Unfortunately for anyone purchasing lenses, there are a myriad of lens qualities, specifications, and characteristics which should be considered. Even if you’ll be purchasing nothing but the absolute best quality pro lenses, you will be weighing choices and making compromises.
You’ll have to decide the relative importance of specifications and characteristics for you photography to make your lens choices. This article should help you familiarize yourself with what you need to consider, and why, when purchasing your lenses.
- Before DSLR cameras, all “35mm” SLR cameras had the same size film-frame area on which the image was captured. When the first DSLRs were introduced they had sensors which were smaller than “35mm” SLR film-frames. Nikon calls them DX. Canon calls them APS-C.
Some lenses are specifically designed for the smaller sensor based cameras. If placed on an FX (Nikon’s designation for a “full sized” sensor, the equivalent of the “35mm” SLR film frame.) the outer portions of the image will be black as the shaft of light coming out the back end of DX/APS-C lenses will not fully cover an FX sensor.
If you're planning to move to an FX sensor based camera, consider sticking to non-DX/APS-C lenses. Otherwise, don’t hesitate to get a DX/APS-C lens. One of my favorite lenses on my Nikon D200 is a DX lens.
In Lenses for travel photography - Part II (Important Specifications) I discussed the basic specifications of lenses such as: focal length, maximum aperture, magnification, vibration reduction, and auto-focus. You should go back to Part II for a more thorough discussion of those attributes. Some of what was discussed will be repeated below.
- Mount Type: For example, Nikon SLR/DSLR cameras have a different lens mount than Canon models. Make sure that the lens you’re purchasing has the right mount for your camera.
- Focal Length: The focal length or focal length range (zoom lens) of your lens is indicative its type of lens (lenses); wide angle, normal, telephoto, etc.
- Zoom Ratio: This ratio between the largest and smallest focal lengths a zoom lens has.
- Maximum Aperture: The largest (fastest) aperture opening a lens has (may be a range for zoom lenses).
- Minimum Aperture: The smallest aperture opening a lens has. Using this aperture will give the longest depth of field of the lens, but to be effective at this setting, the scene must be very bright.
- Angle of View (for zoom lens minimum and maximum): Will depend on the sensor size of the lens. For DX/APS-C sensors, this angle will be narrower than for FX, full sized sensors.
- Compatible Formats: This has to do with which sensor or film sizes with which the lens will be compatible. Lens which work with “35mm” film SLRs and FX sensor DSLRs will work with DSLRs having a smaller sensor, however, compatibility doesn’t work in reverse.
- Auto-Focus: A lens may be listed as an auto-focus lens, however, that doesn’t mean it will auto-focus with all auto-focus cameras. Not all lenses contain an internal focus motor, and some cameras don’t have their own motors to auto-focus lenses without an internal motor. AF-S/HSM (Nikon/Sigma) are examples of lens designations which have internal motors, which are fast and relatively silent, to provide lens auto-focus. Internal auto-focus motors are generally faster and superior to having the camera provide auto-focus.
- Focus Limiters: Some lenses don’t focus quickly and if the subject is well out of focus “hunt” for the correct focus. A focus limiter sets a narrow range of focus to force quicker focusing, and limit hunting. Typically the full focus capability is divided into two focus ranges, if this is present.
- Minimum Focus Distance: This is the closest a lens can be to the subject and focus.
- Filter Size: Typically SLR/DSLR lenses can have filters screwed into the front of the lenses. This is the size of such filters for the lens.
- ED/APO: This indicates the lens has special low-dispersion glass designed to minimize color aberration.
- VR/OS (Vibration Reduction/Optical Stabilization): This indicates the lens utilizes a built-in mechanism which compensates for camera shake. It is especially important for camera/telephoto lens combinations being handheld.
- IF: This means the lens has internal focusing so that the length of the lens doesn’t change as the lens is focused. This is especially important with macro lenses so the lens doesn’t bump into the subject while being focused.
- N: This means the lens has a nano-coating to reduce ghosting and lens flare problems.
- Build and Quality: Lenses vary considerably in terms of their quality of construction. You can often notice the difference in this simply by picking two lenses up and feel the weight of them. One may contain a lot more plastic than the other.
- Soft Focus: This is often more of a problem for zoom lenses which generally have more complicated optics. This can occur only in the corners of the image, or more across the image. The f/setting the lens is at can affect the quality of focus.
- Lens Flare and Ghosting: Flaring is when light is scattered manifesting itself as starbursts or rings, while ghosting is when you get a second lighter image out of sync with the actual image.
- Bokeh: This is the blur, or really the aesthetic quality of the blur, in the out-of-focus areas of an image, actually the way the lens renders out-of-focus points of light. This is especially important for portrait and wildlife photography where you want to ensure the area surrounding the main subject doesn’t detract from the subject.
- Vignetting: This is the underexposure of an image in the corners.
- Spherical Aberration: This is when there is an increased refraction of light it strikes a lens near its edge, in comparison with the center and results in “halos” in the image.
- Chromatic Aberration: This is caused by light rays of different wavelengths coming to focus at different distances from the lens, and results in both blurring and color fringing, generally a high contrast points in an image, and along what should be sharp edges.
- Distortion: These are different effects due to the design and construction of the lens which result in a distorted appearance. For an example, an rectangle may appear as a barrel or pin cushion-shaped object. This is often seen in the two focal length ends of zoom lenses, which in the middle focal lengths these effects are absent.