The Truthiness of Normal Lenses

When describing items that are available in different shapes, sizes, flavors, and/or intended uses, you need to establish frames of reference, or qualifying attributes, in order to be able to differentiate between them. These attributes may include descriptives such as ’round’, ‘square’, ‘soft’, ‘hard’, ‘long’, ‘short’, ‘fruity’, ‘nutty’, etc.

In the case of camera lenses, the defining points of reference include the focal length of the lens, typically measured in millimeters, and the physical size of the imaging sensor, which determines how much of the total lens coverage is actually being recorded by the camera, i.e., the camera’s crop factor.

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Defining the term ‘normal lens’ is not as simple as it may seem, especially if your frame of reference is the human eye, which in concert with the brain pretty much smokes any camera/lens combination you’re likely to use in this lifetime. Camera lenses see what our eyes see, but the information is collected and processed within a far more complex set of dynamics.

 

By definition, a normal lens renders a scene the way our eyes perceive the scene, i.e., the vanishing points, overall perspective, and perceived distances between people and/or objects closer and further to and from the lens are neither exaggerated, as they are with wider-angle lenses, nor compressed as they are with telephoto lenses. In a word, pictures taken with normal lenses appear… well… ‘normal’.

The similarities and differences between the way a camera lens captures a scene and the way we perceive the same scene with our eyes part ways as soon as you take into account most of us do not view the world through a singular lens. Rather, we view the world through a pair of eyes, which are positioned about 2-1/4” to 2-1/2” apart.

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This dual eye arrangement enables us to accurately judge depth perception as well as the spatial relationships between objects and/or people around us as we make our way through the day. If you want a lens that will record your subject that perceptually speaking will render the scene you are photographing that is perceptually similar to what you are seeing in your mind’s eye, you want a normal lens. And this is where the fun begins.

Even though camera manufacturers have long been labeling 50, 55, & 58mm lenses as being ‘normal’, according to the math, the angles-of-view of 50, 55, and 58mm lenses are off the mark by about 16 to 30%.

The focal length of a normal lens for any camera system is equivalent to the diagonal dimension of the camera’s sensor (or film frame), which in the case of 24x36mm imaging sensors is 43.3mm. Similarly, the angle-of-view of a true normal lens – regardless of format size, is about 53°s, not the 46-or-so° angle-of-view we’ve been living by all these years.

Though manufacturers march out the occasional 40mm and 45mm lens, in terms of mathematically accurate normal lenses, Pentax, a company that ironically hasn’t manufactured a 135-format camera since they ceased production of film cameras, produces the Pentax SMC-FA 43mm f/1.9 Limited, which is designed to cover a 24x36mm sensor.

Rumor has it Pentax might some day introduce a 135-format camera, but for now if you want to use this lens on a 135-format camera you’ll have to invest in a lens adapter for a Sony A7-series or Leica M-series camera, the only full-frame (135-format) cameras that enable you to take advantage of what this lens has to offer.

Another thing to keep in mind when comparing eyes and lenses is that we don’t see the world through a lens – we view it through a pair of eyes, and because our eyes dart about and record image data at about ten images-per-second, our eyes see a much wider image field compared to the singular perspective of a camera lens.

Our eyes also take in wider fields-of-view than normal lenses. Though we perceive spatial relationships as they appear through a lens of about 43.3mm (approx 53° AoV), our eyes actually take in the approximate field-of-view of a 24mm lens on a full-frame camera, which is approximately 84°.

Now factor in the slightly overlapping 84°AoV of the second eye, which is located across the nose from the first eye, and you now have a combined semi-overlapping angle-of-view of about 135°, which is roughly equivalent to the AoV of a 12mm rectilinear (non-fisheye) ultra wide-angle lens on a full-frame camera.

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A full-frame camera with a 40mm ‘normal’ lens will record the scene with perspective and spatial relationships between the various visual elements within the frame similar to the way our mind’s eye processes the scene.

Even though our eyes view perspective from the view point of a 43.3mm lens, each of our eyes takes in the approximate field-of-view of a 24mm or 25mm lens, which is about 84-degrees, albeit without the spatial distortions inherent to wide-angle lenses.

Even though our eyes view perspective from the view point of a 43.3mm lens, each of our eyes takes in the approximate field-of-view of a 24mm or 25mm lens, which is about 84-degrees, albeit without the spatial distortions inherent to wide-angle lenses.

Though each of our eyes take in the field of view of a 24mm or 25mm lens on a full-frame camera, because we have two eyes set about   2.25" to 2.5" apart from one another, our eyes take in a field-of-view closer to a 15mm to 12mm lens (114-degrees and 121-degree AoV respectively).

Though each of our eyes take in the field of view of a 24mm or 25mm lens on a full-frame camera, because we have two eyes set about 2.25″ to 2.5″ apart from one another, our eyes take in a field-of-view closer to a 15mm to 12mm lens (114-degrees and 121-degree AoV respectively). Because the angle-of-view is wider, the image’s inherent spatial distortions are proportionately more exaggerated.

Unlike 12mm ultra wide-angle lenses, which greatly exaggerate perspective and spatial relationships, our eyes render perspective from the point-of-view of a wider-field normal lens. So how do you visualize the field-of-view of human vision photographically.

Perhaps the most accurate way to capture the field-of-view we see through our eyes would be a photograph captured using a camera set to ‘Panorama’ mode using a lens that captures an AoV of about 53° (or thereabouts) based on the size of the camera’s imaging sensor. In the case of a full-frame camera, you’d want to capture panoramas with a lens as close to 43.3mm as you can find. For APS-C cameras you want a lens of about 27mm to 29mm, and for Micro Four Thirds you want a lens with a focal length of about 21mm to 22mm.

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Perhaps the most accurate emulation you can capture that will portray a scene the way we see it with our eyes is to capture a sweep panorama image with as close to a 43.3mm lens you can find on a full-frame camera. This picture, taken with a 40mm lens captures an image that on the horizontal plane, is fairly close to the way our brain interprets the same scene through our eyes.

Two discontinued, but widely available used  film-based cameras that came close to capturing photographs similar to the way our eyes see our immediate surroundings are the Hasseleblad X-Pan (with 45mm or 30mm lenses) and Mamiya 7 (with a 43mm lens).

The X-Pan was a novel 35mm camera designed by Fujifilm that captured pictures in a choice of standard 24x36mm or a panoramic 24x65mm. When used with the camera’s wider-angle 30mm lens, the diagonal field-of-view was about 94-degrees, which is comparable to a 17mm lens on a full-frame camera albeit without the exaggerated, wide-angle perspective of a 17mm lens.

The Mamiya 7 (or7II) with an optional 35mm panorama film insert(also 24x65mm) and a 43mm wide-angle lens will capture images comparable to Hasselblad’s X-Pan with a 45mm lens.

Both of these unique camera systems and their respective lenses are still widely available on the used market

Note – To save you the math I’ve precompiled a list of mathematically-correct normal lens sizes for the more popular camera sensor format sizes. Note these focal lengths are approximate and as such may or may not be available in these exact focal lengths from some or all of the camera and lens manufacturers.

• APS-C (1.5x) 29mm
• APS-C (Canon 1.6x) 27mm
• Micro Four Thirds 21mm
• 1/2.3” 7.66mm
• 1/2″ 8mm
• 2/3” 11mm
• 1” Sony 15.86mm

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