In the words of Scott Bourne, a professional photographer, author, teacher and pioneer in the digital imaging field ...
First, you should know the criteria for choosing a lens. Some of the criteria are subjective like…Is this lens appropriate for the intended purpose? Is it versatile enough? Do you like the look and feel of the lens? These are important, but hard to measure criteria.
You should also consider more measurable criteria, such as:
- Lens sharpness
- Lens speed (F/2.8 is a faster lens than F/5.6 and faster is usually better)
- Minimum focus distance
- Number and type of lens elements
- Manufacture for film, digital or both (Lenses made specifically for digital cameras will often not work properly on film cameras)
- Amount of lens flare
- Amount of chromatic aberration
- Bokeh (How the out of focus background appears)
- Light fall off (Corner to corner illumination)
- Lens weight
- Lens construction and build standards
- Lens focus speed
Let’s take a look at each of these criteria, and where possible, how you should test them:
1. Sharpness is mostly about contrast. Old timers might say, “That sure is a nice contrasty lens.” They mean it’s sharp. How do you test this? Put a piece of newspaper on the wall and illuminate it with two high power lights, one on each side. Mount your camera on a tripod and make an exposure of the newspaper based on the distance scale on the lens or using the lens’ autofocus. Use as fast a shutter speed as you can for the light conditions and open the lens all the way to its widest aperture. (The smaller number.) Then make on exposure at each aperture and look at the results.
You want to see how sharp the text in the newspaper appears at a 100% enlargement. Pay particular attention to sharpness at the center of the frame versus the edge of the frame. Even bad lenses are typically somewhat sharp at the center but perform poorly at the edges. Look at the results for each aperture. They will be different. While this is less than scientific, it will help you evaluate sharpness in a meaningful way.
2. Lens speed is relevant only insofar as you need good low light performance. But it can also be an indicator of a high quality lens. Most manufacturers put their best glass into their fastest lenses. If you have the money and want the best, buy fast lenses. Also, note that zoom lenses with constant apertures (Say a Sigma 300-800 zoom which is set at F/5.6 regardless of focal length) tend to outperform variable aperture zooms where the lens speed changes with the focal length.)
3. Minimum focus distance is important but because only you know how close you will be to your subjects, only you know how much or how little minimum focus distance you need. Higher quality lenses tend to have closer minimum focus distances, but not always. To test a lens’ minimum focus distance, use a tape measure between the focal plane and the subject. Usually the manufacturer’s estimates are close but not always.
4. Lens element quality is critical to lens performance. I try to use only lenses with low dispersion glass. These typically eliminate chromatic aberrations that can denigrate image contrast and sharpness. Generally, I also look for lenses with Apochromatic and Aspherical elements which reduce distortion, and add contrast and sharpness. The more of these types of elements in your lens, the better. Of course you will have to trust the manufacturer on this point since you can’t take the lens apart to see for yourself.
5. Digital lenses don’t work on film cameras and don’t even work on all digital cameras. Make sure the lens you are buying will work on your cameras. Some lenses will ONLY work with digital cameras featuring an APS sized sensor. On film cameras, these lenses are effectively worthless. Likewise, they don’t work on full frame digital single lens reflex cameras. So if you shoot both digital and film, stay away from special digital lenses.
6. Lens flare is more than little star-shaped rectangles appearing in backlit pictures. Lens flare is often unseen or unnoticed in the viewfinder. Its impact is to reduce contrast. Even when you don’t see a starburst, you may be the victim of lens flare if you use inexpensive lenses. Pro lenses have coatings to reduce lens flare and may even include special lens hoods also designed to reduce flare. Test your lens by shooting backlit subjects and then look for washed out contrast in the picture.
7. Chromatic aberration is color fringing or halos around high contrast objects. It makes pictures look soft and is more prevalent in lenses without LD glass. It’s also easier to spot at wide apertures than smaller apertures. To test your lens for CA, shoot images wide open in high sunlight focusing on high contrast edges. The CA will show up on the edges if you magnify the print. All lenses have SOME CA. What amount is acceptable is up to you. Note than you can correct CA in Photoshop but with most pro lenses; the correction is so minimal as to be unnoticeable.
8. Bokeh is the rendition of out of focus points of light. Shoot any shot with a forest or greenery at least 15 feet from the camera, wide open. Look at the background. If you see well defined edges in the out of focus circles of light, you have a bad bokeh. This is mostly subjective and like everything else, the faster and better the lens, the better the bokeh.
9. Light fall off can be simply measured by shooting any photo wide open that contains sky. The illumination should be the same from corner to corner in a lens that has good light fall off characteristics.
10. Lens weight is the easiest criteria to measure. Weigh the lens and decide if it is lighter than other lenses in its category. You usually want the lightest lens you can find, assuming it meets your other criteria.
11. Lens construction and build standards will help you decide whether or not the lens seems likely to last under day-to-day use. Cheap plastic lenses are more likely to fail than magnesium lenses. Lenses with solid metal lens mounts will not warp as will plastic lens mounts. Sealed lenses will not be as likely to suffer damage from dust and moisture as unsealed lenses. If a lens feels cheap, it probably is. You don’t want a heavy lens, but you do want one that is sturdy and well built. Fit and finish are also areas to consider since they portend overall manufacture quality.
12. Focusing speed can be critical to some types of photography. Since most 35mm camera lenses are autofocusing, you want one that acquires focus quickly and accurately. To test this, shoot the types of objects you normally photograph with the lens you are testing. Have someone next to you with a stopwatch. Set your camera to beep when focus is acquired. Ask them to watch you press the shutter and start the stop watch.
Stop the watch when you hear the beep. Measure the time between the shutter press and the beep and you now know the time it took to acquire autofocus. The less time the better.
13. Image stabilization is increasingly available on higher end lenses. If you use long lenses in the 300mm plus range, stabilization can be very valuable. But not all stabilization is created equally. Canon has three versions of stabilization. Nikon has two. Some work on tripods while others do not. Some work in panning mode while others do not. Make sure you know what you are getting here before you buy a stabilized lens.
14. Ergonomics refers to the usability of the lens. Do the focusing rings turn easily? Are they well placed? Does the lens fit your hand? Can you still turn a polarizing filter when the lens hood is attached? Ask these questions when testing a lens. The answers will impact you every time you use that lens to make a photograph.
15. Price is perhaps the most important factor for every photographer. If you cannot afford a lens, it doesn’t matter how good it is. Compare all the features and criteria mentioned here and then mitigate each by its cost. While it’s great to have a lens with three pieces of low dispersion glass, that costs more than one with two. Maybe two pieces of LD glass are enough for your type of photography.
Always evaluate price from the standpoint of knowing what you are paying for and whether or not you need it.
Just carefully examine each of the criteria in this article before making your choices and you will be armed with the best information you can get when picking a lens. And if one of the geniuses on the Internet photo forums pans your choice of lens, ignore him unless he can articulate the 15 criteria mention in this article.