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photo:exposure

exposure

photography basics - exposure

  • simplistically, exposure is how light or dark your photo will be, if it is too dark for your liking then it is under-exposed, conversely, if it is too light, it can be said to be over-exposed.
  • exposure is important in cameras because cameras cannot record detail dark regions as well as the brightest regions in the same way that our eyes can.
  • in other words, the camera as a recording device has a limit in its dynamic range of light values it can record in the one photo and if you try using Photoshop to lighten or darken the photo, you will find that either the darkest region when made lighter will just produce image noise and not much if any detail, or if you try to darken a photo, the lightest areas tend to stay light or became a strange colour but you do not get much if any detail in these areas that have been “blown out” by over-exposure.
  • so what we usually aim to do is set the exposure so that the most important part of the scene (eg. skin) is displayed well and as similar as possible to our usual visual experience.
  • camera manufacturers provide automatic exposure settings on cameras which are designed to adjust the camera's various parameters which modify exposure to produce a result that most of the time is acceptable in terms of exposure. This setting is usually marked “AUTO” or “PROGRAM” on the exposure mode dial. The camera has light sensors which measure the incoming light and then adjusts aperture and shutter speed, and in some cases the ISO (camera sensitivity to light).
  • of course, sometimes we don't want the scene or skin to be as we see it but darker or lighter, and we may need to over-ride this by either setting the exposure manually or adjusting the “exposure compensation” setting - this is particularly the case when using a flash as we often prefer to soften the effect of the flash by reducing its exposure in which case we may need to either use a manual flash mode or adjust the “flash exposure compensation” setting.
  • these compensation settings are measure in terms of EV (exposure values or “stops”)
  • 1 EV or 1 stop difference represents a difference of half (if minus 1EV) or double (if plus 1 EV) the exposure.
  • exposure can be doubled by either:
    • doubling the duration the shutter stays open - eg. 1/125th sec instead of 1/250th sec
    • doubling the ISO (sensitivity of the camera or film) - eg. ISO 200 instead of ISO 100
    • increasing the diameter of the lens aperture by 1 f-stop or stop
      • aperture is measured in f stops and for the beginner, this is perhaps the most confusing aspect of exposure.
      • apertures with differences of 1 stop between them are f/1.4, f/2.0, f/2.8, f/4.0, f/5.6, f/8, f/11, f/16
      • thus you can double the exposure by using f/2.8 instead of f/4.0
      • the f ratio number actually comes from the focal length of the lens divided by the lens diameter
      • the camera can close the aperture down at the time of the exposure to a designated f stop setting
      • when the term “stop down the aperture” is used, it means use a smaller aperture (ie. a larger f number).
  • conversely, exposure can be reduced by doing the opposite of the above, and in addition, it can be reduced by placing a filter in front of the lens which reduces the amount of light coming in (eg. polarising filter or neutral density (ND) filter).
  • you can guess the exposure settings in bright sunlit using the f/16 rule approximation:
    • f/16 aperture with shutter speed set to 1/ISO th sec. (thus if using ISO 100 then set shutter to 1/100th sec).
    • see also light values and exposure metering for more information on actual exposure amounts in given situations.

So what's wrong with just leaving your camera on auto exposure mode?

  • well, for most people using point and shoot cameras, there is nothing much wrong for most situations and you will get OK or even very good results most of the time.
  • if you use a 35mm film camera or a digital SLR, then the auto mode is more than likely NOT going to be your best option as the camera will make decisions which you should be making that will affect how the image will be created, even if exposure is spot on.
  • what's the difference between these cameras that makes using AUTO not such a great idea?
    • the main difference is that the point and shoot cameras have a very small sensor which generally produces images with most of the scene in acceptable focus if objects more than about 2m-3m away (the amount that is in focus is called “depth of field” which is a very important consideration in photography).
    • exposure is kept constant by changing the shutter speed and the shutter speed stays fast enough to avoid camera shake or subject motion causing blur.
    • in SLR cameras, the situation is VERY different as they tend to have much shallower depth of field (ie. less of the scene is in focus, especially at wide apertures, close distances or with telephoto lenses). Changing the aperture can have a dramatic effect on how much is in focus and how much is blurred and how blurred it is. This allows you to isolate the subject of interest by using a wide aperture and focusing on the subject while blurring out the background and the foreground. On the other hand you may want as much of the scene to be as sharp as possible, in which case you may choose a smaller aperture.

what exposure mode do I use 90% of the time?

  • you may have guessed it - I use “Aperture Priority” or “A” on the exposure dial.
  • this mode allows me to actively decide which aperture to use to let me decide the amount of blurring I want in my photo.
  • if I am taking a portrait with a short telephoto, I will often use f/2.0 to blur the background
  • if I am doing a macro shot of insects, I will often use f/8 or f/11 to give me enough depth of field if I have sufficient light (eg. a ring flash)
  • if I am doing a landscape shot and I want nearly everything in focus and as much resolution as possible, then I will use f/8.0 or f/11
  • often I will be forced into a compromise and then use f/4-f/5.6 to ensure a reasonable depth of field with a reasonably fast shutter speed and a reasonably low ISO (ISO 400 or less to minimise digital noise).
  • the camera then determines the correct exposure by adjusting the shutter speed.
  • in general, the slowest shutter speed you can use carefully hand held without causing blurring is 1/focal length sec, thus if you are using a lens with a focal length of 200mm in 35mm terms, the slowest shutter speed you should be using is 1/200th sec.
    • this may be several stops slower if you are using an image stabiliser or anti-shake mechanism.
  • if I am concerned the shutter speed will be too slow to hand hold the camera or for the subject motion, then I will consider increasing the camera's ISO setting until i get the best compromise between:
    • amount of blurriness due to depth of field (ie. the aperture)
    • amount of blurriness due to camera shake or subject motion (ie. shutter speed)
    • amount of digital noise (ie. camera ISO - the higher the ISO, the more noise and less dynamic range available).

when do I use "MANUAL" exposure?

  • the main times I use manual exposure is when the camera's light metering keeps giving me variable results such as when I am shooting into a light source or there image area is dominated by background which is very dark or very light (you could use spot metering in this situation).
  • I also tend to use it for astronomy or night photos
  • finally, I always use Manual exposure when using studio flashes or off-camera flashes:
    • set the shutter speed to the flash sync speed (eg. 1/180th sec for digitals) or slower.
    • set the ISO to desired ISO - usually ISO 100
    • set the aperture to that needed to match the flash (& ambient light but that is a bit more advanced)
  • digital cameras give you the wonderful opportunity of trial and error, set your exposure, take a photo, check the result then adjust the exposure as needed.
  • even better is the RGB histogram that most cameras can display which enables you to see if you are blowing out your highlights (see below).

what about the "SHUTTER PRIORITY" or "S" exposure mode?

  • the main time I use this setting is for sports photography or moving water (eg. waterfalls)
  • I set the shutter speed to the level sufficient to give the motion effect I desire eg. 1/500th sec so that a moving football has motion blur but the players are relatively sharp.
  • then set the ISO to a level that allows the aperture to be within the range of the lens aperture values.
  • the camera will then hopefully determine the best aperture.

Digital exposure using the histogram method:

  • most digital cameras display RGB histograms after the photo has been taken - that is a graphical display of how much of the image is recorded at each brightness level for each of the 3 digital color channels Red, Green & Blue.
  • the main exposure problem in digital cameras is the loss of detail in over-exposed regions which cannot be retrieved by later processing (although if you shoot in RAW mode, some of it can be retrieved).
  • thus, most digital photographers prefer to check the histograms to ensure none of the main image peak goes past the right side of the histogram in any of the 3 channels - although, inevitably with high contrast situations where there are 2 peaks, the right peak for very bright sky, reflections or the light source itself, will usually be to the right of the limit of the histogram and is thus ignored, while the right edge of the left peak representing your subject does not reach the right limit.
  • some cameras such as the Canon 1D Mark III even display this RGB histogram in live preview mode allowing you to optimise your exposure manually before taking the photo, rather than just adjusting your exposure and re-taking the photo.
photo/exposure.txt · Last modified: 2011/09/06 14:24 by gary