Toolkits

Digital Camera - DSLR - Imaging Basics

 The core principles of photography are not especially complex, but they often remain a mystery to the typical layperson as they have not had the concepts and terminology explained in a useful or practical manner.  Photos will come out of the camera too dark, too blurry, or otherwise unusable and it will seem that nothing can be done to remedy the problem.  The basic concepts behind taking a photo are discussed below, with an additional section dedicated to looking at selecting a lens and working with the patient.  This material may benefit from the reader being familiar with the concepts introduced in the Technology Overview.

This portion of the toolkit includes the following sections:

Exposure

The concepts behind proper exposure of an image can seem fairly daunting, as there are multiple elements that can be adjusted and shifted to change how an image is exposed.  Fortunately, these concepts are fairly easy to grasp once the parts are broken down and the relationships between settings is explained.  Even better yet, many DSLR cameras have a decent auto-exposure setting that will take out a lot of the guesswork in capturing an image.  If using manual controls, some of the additional information will be more useful.  Even if shooting on automatic settings, this information will help explain why and how the camera images the way that it does.

There is a basic relationship between the three key elements of exposure (ISO, Aperture, and Shutter Speed).  Doubling any one of those elements will allow reducing another setting by half (or a combination of the others).  For example, doubling the ISO will allow the shutter speed to be twice as fast.  The exact mechanics of doubling and halving the settings will be explained below.

DSLR cameras will typically have an internal light meter that is visible through the viewfinder, appearing as a little series of hash marks with a moving indicator that changes location depending on how well exposed the scene is.  Changing the various exposure-related settings or the lightness of the scene being captured will change how well exposed the meter considers the scene to be. 

DSLR - Illustration - Internal Light Meter Properly Exposed
DSLR - Illustration - Internal Light Meter Under Exposed
DSLR - Illustration - Internal Light Meter Over Exposed

Some cameras will also include an aperture-priority exposure setting, which allows a photographer to specify an aperture or f-stop to use.  There may also be an option for a shutter-priority exposure, which allows the photographer to specify the shutter speed for the image.

Back to Top

ISO

The ISO settings refer to how responsive the sensor is to light.  Initially, when used in film, there was an actual difference between the light-sensitive materials that were exposed to light.  Changing the ISO no longer changes the physical medium of the film, but instead changes how quickly the sensor “reads” the light.  The higher the ISO, the more responsive the sensor is to light.  Cameras often provide a range of ISOs from 100 through 6400.

Unfortunately, increasing the ISO has an impact on the “noise” present in the image.  This will appear as a sort of static or grit in the image.   Some cameras have a better algorithm for processing high-ISO images and reducing noise, but none will remove it completely.  The noise is more prevalent in darker or underexposed sections of the image.  

DSLR - Illustration - ISO Composite Short

ISO Noise Example Image – ISO 100-12800

Doubling and halving the ISO is a simple matter of doubling and halving the number.  An image captured at ISO 400 will be twice as responsive to light as an image captured at ISO 200.

Back to Top

Aperture

The aperture is a mechanism within the lens that limits how much light reaches the sensor.  There is an internal iris that blocks out portions of light, rendering the overall light coming through the lens much lower.  The aperture settings are typically referred to f-stops, and are a common element in lens labeling.  The higher the f-stop, the less light is allowed in (or the more closed the iris becomes).

Aside from adjusting how much light comes through the lens, the aperture also directly impacts the depth of field in the image.  While different lenses will have their own various depths of field, the general rule is that the higher the f-stop, the more of a scene will be in focus.  This is mostly only a concern below f/4, as lower f-stops may leave only mere inches of the scene in focus, rendering some parts of an image blurry.  Extreme cases of this will bring the depth of field down to a fraction of an inch, meaning that very minor movements will render the subject blurred and out of focus.

Doubling and halving the aperture is a little strange, as the numbering system appears to be inconsistent at first glance.  There are set numbers that relate to doubling and halving the light – 1, 1.4, 2, 2.8, 4, 5.6, 8, 11, 16, 22, 32, 45, and higher (though most lenses do not go higher than 22).  Most cameras and lenses will support partial f-stops.  For instance, there may be options for 3.2, 3.5, and 3.8 between 2.8 and 4.

Back to Top

Shutter Speed

The shutter speed related to how long the sensor is exposed to light.  In most DSLR cameras, this will be the result of a shutter mechanism temporarily moving to allow the sensor to be exposed.  The raising and closing of the shutter will be a single click in the faster shutter speeds, while slow shutter speed will have distinct raising and closing sounds.  

Shutter speeds that are too slow are prone to increased burring in the image from small hand movements of the photographer or movement of the subject.  The effect begins to be fairly pronounced below 1/60th of a second, though extremely fast movements may blur with even faster speeds still.  The shutter speed can also be a problem when set too fast, as some flashes will have a hard time syncing with rates over 1/200th of a second.  

DSLR - Illustration - Flash Sync - Shutter Too Fast
DSLR - Illustration - Slow Shutter Dramatic
DSLR - Illustration - Slow Shutter Detail

The doubling and halving of an exposure are fairly straight forward – simply double or half the time that the sensor is exposed to the light.  The numbers on the camera can be a little strange, though, and would warrant from some discussion.  Longer shutter raises, or slower shutter speeds, are indicated by a number followed by the second hash mark (“).  This means that an exposure of 1 second will be shown as 1”.  Faster speeds will simply show the lower number of a fraction, where 1/100th of a second will appear as 100.

Back to Top

White Balance

Many modern DSLR cameras do a very good job with white balance, or determining what colors are actually white.  They also often provide some manual settings for incandescent, fluorescent, daylight, and other light types.  Our eyes typically perceive slightly-colored light as white, adjusting as necessary for us to not be overly distracted by different light sources.  However, in thinking about lights, people may often be aware of the “warm” yellow color of an incandescent light or the harsh “green” color of some fluorescents.  White balance is the camera’s way of compensating for these differences.  This is an especially important topic in clinical imaging, as differences in colors may change the overall diagnosis of various conditions.  Proper white balance can be harder to ascertain if there are multiple light sources of different types, and often requires making a best judgment depending on the scenario.

On the subject of white balance, though somewhat separate, is the issue of background color.  This can drastically change both how the camera interprets color and how the eye views it.  Below are examples of different-colored backgrounds with the same subject being captured.

DSLR - Illustration - Hand All Backgrounds
DSLR - Illustration - Hand White Background
DSLR - Illustration - Hand Black Background
DSLR - Illustration - Hand Gray Background
DSLR - Illustration - Hand Blue Background
DSLR - Illustration - Hand Beige Background

Back to Top

Composition and Shooting

For most imagers, using a DSLR camera can be technically more challenging than utilizing a point and shoot camera for clinical photography. Clinical photography is a learned skill and an art form. That being said, there are some basic tenets that an imager can adhere to that will improve their clinical photography skills. Taking into the consideration the principles of subject, perspective, scale, background selection, lighting selection and proper lens selection will help any imager move towards the goal of composing and shooting great clinical images.

Subject

The subject should take up the majority of the frame in a clinical image. Some governing bodies and individual clinicians have guidelines established framing guidelines for defined subject shots. Even when an image is clearly depicting a subject, additional reference shots may be required or requested by reviewing clinicians. The goal when considering the subject in a clinical image is ensuring that the subject is clearly apparent the person who will be potentially viewing the image. When shooting clinical images of human subjects, this means removing all jewelry, glasses and background distractions. For larger subject areas, appropriate draping to show only the part of the body that is the subject of the photograph is necessary.

Perspective

Taking the photograph with the appropriate perspective will help to take the guesswork out of obtaining the optimal depth of field and focus. The camera should be positioned parallel to the subject matter surface as well as at the height of the focal plane.

Scale

In clinical imaging it is important to provide some type of scale for the imager reviewing the image. In some subject shots where you don’t want to sacrifice image quality, and distance and focusing are crucial, you may need to obtain a more general shot with an object present to help determine scale.

Lens Selection

Choosing a lens for a telehealth program is not always a clear decision.  While some lenses can be fairly clearly ruled out – lenses on the extreme ends of the focal length spectrum (those that are especially “wide” or “long”) are not especially useful in clinical settings – there is still a rather large list of lenses that must be considered.

Some things to think through and what they imply:

  • What subjects will you be shooting?
    • Small details – lesions, wounds, teeth, hands and feet, etc
      • Imaging smaller details can benefit from either macro lenses, such as the fairly common 50mm and 100mm varieties, or moderate telephoto lenses that range between 80mm and 120mm.  Beware that using a very wide lens may distort some features, causing nearer objects to be larger than those in the distance. For instance, even a small nose may appear prominent with a 14mm lens when shot in close proximity, but be appropriately-sized with larger lenses. 
    • Appendages and sections of the body
      • The lenses that are used for larger sections of the body may need to be a little wider, depending on the space available in-clinic for imaging.  The 50mm macro and “normal” lenses around 50mm will likely meet these needs, though the macro functionality may not be as useful as it is in imaging smaller details.
    • Full body, extended limbs
      • Shooting the full body often requires a fairly wide lens, and/or some substantial space in which to image.  A macro lens is likely unnecessary at this point, and a telephoto lens is not going to be wide enough to capture the full subject.  A lens that is in the wide-to-normal range will suffice, perhaps between 24 and 55 mm.
    • Multiple subjects
      • If you are shooting a wide array of subjects, such as dermatological lesions and images for tracking range of motion improvements for physical therapy, a single, set lens may not be enough.  Zoom lenses that offer ranges of between 20mm and 80mm will cover most needs.  Advanced users may desire to have multiple lenses available to meet their needs.  Remember also that the modern DSLR is capable of providing images at a very high resolution, which means that an image appearing to be taken from too far away can often be zoomed-in during the image review process.
  • How much room do you have in your clinic for imaging?
    • Under five feet
      • If you are planning on shooting full-body images at this range you are going to struggle in finding a lens that meets your needs.  Extremely wide lenses will distort the subject, and longer lenses will not provide enough coverage.  Most other imaging can be done, though some of the partial-body and appendage imaging may benefit from a slightly wider lens.
    • Between five and ten feet
      • Full body imaging can be accomplished with a fairly wide angle lens when more than five feet of distance is available between the subject and the camera.  Some of the longer telephoto lenses may have some application for partial-body or detail images, and the 100mm macro can be used for capturing more of a subject.  Wide angle lenses will provide less distortion when shot at greater distances.
    • Over ten feet
      • At distances greater than ten feet, wide-angle lenses will provide minimal distortion (though super-wide “fisheye” lenses will still be inappropriate for use).  The 100mm macro may be used for full-body imaging, though crop factor may be a problem in some cameras, and especially tall patients may still prove difficult to image without the wider lens.
  • How comfortable will your patients be with having the lens of the camera in close proximity to their face and/or body?
    • Very comfortable – distance is not a concern
      • Macro lenses and wide-angle lenses will be fine for this patient population.
    • Moderately comfortable – some patients would be uncomfortable with a lens too close to them
      • Shooting full- or partial-body images will likely be acceptable with the recommended lenses, as there is enough distance required to appropriately frame the image.  Some imaging of details with wide angle lenses may prove a little close for comfort, and the 50mm macro lens for close-up images is likely to feel somewhat intrusive.
    • Moderately uncomfortable – some patients have strong personal space requirements
      • Wide-angle lenses used for imaging small areas are likely to feel intrusive, as will the 50mm macro for close-up imaging.  It may be beneficial to plan on using slightly-longer lenses in a larger space, if possible.
    • Very uncomfortable – patients are very uncomfortable with cameras and /or have a strong need for personal space
      • Some populations may feel very uncomfortable with having a camera and doctor too close, requiring the use of longer lenses where they might not otherwise be normally used.  A telephoto lens can be used to image small portions of the body at a greater distance.  Wide angle lenses and the 50mm macro are likely to be inappropriate for these patients, unless being used for full-body imaging.
  • How much light is available?
    • Ample lighting – the imaging room is bright and evenly lit
      • Most lenses will meet your needs.  There is no special requirement to buy a lens with an especially low f-stop.
    • Fair lighting – the imaging room is sufficiently lit, but there may be areas that are less bright or the light may be a little dim
      • Most lenses will still meet your needs, but it may be beneficial to consider either using these lenses with a flash or to purchase the more expensive lenses that have a lower available f-stop.
    • Poor lighting – the imaging room is insufficiently lit for photography, either due to areas of strong shadow or overall dimness of light.
      • Consider purchasing a more expensive lens that has a lower aperture.

Working with the Patient

It is important to explain to the patient the methods that you will be using when obtaining clinical images. Explaining the equipment that will be used, the positions that a patient will have to assume and the methods for draping certain subject areas can be very important in reducing any additional fears and/or anxiety that some patients may have around being clinically imaged. Explaining why you are obtaining the images is important, including why they have been ordered by the patient's care provider. Briefly explaining the benefits of utilizing telehealth may also be important to some patients. Ensuring the privacy of patients during imaging sessions and with the handling of images after imaging is very important.

Back to Top