Optical Distortion

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My photography friends often asked, why do I need a 50mm lens in my camera bag?  What are they good for?

As a beginner in photography, having the flexibility of a zoom can be very helpful and feels a lot safer, but it also has many disadvantages:

• Usually poor image quality (not very sharp and poor distortion characteristics)
• Very slow, in the aperture range of f/4 –f/5.6. Just forget about low-light and indoor photography
• Slow aperture. Suffers from poor depth of field control resulting in a loss of separation of your subject from your background.

The best thing about a kit zoom is its optical range.  Essentially, you get a reasonably wide-angle with a standard 50mm lens and a conservative telephoto lens in a single compact package for a fair price.  So, it is a good starting point for most beginners.

However, as you build your capabilities as a photographer, you should consider at least one prime lens to be in your camera bag.  I think the 50mm prime lens is the obvious choice.  It is said that the 50mm focal length is the closest approximation to the vision distances and angles that humans possess.

Lens distortion can be a big problem in photography.  There are two different types of distortion in photography: optical and perspective.  This article is focused on optical distortions.  Optical distortions are the result of the construction and the quality of the optical elements and groups of elements within the lens itself.  Whereas, perspective distortion is the result of angles and proximity to the subject and the type of lens used.

For the optical evaluation, a simple brick wall test was performed.  The day of this test was January 1, 2019, in Toronto, Ontario, Canada.  The images were taken one after another over a 60 minute span, so the sunlight changed slightly over this period.  The camera was mounted on a Gitzo carbon fibre tripod with a Manfrotto 222 ball head.

The ambient air temperature was just +2.0° Celsius in unobstructed winter sunlight.  There was high cirrus cloud to defuse the sunlight.  With a harsh brisk wind from the south, it felt like -15° Celsius.  So, not exactly ideal conditions for this test, but I promised to do it for a friend, so I am good to my word regardless of the bone-chilling hands and potential negative impact to the equipment.

The camera and lenses were all left overnight in a secure environment to get them close to the right ambient temperature, so as to not shock the equipment from the cold air.  If they were carried directly outside from indoors the difference in temperature would be problematic.  Glass and metal react and contract differently to cold, so it was important to have them near the ambient temperature in advance of the test, or else face environmental factors that could unduly affect the results.

With that said, all of this test effort is only meant to be an ‘indicative’ evaluation and not a real science project in formal controlled test conditions.

When judging a lens, we consider several key parameters.  These characteristics can include the following parameters:

• colour distortions
• light distortions
• optical distortions
• perspective distortions
• and more….

So, for this effort, the test was principally concerned with optical distortions although other characterizations can be seen in the results.

Optical Distortion

Every lens has varying degrees of distortion, some more than others.  Fisheye and wide-angle lenses have the most as they have a ‘wider’ view.

The distortion definition is simple.  It comes from the optical design, or rather, how the lens was made.  Optical distortion is down to the lens.  This is why we often call it lens distortion.

As mentioned, perspective distortion happens because of the position of the camera relative to the subject.

Optical lens distortion in photography is also known as an optical aberration.  The straight lines in the photograph become bendy and wavy.  This is why they are also known as “curvilinear”.

This distortion occurs due to the lens design, which is actually a lens error.

There are two types of optical distortion that could be looked at here. These are Barrel (Convex) and Pincushion (Concave).

To judge for optical distortions in a lens, it is best to shoot some sort of grid.  Below I have photographed the same brick wall as a common reference to compare several lenses in, and at, the 50mm focal length to compare the quality of the optics.  This grid image is a great way to define distorted images and distortion photography.

Depending upon the display you are using to read this article, the distortion will be more or less pronounced.  A computer display is best as it offers the best brightness, contrast, linearity, and size.  Portable devices like smartphones and tablets will not reveal as much due to the limitations to their displays.  Do not forget that the display can affect the distortions too, so a calibrated monitor is ideal.

‘Perfect’ lenses or lenses with no picture distortion are rare.  Most lenses will suffer from one of these distortions.

Some lenses are made with elements that significantly reduce the aberrations.  So much so that they are not noticeable to the naked eye.

All four of these images were taken with the new Nikon Z7 mirrorless camera.  This is a full frame still camera with a 45.7 MP sensor.  With the two F-Series lenses, a Nikon FTZ adapter was used to permit the mounting of the F-Series lens to the larger mirrorless S-Series camera mount.

Nikkor Z 24-70mm f/4.0 S-Series

This new 24-70mm f4.0 S lens holds up fairly well.  It seems to be sharp from edge to edge and equally sharp to the centre.  The curvilinear is not noticeable.  For a kit lens, it is a good performer.

Nikkor Z 50mm f/1.8 S-Series

The exposure is noticeably different with this lens compared to the kit zoom lens above.  It seems hotter in the centre compared to the edges.  The curvilinear is fine.

Nikkor AF-S 58mm f/1.4G F-Series

This new lens receives a lot of criticism for its high price, but it is an aspherical lens, so the light is aligned to the sensor target unlike other spherical lens at lower prices.  The exposure is good and the curvilinear seems fine to me.

Nikkor 55mm f/1.2 F-Series

(Vintage Lens, circa early 1970s)

Just for fun, I took along an older lens from my film days.  This original Nocto lens has a stunning f/1.2 aperture, so it is ultra fast.  However, it is easy to see how optics have improved over the decades.  This image is hotter in the centre compared to the edges.  I see some curvilinear distortion too.  So, even though photography friends often say how they miss the old days and the Japanese glass lenses, this simplistic test shows that the older glass is not performing as well as the modern optics made by Nikon in China.

The best lenses are free of optical distortions and do not negatively impact the image results.  It is clear to see that the older lens does significantly influence the image outcomes due to non-linearity in exposure and optical distortions.  With the advent of these amazing new advanced digital cameras from a myriad of high-end vendors like Nikon, Canon, Sony, Fuji, and others, the role of the lens has become much more important.  This generation of sensors can now outperform low quality plastic optics.  Therefore, it is smarter to buy the very best lenses that you can afford.  You will see the benefits for many years with resulting images that are sharper, properly exposed, and free of curvilinear distortions – basically more truthful.

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References:

Hull, C. (2019). What is Lens Distortion? Expert Photography. Retrieved on January 1, 2019 from, https://expertphotography.com/what-is-lens-distortion/

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About the Author:

Michael Martin has more than 35 years of experience in systems design for broadband networks, optical fibre, wireless and digital communications technologies.

He is a Senior Executive with IBM Canada’s Office of the CTO, Global Services. Over the past 14 years with IBM, he has worked in the GBS Global Center of Competency for Energy and Utilities and the GTS Global Center of Excellence for Energy and Utilities. He was previously a founding partner and President of MICAN Communications and before that was President of Comlink Systems Limited and Ensat Broadcast Services, Inc., both divisions of Cygnal Technologies Corporation (CYN: TSX).

Martin currently serves on the Board of Directors for TeraGo Inc (TGO: TSX) and previously served on the Board of Directors for Avante Logixx Inc. (XX: TSX.V). 

He serves as a Member, SCC ISO-IEC JTC 1/SC-41 – Internet of Things and related technologies, ISO – International Organization for Standardization, and as a member of the NIST SP 500-325 Fog Computing Conceptual Model, National Institute of Standards and Technology.

He served on the Board of Governors of the University of Ontario Institute of Technology (UOIT) and on the Board of Advisers of five different Colleges in Ontario.  For 16 years he served on the Board of the Society of Motion Picture and Television Engineers (SMPTE), Toronto Section. 

He holds three master’s degrees, in business (MBA), communication (MA), and education (MEd). As well, he has diplomas and certifications in business, computer programming, internetworking, project management, media, photography, and communication technology.