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HelpLine By
Michael Guncheon
35mm Compared To Digital
Focus On Focal Length
F-Stop Mania
Digital Flash Sync
Some readers have questions that deal with comparisons between digital
cameras and 35mm SLR cameras. The readers are familiar with SLRs
and are trying to understand how the terms they've lived with over
the years apply to digital cameras. As this is PCPhoto's
Buyer's Guide issue, I thought this might be a good time to answer
a few of these 35mm-to-digital questions.
Focus On Focal Length
I've been reading much about digital cameras and I'm getting
ready to make the investment in a digital camera, but I'm a little
confused about the lenses. I have some experience with SLRs and
have a couple of lenses for my camera—a telephoto and a wide-angle.
But when I look at digital camera information, there seem to be
two measurements listed. Why is there an "equivalent" number? Isn't
a lens, a lens?
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Walt
M.
Rochester, New York
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Digital camera shoppers have noticed that when they look at the
camera's specifications, the digital lens might be a 7.7mm lens,
but have a 35mm equivalent of a 50mm lens. I can understand why
Walt asks, "Isn't a lens, a lens?" Before I jump into a quick answer,
we need to understand what's meant by focal length. Let's consider
a simple lens that isn't attached to any camera, digital or film.
Light enters the front of the lens, goes through various optical
elements and leaves the back of the lens. As it continues at some
distance from the back of the lens, the light rays are in "focus."
This magic spot is called the focal point of the lens and the location
of film or an image sensor. If you measure the distance from the
optical center of the lens to the focal point, you get the focal
length of the lens.
What's the difference between a 35mm camera and a digital camera
that would change this focal length? I could take the simple way
out and say it's the image sensor that changes the focal length,
but that's wrong. If you remember the definition of focal length
above, you noticed it's strictly about the distance from the lens
center to the focal point.
So what changes? The size of the sensor or film. At the point where
the image is in focus, you could have a variety of sizes of light-sensitive
material. If you have a smaller surface to pick up the light, you
end up capturing a smaller picture as the sensor sees a smaller
field of view. Larger surface, larger picture. But both are working
with the same lens focal length.
Which brings us to equivalent focal length. The imaging sensors
found in digital cameras are significantly smaller than a piece
of 35mm film, so the field of view is smaller. When manufacturers
talk about an "equivalent" focal length, they're trying to help
those who have experience using 35mm cameras with interchangeable
lenses. That way, users aren't surprised by the field of view they
think they should be getting. We know what kind of field of view
we can expect with a 50mm lens or an 80mm telephoto or a 24mm wide-angle.
The equivalent focal length is an attempt to compare the field of
view of a 35mm camera lens to a digital lens.
F-Stop Mania
I'm curious about how f-stops on consumer digital cameras relate
to 35mm cameras—for instance, the idea that f/8 is an ideal
setting for lens sharpness. The settings on my Nikon Coolpix 995
in aperture-priority mode range from 4.6 to 10, with no f/8 between.
Is there some way to compare these to 35mm?
Brook
Thompson
Via e-mail |
This brings up an interesting point that may or may not be important
today. In the past, lenses were always at their best stopped down
a couple of stops from wide-open and opened up a few stops from
the smallest aperture. This was often around ƒ/8, so many people
just used that number. It was safe and almost always in the range
of the best results for a lens. Modern lens design has made this
less of a factor. The image quality of a lens wide-open, as well
as stopped down, is excellent. There's some improvement in the mid-range
of ƒ-stops, but it's doubtful most people will see much of
a difference in average photography. You're most likely to see the
difference stopped down vs. wide-open with less expensive lenses.
With digital camera lenses, it's highly unlikely you'll see much
of a difference through the ƒ-stop range other than standard
depth-of-field effects. The reason lenses don't stop down as much
as 35mm lenses (to ƒ/16 or ƒ/22, for example) is because
of the size of the lenses. As I mentioned above, the image sensor
size is significantly smaller than 35mm film, so the lenses can
be made smaller, making the opening in the lens proportionately
smaller, too. An ƒ/16 for most digital cameras would require
such a small opening that diffraction effects would strongly kick
in, quickly reducing image quality.
Digital Flash
Sync
I spent many years as a wedding photographer using standard 35mm
equipment. I recently purchased a digital camera and the question
of flash sync came to mind. Is flash sync still a part of digital
photography, or does the shutter sync at all available speeds on
a given camera?
Flash sync refers to the timing of the flash as it relates to the
shutter in the camera. In its simplest mode, flash synchronization
works so that the flash is triggered at the exact exposure time.
The amount of time the flash is lighting up your scene matches the
amount of time the shutter is open.
A digital camera doesn't have film that needs to be kept in the
dark, so why would it need a shutter? It turns out that an imaging
sensor needs to be reset so that it can capture an image. Depending
on the camera, this is accomplished with either a mechanical or
an electronic shutter.
On basic digital cameras, your options for using flash are "auto,"
"on," "off" and "red-eye reduction." None of these options changes
the flash sync. The flash will generally sync with all standard
shutter speeds on most digital cameras. This is why fill-flash can
be an easy and great technique to use with a digital camera. Turn
on the flash and set it to always flash, then use it during the
day to fill in shadows in people's eyes and under hat brims.
On some cameras, you can add an external accessory flash for more
versatility. On some low-end cameras, this isn't possible because
the connections aren't there. You can't even use a typical flash
with a slave that sets off the unit when the camera flash fires
because the camera flash will typically have two instantaneous flashes—one
to set exposure, one for the picture—and the slave will go
off with the first flash, so it will be "out of sync." Some flash
units will allow you to set off the unit on the second flash.
It's with the more advanced cameras that you'll find settings like
"slow sync." With slow sync, the shutter will be open for a longer
period of time than the flash duration. In this way, the image being
captured will be lit by both the ambient (existing) light in the
scene and the flash. This is a great technique to allow a more natural
look to the photograph, with less of the harshness of a straight
flash. If there's movement in the scene, however, you'll notice
the blur caused by the long exposure, but when the flash fires,
it will stop the movement at the time of the flash. This can be
a very neat effect.
As you move into even more advanced digital cameras and SLRs, you'll
be able to control the flash sync in another way. You can choose
whether the flash "syncs" or triggers at the beginning of the exposure
or the end. This allows you some creative control over the type
of blur that appears from the movement during the ambient light
time of the exposure.
If you have any questions, please send them to HelpLine, PCPhoto
Magazine, 12121 Wilshire Blvd., Ste. 1200, Los Angeles, CA 90025
or [email protected].Visit
our Website at pcphotomag.com
for past HelpLine columns.
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