Understanding Lenses Part II

Focal length and type of photography

In the strictest term, there isn’t a rule that states a certain lens can only shoot a certain type of scene. Traditionally, the wider the focal length, the more it is recommended for landscape and scenes with a lot of environmental elements; while longer focal lengths are used for isolating subjects from a distance such as portraits or wildlife. However, there are a lot of examples of great landscape photographs taken with a long telephoto, while there are many eye-catching portraits taken with wider angle lenses, when used correctly.

There are general guidelines that certain focal lens are more suited for certain subjects though. The reason would be how humans normally perceive a certain image type. For example, we are familiar with how a “normal” human face should look like in terms of perspective, size ratio between facial features such as eye distances, nose size, and face width, to name a few.

Using focal lengths or lenses that distort this perceived universal photo characteristics often would lead to unusual interpretation or confusion to the viewer. These “norms” assign certain lens focal length to be ideal to certain types of photography, but not absolute rules. Here are some common classifications on lenses:

“Normal” Lenses

The most common focal range for cameras are between 35mm to 50mm, which are considered to be “normal” focal ranges as these focal lengths replicate the human eye’s field of view coverage. Virtually any scene is suitable for a normal angle lens, which makes it very versatile.

Wide Angle Lenses

A wide angle lens often covers a viewing angle wider than 40 degrees or so. This type of lens is often used for sweeping landscape or when a wide span of area needs to be captured in a single frame while still maintaining proper linearity. These lenses are often between the focal ranges of 18-28mm.

Ultra-wide lenses are available as well, offering an even wider view with focal lengths between 15 to 20mm range for even wider view, but distortion will be evident in most photographs taken with an ultra-wide angle lens.

Fisheye Lenses

Fisheye lenses create severely rounded images that cover almost 180-degrees of viewing angle, allowing unusual visual representation of a scene. These lenses are rarely used in normal scenes but could provide a fresh interpretation and rendition of an otherwise mundane scene.

Telephoto Lenses

A lens that has a focal length of over 70mm is often considered as a telephoto lens. These lenses are most often used for portraiture as the lenses are virtually free of distortion, which allows human facial features to appear more natural and flattering. As focal length increases, the depth-of-field also decreases, hence allowing better background-to-subject separation as well.

Longer telephoto lenses (200-300mm) are often used to shoot subjects from a distance, such as sporting events, stage performances, and even wildlife.

Super Telephoto Lenses

These are often large, heavy, and expensive lenses that have focal ranges beyond 300mm. The usage of these lenses are rather limited to outdoor photography as these lenses require a lot of physical space to work with. These lenses are best suited for wildlife, aviary, and astro-photography purposes.

Macro Lens

Macro lenses utilize additional optics that allow the lens to be positioned much closer to the subject than normal lenses. Macro lenses can magnify small objects beyond their life-size dimensions on the sensor. Macro lenses are great for shooting insects and flora. Using a macro lens often creates a wide range of unseen and interesting subjects that we may not see with our day-to-day visual scenes.

Tilt-Shift Lens

A tilt-shift lens is a very specific piece of equipment often used for architectural photography to keep perspective and angles in check. A tilt-shift lens allows the photographer to physically correct perspective before an image is captured by moving the body of the lens in different vertical and horizontal adjustments.

Tilt-shift lens can also be used to create extreme depth-of-field control that results to miniature-like subjects.

Lens speed

As discussed in our previous article, the lens speed is dictated by the maximum aperture opening of the lens. The larger the opening, the more light passes through the lens, which allows the photographer to use faster shutter speeds. The ability to use faster shutter speeds in low-light situation reduces problems such as camera shake and excessive subject movement.

Large aperture lenses allow more light through, but they also allow the photographer to have more control on the amount of depth-of-field in their photographs than smaller aperture lenses. Large aperture lenses can provide really thin depth-of-field which in turn allows more visual separation between subject and background.

Blurring the distracting elements makes the subject stand out in a photograph and forces the viewer’s attention towards the subject and nothing else.

Remember that we can always reduce the aperture size of a lens but we can’t open the aperture beyond its maximum setting, so having a large aperture lens allows us more options when it comes to controlling depth-of-field.

Image quality

At the end of the day, a lens has to produce sharp, high-contrast, artifact-free images for the sensor to capture. Lenses consist a number of glass elements inside the lens barrel itself to bend and manipulate light to create a final image. In general, the less elements in a lens’ construction, the better the image in terms of sharpness and contrast because there’s less light degradation as it passes through the elements. With all things equal, prime lenses (lenses with a single focal length) often produces better image quality than zoom lenses due to their simplistic design. However, modern computer-aided designs have closed the gap between prime and zoom lens image quality considerably, and it’s not uncommon to find modern professional zoom lenses outperforming old prime lenses.

The image quality of a lens is judged by several common factors such as sharpness, contrast, distortion, chromatic aberration, and vignetting. The ability of a lens to control the negative and enhance the positive attributes of how the light is processed differentiates a good lens from a bad lens.

Figure 11 shows our baseline image, the image is relatively sharp, good in contrast, and not a whole lot to complain about, so let’s take a look at common lens problems that we may encounter with some lenses.

Distortion

Distortion is when a lens causes a straight line to appear bent or curved.

If we take a closer look at Figure 12, we can see that the center portion where the engravings are present are bulging from the center instead of flat like in your baseline image. Problems with distortion are common and expected with wider focal lengths that exceed 24mm or so. Fisheye lenses will always have a bulging effect due to lens design.

With more common focal lengths, however, a lens should have little to no distortion as this will cause straight edges to be crooked or human faces to be enlarged.

Zoom lenses and long telephoto are more prone to having distortion on either end of its zoom range as there are more glass elements inside the lens itself that can cause light to bend incorrectly.

In order to test the severity of your lens distortion, simply take a photo of a rectangular grid and see if both vertical and horizontal lines are parallel with the adjacent lines.

Figure 13: Distortion Chart

A small level of distortion is expected on most lenses, but obvious distortion should never be present with lenses unless the lens is a fisheye or ultra-wide angle lens.

Vignetting

Vignetting is when the edges of a frame have a different level of brightness than the center of the frame. This is a common effect added onto a photograph especially when the photographer wants to emphasize and direct the attention of the viewer to the center of the frame. When a vignette is visible through the viewfinder or unprocessed photographs, however, this becomes a lens shortcoming, and all the photos taken with the lens would require post-processing correction to fix the resulting vignette.

Figure 14 shows an image with a dark vignette. The edges are a few stops darker than the center of the frame. This is not ideal especially when you have subjects or details across the length of the image. A light vignette is less common, where the corners are brighter than the center.

Many large-aperture, wide-angle lenses exhibit more vignetting than smaller aperture lenses. Smaller aperture lenses or when a lens’ aperture has been “stopped down” will often have less vignetting.

When testing for lens vignetting, make sure that the lens being tested doesn’t have any filters or lens hood attached as filters that are too thick or an improper lens hood may cause light to be blocked from the edges of the lens causing the vignette.

Chromatic Aberration

Chromatic Aberration (CA) occurs when the different spectrums of light fails to hit the sensor at the same angle causing halo-like artifacts around the hard edges of the image captured. In Figure 15, we can see purple and cyan fringing around the hard edges of the subject. This phenomenon is most visible when the subject is shot against a contrasting background (dark subject on light background, or bright subject against dark backgrounds).

Manufacturers often employ special coatings and lens treatments in their lens elements to ensure that the different color spectrums align themselves in perfect focus when hitting the sensor, however, some camera sensors itself are more prone to CA due to the microlenses used in the design of the sensor. With that in mind, some lenses would probably exhibit a certain amount of CA when mounted on certain cameras but may not show any CA when mounted on a film camera, for example.

Low Contrast

Contrast is the difference in brightness and color between elements within the same scene. Black and white would translate to a high-contrast scene, while a white and light gray would be considered as a low-contrast scene (see Figure 16)

Compare Figure 17 with our baseline image, we’ll see that the blacks are less defined in the low-contrast image and there’s less tonal gradation from pure black to pure white.

Take note that when we check for lens contrast that the scene we are shooting actually has a good degree of contrast itself. There’s no way to capture a high-contrast photograph if the scene itself has a low contrast such as a fog or smoke-filled scenes.

Soft Image

Perhaps the most scrutinized performance of lenses would probably be sharpness, or the lack of it. When an image is soft and hazy, it is usually caused by a lens that utilizes too many inferior glass elements or its inability to focus all the light rays onto the exact same area on the camera’s sensor, resulting to an un-sharp image.

Soft images are rarely useful as most photographs require a certain amount of details captured to show the intricacies of the subject being photographed. With a soft lens, these details will not be registered with sharp details but rather a group of hazy, blurry patterns.

We can see in Figure 18 that the intricate markings on the edge of the subject are no longer rendered. Remember that we can always blur an image through post-processing but we cannot add details that were never there during the time the image was captured.

Summary

Keep in mind that a lens is the first piece of equipment to capture light rays in order for us to create a photograph. Choosing a lens is very important as regardless of camera body advancement, a good lens will often be able to provide great images regardless of camera used. However, a substandard lens will never be capable of providing decent images regardless of how sophisticated the camera may be. Garbage-in, garbage-out.

Pick a lens that suits your budget and subject preference wisely. If you find yourself shooting wildlife most of the time, concentrate your options to getting a good telephoto lens. On the other hand, if you know you’ll be shooting macro often, there’s no reason to even consider a fisheye lens or a super telephoto, for example.

Lenses are great photographic investments as their value rarely depreciates as fast as camera bodies. So picking a great lens can serve you for decades even if we change cameras multiple times in the future.

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