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Focal length issues in pano shooting

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  • Frank Lopes
    started a blog post Focal length issues in pano shooting

    Focal length issues in pano shooting

    Since I shoot with Nikon cameras, most of these lens references are based on my personal experiences with Nikon equipment. The general concepts however apply to any manufacturer.


    The focal length of a camera lens is the distance between the center of the lens and the film/sensor when an in-focus image is formed of an object far away.

    This fact causes the angle of view of the camera to be inversely proportional to the focal lens. Another way to look at it is: the closer this point is to the film/sensor the less the camera captures.

    (Wikipedia image)

    Since most digital cameras come with all purpose 18-55mm, 28-70mm, 18-153mm or 18-200mm zoom lenses, what focal length should one use when shooting panos?

    Should you use 18mm so that with just a couple of images you cover the whole scene?
    Or should you use a longer setting, 70 or 135mm, to minimize distortion in your images?


    First a primer on modern zoom lenses.

    A camera lens is composed of a series of glass elements that focus light on the camera sensor or film. If the lens is not a zoom lens (a lens that does not allow you to change the focal length), the lenses are fixed and only certain elements move to allow for focusing. These lenses are typically called "primes".

    A zoom lens is mutch more complicated since it has banks of glass elements that need to move in an away from the sensor thus increasing or decreasing the distance between the optical center of the lens and the sensor causing the angle of view to decrease or increase.

    Since zoom lenses have that much more to deal with, they are most of the time a compromise between performance, price and weight. Just like anything else in life, you can get 2 out 3 but never all three. There is no such thing as a lens that is cheap, perfect and weights only a few ounces.

    Because consumer zoom lens manufacturers make compromises regarding lens performance (to keep the price down), what do you lose when shooting panos with a variable zoom lens?

    All lenses suffer in different degrees, but specially so zoom lenses when used in wide angle mode, from vignetting and chromatic aberration.


    Vignetting


    (from Wikipedia: http://en.wikipedia.org/)

    Vignetting is a reduction of an image's brightness or saturation at the periphery compared to the image center. This will cause the stitching software to have a hard blending the individual photos since the periphery of the photos doesn't match the center of the photo.


    Chromatic aberration


    (from Wikipedia: http://en.wikipedia.org/)

    Chromatic aberration is caused by a lens having a different refractive index for different wavelengths of light ie the dispersion of the lens. In layman's terms this means a photo shot with a lens with poor chromatic aberration, will show obvious signs of color fringes around high contrast areas.

    In addition to these two problems, there something else that you will need to deal with regardless of the quality of the lens when shooting with a wide angle lens 28mm or less: The distortion on the periphery of the image. Due to the lens having a wider angle of view, the periphery of the photo tends to have circular distortion. Remember, the bigger the angle of view, the bigger the distortion.

    When I tried my first panos with the lens that came with the camera, I used to set the lens to 18mm assuming that was the best setting. The final work looked ok but great. This was until I tried a prime lens: 50mm f1.8

    This is a fairly inexpensive lens (less than US $100) and very fast allowing for low light situations. It is very sharp and has no discernible vignetting or aberrations. Also, because it is not a wide angle lens, it causes no distortion in the periphery of the photos.

    Off course the penalty is, since it has a narrower angle of view, I could not longer shoot only 3 or 4 photos to capture the whole scene but now I would have to shoot 6 or 7.

    Eventually I bought its faster brother, the 50mm f1.4 It is much more expensive (US $275), but it gave me what I was looking for: a prime, non wide angle, very sharp, very fast lens that I could use both for panos as well as general photography.

    What if you want to shoot panos with the zoom that you have?

    Here are a couple of ideas.

    Try not to shoot at the lenses' widest settings: even still it is tempting to do so, you complicate the life of the software and your results will have many more compromises.
    Try between 35 and 50mm. Much more than 50mm and you will probably have to shoot way to many shots to capture the whole scene. Much below 35mm and you will have to deal with the issues mentioned above. Specially vignetting.

    When shooting, avoid the extremes in aperture: all lenses have a range of aperture at which they are the sharpest. If you don't know what it is for your camera, try to shoot halfway between the two extremes. Experiment with f8 to f11 and go from there until you are happy.

    A terrific site that demonstrates these performance issues issued with aperture and focal length using a typical zoom lens is kenrockwell.com
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