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While many perspective control
(PC) lenses from multiple manufacturers are
available to users of the 35mm format, the best
known and most widely available are the Canon series
in 24mm, 45mm, and 90mm focal lengths, and the Nikon
line with newly announced lenses in 24mm, 45mm and
85mm focal lengths (Nikon's older versions, perhaps
more readily available at this time, are available
in 28mm and 85mm focal lengths). These lenses allow
you to easily make composite images without having
to invest in panorama gear.
Panoramic image made by
stitching two horizontal images together
PC lenses (tilt/shift or shift
only) have a housing that allows the user to move
the front elements and barrel sideways or up and
down in order to take advantage of their large image
circles. Shifting the lens projects a different part
of the larger image onto the sensor. The lens “sees”
the entire image but the sensor only sees whatever
slice is projected onto it – the rest is wasted
space unless used in this fashion. Because the
camera is stationary, and the images overlap,
aligning the frames is usually quite easy if the
camera itself is level. LEVEL is the key word. Being
off level makes the stitch process a huge headache.
Why Stitch?
First and foremost, you can
combine two horizontal images into a traditional
panorama format with an aspect ratio of 1:2½. This
is precisely the aspect ratio of the traditional
4x10” sheet film cameras, and in between the
traditional 6x12 and 6x17 cm roll-film cameras. The
aspect ratio is pleasing and allows cropping to
other sizes. Those panorama shooters who switched to
digital early on had only two choices, give it up,
or stitch.
Secondly, you effectively
double your sensor real estate. A 13mp sensor
effectively becomes 26mp. While 13mp is overkill for
most publications, the larger file will give
improved print quality at larger display sizes.
Third, use of three vertical
images to make a horizontal may allow an image that
otherwise would not exist. This happened to me at
Cirque of the Towers in the Wind River Range just
recently. My widest angle was a 24mm, and the comp I
wanted wasn’t possible with that lens. Stitching to
the rescue, as the accompanying image illustrates. I
haven’t calculated the effective focal length, but
it’s wide.
Fourth, there are subtle
differences in the image created by combining two or
more frames together from the one created with a
single frame and a wider angle lens. For the
photographer looking for a creative edge this
enhancement should not be ignored.
Before You Stitch
Previsualizing your final image is
key, particularly if creating a horizontal image from
verticals. Be certain your camera is level. If you aren’t
going to rotate the tripod base, then a level base is less
critical, but the camera itself has to be as level as
possible, or a skewed image will result. You will then have
severe limitations on how to crop your final image. Shift
your lens freely and rotate the tripod head to plan where
the boundaries of your image lie. Compose loosely – it’s
hard to tell where the final edges will be when combined
into a composite. The center image may not be a stand alone
image because it has to be off center a little, but one of
your images should be pretty strong by itself. If not,
rethink the composition. Horizontal panos in more traditional forms
can usually be made from two images because there is enough
overlap. Two such images end to end would give you a 1:3
aspect ratio, so you lose some in the process. Usually there
isn’t so much softness at the edge that it becomes apparent
in the finished image. Not so true when stitching verticals
into horizontals. Three images overlap about 50% each, so
you are using much more of the sweet spot and avoiding two
stitched edges down the middle of the finished image.
Making the Exposures
Auto “anything” won’t work. Your meter
will go nuts when you shift the lens and give wildly
inaccurate readouts, and auto white balance will shift your
color on you also. Set your meter to manual, pick a white
balance that you like (5000k is approximately daylight) and
make your exposures based on the lens being at neutral,
i.e., centered. This ensures that each frame is exposed and
color-balanced identically, and the locked-in exposure
settings are correct for the shifted frame. You can always
adjust color balance during or after raw conversion.
How to Stitch
Until Photoshop CS3 came out,
stitching was a bit of a pain, at times almost
impossible. Often the earlier versions of photomerge
wouldn’t combine the images properly, or at all.
This left manual superimposition and use of masks to
blend to exposures together. Several proprietary
programs came on the market to accomplish this task,
apparently pushing Adobe to improve their product.
Currently CS3 does a superb job of locating just the
right points to join and makes a breeze out of the
process. This can be done from the raw converter
interface or after conversion. If done after
conversion, all files should be converted with the
exact same settings. I have experienced subtle
differences in the files after conversion that made
the process more difficult. The same files combined
in photomerge straight from the Adobe Camera Raw
program resolved those issues, don’t ask me why. The
stitch line in the un-flattened file may be visible
at certain scaled image sizes but will almost always
vanish when the file is flattened, so don’t wig out
if you see the joint at first glance.
Pros and Cons
So, rush right out and buy a
fistful of PC lenses? Well, maybe – maybe not. They
are fixed focal-length lenses with very high quality
optics (and priced to match). The tilt feature is
great for macro and other applications, and you can
often compose to shift lens flare out of the frame.
The Canon 24mm T/S suffers from some chromatic
fringing but this easily fixed either in Camera Raw
or by easy moves in LAB color space. The main
drawback is cost, though zoom lovers will also miss
the freedom that zooms afford. Panoramic gear costs quite a
bit less, at the expense of more involved set-up,
bulk, and weight. Both are tools to help us achieve
our personal vision – PC lenses offer a powerful
option for us in many ways. |
