‘85’ filter to balance tungsten film in HMI lighting is essential, particularly
in a blue screen scenario, but use of a chocolate or pink grad filter would be
totally out of the question.
Obviously it is very important that VFX shots, particularly of the invisible variety,
match exactly those surrounding them. Thus if all of the shots in a sequence
are made with a chocolate filter then the effects shots themselves should maintain
this style and demonstrate a colorimetry slued in exactly the same way. But
shooting, say, a blue screen through such a filter would be totally inappropriate
if a good key is to be created and therefore we are faced with a dilemma. The
solution for all such situations is to shoot completely ‘clean’ without any distortion
and then add a matching color manipulation in post-production after the
effect has been accomplished. In the example of a chocolate filter it should be
left off for the blue screen and preferably background photography and then,
once the combined image has been created, the chocolate filtration should
be added to the entire composite. This has the double advantage of not only
allowing maximum quality for the matte process but also of creating the effect
across the entire image including the demarcation line between foreground and
background and thus contributing to its concealment.
‘Shoot clean, add image enhancement after’ is one of the major rules of
effects photography but it will only work if post-production can successfully emulate
the camera effect being deferred. It is therefore extremely important that sufficient
information is provided to enable the effect to be recreated in post. To do
this, examples of shots with and without the effect should be provided and, with
a colored filter, a reference shot of a white card with and without the effect.
These before and after shots should include both the background and foreground
elements of the composite as well as any other appropriate scenes. This
avoidance of filtration should be taken very seriously since it must be realized
that many well made filters actually stop almost all light of a particular wavelength
from passing through and therefore the damage cannot be undone in a
telecine or scanning bay. For instance a chocolate filter will actually stop a lot of
blue light from passing through and so to achieve a blue screen post-production
color grading may not be able to restore an adequate amount of blue since
there is nothing there to actually enhance. Remember, filters ‘remove’ light, they
do not add it. A red filter removes cyan, it does not add red!
The other popular form of filtration used in principal photography is diffusion.
Many cinematographers use this to enhance the ‘look’ of romantic scenes to
help ageing actresses and to ‘blow out’ in-shot illumination. Although many
modern keyers, both hardware and software, are said to be able to deal with soft keys, and indeed despite the fact that keys are often softened to make them more effective, it is not a good idea to shoot scenes to be composited with a softening filter on the lens. Using our example of a blue screen the fact that edges are softened will mean that there will be no definitive setting for the crossover. A soft foreground subject edge will mean that at some point it will be not quite blue enough to key but still bluer than normal, resulting in a range of settings making the matte bigger or smaller but never exact
As with colored filtration the best approach is to shoot clean and sharp and
then add a matching diffusion across the entire composite once it is combined.
More so than in the case of coloured filters, this will do a great job in covering
up the join between foreground and background. Even if a reasonable key can
be created with a diffused foreground, the relative hardness of the key line compared
with the images to be combined, could show up as a strange and unrealistic
effect highlighting the subject. A bright light on the background, which
would spill across onto the subject if it were really there, would simply stop at
the matte line whereas, if combined in post, would cross over just as it would if
the entire image were real.
It goes without saying that good reference material should be provided,
such as a white grid on black and some shots of bright points of light with and
without the diffuser.
Other image enhancements
Just as with filtration any other methods used in principal photography to skew
the image should be avoided in the shooting of elements for composite photography.
Thus, for example, bleach bypass/ENR/silver retention should not be
used when shooting material which is colour-reliant, since the desaturation and
colour skewing which might result could endanger the successful execution of
the effect. Other chemical processes such as cross and push/pull processing
and optical systems such as flashing (in camera or lab) should also be avoided
since all of these effects could complicate keying but can be successfully
emulated in post-production.