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Is Your Lens Digital?
By Peter Kotsinadelis
The rapid growth of digital cameras has forever changed the way we look at photography.
While digital SLRs have become significantly better with each passing generation, many
photographers question if they need to buy lenses optimized for digital photography
to reap the full benefit.
Understanding Digital Lens Designs
To understand digital it is best to first understand how film accepts light. Film itself
is a flat surface that collects light regardless of angle. When light reaches film, a
photochemical reaction takes place. In the case of color film, the light reproduces
an image onto each of the color sensitive layers, typically red, green and blue.
These layers are then chemically processed to amplify them and produce a color negative,
which in turn is used in the creation of a color print.
Resolution of Digital Lenses
We ran a real world test comparing three designed-for-digital wide-angle zoom
lenses from independent lens manufacturers. They are the Sigma 12–24mm f/4.5-5.6,
Tokina 12–24mm f/4, and the Tamron 11–18 f/4.5–5.6. All operated without issue,
however, the Sigma is the only lens that can be used on full-frame Digital SLRs
such as the Canon EOS 5D or Nikon D3 as well as those with smaller sensors like
the Nikon D300. The Sigma and Tamron offer a variable aperture from f/4.5 at the
wide end to f/5.6 at the long end while the Tokina provides a constant f/4 aperture
throughout the entire zoom range.
We tried each on a variety of outdoor subjects and all proved to be sharp lenses.
As we enlarged each of the digital images we made with the lenses, it began to
pixilate before we could see a really significant difference between them in
sharpness. What we did notice was the Tamron lens exhibited a slightly warmer
coloring compared to the others. We also noticed that the Tokina had a slightly
higher contrast, which did provide somewhat better detail when we enlarged our
images.
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In digital cameras, an image sensor is used in lieu of film. This device is comprised
of millions of individual photodiodes, or pixels that can be likened to small buckets.
They convert light into electrical signals and subsequently into a file you can view.
Much like the color layers in film, these photodiodes are specifically made to accept
red, green, or blue light as it strikes the sensor. To get the best quality image possible,
light reaching the sensor must be as perpendicular to the photodiodes as possible.
This maximizes the light entering the sensor and consequently will improve the final
image.
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Newer lenses designated as designed for use with digital photography attempt to bend
the light rays to make them more perpendicular by the time they reach the image sensor.
A good analogy of this would be rain filling a bucket. If the rain falls straight down,
the bucket fills quickly. If the rain comes in at an angle, you lose some rain since it
will also hit the side of the bucket and run off. This light loss is mainly seen when
using wide-angle lenses with digital photography since the angle of light that reaches
the film plane or image sensor is greater than with telephoto lenses. While this is not
a problem for film as it is a flat surface, it is much more of a problem for digital image
sensors since some of the light will be lost. Photographers using conventional wide-angle
lenses--designed for film cameras--on a digital SLR may see a somewhat softer image
compared to that taken with a designed for digital lens of a comparable focal length.
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The Pitfalls of Digital
A number of newer generation digital lenses are designed specifically for smaller image
sensors such as those found in the Canon EOS 50D and Nikon D300. These lenses improve
on the angle light that reaches the image sensor. They also provide the added benefit
of being generally smaller in size and are often less expensive than the equivalent
lens designed for a full-frame Digital or 35mm SLR. While the savings and lighter weight
are attractive attributes, these smaller lenses will cause vignetting when used on SLRs
with larger sensors, such as a Canon EOS 5 or Nikon D3. With the exception of the Nikon
D3, which has a feature allowing you to use a cropped portion of the full-size image
sensor, only digital lenses designed for full-frame sensors can be used without issue
on digital or film SLRs regardless of sensor size. The drawback is that full-frame
lenses like these may be a bit larger and more costly.
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Sometimes a lens that was originally designed for film cameras will have an issue with
auto focus on newer digital SLRs. Some photographers report that they have experienced
a back focusing issue, where the focus point appears to be in back of the subject, even
though the photographer had the focus point on the subject. This happens more with fast
lenses and may be due to the image sensor mounting as opposed to a problem with a lens.
Sometimes a lens may work perfectly on one camera body but not on another. To check your
camera’s AF use a flat subject and take several shots of it with one or more lenses wide
open. If you are experiencing a problem with focusing, the remedy may be to send the
camera body in for recalibration. If it is with a specific lens, then you may need to
send that lens back to the manufacturer along with the camera body and ask them to
recalibrate the camera body using that lens. This will generally correct this issue.
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Other times there are unexplained reflections, sometimes seen as “ghosting” (reflections
shaped like the lens diaphragm), appearing in an image due to the highly reflective
surface of a digital image sensor. These reflections strike light back onto the rear
elements, which in turn reflect that light back onto the image sensor as it captures
the image. Newer digital lenses compensate for this by using new coatings on the rear
elements to significantly reduce the potential of something like this from occurring.
Additionally, some lens manufacturers have also changed the design of the rear of the
lens to minimize this. This is one reason why a newer version of the same lens may have
a somewhat different looking back than an earlier version.
How Wide is my Wide Angle Lens?
Some digital SLRs have something called a focal factor. Most camera manufacturers specify
this as a specific multiplier for a DSLR. For example, the focal factor of a Nikon
D300 is 1.5x while that of a Canon EOS-1D Mark II is 1.3x. Therefore, a standard 50mm
lens on the Fujifilm S3 will be the effective equivalent of a 75mm lens, but on the
EOS-1D Mark II it is 65mm. Sports photographers relying heavily on telephoto lenses
can take advantage of this since a digital SLRs with 1.5x focal factor will effectively
turn their 300mm f/2.8 into a 450mm f/2.8. However, wedding photographers who rely on
wide-angle lenses will see a disadvantage. As their favorite 24mm wide-angle is now
effectively a not so wide 36mm lens. To get the equivalent of a 24mm lens, they would
need to purchase a 16mm lens. With a 1.5x focal factor the 16mm effectively becomes a
24mm lens. The dilemma here is that although you may have the equivalent of a 24mm
focal length, the lens will retain all the properties of a 16mm lens, including any
perspective distortion.
Decisions, Decisions
Newer digital lenses do offer some advantages to photographers looking to maximize their
image quality. A good understanding of just what a digital lens can offer is one way to
ensure you are making the right decision when purchasing one.
Peter Kotsinadelis is a writer/photographer living in Pleasanton, CA. He may be
reached at peterk727@gmail.com.
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