The digital darkroom, part 2: Digital photography basics

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S c i e n t i f i c F o r u m

The Digital Darkroom, Part 2: Digital Photography Basics

Gregory M. Galdino, MD; Paul N. Manson, MD; and Craig A. Vander Kolk, MD

We summarize here the second part of a 3-part series on the digital darkroom. Inthis part, we review the basics of digital photography, including (1) the variables inimage capture, processing, and storage that affect picture quality and (2) the vari-ous types of digital cameras currently available. The full text of the complete series,including expanded illustrative material and complete bibliographic documentation,is now available on our journal Web site at http://www.mosby.com/aesthetic. Pleasesee page 476 for instructions on how to access Aesthetic Surgery Journal Online andview the entire series.

Digital photography is gaining popularity as a replacement for standard 35-mmphotography in plastic surgery, offering such benefits as an overall reduction incosts, the ability to easily archive and retrieve images, flexibility in communica-

tion, enhanced patient education, and computer simulation.1-7 Most photographic prin-ciples related to standardization for medical photography, such as those pertaining tolighting and patient positioning, also apply to digital photography.8-17 However, thetechnology of digital photography differs significantly from that of conventional photog-raphy; a digital camera is more akin to a scanner or video camera than to a standard35-mm camera. The user must understand the basics of digital cameras and image cap-ture to take reproducible, standardized photographs while reaping the benefits of thisnew technology.

Image Capture

The keystone of digital photography is the image sensor. The most widely used sensor isthe charged-coupled device (CCD), an electronic chip composed of individual light-sensitive electrodes, each of which represents a pixel (picture element) in the final image.18

Light travels through the lens and strikes the electrodes, causing an electronic signal tobe created; the strength of this signal is proportional to the amount of light striking eachelectrode. The signal is converted into digital form by a digital converter, processed bythe camera’s microprocessor, and displayed as a color image. Because the CCD is notsensitive to color, only a black-and-white image is created. However, by placing individ-ual color filters over each electrode, corresponding to red, green, and blue light, theprocessor is able to add color to the final image.18,19 Most cameras available today cap-ture 24-bit color (16.7 million colors) and provide image resolutions ranging from 1.5 to3.5 million pixels, even though the human eye cannot delineate nearly that many colors.

From the Johns Hopkins Hospital,Baltimore, MD.

Accepted for publication July 7,2000.

Reprint requests: Gregory M.Galdino, MD, Division of PlasticSurgery, Johns Hopkins Hospital,8130A McElderry, Baltimore, MD21234.

Copyright © 2000 by The AmericanSociety for Aesthetic PlasticSurgery, Inc.

1084-0761/2000/$12.00 + 0

70/1/111557doi:10.1067/maj.2000.111557

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Variables directly affectingimage quality

Variables that directly affect the overallquality of the final digital imageinclude lens quality (focus, zoom, andmacro capability), viewfinder type,image sensor, resolution, compression,and image processing quality.20

LensesAs in 35-mm photography, the betterthe quality of the lens, the better thepicture quality. In some professionaldigital cameras, standard 35-mmautofocus lenses can be mounted on adigital camera body, allowing inter-changeability of existing lenses.

Focus

A digital camera can have fixed-focus(nonadjustable), autofocus, or manualfocus capability. Fixed-focus camerasare not recommended for medicalphotography. Some digital camerasoffer both autofocus and manualfocus modes, which are usually select-ed by means of a switch on the cam-era lens or body. Except for somenewer professional models, such asthe Nikon D1 (Nikon Corporation,Tokyo, Japan), the focusing mecha-nism in a digital camera is often slow-er than that in a standard 35-mmsingle-lens reflex (SLR) camera. As in35-mm photography, the focusingmechanism looks for contrast and ver-tical lines to adjust the focus sharply.Thus, focusing on the eye for a facialshot or the nipple/areola for a breast shot usuallyensures a sharp picture; the subject is then adjusted inthe frame after focusing. Focusing on areas lacking con-trast and lines, such as the cheek or abdomen, can resultin out-of-focus pictures.

ZoomMagnification of the subject that is performed opticallythrough the lens is called optical zoom. Magnificationthat is controlled digitally, by allocating a smaller portionof the CCD to scene capture and then interpolating (add-ing) extra pixels to reach the final zoom desired, is called

digital zoom.18,20 Digital zoom often results in poorerquality magnification; in addition, the resolution (seebelow) is always reduced, because only a portion of theCCD is used.18,20 Zoom functions used in medical pho-tography should rely on optical zoom, not digital zoom.However, digital zoom is useful for checking the clarityof the image once it is captured.

Macro modeMacro capabilities differ between digital cameras. Somecameras can photograph a subject as close as 6 in with-out a macro mode. Other cameras offer a specific macro

Figure 1. This digital photograph illustrates the macro capabilities of digital cameras withoutthe need for additional attachments to the camera. It was taken with the Nikon 950 Coolpixon macro mode through use of the on-camera flash and softbox lighting. The inset shows theflower icon commonly used to indicate the macro mode.

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mode (often indicated by a flower symbol or icon) orsuper macro mode in which the camera can focus from0.8 to 20 in, depending on the camera (Figure 1).

ViewfinderThe optical viewfinder is typically one of the followingtwo types: (1) parallax (rangefinder), seen in many point-and-shoot 35-mm cameras, and (2) through-the-lens, orSLR. Parallax viewfinders may exhibit parallel distortion(inaccuracies in subject/lens relationship). This is morenoticeable the closer the camera is to the subject; it disap-pears or becomes less noticeable at greater distances fromthe subject (Figure 2).18

Image sensorsCCDs are generally smaller than film and thus havedifferent focal lengths. The focal length of the lens is

dependent on the size of the sensor. For this reason,manufacturers often give focal length in terms of 35-mmequivalency when reporting the zoom capabilities of thelens. (For example, a 60-mm digital macro lens may beequivalent to a 90-mm standard 35-mm lens.18) In gener-al, digital cameras require more light than 35-mm cam-eras.22 Sensor sensitivity is expressed in terms of effectiveISO rating. Most cameras have an effective ISO ratingbetween 50 and 200 ISO. Adjustable ISO settings allowphotography in lower light conditions but introducenoise (lessen image quality) at higher ISO settings.

ResolutionUnderstanding resolution is paramount to understandingdigital photography. The best way to approach resolu-tion is to determine what the image will be used for. Forexample, when one is photographing patients, it israrely, if ever, necessary to enlarge the image to 8 × 10in for printing; usually, images measuring 5 × 7 in oreven smaller are sufficient. Most cameras with totalresolutions of 1.5 to 2.7 million pixels capture sufficientresolution to print excellent 5 × 7-in and 8 × 10-inphotographic-quality prints. If the images are going tobe displayed only on a monitor, an even lower resolutionis sufficient, because most monitors display a maximumof 72 pixels per inch (PPI) at 640 × 480 or 600 × 800resolution.21 A general rule of thumb is to always shootat a slightly higher resolution than you think is necessary(according to what the image will be used for).21 Photo-graphing at low resolutions (less than 1.5 million pixels)can result in low-quality images. File size should beadjusted with compression (discussed below) instead ofresolution if the resolution setting falls between 1.5 and2.7 million pixels.

CompressionMost digital cameras capture on one of two file formats:(1) TIFF (Tagged Image File Format) and (2) JPEG (JointPhotographic Expert Group). The TIFF format retains allof the information in the original image and is thus toolarge to be used efficiently for patient medical photogra-phy. The JPEG format uses a mathematical algorithm tocompress image file size. The compression ratios mostcommonly used in this format produce too little imagequality loss to be detected by the human eye.22 Degra-dation in image quality is noticeable with large compres-sion ratios and repeated resaving of the image in JPEGformat by means of image editing software.22 Compres-sion is usually indicated on the camera as the quality set-ting—super, high, normal, or low.

Figure 2. Macro images taken with the Nikon 950 Coolpix illustrateparallax error using the parallax viewfinder. This error is maximized inimages taken close to the subject. A, The view through the viewfinder.B, The resulting image. To avoid this error, the LCD display should beused to frame subjects at close distances when one is using a cameraequipped with a parallax viewfinder.



Image processing

Because different manufacturers use different sensors andprocessors to capture and process the image, all digitalcameras are not equal. This is especially evident withregard to dynamic range—the camera’s ability to capturedetail in the lightest and darkest portions of a photograph(Figure 3). Different cameras capture the same image dif-ferently. To maintain consistency in preoperative and post-operative photographs, the same digital camera should beused for both.

Variables indirectly affecting image quality

Lighting

Most of the principles of conventional photography thathave to do with lighting setup apply to digital photogra-phy. There are several publications illustrating propersetup.1,3-6, 9,10 Digital photography may use an externalflash via a “hot shoe” connection, the on-camera flash, orexisting light (depending on the lighting conditions) toallow proper exposure. Because the performance of mostdigital cameras decrease as the amount of light decreases,adequate lighting is essential for good-quality digital pho-tographs (Figure 4).18,22

Metering

Most light meters are programmed to expose the photo-graph for 18% gray (reflecting 18% of the light hittingit).18 The matrix metering setting (multisegment; portionsof the entire frame contribute to the meter reading) is themost useful and accommodating setting. It should beused at all times, except when there is very bright light ina portion of the photograph (eg, in the operating room)or a very bright or very dark backdrop, in which casespot metering may be more useful to avoid inaccurateexposure.

Depth of field

In digital photography, as in 35-mm photography,depth of field is dependent on the focal length of thelens and the size of the CCD.18 Many digital camerashave f-stops only in the range of f/2.2 to f/11; thus, thedepth of field is not as adjustable as it is in conventional35-mm photography.

Color temperature

The term color temperature refers to the inherent color ofthe light (in degrees Kelvin): the lower the temperature,the warmer (slightly reddish) the light; the higher the

Figure 3. A, An image taken with the Sony DSC-D700. B, The same image taken with the Olympus 600-L. Both imageswere taken under identical lighting conditions and color balanced with an 18% gray card (eliminating color shift). Thisexample shows how digital pictures differ from one camera to the next on the basis of the different image sensors andprocessors used in the cameras.

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temperature, the colder (slightly bluish) the light.22 Inas-

much as digital cameras do not use film, the sensor records

these color temperature casts in the image. Thus, the

color cast can be compensated for by using the white bal-

ance control, specific for color temperatures, such as

incandescent and flash. However, for the many condi-

tions in medical photography that involve mixed lighting,

the auto white balance setting should be used.

Output

Monitors, liquid crystal display (LCD) projectors, and

television sets can all be used to view digital images.

Because each source might have different methods of dis-

playing color, have different sets of directions according

to which it instructs the device on how to display color,

and be adjustable with regard to brightness, contrast, and

color, digital images may appear slightly different on dif-

ferent devices. Software is available to help calibrate the

monitor display to what is printed or to standardize the

display between computers.

Camera Performance

Factors that can improve the performance, enhance theefficiency, and maximize use of the digital camera includepower, storage capacity, and image transfer capability.

Power

The source of power for the camera has a profoundimpact on the length of time the camera can be used, themaximum number of pictures that can be taken continu-ously, and the use of the flash and LCD. Digital camerasare generally powered by one of the following types ofbatteries: standard alkaline (usually AA), nickel cadmium,nickel metal hydride, and lithium ion. The best source ofpower is rechargeable batteries, specifically lithium ion ornickel metal hydride, which lose their charge faster thandisposables but can frequently be reused more than 100times.18

Storage

Although digital cameras are “filmless,” they do use atype of film known as a memory card. Removable

Figure 4. A, A picture taken in low light conditions. B, A photograph of the same subject taken in adequate lightingconditions. Note the loss of image detail in the image captured under low light; most professional lite digital camerasperform best in high light (low ISO) situations. These images were taken with the Olympus 600-L through use of soft-box only (A) and softbox with additional photoflood lighting (B) only (no flash).

A B



memory (also known as flash RAM) from digital camerasgenerally costs $1 to 5 per MB and comes in 4 standardformats: Smart Media, Compact Flash, PC Cards, andMemory Stick.18 An adapter may be required to transferimages to the computer by means of a card reader or lap-top via PCMCIA card slots. A good rule of thumb is tohave a card large enough (32 to 48 MB) that frequentdownloads, which limit picture-taking, can be avoided.Regularly downloading images into a database can helpyou keep track of patient records and information in anefficient manner.

Image transferImage transfer affects digital camera performance andcan have an impact on efficiency. Serial port connectionsare generally too slow for a busy practice and are beingreplaced by USB connections, which are more than 100times faster. Removable memory generally requires anadapter to input the images directly into the computer orinto a card reader. The fastest transfer is accomplishedwith an IEEE-1394 (FireWire [Apple Computer, Inc.,Cupertino, CA], iLink [Sony Corporation, Tokyo, Japan])connector; these are starting to appear on several newercamera models.

Choosing a Camera

Our experience has been that the Nikon 950 Coolpix, theNikon 990, and the Nikon D1 have macro capabilitiesfar superior to those of other cameras in their pricerange. Nikon, Olympus, Minolta, and Sony have devel-oped cameras targeted for higher-end uses; Fuji andCanon have also released SLR digital cameras. The onlyreasonably priced true SLR digital cameras are the SonyDSC-D700/770 (Sony Corporation, Tokyo, Japan), theOlympus 2500 (Olympus Optical Co., Ltd., Tokyo,Japan), the Nikon D1, and the recently released SLRsfrom Fuji and Canon. However, a smaller camera withthe LCD back, such as the Nikon 950/990, is advanta-geous for intraoperative photography and is the mostportable.

High-end SLR cameras like the Nikon D1 range in pricefrom $4000 to $5000 and will probably not be availablefor less than $2000 in the next few years. These camerascapture outstanding image quality and are completelycustomizable. Most permit the use of interchangeablelenses. Prices for most of the other higher-end camerassuitable for photography in plastic surgery range from$500 to $1500. For those who would like to try digital

photography without spending a lot of money, the Nikon950 Coolpix is a good choice; it is available for less than$600. �

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The digital darkroom, part 2: Digital photography basics