The Lense

The eyes of the camera.  This is the input device the brain uses to gather the data it will process into an image.

Most DSLRs allow for interchangeable lenses.  This is not to be confused as a prerequisite of a DSLR.  Remember that SLR is an acronym for the type of reflex action the camera uses (how it exposes the imaging sensor).  It has nothing to do with whether the lense can come off or not.  That said, however, this is a common feature with DSLRs.  There are several lenses available for most DSLRs, and using these specialized lenses allows us to direct and control the incoming light as we see fit.

On digicams, the lense is part of the camera as a whole.  Many allow attachments that modify the characteristics of the integrated lense, but the main lense is non-removable.  These attachments function much the same way as switching lenses on an interchangeable lense system.  Adding attachments, however, tends to reduce the quality of the final image.  Whereas changing the lense on a DSLR introduces a completely different light path optimized for that particular lense, adding optics to a lense modifies the existing light path for a lense optimized for a different shooting situation.  The result is some image degradation.

These are lenses that have no zoom features.  A 50mm prime has a 50mm fixed focal point.  Prime lenses require significantly fewer elements than a complex zoom lense.

Some Advantages:
•  Lighter weight
•  Compact
•  Higher quality optics
•  Allows more light through

The end result is a sharper, cleaner image with greater contrast and better color rendition.

These are lenses that stay relatively close to the base focal length of 50mm.  It is common to hear terms like mid-range telephoto.

Macro lenses have short focusing ranges and are useful for taking photos of subjects close to the lense.  Most macros allow for 1:1 magnification.

A 1:1 magnification factor means that an object that is 1cm high in front of the lense will appear 1cm high on the imaging device.  In other words, “life size.”  When this image is actually printed, it becomes larger than life size because the image captured on the imaging surface is enlarged onto the printed medium.

A 2:1 magnification factor means the image on the sensor is two times larger behind the lense than it is in front. In specialized applications of microphotography, the camera body is attached to a microscope, and the microscope becomes the lense.

For reef tank photography, this is the key lense, as many of the shots we will want to capture are close-ups of our favorite corals.

These lenses allow for a wider view than we would normally see with our eyes.  This type of lense is especially useful in landscape panoramas and similar applications.   In reef photography, use of a wide angle lense will allow for the capture of the entire reef tank when space in front of the display is tight.

Wide angle lenses are prone to distortion where horizontal and vertical lines bow outward towards the edges of the frame.  This is called barrel distortion, since a photo of a cube starts to look like a photo of a barrel.

Extreme wide angle lenses are referred to as “fish eye” lenses.  The barrel distortion on these is so great, the image looks as if we were looking through the eyes of a fish.  Those rounded mirrors at corridor intersections that allow us a 180° view are of a similar concept.  The reflection in these mirrors is what the world looks like through a fish’s eye.

These allow the photographer to get “closer” to the subject without physically moving closer.  Telephoto lenses also change the angle of view.  A telephoto lense is not necessarily a zoom lense.  Many prime telephotos are commonly available.  One common lense in the portrait segment is the 85mm telephoto.

Long telephotos, lenses with “long” reaches, have larger focal lengths.  A telescope is basically a really long telephoto lense.

A 50mm lense on a 35mm based system has approximately the same field of view as the human eye.  A 100mm lense will get us closer to our subject, though not necessarily twice as close.  Similarly, a 150mm lense will allow us to get closer still.  This increase in focal length is referred to as a 2x and a 3x zoom, respectively.

Zoom lenses tend to be complex, heavy, slow (letting less light through), and pokey (taking longer to focus).  The longer the reach, the worse the effects.  Inexpensive zooms tend to be lighter in weight, have poor build quality, second rate optics, and fewer optical coatings.

The disadvantages of zooms stems from the optics involved.  In order to change the actual focal length of a lense requires several groups of lense elements, including aspherical elements which are costly to manufacture and degrade image quality.  The more glass light has to travel through to get to its destination, the more it will be degraded.  Comparing a tank with thin acrylic walls to one with thick walls, we will notice the thicker acrylic tends to have more fogging and less contrast.  The same holds true for lense elements.

Lense Technologies

Optical Coatings (ED/L)
The type, quality and application of coatings can greatly enhance the performance of good quality glass.  More expensive lenses use processes that allow for consistent application of these coatings.

Light bends as it enters and exits the air-glass interface.  The amount that it bends depends on its wavelength.  Therefore, like a prism, lenses will separate light into different colors resulting in a rainbow-like effect.  This is referred to as chromatic aberration.  The purpose of multi-coating a lense is to offset this effect.

Extra-low Dispersion (ED) is a special multicoating process used by Nikon to minimize chromatic aberration.  Canon employs fluorite lense elements (rather than glass elements) in their L-series lenses. These fluorite elements tend to expand at different rates than the glass elements when heated quickly. This causes focusing issues, and as such, L-series lenses are painted white to reduce their absorption of solar radiation.

Silent Wave/Ultrasonic Motor (SWM/USM)
In order to focus a lense, the elements contained within must move.  In older and/or less expensive lenses, movement of these elements relies upon stepper motors contained in the camera body.  The stepper motor is physically connected to a spindle on the lense, which are connected to the elements that need to be moved through a series of gears. 

The mechanical nature of this method is not without its consequences.  First, to move big elements quickly requires the motor to put out great torque.  In professional cameras, the inertia from these high torque motors can be felt as a shifting of the camera body during focusing.  Second, the inefficiency of these motors results in greater battery consumption.  Third, the winding of motors and spinning of gears tends to create noise and high pitched whines that could attract attention to the photographer potentially resulting in a lost shot.

A more efficient mechanism of moving lense elements silently and rapidly is by using ultrasonic soundwaves.  In these lenses, ultrasonic soundwaves (soundwaves above human hearing threshold) push the elements into position at the speed of sound.  Additionally, these ultrasonic motors are integrated into the lense itself, have very low power requirements, and fewer moving parts.

Internal Focus (IF)
When the focus ring is turned, or autofocus activated, on many lenses the front element will spin and the lense will grow and shrink in length.  This is a particular nuisance if a directional filter, such as a polarizer, is employed.

Internal focus lenses accomplish focusing by moving lense elements inside the body of the lense itself, so there are no moving parts externally.  Higher quality lenses will have this feature.  Additionally, the nature of lenses that employ ultrasonic motors dictates, they too, focus internally.

Internal Zoom (IZ)
Zooming a zoom lense will cause it to telescope in (wide) and out (tele).  This is not only cumbersome, but also degrades optical quality.  By zooming in and out, air must be displaced by this pumping action.  Along with the air come all the particles suspended in it.  These particles eventually settle on the lense elements, fogging the glass, degrading the light path, and consequently affecting image quality.

Internal zoom lenses accomplish zooming inside the lense body.  Keep in mind, however, that an internal zoom lense is not necessarily an internal focus lense and vice versa.

Digital Zoom
Found on most digicams, this is a pretty useless feature.  A digital zoom simply crops the center of the image and interpolates it up to make a "bigger" image.  Unlike an optical zoom, where zooming in allows the sensor to capture finer details (limited only by the quality of the lense), a digital zoom adds nothing in terms of detail.

In fact, too much digital zoom can actually destroy detail in a captured image.  It is far better to crop and interpolate up, should it be required, using software.  Imaging programs typically employ powerful algorithms to upscale an image.  This type of processing power is beyond the capabilities of most digicams.

Vibration Reduction/Image Stabilization (VR/IS)
At long telephoto ranges and low light situations, even the slight movement caused by depressing the shutter release can cause motion blur in a photograph, resulting in a soft image.

Image stabilization technologies, based on a gyroscopic principles, dampens these small vibrations.  This allows for handheld shots at less than ideal shutter speeds.

In the body of the lense, angular speed sensors for pitching and yawing motions pick up movement in the camera body once the pre-focus sequence starts.  The degree and magnitude of these motions are transferred to voice coil motors (the same type of motors that drive our subwoofers and tweeters), that react quickly and accurately to counter the motion.  These motors are constantly updated and repositioned as movement continues.

Some newer digicams employ vibration reduction also.  But, rather than stabilizing lense elements, they work by stabilizing the imaging sensor.

DX Lenses
These lenses are "optimized for digital photography," according to Nikon.  Translation:  "These lenses are cheaper."

Nikon DSLR imaging sensors are similar in size to Advanced Photo System (APS) film.  Since an APS-sized sensor is smaller than a 35mm-sized sensor, lenses designed for use on traditional 35mm systems are larger than necessary.  With a reduced imaging circle, the entire APS-sized sensor area can still be covered without as much glass.  As such, Nikon has designed a series of F-mount lenses specifically designed for use with APS sized sensors.

The image to the right was shot with a Nikkor 18-70mm DX lense mounted on a camera utilizing the entire 35mm frame.  Although this shot was taken at 70mm, the vignetting seen is a function of the exit pupil.  As such, the focal length and aperture have no effect on the imaging circle.  Roll the mouse cursor over the image to superimpose the coverage area of a DX sized sensor.  Click to see a larger version.

There are several advantages with DX lenses:

• Smaller - lense elements can be smaller in diameter and thinner.
• Lighter - as a result of the reduced glass volume.
• Cheaper - less glass = reduced cost of production.
• Plastic - use of plastic housings further reduce cost and weight.

There are also some disadvantages:

• Cannot use the lense on 35mm cameras (film, or digital) without getting the vignetting seen above.
• Plastic - not quite as durable as metal.