• Going To Forest Safari. What Zoom Lens Should I Take?

    by  • November 9, 2011 • Tips & Tricks • 2 Comments

    Amateur photographers cannot help but browse through Amazon.com for hours altogether looking for the biggest telephoto at the smallest price. Hours are spent debating the demerits of a Mirror lens versus the price of a Sigma 150-500mm lens or god forbid – the Canon L Series!. The question is, do you even need the 500mm telephoto? Or is it a must have before your trip to Africa?

    The answer lies in a simple mathematical equation involving one multiplication and one division.

    Before we delve into the maths, you need to keep in mind the following:

    • Lenses do not perform their best at the extreme levels of Aperture and Focal Length. For ex: A lens with aperture 1.4 – 16 will produce soft images at f.4 and f16. It’s sharpest images are likely to be at two stop smaller than maximum. i.e. f2.8. Similarly, a 70-300mm telephoto zoom lens will produce slightly soft/distorted images at 70mm and 300mm focal lengths. So either buy a telephoto lens with a range higher than your requirement or use other techniques such as smaller aperture and tripod support to achieve sharper images.
    • The more the telephoto range, the bigger the front element and the smaller the aperture. In plain English, telephoto lenses have big glass elements, hence heavy. Also, as the magnification power increases, the aperture decreases to maintain focus and thus light falling on the sensor decreases. As a result, you need to shoot with slower shutter speeds and take care to avoid image blurring.
    • Research in optical photography technologies has resulted in evolution of new generation of lenses capable of performance similar to theoretical expectations. In plain english, throw enough money and you can buy high quality lenses (such as Canon L series) which give you terrific focal length as well as large apertures.

    The ground being set, let us proceed.

    In mathematical terms, the focal length (F) required is dependent on the distance of the object from lens, the height/width of the actual object and desired height/width of the image formed on the sensor.

    • F = (distance x height of image) / height of object
    • F = (distance x width of image) / width of object

    Case Study: Kid at soccer game

    Assuming that you are photographing with a Nikon D5100, which has an APS-C sized sensor (23.6 mm wide x 15.6 mm high). Your kid is four feet tall (1219.2 mm).  The soccer pitch is 70 m wide and you are sitting near the boundary. You want to take shot when your kid is near the center line (distance from you = 35 m or 35000 mm). Let us also assume that in the frame, you want to allow for a little background and do not want to frame the shot very tightly, i.e. your kid’s head and feet are not near the edge of the frame, a fair bit of the playground is visible.

    Since we are primarily working with height (kid is 1219.2 mm high), let us assume that you will frame the shot such that he will occupy 75% of the height of the image. i.e. 11.7 mm (75% of 15.6 mm, the height of the APS-C sensor)

    Applying the formula: F = (35000 mm x 11.7 mm) / 1219.2 mm = 335 mm. Hence you need a telephoto lens that does at-least 335 mm (perhaps the Sigma 150 – 500mm?)

    You could use a smaller telephoto lens. In that case, your kid’s image in the frame will be smaller and more of the background will be visible.

    Case Study: Parrot on tree

    Assuming that you are photographing with a Nikon D5100, which has a APS-C sized sensor (23.6 mm wide x 15.6 mm high). The parrot is 8 inches high (203.2 mm) and the branch it is sitting on is approximately 40 feet away (assuming the distance from you to base of tree. Ideally, we should also consider the height of the branch and use trigonometry to arrive at accurate distance, but I promised to keep this simple). Also, we want to frame very tightly and want the parrot to fill 90% of the height of the image (14.04 mm).

    Applying the formula: F = (12192 mm x 14.04 mm) / 203.2 mm = 842.4 mm.

    Clearly you need a huge telephoto range. Conventional lenses providing this range are pretty expensive and mirror lenses (aka reflex lenses) with focal doubler (for ex: Opteka 500mm mirror lens with 2x Tele-convertor) maybe better for amateurs. But mirror lenses tend to be quite dim (aperture fixed at f8, f16 with tele-convertor), require very careful manual focusing and sturdy tripods for the long shutter speeds.

    Lets us consider that you are willing to risk your life and limb (from being attacked by a parrot) and approach the parrot so that is now only 10 feet (3048 mm) away.

    Applying the formula: F = 3048 x 14.04 / 203.2 = 210.6 mm. This is well within the realm of a 70-300mm Telephoto lens. Perhaps you should step back a little and stop threatening the parrot.

    Case Study: Crocodile at zoo

    Assuming that you are carrying a Point & Shoot camera like the Canon SX40 HS (featuring 24mm – 840mm telephoto), you are at the zoo and want to take a photograph of this African crocodile lazing in the sun.

    The crocodile is about 15 feet (4572 mm) long (hardly any height to consider) and about 50 feet (15240mm) away from you. You want a tightly framed shot. The question is can you get the shot you want?

    Let us find out.

    The Canon SX40 has a typical Point & Shoot sensor size of 1/2.3″ (6.2 mm wide x 4.6 mm high). Since the crocodile is far longer that it is higher, we are bothered about the width of the object rather than height. Let us fill 95% of the width of the sensor (5.89 mm) with the crocodile.

    Applying the formula: F = 15240 x 5.89 / 4572 = 19.63 mm.

    Since the Canon is only 24 mm at it’s widest, you will have step back or the crocodile will not fit.

    Assuming that you stepped back to 75 feet (22860mm) distance, applying the formula:

    F = 22860 x 5.89 / 4572 = 29.45 mm. Now you are well within the range of your camera.

    Case Study: In Africa, face to face with lazing lion

    More often than not, we purchase the equipment first (because of monetary limitations) and then worry about what we can photograph with it.

    Assuming that you bought the excellent Canon 5D Mark II full-frame DSLR (sensor size: 36 mm wide x 24 mm high). You also splurged on the Sigma 150 – 500mm lens. Now you are in Africa, sitting in the Guide’s Jeep, about 100 feet (30480 mm) away from a Lion and wondering what sort of an image you are gonna get.

    Note that the formula F = (distance x width of image) / width of object, can also be interpreted as:

    Width of Object in Image = (Focal Length x Width of Object) / Distance to Object

    Assuming we want a tight shot (95% of sensor width, 34.2 mm), the lion is approx 6 feet long (1828.8 mm) and we are applying the maximum telephoto of the lens, applying the formula:

    Width of Object in Image = 500 mm x 1828.8 mm / 30480 mm = 30 mm. That’s 86% of the sensor width! Not bad at all.

    We can also arrive at the distance required to shoot using another interpretation of the same formula:

    Distance from Object = Focal Length x Width of Object / Width of object in Image

    If we want the Lion to fill the entire 36 mm sensor width, applying the formula:

    D = (500 mm x 1828.8 mm) / 36 mm = 25400 mm = 83.33 feet. That’s closer to the lion by 17 feet!

    Till Then, Stay Safe:

    You will realize that arriving at what telephoto lens you need, or what distance you need to shoot from or simply what can you realistically expect from your lens is only a matter of multiplying and dividing. The formulas illustrated here were invented centuries ago. How I wish camera manufacturers either included a calculator function in the modern day cameras or at-least threw in a cheap one that could be stored in the camera bag and fished out when you desperately need, to arrive at the distance between you and an angry lion, based on size of the lion in the image you just shot.

    Best Selling DSLR Cameras

    Best Selling Lenses for Nikon cameras (Canon mount/equivalent also available)



    About

    Based in Hyderabad, India.

    http://www.rajib.com

    2 Responses to Going To Forest Safari. What Zoom Lens Should I Take?

    1. Sheeba
      December 13, 2011 at 12:42 am

      Have you reviewed the Canon PowerShot SX 40 HS ? Considering a purchase, but haven’t found too many independent reviews.
      My SX20 IS, again, has the ‘lens error’ message as last year; since it is out of warranty, plan to take it to a reputable camera shop a few towns over.
      Thank you for your help.

      Sheeba C

      Reply
      • Rajib Ghosh
        December 13, 2011 at 9:55 am

        Dear Sheeba,
        I have not persoanlly tested the SX40 yet. But from what I have read, it is a worthy successor to the SX20 and SX30.

        Apart from the 35x optical zoom (same as SX30), it features a CMOS sensor (versus CCD sensor of SX20 and SX30). The CMOS sensor allows images with lower noise, particularly in low-light situations.
        Also, the new Digic 5 processor enables the camera to capture Full-HD videos (1920 x 1080 pixels) versus the HD videos (1280 x 720 pixels) of it’s predecessors. In a review of image quality (in a reputed Photography mag), the reviewers lamented the lack of fine detail in (when compared to a DSLR) but rated the camera top overall among it’s competitors (Nikon P500 and other super-zoom cameras from Olympus, Fujifilm).

        I had found little reason to upgrade to the SX30 when it was launched, but should my SX20 die (of lack of use), I will surely pick up the SX40 without debate. FYI, I also purchased a Nikon D5100 DSLR but the SX20 remains my preferred travel camera.

        HTH

        Reply

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