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photo:available_light

available light - clinical photography without using a flash

Introduction

  • most doctors are unlikely to bring external flash systems to work and on-camera flash often results in poor clinical images due to either unwanted reflections, shadow from lens or over-exposure at macro distances, or just flat lighting.
  • unfortunately, the lighting sources in most EDs are quite variable in their colour temperature and the camera auto white balance or even presets can be very inaccurate in portraying the true skin colour.
  • furthermore most ED lighting is of “office level” illumination and you may be better suited to using a digital SLR to manage the lower light levels (eg. better image quality at ISO 400, image stabilisation for hand held images, better autofocus at low light and wider apertures to allow more light in).

Setting your camera for ease of use

  • set ISO to 400 to ensure a relatively fast shutter speed to minimise camera shake
  • ensure image stabilisation (IS) is turned on if the camera has it
    • on Olympus dSLRs:
      • press the IS button on the rear and ensure it is set to IS 1
      • ie. ALL lenses are effectively image stabilised with the E510/520/E30/E3 dSLRs
    • on Panasonic GH-1:
      • set switch on the lens to ON and CAMERA menu item STABILIZER should be on MODE 1.
    • on Canon/Nikon dSLRs:
      • IS button is on the lens if it is equipped with it
  • make custom white balance (WB) easy:
    • with Olympus dSLRs:
      • on the menu, allocate the Fn button to custom WB
      • whilst aiming camera at the white sheet, press the Fn button (you have already programmed this for custom WB) and press the shutter to take the test image, then choose OK to accept it
    • on the Panasonic GH-1:
      • just press WB button on rear, select CWB1 or CWB 2, then aim camera at a white piece of paper and press the up arrow to record it and then SET button to save it.
  • choose a lens with a relatively wide aperture and ability to focus as close as you will be needing
    • on an Olympus dSLR:
      • ZD 50mm f/2.0 macro (gives 35mm eq. focal length of 100mm) is perfect for most applications
      • ZD 50-200mm f/2.8-3.5 lens is a lot bigger, heavier and requires you to be at least 1.2m from subject but offers more flexibility and versatility.
    • on the Panasonic GH-1:
      • the 14-140mm HD kit lens will suffice for most purposes but maximum macro covers a horizontal field of 9cm, you may need a dedicated MF macro lens for closer work (eg. Olympus OM 50mm f/3.5, Olympus ZD 50mm f/2.0 for Four Thirds) until they come out with a MFT macro AF lens.
      • consider using a Lens Baby for procedural documentation when you wish to blur faces out - especially good for HD videos
  • select jpeg size:
    • if your desired output is just for teaching on computer or display on the web, you may want to choose a smaller jpeg setting such as 1024×768 with 1/8 compression (yes - 1 megapixel is ALL you need for this and 10 megapixel is all you need to print to 20“x30” - you do not need more it will just make your life more difficult)

Taking the photo

  • set custom WB (see camera manual for exact details for your camera):
    • aim light source at a white sheet and set custom WB as per camera manual (see also above)
  • aim light source at subject - if necessary, angle it to give the best illumination or shadowing for 3D effect if appropriate.
  • select aperture if using aperture priority metering instead of Program mode:
    • choose an appropriate aperture small enough to give adequate depth of field (amount of subject in sharpness) whilst allowing a fast enough shutter to minimise camera shake (eg. 1/focal length of lens)
    • example of office level lighting would give f/4.0, 1/80th sec at ISO 400 and with IS on, this should give reasonable compromise of all factors, although with care to minimise camera shake f/5.6 1/40th sec may be possible.
  • focus
    • AF may not work well on low contrast skin so you may need to choose a more contrasty part at the same distance to camera and recompose
    • for macro work, you may wish to set the camera to manual focus and then just move camera towards or away from subject until focus is achieved in the viewfinder
      • tip: using this technique, you can keep the magnification constant by choosing a particular focus distance on the lens.
  • take the photo
  • put WB setting back to auto WB (AWB) in case you forget it was on custom WB and your other photos come out with a colour cast.
photo/available_light.txt · Last modified: 2009/07/26 22:21 (external edit)