There&#;s a lot of talk about cameras and, consequently, lenses get overlooked. With competing priorities when you buy a lens, here are some things that could affect your decision on which lens to buy.
In photography, there is always a payoff. Whatever advantage a particular choice brings, there will be a disadvantage. Also, any rule in photography should be treated as if we mean &#;as a rule,&#; and not a hard fact. There are always exceptions. So, as a rule, if we consider buying a lens we want it to help us get the best quality photo possible. So, we&#;ll start by thinking about what we want to avoid.
An aberration is a lens defect that prevents the lens from forming a perfect image. Often, this will mean that some of the light entering the lens will be focused too near or too far from the focal plane. Sometimes this may be light of a particular wavelength, so light is split, causing colored fringing most visible around high-contrast edges. That effect is called chromatic aberration.
Shot with a Nkon bridge camera back in , and nowhere near the optical standards we expect today, colored fringes are clearly seen on the edges against the sky and the focus is off. Click on the image to see a larger version.Visible lens flare is an aberration where light enters the lens and bounces off internal surfaces. This is usually unwanted. However, it can be used as a creative effect. Think of JJ Abrams&#; films and the horizontal lines of lens flare resulting from the anamorphic lenses used in cinema. Good quality lenses have coatings on the internal lens elements that prevent light from being reflected from their surface.
The lens flare seen here was caused by a scratch on the front element. Shortly before this shot, I had removed sea spray from the lens. Despite my care, a grain of sand scratched the glass.There&#;s a second kind of lens flare where the image's contrast is reduced by the light falling on the lens' front element. This can be avoided by using a lens hood that shades the lens.
Barrel distortion is a lens aberration that stretches the image around the edges. Thus, a square becomes barrel-shaped because magnification reduces towards the edge of the lens.
Barrel distortion to a rectangle.The opposite, pincushion distortion, is more likely to be seen on a telephoto lens. In that case, the scene's edges are distorted inward, towards the center.
Pincushion distortion.Vignetting is darkening at the corners and edges of the frame. It is caused by light at extreme angles being blocked off by the lens barrel. It can sometimes be avoided by zooming in, thus cropping the vignette, or stopping down the aperture. The smaller aperture reduces the overall amount of light and the angle at which the light enters the lens is also reduced. Thus, the light from the center of the lens is visible at the corners.
Many fast fixed focal length lenses have vignettes that rapidly disappear as the aperture size reduces. However, one must also be aware that small apertures can result in softer pictures as the light refracts (bends) around the aperture blades, scattering the light before it hits the sensor.
I couldn't find any images in my catalog with a vignette, so this was added in the image development as an illustration.Lens breathing is an unwanted change in focal length as the focusing distance changes. Even expensive prime lenses can exhibit it, but cheaper lenses tend to exhibit it more, thus making photo stacking more difficult.
Beauty defects are visible cosmetic flaws in lenses that don&#;t adversely affect the image. When you buy second-hand lenses, retailers will mention that dust is visible inside the lens, but this won&#;t affect image quality.
When buying gear, price may be an important factor for you. Nevertheless, the more you pay the better it is. Which bracket should you choose? Unless you have a blasé attitude to image quality and are just snapping away selfies for social media, get the best you can afford.
If you are paying big bucks for a lens, it had better be good. This Nikon 800mm lens costs $16,296.95.Lenses usually fall within one of three quality brackets. The low-end hobbyists&#; lenses are cheaper and don&#;t perform as well as the mid-range enthusiasts&#; lenses. Those, in turn, are not as good as the top-quality professional lenses. What goes into producing a top-of-the-range lens is a far cry from the mass-produced kit lenses that come as standard on entry-level cameras. That doesn&#;t mean those cheap lenses are necessarily bad, though some are, but more expensive lenses are better.
Although it wasn&#;t their original definition, prime lenses are now considered to be those with a fixed focal length. Zoom lenses can vary their focal length. The quality of zoom lenses is far better now than twenty years ago and can have superb image quality. However, prime lenses in the same quality bracket will usually give better image quality, but they don&#;t have the versatility of a zoom.
A Sigma 16mm f/1.4 prime lens.Optical quality is usually an important consideration for photographers. Good lenses minimize distortion and aberrations and so deliver sharp, clear images. To achieve this, lenses require meticulous design and manufacturing. That is why the best lenses are often the most expensive. Top-grade lenses have aspherical elements made with higher-quality glass. That glass requires ultra-precise grinding and polishing. They will probably contain one or more extra low-dispersion elements that reduce chromatic aberrations and high-refractive index elements that help manufacturers make lenses smaller and lighter. This glass has several different proprietary names depending upon the manufacturers, but you will usually see them mentioned in the lens specifications.
Good quality lenses should be fast to focus and not exhibit lens flare, chromatic aberrations, or other defects even when shooting in challenging conditions such as contre-jour.Looking at the specifications, you may notice that lenses have different numbers of elements. Fewer elements should give better light transmission. However, that doesn&#;t necessarily translate into better image quality. More elements can help correct aberrations. Furthermore, more complex designs, such as those found in zoom lenses, require more elements to work.
The focal length is the distance from the lens' optical center to the sensor when the lens is focused on infinity. Without getting too hung up on the physics, the longer the lens, the narrower the field of view, and the closer objects will appear. Thus, distant objects seem more magnified with a long lens because they fill the frame. Wider-angled lenses mean a broader view appears in the picture, so subjects appear smaller.
Using a 12mm wide angle lens, distance appears exaggerated and the photo appears to have more depth. The island seems far away compare to the next image despite it being taken physically closer. In this shot is it approximately a mile from where I stood. The vignette was added in development.The focal length changes the appearance of a scene in other ways too. A wide angle lens will make objects close to the lens seem far larger than those just a little way back. You&#;ve probably seen the pictures of kittens and calves where their noses seem unnaturally large. These were shot with short wide angle lenses close to the animal's face. It's cute with baby animals but not a good look on humans. Meanwhile, long telephoto lenses appear to compress the image and can flatten subjects so they look like cardboard cutouts. They make distant objects appear much closer to objects at the front of the scene.
Using a longer lens, the island is more than 1.8 miles from where I stood but appears much closer because of the 500mm focal length.There&#;s no hard and fast rule here and the numbers may be different depending upon the sensor size of the camera you use. But as a generalization, a typical wide angle lens might be somewhere in the range of 7-30mm, depending upon the camera. They are great for landscapes. Telephoto lenses are often 60mm or longer, although usually in the low to mid-hundreds. They are used for photography where the subject is farther from the camera, such as sports and wildlife. In the middle are standard lenses, normally employed for portraiture.
Standing at about the same location where I took the first island picture above, this time my lens was set to 25mm.A mistake many beginners make is buying cheap, long lenses. Big is better, right? Wrong! The longest possible lens isn&#;t necessarily the best solution. Unless you pay lots of money, long lenses are slower and have poorer image quality. Getting physically closer to your subject will always give you better results. Also, zoom lenses with a limited range perform better across all focal lengths than those with a smaller range. A 70-200mm lens will be better than a 70-300mm.
The size of the aperture determines how much light enters the lens. Wider apertures (i.e., lower f-numbers) allow better low-light performance, therefore faster shutter speeds at lower ISOs are possible. They also give you more latitude for controlling the depth of field.
Bokeh with balls of light reflected odd the water's surface.The number of blades and their shape will affect the nature of the out-of-focus area, or bokeh. A smooth, creamy bokeh usually results from a high-quality lens with many rounded aperture blades. That too will create round balls of light in the bokeh, whereas six-bladed apertures will produce hexagons. What's seen as undesirable are asymmetrical light balls.
Construction and durability are important factors. I recommend lenses that have at least a metal bayonet mount. All-metal lenses withstand wear and tear better, ensuring longevity. They are usually more expensive than their plastic equivalents. However, plastic lenses are lighter.
Autofocus performance is important too. Fast and accurate autofocus enhances usability. Cheap bottom-end lenses are not great at tracking fast-moving subjects.
Older lenses used a screw-drive mechanism for adjusting the autofocus. Cheaper lenses replaced those with a micromotor, which uses tiny gears to drive the lens. Higher-quality models use faster piezoelectric motors. These come under a variety of names such as Supersonic Motor (SSM), Voice Coil Motor (VCM), Ultrasonic Motor (USM), Silent Wave Motor (SWM), Hyper Sonic Motor (HSM), and many more. This mechanism type is both fast and quiet. Although their design can vary, they are all based on piezoelectric materials that change shape when they receive an electric current.
To capture action like this tern emerging from the water after its dive you need your lens to focus quickly.Do look at splash and dustproofing. There&#;s no point in buying a weather-sealed camera if the lens isn&#;t similarly protected. However, with one exception, there is no legal definition of weather-sealing and I know people whose supposedly splashproof cameras and lenses have died after a rainfall. Ideally, you want a camera system with an ingress protection (IP) rating, but only one manufacturer offers this.
For more information, please visit Hongsheng.
With my camera standing on a tripod in the torrential downpour, I needed to be sure of its IP53 weatherproofing.There are numerous manufacturers of third-party lenses. Sigma and Tamron make some consumer-level and high-quality lenses, and the former is often subcontracted to make lenses for camera manufacturers. Generally, their equivalent lenses are cheaper than the manufacturers' own models, but may or may not have quite the same performance, so do your homework.
Other manufacturers such as 7Artisans, TT Artisans, Meyer-Optik Gorlitz, Lensbaby, Samyang, and SLR Magic produce a wide range of lenses. Some are high quality, while others are quirky and good fun.
You can also buy adapters to fit vintage lenses from film SLRs onto most cameras. These give a completely different feel to your photos.
The Lensbaby Composer Pro IIEnsure the lens fits your camera&#;s mount and sensor size. Lenses from the same brand don&#;t necessarily fit all the bodies from that brand, and the recent shift to mirrorless cameras from DSLRs caused much confusion for people.
Finally, decide what you need and research whether a lens meets your needs. Then, if you can, take your camera to a camera shop and try it with that lens. Take some test shots and see how it performs before you part with your cash.
If you are thinking about buying a camera for the first time, please check out my article about that.
TV distortion values are typically much lower than geometric distortion values (See Figure 1 on the previous page). We choose to present a geometric distortion value on our data sheets because it provides a much truer indication of lens performance than TV distortion, but that lower distortion number on the competitor&#;s data sheet sure looked attractive.
It almost worked, and there&#;s the rub!
The technical data sheet &#; the most basic form of communication about lens specifications in our industry &#; should provide an objective and uniform key to helping buyers compare lenses. But it doesn&#;t. Data sheets can be very misleading. The comparisons are not apples to apples because the format and the data presented in data sheets are not held to an international standard and are not overseen by a governing body. Additionally, even when certain lens parameters within a data sheet are tied to an ISO standard, the standards themselves are poorly understood by the engineering community.
Have you ever looked at data sheets from Japanese, German, and U.S. lens manufacturers side by side? They have almost nothing in common. But the problem isn&#;t rooted in cross-cultural issues. Compare data sheets from three lens makers in the same country, and the result is the same. Each company uses its own format and nomenclature. Each company essentially is reinventing the wheel, but it is the buyer of the lens who, as we&#;ll see, faces the prospect of a flat tire.
In order to help optical engineers and system designers view lens data sheets more critically, better circumvent traps hidden within, and ask the right questions of lens providers, we suggest focusing on the following key datasheet parameters:
1. Modulation transfer function (MTF)
2. Focal length
3. f-stop
MTF: A Convenient Way to Mislead
The single most important thing for a buyer to consider when trying to decide if a specific lens will meet their system needs is to look at lens performance. The data-sheet entry that best approximates performance is modulation transfer function, better known as MTF.
What most optical engineers and designers don&#;t realize is that there are many different types of MTF, each with varying values that can artificially inflate an estimate of lens quality. Because there are no standards on how lens makers report MTF on a data sheet, it is left to the buyer to ascertain which kind of MTF he or she is assessing. This quandary is made even more difficult by the fact that many vendors use the term MTF generically on their data sheets.
Don&#;t fall for it. All MTFs are not created equal.
The generic use of MTF is a convenient way of misleading the reader. Used generically, MTF is commonly shorthand for &#;geometrical MTF,&#; a value that does not take into account diffraction, and thus provides a higher, seemingly more attractive number. The most rigorous and useful type of MTF &#; one that is the best indicator of the lens&#; true performance potential in a vision system across wavelengths &#; is called &#;polychromatic diffraction MTF.&#;
Calculating polychromatic diffraction MTF is a better way to evaluate lens performance than geometric MTF because it better reflects how a lens will be used in the real world. Namely, it takes into account the physics of optics, which includes diffraction, something not accounted for in geometric MTF. Again, you can see from the calculations that polychromatic diffraction MTF is a higher number, and, to the untrained eye, that means &#;less good.&#; Not so. (See Figure 2 on the next page).
Geometric MTF, a much easier calculation than polychromatic diffraction MTF, represents how MTF was calculated 30 to 40 years ago, i.e., before computers. For engineers building high-end optical systems to operate in pre-PC environments, geometric MTF is just the ticket!
We&#;re being cheeky here, of course, but there is nothing funny about not labeling what kind of MTF a lens manufacturer is including on its data sheet. To pass off geometric MTF as true MTF and hope the buyer doesn&#;t look closely is not illegal, but it&#;s not right. Unfortunately, this is an area where the lack of transparent, highly understood data-sheet standards has hurt the experienced engineer and neophyte alike.
To be clear, manufacturers who do this aren&#;t breaking the law. The MTF number they are providing is accurate, but by leaving out an MTF descriptor (&#;geometric&#; in this case), the manufacturer creates a circumstance where the buyer is likely to stumble upon a significant gap in performance that will slow down, add to the cost of, or imperil the development of their vision system.
At Schneider Optics, we often answer questions from prospective buyers about why our MTF values don&#;t compare favorably with various competitors. Nine out of 10 times, they are shocked to learn not only that there are multiple kinds of MTF, but also that they haven&#;t been comparing apples to apples. Now they know. And so do you. Focal Length: When 100 &#; 100 Even when there is an International Organization for Standardization (ISO) or Deutsche Industrial Normen (DIN) standard associated with a lens specification, many buyers are unaware of the standard and, of even greater consequence, the potential ramifications of that standard on their vision-design system. Lens focal length (f') provides a telling example.
Fig. 2: Diffraction MTF vs. Geometrical MTF for the same lens
Diffraction MTF Polychromatic
Geometrical MTF Polychromatic
The focal length of a lens is pretty straightforward, right? Whether you need a 100mm lens for a camera, or 20 100 mm lenses for a vision system monitoring a manufacturing process, the data sheet states unequivocally that the buyer is getting a 100mm lens. But did you know that according to DIN standard , section 11, the standard for engraving a lens for focal length is ±6 percent? The majority of lens buyers don&#;t &#; this isn&#;t taught in engineering school!
For that company buying 20 100 mm lenses for its web-inspection system, say for measuring defects over a large swath of material, the impact of the DIN standard for focal length is significant. It could mean that instead of buying 20 uniform 100 mm lenses, they are actually getting lenses of varying focal lengths, running from 94 mm to 106 mm (where the delta between what they buy and what they get = aggravation, extra work or rework, and, at its worst, a system failure, exposing the engineer&#;s ignorance of the lens-buying process). In practical terms, this would mean every single lens and camera would need to be set up at a slightly different distance to give all the same magnification and field of view. Think, too, of the poor stereo-vision designer!
One way to fight back against this imprecision is to ensure you know exactly what you are buying. And to do so, you have to ask questions. A reputable manufacturer of quality lenses will be able to answer your questions about focal length and tolerance from lens to lens, lot to lot, and year to year.
An empowered buyer needs to know more than the data-sheet spec: he or she must drill down on the presented values and only work with manufacturers who really know their wares, produce lenses at tight tolerances, publish them with accuracy, and work collaboratively with you to ensure you are buying the best lens for your system.
f-number: Seeing the Light
Similar to focal length, the f-number (f/# or f-stop) &#; what controls the amount of light going through the lens &#; is addressed by an international standard, but, similarly, most buyers don&#;t know the standard. And, again, what they don&#;t know can hurt them.
We think you will be surprised by what the ISO standard (ISO 517) dictates in terms of f-stop tolerances. When the f-stop is 5.6 or slower &#; for example, settings of f/5.6, f/8, f/11, f/16, or f/22 &#; the f-numbers need only be accurate to ± one-half a stop, or ± 25 percent of the light. That is a huge difference! Imagine trying to set up 20 lenses in a row in your vision system with each turned to f/8, knowing they can all be off by± 25 percent! And when the lens is faster, set to f/4, f/2.8, or f/2, the tolerance is ± one-third of a stop, or ± 16.6 percent of the light. You could drive a bus through these gaps, which certainly doesn&#;t help when you are trying to set up multiple lenses in a row for your vision system.
Some might consider a simple workaround to these variances. For example, couldn&#;t the f-number be adjusted slightly for each lens, so each had the same amount of light in the system? Yes, but remember what the lenses are being set up to do. The lenses would no longer be working at their proper f-number, affecting the depth of field and, consequently, the focus!
This topic of workarounds brings us back to our aerospace engineer mentioned earlier. The lens specification that tripped him up was distortion, another spec without a standard. The real distortion of the lens he almost bought would have crippled the system being designed, something that could have delayed the launch of a satellite and cost people their jobs. The error surely would have been detected downstream before a more catastrophic consequence, but that&#;s not good enough. Engineers work in a world of exact values and need the means to get them before they buy.
Conclusion
Experience is a great teacher. Through many years of experience, we have learned a lot about the shortcomings of data sheets and the rules, or lack thereof, that govern them. Here&#;s a quick summary:
1. Technical data sheets for optical lenses are not governed by standards, enabling each vendor to present the data it wants in the way it wants, which can lead to misleading performance claims and hamper apples-to-apples comparison.
2. Generic specification terms, such as MTF, often gloss over exactly what they are measuring and the variety of ways they are calculated, again leaving it to the buyer to do the detective work.
3. Even when specs are regulated by an ISO or DIN standard, the standards are often: a) not well understood by lens buyers; and b) typically wide enough to drive a bus through them. Efforts to bring industry players together to develop common standards have failed and been abandoned. This puts the onus squarely on the buyer to become better informed and more inquisitive. It also leaves it up to reputable manufacturers to move away from subterfuge and present their data clearly &#; calling out the type of MTF, for example.
And where these two parties &#; buyer and seller &#; will continue to intersect is on the . Informed buyers need to ask the right questions, digging into the details behind key specs. In response, quality providers need to wow their buyers with a thorough knowledge of their lens tolerances and willingness to share them in a helpful and transparent way.
In the meantime, remember, you get what you pay for. Caveat emptor.
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