Sharpness

21 Feb

Photography at its core revolves around the challenge of obtaining a sharp and colour-accurate image of a scene. Once we’ve achieved sharpness, we can modify the image in any way we like, even removing the sharpness in creative ways. However, we cannot yet easily move in the opposite direction. Therefore we’re obliged to obtain a sharp image as a first step.

What, then, are the factors that determine the sharpness of the image we obtain?

  1. Lens: Lenses differ in “optical quality”, that is to say, how much heterogeneity in surface structure and material composition there is in the glass. Heterogeneous glass will render blurry images, whereas glass that adheres exactly to the intended design will give sharper images. For this reason, many photographers advocate starting with good glass and a cheap camera body, but this again depends on the intended application – for sports and wildlife photography, you will want to have a camera with a high frame rate in order to avoid missing crucial moments in your scene. This may limit your options to mid- to upper-range camera bodies, and take away from your lens budget.
  2. Sensor: The other piece of hardware that matters is the sensor. As part of the sensor assembly, there will usually be an “anti-aliasing filter”, which adds a small amount of blur to the recorded image to avoid the appearance of aliasing and moiré. Recently, camera manufacturers have tended towards giving a weaker AA filter and render a sharper image. It is partly a matter of personal preference whether to use a stronger AA filter, but keep in mind that this, as well as any other filter you use in front or behind your lens, will influence image sharpness.
  3. Focus accuracy: It is obvious that the autofocus accuracy of a camera will determine sharpness (or focus placement, if you will) when using autofocus. When using manual focus on an SLR, much will depend on your skill, but also on having a large and bright viewfinder that allows you to see your scene accurately as it will be rendered on your sensor. On better cameras, the viewfinder will be adjustable to your preferred way of holding the camera up to your eye, or to your level of near- or far-sightedness. The more customisation your camera offers in this regard, and the more accurately you adjust it, the better your focusing ability will be. When using LiveView, you only need to be able to see whether the image on the screen is sharp. Many now recommend using a camera with “focus peaking” to help with this, which highlights any sharply focused edges in your image.
  4. Stabilisation: If you are using longer exposures (weak light or small aperture), and particularly if shooting at greater zoom lengths (telephotography), you’ll have to consider the merits of stabilising your image to avoid blur, either by using a stabilising aid such as a tripod, monopod, gorillapod, beanbag, window mount, etc., and/or by having a camera or lens that support sensor or optical stabilisation, respectively. So-called digital stabilisation is generally less preferred than the other methods, as it typically “slices” the image into sub-images and re-combines them into your final image following alignment. This process is slower both in the taking and processing of the image, and may increase noise in your image. A widely touted rule of thumb says that optical stabilisation should be preferred for larger focal lengths (zoom distances).
  5. Exposure (shutter) time: As mentioned in the previous section, short exposures require less stabilisation. This is because you’re less likely to shake your camera a sufficient amount during short exposures. On some, especially older, camera models the shutter itself will cause the camera to shake, and in some cases the image stabilisation mechanism is in turn shaken in such a way as to add blur rather than subtract it. It’s good to be aware of such settings and either avoid those exposure timings, or disable the image stabilisation. This will vary from camera model to camera model, so you must do some research on how this relates to your particular camera. Notwithstanding these observations, selecting a short exposure is generally advantageous to the quality of your image, with the following caveat.
  6. ISO level: On digital cameras, you can manually select a virtual “film speed”, which corresponds to what in the old days determined how grainy your film was (grainier film catches light more easily on each grain, but with a loss of detail because the “colour dot” (grain) is larger). Higher ISO levels will cause you to lose detail, and will increase noise in your image. You may then want to remove this noise, which, in addition to the loss of detail already inherent in the high ISO level, will cause your image to become even blurrier. Selecting higher ISO levels will allow you to shoot fast action better, but due to the loss of image detail, you should not always shoot at a high ISO setting, or your sensor will perform far below the capability of the sharp lens I hope you bought.
  7. Aperture: This is a slightly advanced topic, but all lenses are sharper at some aperture settings than others. As a general rule of thumb, f/8 will be quite sharp, but the exact sweet spot will vary from lens to lens. The fall-off in sharpness also varies – some lenses can still be very sharp at f/2.8, and most also generate very acceptable images up to f/11. Changing the aperture is, of course, how we control our depth of field, that is, how “deep” into our image we will reach with the sharpness. A small aperture will give greater depth of field, but in order to obtain a nice “bokeh”, i.e. background smoothness, we need to select smaller apertures. Depending on our personal preferences, a truly great lens might be one that allows us to have a part of the image pin-sharp while rendering great bokeh in the background. The shape of highlights on your bokeh, by the way, is determined by the shape of your aperture. Six aperture blades will give you hexagonal highlights. You can make your own.
  8. Adding light sources (flash): Flash may be a very old and time-tested way to improve sharpness in a moving scene, but I mention it last because flash will definitely change the way your scene appears. When using flash, you will always capture an approximation of what you were looking at in real life. Flash can be a creative medium, and especially in a studio setting, where you have more flexibility over the placement of your light sources, these become an irreplaceable part of your toolkit. However, a camera-mounted flash (with the exception of ring flashes in macro photography) will, in most scenes, cause unsightly shadows to appear behind your subject, so camera-mounted flash should be used cautiously and in small doses when possible. You can offset this effect somewhat by “bouncing” your flash off a nearby matte surface with sufficient reflectivity, which will serve to scatter the light from your flash and make shadows less harsh. However, to cut to the short of it, the added light from a flash allows you to lower your exposure times, giving less shake in your image.
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  1. What’s the deal with the Canon 70D? | breakfastographer - July 2, 2013

    […] of the resulting image. Image quality could be said to consist of a combination of resolution, sharpness, dynamic range, and colour and brightness fidelity. Unfortunately, with the camera having just been […]

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