Choosing A Lens - More Than Sharpness
When many people talk about lenses, the conversation seems to gravitate toward sharpness and stay there. Although sharpness is important, it's not the only consideration by far. If you want better results from your camera and lenses, you need to think about lenses in greater terms than just their sharpness.
This page describes many different aspects of a lens you might consider. Throughout the article, I also try and describe when and why a particular aspect might matter.
How To Test
The best way I have found is to
rent a lens you are thinking of buying for a week and use it as you would in "the real world". It's also good to review the qualities listed below and explore the ones you think you might run into in your type of shooting. I find
Lens Rentals to be a reliable place to rent from. Another approach is to borrow a lens from a friend or coworker.
E-bay and
craigslist are also nice options if you are comfortable buying and selling.
Lens Qualities
I cover a
lot of material in this article. I thought about breaking it up but decided there is value in being comprehensive. To support a a quick read, I try to summarize the key points of each section in the first paragraph.
Specification aspects are aspects you can find in a lenses specification sheet and are often present in the title of the lens.
This is a set quality of a lens which specifies how large the
aperture can be.
Aperture is usually measured in f-stop, which is a ratio of focal length and aperture diameter. For example, a 50 mm lens with an aperture opening of 25 mm is set to 50/25 = f/2. Terminology can be confusing for beginners; here are the basics:
- "Large Aperture" = "Fast Lens" = small f-stop number
- "Small Aperture" = "Slow Lens" = larger f-stop number
- "Stopping Down" means making the aperture smaller (increasing f number)
- "Wide Open" means the lens is set at it's largest aperture (smallest available f number)
Subjectively, a "fast" lens would offer a max aperture of f/2.8 or faster while a "slow" lens would be f/4 or slower.
A larger max aperture allows for
less depth of field and, as a directly related effect,
faster shutter speeds for the same lighting conditions. A large maximum aperture can make the difference between getting enough shutter speed in lower light and not making it.
Larger max
apertures also allow for
more precise focusing (unless there is
focus shift, which is described later).
For landscape and macro application, max aperture is less important. It can even be a downside as the lenses will be bigger and thus harder to carry. The larger aperture might also bring along a need for larger filter sizes and increased flare issues.
Nearly all "fast" lenses have
more aberrations (described later in this article) "wide open" than stopped down a little. Many modern lenses are trying to combat this with complex designs and finding some success, but there are always trade-offs. One interesting one is that correcting for
spherical aberrations too much can lead to less-pleasing
bokeh - dang!
Larger max apertures, in theory, give a brighter image in optical viewfinders. In most cases, this is not true however, because modern optical viewfinders use
Fresnel designs that collect and focus light rays into our eye. The trade-off of a Fresnel lens viewfinder is that brightness (and focus accuracy) benefits for very bright lenses disappear. Some (fewer by the year) cameras allow these screens to be interchanged for screens with weaker (or no) Fresnel lens. These
focusing screens can focus brighter lenses more accurately, but give a dark image with slow lenses.
When using an electronic viewfinder or live view, a larger max aperture can allow for a
cleaner image with a higher refresh rate when the light get low. Unfortunately when the light is bright, many cameras automatically stop down their live view aperture for what I assume are heat-related issues; this makes depth-of-field larger than it will be in the captured image; it also makes focusing less precise.
Focal length (along with sensor size) determine the
field of view a lens will have on a given camera. The shorter the focal length, the wider the field of view.
Focal length is a number measured in millimeters. For a prime lens, it's a single number (such as 35 mm). For a zoom lens, it's a number range (such as 24-70 mm). I cover the meaning of focal length in detail
here.
The focal length range of zooms is convenient but note this is invariably trades for other desirable aspects. Common trade-offs include distortion, max aperture, field curvature, and bokeh quality (but usually not sharpness, since it's a primary conversation point for people).
Another drawback of zooms is that trade-offs often vary by focal length. For example, distortion might be fine at 50mm but poor at 17mm. Or bokeh might look good at 35mm but not as nice at 24mm. The list goes on... These can make a zoom lens more difficult to fully master and predict, verses a prime.
Some people are under the misimpression that moving the camera closer to an object is similar to "zooming in" with a zoom lens while standing still. They are not the same. Moving the camera changes perspective where objects in the scene will change shape and relative position to one another. While zooming, the perspective does not change. Zooming instead has the same general effect as center cropping, but without the loss of pixels.
Maximum magnification relates to how closely a lens can focus.
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| Only "macro" lenses can focus this closely |
Maximum magnification is specified as a ratio (such as 1:1, 1:2) and represents how large of an image a lens can project in-focus. A 1:1 ratio means that a "life sized" image can be projected on the sensor. Such a lens is a macro lens. Most non-macro lenses do not do nearly this well. For example, a Canon 50mm f/1.4 has a maximum magnification of about 1:6.
If you plan on photographing flowers or other small objects, maximum magnification is an important aspect to consider.
These aspects represent imaging performance qualities of a lens that designers can control (often trading off one for another). These are not obvious in spec sheets, although some aspects can be estimated if you read a published
MTF chart.
Center sharpness is the measurable detail captured at the center of an image.
Under test conditions, center sharpness represents the
resolution of a lens. For human perception, a combination of resolution and local contrast collaborate as "perceived sharpness".
Resolution capability varies by aperture with wide apertures
limited by aberrations and narrow apertures
limited by diffraction. Both of these limits are explored later in this article.
Center sharpness is where most people start and, unfortunately where many end when evaluating a lens.
Resolution is revealed when the image is viewed very large and can be invisible at smaller sizes or greater viewing distances. Contrast-based sharpness can easily be added by software. It appears as sharpness at smaller sizes and can appear as
sharpening artifacts under magnification.
Resolution measurements are
loaded with complications which you should be aware of when researching them:
- Lens testing sites will often list sharpness as pure numbers but don't really give you a feel for what the difference is. For example, if one lens can do 2500 lines per picture height and another can do 2700, under what conditions will this even be detectable? It's generally left unexplained...
- Changing a camera sensor changes the resolution numbers for the exact same lens:
- It's intuitive to think of a lens-sensor system as a "weakest link" system where the worst component will limit the resolution. This intuition is incorrect. Resolution will increase with a higher-megapixel camera, even for even for cheap lenses. As an example, check out this same lens tested on three camera bodies and note how higher sensor resolution makes overall resolution numbers better too. The mathematics are described here.
- Some sensors use a strong antialiasing filter, while others have none. Antialiasing filters help prevent false color artifacts in well-focused fine-detail objects (like feathers and fabric), but they lower resolution as a result. To see the trade-offs of antialiasing filters, I prepared this link. The D810 sample in upper right image in the test has no antialiasing filter and shows higher resolution in trade for false color.
- Transforming a RAW image into a visible image often involves a contrast-enhancing sharpening step (adding the illusion of resolution via local contrast). If a tester is not controlling this variable, it can influence the results greatly.
- Finally, sharpness varies per copy of the same lens, sometimes widely. When you see a test number on a website, was it a good copy or a bad one? For additional context, see Roger Cicala's excellent article on the subject of testing and variance. For this reason, I'm skeptical of resolution results from single lens copies.
Quick Test: You can check lens testing sites such as
DXOMark,
Photozone or
Lenstip, but understand the limitation described above. Another test is to simply take pictures under good lighting and see for yourself.
Pretty much all non-damaged lenses have less measured resolution in the corners than in the center. This is less a real-world problem than you might guess. For example, corner sharpness rarely matters in portrait photography. As a test, consider looking through your recent photos to determine how much it matters for you.
Again, with landscapes, corner sharpness is mostly irrelevant until f/8 (full frame).
Quick Test:
I usually do an infinity test where I focus on a distant tree (pine trees work nicely)
Some people do non-infinity testing, but this requires very precise alignment to avoid invalid and misleading results.
I first take a photo with the tip of the tree centered. If enabled, auto focus should be turned off after taking the first shot. Shutter speed should be at least 3x the focal length (e.g. 1/150 or higher for a 50mm lens) or use a tripod. After the shot, confirm good focus with magnified review.
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| The upper left image has a distant pine tree tip centered and focused. In the shots that follow, the pine tree is placed in each corner. |
To complete the test, I pan and take a photo with the tree tip in each corner. Remember, auto focus must
not be enabled or a focus change could invalidate the result.
I do this test
wide open and
stopped down to f/5.6 or f/8.
I evaluate results at a computer, comparing magnified views of the the center with the corners. The center can be sharper than corners on a perfect lens copy (due to it's design). If the corners have different sharpness relative to one another, the lens might be
decentered. If you find a difference, I recommend testing again later to prove consistency. If you have other lenses or cameras, it might be good to try mixing them in to eliminate possible testing errors as well. If you still have issues, read
this, then make your choice about returning the lens :)
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| Now the brutal test of comparing each corner at 100%, not for the faint of heart! :) |
A lens with poor
flare resistance
readily shows flaring artifacts when there is a bright light source in the frame (such as the sun).
A lens with good flare resistance can still show flare, but in more extreme cases, like a bright light in a frame corner.
Most (if not all) lenses have better flare resistance to lights in the center than the corners. Flare resistance is an important aspect in landscape photography and, unfortunately, many landscape-oriented lenses do more poorly here than one would hope. Last year, I rented a $1,500 landscape lens which I will never be tempted to buy - the sole reason is the disappointing flare results I saw.
Flare manifests itself in three main ways.
One is
reduced contrast, either in part of or all of the frame.
Another is a
series of colored discs (or other shapes) in the image. Sometimes these can be quite pronounced.
Finally, you may see
"beams" of light emanating from a bright light source, such as a the sun,
Flare is a bit tricky to test as no lens is fully immune to it. That said, it's definitely more of an issue on certain lenses.
Flare is clearly also usable in pleasing artistic ways but it can be annoying if that's not the intent or if the flare renders unfavorably (e.g. a strong contrast reduction or giant purple discs).
A classic way to reduce flare is to use a lens hood, although this works best when the light source is out-of-frame. Another, less satisfying, approach is to adjust composition. Sometimes very slight adjustments can make a big difference.
Quick Test: If you have a somewhat wide angle lens, take a few pictures with the sun in different parts of the frame. Longer focal lengths can be used too but note that the sun will be more intense and can put your equipment and eyes at greater risk. For longer lengths, try the moon or a bright street light instead.
Some lenses (especially "fast" lenses) can suffer from
focus shift. Focus shift is where the focus plane changes based on chosen aperture. Since many cameras focus wide-open, this can create a difficult-to-resolve focus problem.
Focus shift can especially manifest itself when an afflicted lens is slightly stopped down. In other words, if you have a f/1.4 lens with focus shift, it might focus perfectly at f/1.4, look a little off at f/2.0, then be fine again at f/4.0 (where the depth of field is large enough to hide the slight change). Avoiding the "danger zone" aperture settings can be an effective workaround, but an annoying one.
Quick Test: A Google search for the lens model plus "focus shift" should be enough. If you feel like experimenting, try photographing a wall or focus chart "wide open", then stopped down a slight amount. Remember to use manual focus or the test results will be invalid.
Coma is a quality where the
out-of-focus "circle of confusion discs" stop looking like circles. It contributes to muddy-looking corners, especially if the corners are not in focus. Many bright and wide angle lenses show coma, even "perfect" copies due to the large menu of trade-offs involved in designing these optics. Damaged lenses with misplaced elements can show it as well.
Quick Test: Lenstip does coma tests; see if they tested your lens
. You can alternately take a photo with the corners slightly out of focus. Do the corners look natural or muddy?
This is an effect where
horizontal and vertical light rays don't focus on the same plane. In specialized tests such as lasers entering the lens, you would be able to see how lasers coming in horizontally and vertically would focus in different places. With general light that's coming in from all directions, the image will lose it's "crispness" and be unable to focus sharply at any setting.
Astigmatism is usually not a big issue with non-damaged lenses. Stopping down the lens is an effective way to limit the effect if it's showing up.
Quick Test:
Lenstip measures and reports on astigmatism.
Lenses bend light using
refraction and refraction naturally creates
dispersion. In simple terms,
different colors of light are refracted (bent) slightly differently by glass. Unless you are using a simple magnifying glass, your lens has additional elements to try and balance this out but all things in lens design involve trade-offs and 100% correction may not be achieved. Higher megapixel cameras tend to make CA's look worse because they scrutinize these details more.
- Lateral: In this case, different colors are projected on the sensor in slightly different places. This creates a fringing effect. This problem is mostly correctable in Lightroom and other tools, even in-camera.
- Longitudal: In this case, different colors are focusing in front or behind the sensor. This is not easily fixed as some of the light has actually lost detail (it's out of focus). Correction software that tries to fix this generally use de-saturation tricks that may degrade the image.
- Secondary Longitudinal: This type relates to colors that are outside of the focus plane. It's very common, especially in very fast and wide lenses, for color fringing to occur in high contrast out-of-focus areas. This will appear as "purple bokeh fringing" in front of the focus plane and "cyan bokeh fringing" behind it.
Quick Lateral CA Test: High contrast lighting brings these out. A classic example is backlit tree branches on a sunny day. Try a few different apertures and view at high magnification. As mentioned above, some forms of CA can be
fully fixed by software, such as Lightroom. Note: if the problem areas are not in perfect focus, it's may not be lateral CA, but longitudinal CA.
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| The entire image, note there are CAs in this image but the image is not large enough to see them. |
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| Lateral Chromatic Aberrations from the top-left corner of the photo above. Note the purple on the branch edges. High contrast scenes such as this one bring it out. |
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| The same image after a "one click" correction in Lightroom |
Quick Longitudinal CA Test:
Shoot a piece of white piece of paper with black text at an angle. Exposure should be set so that the paper is very bright (but not clipping). Try wide open, then stopped down a couple of stops. Compare the results.
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| In this really tight crops shot "wide open", you can see evidence of purple fringing in front of the focal plane and cyan fringing behind. This is secondary longitudinal CA. |
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| When the same lens is stopped down to f/8, the CAs are reduced. |
Some lens coatings/materials/designs reduce chromatic aberrations, flare or other issues in trade for even
spectrum transmissions. For example, they might reduce the intensity of some of the blue/magenta spectrum.
The result can be colors that look "off" and are difficult to correct.
There are "hear-and-there" reports of color issues when using third-party lenses. One argument of consideration is that the third-party lens coatings are not matched with the color filter arrays present in the camera bodies. I'm not saying "never buy a third party lens", as I own one myself, but be conscious that color fidelity issues with these lenses are not unheard of.
Another aspect of transmission is light loss. Due to transmission loss, a lens may be slightly darker than it's aperture specification may imply. In video applications, this is prioritized thus
cinema lenses are marketed in "
T-stops". A t-stop is a virtual f-stop where light losses are factored in. An an example, a f/1.8 lens might be a T2 lens (allowing as much light through as a perfect f/2 lens).
Quick Test: Shooting with flash, manually set your cameras white balance to "flash". Also set JPEG settings to "natural" or "neutral". Now take photos of people, and colorful things (even a color chart if you have one). Does the resulting color look correct or do you have to play with them? (Caveat: the problem may not be the lens and may be your monitor. If the colors look bad, try a few other lenses to compare)
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| An animation of an image from my Tamron 17-50mm (at 17mm) before and after distortion correction |
Distortion is when lines that are supposed to be straight look "curvy". Distortion can be partially addressed in post processing using several tools (including
Photoshop,
Lightroom, and
Hugin). That said, correcting heavy distortion will degrade image resolution.
Shorter lenses will have a fundamentally harder task correcting distortion than longer ones.
Zooms also have a bigger problem with distortion than primes with larger zoom ranges (e.g.
18-300) correlated to more severe problems which vary by focal length setting.
Distortion is important when photographing subject with expected straightness, such as interiors and architecture. Photographs of nature tend to make distortion less obvious.
Quick Test: Distortion is reported by both
Photozone and
Lenstip. If you want to experiment yourself. try photographing a brick wall. Do the lines look straight?
Field curvature is where the
focal plane not a flat surface. All lenses have some of this but it's too slight for relevancy on a well-corrected lens.
I can only image how many perfectly working wide angle lenses have been returned because they have field curvature and someone "tested" them for proper centering by shooting a brick wall.
Field Curvature is also more a problem with shorter focal lengths (and zooms) Macro lenses are specifically designed to try and minimize field curvature (so they can be used for copy work).
Since landscape lenses are often short, this is where field curvature is likely to become an identifiable issue. With these lenses, better overall focus can be achieved by focusing near the corners as opposed to the center.
Quick Test: Start with a Google search. Test by focusing a flat brick wall and comparing center and corner sharpness (review the corner sharpness section above to account for decentering). If field curvature is found, you may still find the lens quite usable if focused carefully.
Vignetting is a darkening of the image toward the corners. It's much more common with larger sensors. It also nearly always worse when a lens is used "wide open", rapidly improving as the lens is stopped down.
I consider vignetting to be a minor issue in the digital age, unless it's excessive. This is especially true as the use cases where vignetting is a concern (such as landscapes) are the same ones where the lens is stopped down. One exception (there's always exceptions) is when
stitching together images, vignetting can create issues with the stitching...
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| The same image with an added Vignette on the bottom. It's much easier to add these than remove them. |
Quick Test:
Lenstip and
Photozone both measure and report on Vignetting. You can test yourself by photographing a bright and uniform object, such as a white wall or the sky.
The limitation below (diffraction) applies to all lenses. It's important to understand because it a limitation that affects even the most expensive equipment.
Diffraction is a general phenomenon that applies to
all wave-like phenomenon (light included) when the waves encounter an obstruction.
No lens design can prevent diffraction artifacts.
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| This image shows a tiny section of a brick wall captured at different apertures. The top row represents 100% crops of a 24 megapixel image. The bottom row represents 100% crops of a 2.5 megapixel image. Note how diffraction prevents high resolutions from being achieved at high f-stop numbers. |
Diffraction appears as "fuzziness" in an image, especially noticeable in areas that should have the "sharpest focus".
Diffraction increases as f-sop increases and is fundamentally linked with depth of field. Due to diffraction,
you have to trade resolution potential for depth of field and this trade-off is the same for all sensor sizes. A work-around for this limit is to angle the focus plane with a
tilt-and-shift lens or
view camera.
The usability aspects below are not directly related to image quality but are instead related to convenience and flexibility.
Some shorter lenses (popular in landscape) have
bulb-shaped front elements. This makes adding filters a serious pain.
ND and polarizing filters are powerful assets in both landscape and product photography. I'll probably cover details in a future article.
The size of filter is also a factor with smaller sizes being much cheaper. For example, a Hoya ND400 filter currently costs $71 in 77mm and $35 in 49mm. If you go with higher-end filters, the price difference will be magnified ever further.
Most cameras offer a "auto focus | manual focus" setting. Lenses with full-time manual focus can be manually focused at any time, regardless of the camera's setting.
This is a convenience feature which makes the camera more pleasant to use. With lens that gets a lot of manual focus (such as macro and landscape), it can be a big factor in usability.
Focus throw indicates how many degrees the focus ring needs to be turned between infinity and full close-up. More throw allows for more precise but slower focusing.
Newer mounts (sony E-mount, Micro 4/3rd) often have the manual focus ring actually attached to sensors. Electronics read these sensors and tell the autofocus motor to change focus. I've seen good implementations of this and really terrible ones (The Sony RX100 and Fuji X100 are both terrible, the Fuji X100T improves on the X100 greatly).
I've yet to use an electronic focusing system that I liked better than a mechanical one. In theory, it's possible by offering a long (or configurable) throw.
One thing you generally lose with electronic focusing is distance scales and hard stops. Both losses decrease the general utility of the lens, especially short lenses and macro lenses.
It's becoming more rare at this point, but some lenses still have focus markings such as scales and depth-of-field guides. These can be quite useful in some types of photography such as landscape and street. With time, you can "learn" the positions that work for certain types of shots, speeding up your work and reducing errors.
Focus markings work best on shorter lenses, where there is a bit more focusing tolerance and zone/distance focusing works more reliably.
Most older lens designs move the outer element (or all elements) for focus. An increasing number of modern designs move hidden inner elements instead. Moving inner elements can offer advantages in terms of weather sealing and general protection.
Image stabilization helps
steady the camera during handheld shooting. It has upsides and downsides. It's a bit complex to go into all of it here, but some basic thoughts are.
- Verses a tripod, it's more flexible and convenient but the results are not as repeatable or reliable
- It makes the lens more complex and prone to be outdated.
- A lens with IS can ironically, blur photo in some situations. For example, a tripod-mounted camera with IS turned on can lead to less "crisp" photos.
- The shorter your lens, the less you need IS and the less you should want it due to the downsides. With longer "action" lenses, IS can be a real asset. An ideal use case for IS is when shooting wildlife handheld.
Most older lens designs dive auto focus by a motor in the camera. Newer designs nearly always have the motor in the lens. There are trade-offs...
The upside of a motor in the lens is that focusing is quiet. Focusing can also be very fast with some "hypersonic ring" designs.
The main downsides are cost and sometimes long-term reliability.
My thought on this subject is that the value of in-lens focus motors vary by application. For wildlife and sports, they are a real asset. For landscape, a full manual lens can be preferred as landscape compositions are (arguably) best focused manually.
Some may consider this minor, but I think its important to mention that some manufacturers sell their hoods as a separate (and overpriced) "
accessory". While, this may be justifiable on inexpensive lenses, I think expensive (> $500) lenses should include a hood in the box.
There are also lenses that come with built-in hoods (retractable, screw-on, etc), many of very nice build quality. Some of these, however, are not removable which can create issues mounting filters.
Some lenses have "weather sealing". When paired with a weather sealed camera body, shooting in a light rain should be less stressful. Some people take this to an extreme and wash their gear under running water. I would not recommend pushing your luck!
Beyond water intrusion, a weather sealed body/lens should do a better job keeping out dust particles.
I'll end with the hardest to precisely define quality of a lens. It comes down to materials, tolerances, smoothness and the general consensus that good copies are common and "bad" ones are a rarity.
Some say that the final image is all that matters and from a certain point of view (the image viewers) they are absolutely correct.
From the photographers point of view, however, there is also the aspect of "image making" and build quality definitely has an effect on how enjoyable and reliable this process can be. When a photographer is enjoying the process and can rely on the gear, better photos can easily result.
It's also nice to order a new lens and be confident that the first one you get will be a nice one. Manufacturers do not all score equally here - currently (2015) Canon
seems to be a slight leader in terms on consistency in their latest lenses (
source), but there is no perfect solution at the prices we are willing to pay.
Summary
When buying a lens there are many factors to consider, so many that it is natural to feel overwhelmed. For this reason, I recommend being aware of different factors, but deciding what matters to you by renting/borrowing the lens and giving it a "test drive".
