So what lens is right for me?

In the last posting, I discussed the various general types of lenses - normal, wide-angle, and telephoto - and the pros and cons of fixed (prime) lenses and zoom lenses. Now, I would like to get more specific about the selection of focal length and add some specialty lenses and lens accessories to the mix.

The focal length of a lens is the distance from the optical center of focus within the lens to the plane of the image sensor in the camera. Focal length is usually measured in millimeters (mm) and the longer the focal length the greater the size of the image on the sensor (sometimes referred to as magnification.) I mentioned that a lens with the field of view of our eyes in approximately 50mm when projected on a full-frame (24x36mm) sensor. However, most cameras use a sensor smaller than full-frame that is called a crop sensor. The camera is designed so that the crop sensor still 'sees' the full image so, effectively, the image is magnified by the crop factor of the camera. Typically, point and shoot cameras have a very large crop factor (from 2 to almost 6) so that the effective focal length of the lens is much greater than the measured focal length (compared to full-frame sensor.) This is why many point and shoot cameras are referred to as 'super-zoom' cameras and their zoom range is defined as the magnification beyond the shortest focal length of the lens (12x, 15x, 20x and even 30x.) Digital SLR cameras do not use this 'magnification' terminology but rather relate the effective focal length to that of a full-frame sensor.

So how does crop factor influence the effective focal length of the lens. Many dSLRs from Nikon, Sony, Samsung, and Pentax have a crop factor of 1.5 while most Canon dSLRs have a crop factor of 1.6. The crop factor is multiplied times the actual focal length to obtain the effective focal length seen on the sensor.  In other words a normal 50mm lens on a Canon 1.6 crop sensor looks like an 80mm lens. So this 'normal' lens is working like a short telephoto (if you were shooting on a full-frame camera.) This effect has a tremendous advantage for a wildlife photographer who always wants a longer lens to create images closer to the subject. Effectively, a 200mm (medium telephoto) functions like a 320mm lens on a Canon crop sensor camera. A 70-200mm zoom functions like a 112-320mm zoom. This sounds like a real bonus but what if you are shooting landscapes with a wide-angle lens? Now your 24mm (medium wide-angle) lens acts like a 38.4mm (barely wider than normal) lens. This is part of the reason that landscape photographers like full-frame cameras - they can maximize their wide-angle ability.

So when purchasing a new lens you must carefully consider the type of photography you will be doing and the crop factor of your camera to select a lens that will give you the effect you desire.

There are also several types of specialty lenses that appeal to nature photographers. The first is a macro lens. A macro lens is specially designed for a very close focusing distance for its focal length. A true macro lens projects an image of the subject at its full size on the camera sensor - a 1:1 image. Macro lenses come in different focal lengths depending upon how close to the subject you can get. Typically, macros range from 60mm to about 200mm. These lenses are designed for ultra-sharp images captured at very close distances and are often used to shoot flowers, insects, and other small critters. Focusing a macro lens is a challenge because the the very narrow depth-of-field - often only a millimeter or two when wide open (f/2.8.)  Depth of field is improved if the lens can be stopped down (f/11- f/32) but this usually means a very long shutter speed or supplemental light using a reflector or flash. Some general lenses have a close-focusing feature they refer to a a macro function. I have used this to advantage on my small Sony camera but it is not a true macro because you cannot focus down to a 1:1 size - mine is more like a 1:2 but it still focuses down to about 3-4 inches from the subject.

Another lens sometimes used by landscape photographers is a fish-eye lens. A fish-eye is an extreme wide-angle lens (6-10mm focal length) that is able to capture nearly a 180 degree view. These lenses cause extreme distortion of the image resulting in curving horizons than are usually interesting and can be very pleasing to view.

With all of the choices of lenses, what is the best general 'carry-around' lens for everyday photography? This is a very difficult question to answer and most photographers will tell you that a 24-70mm zoom is a good all around lens (best on a full-frame camera.) This lens gives you a mild wide-angle to short telephoto perspective and can be fast (f/2.8) and not unwieldly large. I like this lens a lot when I am in town shooting what-ever comes by, but it is not my favorite lens in the wilds. It is generally too short for birds and animals and not wide enough on my crop-sensor camera for great landscapes. It is fast for dawn and dusk shots but I almost always carry 25-30 pounds of other lenses and gear when I hike to take wildlife or scenic shots. Some people like super-zoom lenses (28-300mm) for general work. Often these lenses are slow (f/5.6 or worse) and some lack high quality optics so I have avoided the super-zooms.

Finally, how does one properly accessorize your lens? A lens case is a good thing to keep in the closet at home or to store your lens when you are not shooting, but I never use lens cases in the field because they add unnecessary volume to the gear and it takes too much time to get the lens out of the case. I keep my lenses in my pack with lens covers on front and back to protect from dust and weather. 

There is controversy among photographers about using a UV/haze filter to protect the front of your active lens. Clearly, low quality glass in front of your expensive lens can degrade image quality and introduce lens flare on back-lit images so these should be avoided. I have chosen to use high-quality neutral (UV/haze) filters on the front of most of my lenses. It is a high cost but does keep dust off the much higher cost lens. I have never dropped or scratched a lens so I can't (yet) vouch for the protective effect of the neutral filter. Many (maybe most) professionals advise against a neutral filter over the lens. They doubt the protective effect if the lens is dropped and worry about the negative effect on image quality. Almost all professionals use a lens hood on all lenses. The hood protects the front of the lens from damage, reduces the likelihood of lens flare on back lit images, and has no negative effect on image quality. I always use a lens hood unless I need to rotate a polarizing filter or variable neutral density filter or am using a rectangular graduated neutral density filter.  These filters will be discussed in the next posting.

Selection of essential lenses for nature photography

Now that you have a new camera and know the basic exposure controls, it is time to get serious about selection of lenses for nature photography. First, understand that brand is not as important as quality and functionality. Generally, it is best to stick to lenses made specifically for your camera body - usually by the camera manufacturer, but not always. Most photographers will tell you that the quality of your glass is more important than the quality of your camera. So what questions must be asked to select the lenses that will help your improve you images the most?

Before you can choose a lens you must consider if you want a lens that will work only for your current camera body or will it work or future camera body upgrades. Most companies make a line of lenses that is designed specifically for smaller, crop sensor cameras. These sensors are smaller than full-frame sensors that are the size of 35mm film (actually about 24x36mm.) All entry-level digital single lens reflex cameras currently use a crop sensor. The crop factor is typically 1.5 on Nikon and 1.6 on Canon cameras (more about crop factor later.) Lenses made especially for the crop-sensor cameras (Canon calls these EF-S lenses) are usually smaller, lighter, and less expensive so they are aimed at the entry-level photographer. While they work well on the smaller cameras (and some are of very high quality), they will not work at all (or even fit on) the high level, full-frame cameras. Other lenses are inter-changeable between crop and full-frame cameras.  Canon calls these all around, full-line lenses their EF series. Across the board, the EF lenses are generally of higher quality. Canon also has a special series of EF lenses that are called the L lenses (luxury?) that are identified by a red band around the lens. These are extremely high quality, metal framed lenses, that have improved designs, high quality and exotic glass, and additional weather sealing for professional performance.

The first question in selecting a lens is, what are you going to photograph? If you are shooting exclusively landscapes, your lens selection will be far different than if you are going to shoot birds. Keeping this first question in mind, lets look at some basic issues we all must face when selecting a new lens.

Everything in photography is a trade-off and lenses are no exception. Quality of construction and, most importantly, quality of the glass used is of fundamental importance. There is a huge trade-off because high quality almost always means high cost. Two quality factors easily examined are the exterior construction (plastic versus coated metal) and the lens-to-camera connection (should always be metal, and ideally weather-sealed.) Other factors that must be considered are addressed in the specifications for the lens. Ideally, you want a lens with fast auto-focus and, for longer lenses, image stabilization (or vibration reduction for Nikon lenses.) These features will improve your ability to get well-focused shots with no blur caused by camera shake but also add to the cost of the lens.

The most important feature of all lenses is their "speed" defined by the largest aperture or f-stop.  Fast lenses are necessary to freeze motion, shoot in low light, and control the depth of field (DoF.) Fast lenses have a maximum aperture of f/4, f/2.8, or even larger. Since the f-stop is calculated as the quotient between the focal length and maximum diameter of the light path through the lens, it is easy to see that fast lenses (large maximum aperture) are very large lenses, heavy lenses, and expensive lenses.  For example, a Canon 400mm L-series f/5.6 lens weighs about 2.75 pounds and costs about $1,300 while its big brother 400mm L-series f/2.8 weighs about 8.5 pounds and costs about $11,500. Maximum aperture is a very big deal!

Another issue to consider is choosing a prime lens versus a zoom. Prime lenses have a fixed focal length and are generally sharper, lighter, and simpler in design. They also tend to be much more expensive because of the quality of construction. Zoom lenses are generally more popular, more complex, and not as sharp. Zoom lenses are also much more convenient for most photographers since they can carry fewer lenses and still frame their shot conveniently from one position.  Popularity of zoom lenses makes them sell better and in spite of their more complex design, they tend to be less expensive than their prime counterparts.

Now we come full circle to the first question about photographic subjects and focal length of the lens. The focal length determines the field of view and therefore the 'magnification' of the image. A 'normal' lens gives the approximate view we see with our unaided eyes. The focal length of a normal lens on a full-frame camera is about 50mm (or slightly less.) Lenses with a broader angle of view are called wide-angle lenses and those with a narrower view are called narrow-angle lenses - not really(!), they are called telephoto lenses. Wide-angle lenses are great for landscapes and some close-up work and give the feel that the viewer is immersed in the image. They tend to have great DoF and very close minimal focusing distance. They cause some distortion of the image at close range and are very forgiving in focusing.  On the other hand, telephoto lenses are great for wildlife shots and some distance shots. They tend to have a narrower DoF and flatten the perspective of the image thereby removing the viewer from the subject.  They tend to cause less distortion and are very sharp but are more critical in their range of focus and minimal focusing distance.

Some zoom lenses cross the line between wide-angle and telephoto. This is very convenient because fewer lenses are needed for a wider range of images but extreme zoom lenses, for technical reasons, tend to be of lower quality and have issues with sharpness and distortion.

In the next posting we will take on the issue of crop factor and its effects on the image and general care and protection of your lenses.

Exposure details

In the last two postings I discussed the concept of the exposure triangle and how the interaction between ISO, shutter speed, and aperture create the best exposure for a digital image. In the last post I touched on the sunny f/16 rule and how you can use the concept of stops of light to adjust exposure for desired creative effects. Now we will look at the details and practicalities of setting the exposure in a dSLR camera.

Advanced photographers rarely use the Automatic and Programmed modes of exposure control because they want to be able to control the artistic qualities of the image by manually adjusting elements of the exposure triangle.  In the Automatic mode for exposure control, the camera controls the balance between the three exposure elements. In low light it may increase the ISO, open the aperture to a larger setting and slow the shutter speed - all increasing the exposure of the image. This mode may create a grainy, noisy image if the ISO is too high; it may create a shallow depth of field if the f-stop is too large, and, it may create blur if the shutter speed is too slow. Each of these can ruin a photo. The Program mode allows the photographer to set the ISO to a known and constant value but the camera still adjusts the other two elements in a manner that is not always optimal. So, how to we achieve control over our dSLR cameras?

All dSLRs have (at least) three other exposure control modes. In the Manual mode the photographer controls all three element of exposure - usually by adjusting them according to readings from the internal light meter of the camera. Usually the ISO is set first and then either the aperture (to control depth of field) or the shutter speed (to freeze motion or control blur). If the right combination of aperture and shutter speed cannot give an adequate exposure , ISO is adjusted and the process starts again.  This sounds complicated but it is not and with experience, manual exposure control can be properly set in seconds.

Two other modes help the photographer achieve both proper exposure and creative control of the image. In Shutter Priority (Tv on a Canon) the photographer sets the ISO and adjusts the shutter speed for the best creative effect.  The camera then changes the aperture to achieve proper exposure. If the camera cannot get proper exposure with the settings, a beep and/or flashing alert is seen in the view finder and the shutter release may not activate the shutter. In Aperture Priority (Av on a Canon) exactly the opposite occurs - the photographer sets the ISO and aperture and the camera optimizes the shutter speed. This give creative control for depth of field and a huge range of shutter speeds - usually from many seconds to a fast as 1/8,000th of a second. I find that I personally use the Av mode about 80-90% of the time. I can get large depth of field for scenic landscapes using a small aperture (f/16 to f/40) or a large aperture (f/2.8 to f/4.5) to create a shallow depth of field to blur the background and focus the viewer on the subject.

Each of these exposure control modes is dependent upon the through-the-lens light meter of the camera. The light meter is capable of viewing different parts of the image depending upon the need of the photographer. One of the most sophisticated metering settings is commonly called evaluative metering. With evaluative metering the camera assesses the entire frame of the image to adjust exposure for the best combination of light and dark areas. The camera meter senses the entire image and "recommends" an exposure that is the best average for the entire image. It particularly attempts to avoid over-exposure that will destroy details and cannot be recovered by post-processing software. The exposure can be adjusted manually using the viewfinder light meter or the value from the light meter can be used to adjust Automatic, Programmed, Shutter Priority, or Aperture Priority settings.

Similarly, most dSLRs have the ability to select metering from only a portion of the viewfinder.  These metering modes are called partial (field), center-weighted, or spot metering. Each of these metering modes weights a smaller and smaller portion of the image so that only the lighting of the subject will be assessed in the determination of exposure. While each of these metering and exposure modes is very good in determining the exposure for a perfectly lighted subject made of mid-range color tones, judgment is still needed to properly expose subjects that are very white or very black and in background that are very bright (snow or bright sand) or very dark (shadows.) Here, the experienced photographer manually tweaks the exposure up or down using the Exposure Compensation setting. Typically a pure white subject such as an Egret with a neutral or dark background will yield a blown-out, over-expose bird so the exposure should be turned down 1/3 - 1 full stop.  On the other hand photographing a dark brown bison in the snow will need a positive exposure compensation of 1/3 - 2/3 stops to avoid severe under-exposure of the bison.

Understanding exposure and its control is fundamental to good photography. It require a solid understanding of your camera function and controls and experience to select the proper exposure and metering modes and manually compensate for difficult lighting conditions.

In the next posting we will move on to selection of lenses for nature photography.