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Lecture 5 : Forensic Archiving I Continued

Lecture 5 : Forensic Archiving I Continued. Shutter Speed How Long is the Exposure. Shutter Speed Shutter Speed: The speed the shutter closes. The shutter is the door to the outside world that remains open for fixed periods of time measured in seconds.

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Lecture 5 : Forensic Archiving I Continued

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  1. Lecture 5:Forensic Archiving IContinued

  2. Shutter SpeedHow Long is the Exposure

  3. Shutter Speed • Shutter Speed: The speed the shutter closes. • The shutter is the door to the outside world that remains open for fixed periods of time measured in seconds. • Closes the aperture to outside • Controls amount of light entering camera • Regardless of size of the opening, if the digital sensor is exposed too long, the resulting photograph will over exposed and will not meet forensic or artistic standards. • Role of the shutter is to control light getting to digital sensor, by opening and closing the cover over the aperture.

  4. Typical Shutter Speeds Typical shutter speeds • Older cameras: • shutter speeds set to halve or double light allowed in • 1/500, 1/250, 1/125, 1/60, 1/30, 1/15, etc • Newer cameras • 1/3rd steps • 1/500, 1/400, 1/320, 1/250, 1/200, 1/160, 1/124, 1/100, 1/80, 1/60, etc http://photographylife.com/what-is-shutter-speed-in-photography

  5. Focal Length 12-24mm The distance (mm) from the center of the lens to where the image comes into critical view 18-55mm 55-200mm Object Photo’d Distance from Object Photographed

  6. Concept: Equivalent Exposures: Important for forensic photography • Different camera settings allow SAME amount of light to enter the camera but the resulting photographs are subtly different. • Known as equivalent exposures. • Equivalent Exposures • f/8 – f-stop and ¼ second shutter speed • f/11 – f-stop and ½ second shutter speed • f/16 – f-stop and 1 second shutter speed

  7. Aperture & Shutter Speed Equivalent Exposures The combination f/2.8 and shutter speed 1/125 sec gives a cell with number 10.  If you do two stops on the right and towards the top of the chart, you will get the number 10 again. http://irinaovery.com/wp-content/uploads/2012/03/2124175721_591714ec93.jpg

  8. Suggested Shutter Speeds for Photographing Variety of Subjects • When taking a photo of a moving subject, changing the shutter speed can change the look of an image. • A slow shutter speed captures movement. • Fast shutter speed freezes action.

  9. ISOThird Leg of the Exposure PuzzleDigital Processor SensitivityInternational Organization of Standards

  10. ISO • ISO Rating: Third leg of the exposure puzzle • Measure of the digital sensor’s sensitivity to light. • The higher the ISO number, the more sensitive the sensor is to light. • ISO settings affect exposure:Change in ISO from 100 to 200 effectively doubles the light available to the photograph … a full stop. • More light is not entering the camera • Digital sensor is more sensitive to light, which in essence changes the sensitivity of the camera a full stop.

  11. ISO 100 ISO 3200 http://www.digital-photography-school.com/iso-settings

  12. ISO Considerations • Most people keep digital cameras in ‘Auto Mode’ where the camera selects the appropriate ISO setting depending upon the conditions you’re shooting in (it will try to keep the ISO as low as possible) • Overriding the camera impacts the aperture and shutter speed needed for a well exposed shot. For example – • if you bumped your ISO up from 100 to 400, you can shoot at higher shutter speeds and/or smaller apertures. • Choosing the Correct ISO setting • Light – Is the subject well lit? • Grain – Do I want a grainy shot or one without noise? • Tripod – Am I using a tripod? • Moving Subject – Is my subject moving or stationary? • If plenty of light - AND need no grain in photo – AND using a tripod - AND subject is stationary. Use pretty low ISO rating. ject • If it’s dark – AND want grain - AND don’t have tripod – AND subject is moving. Consider increasing the ISO - Can shoot with a faster shutter speed and still expose the shot well. • Trade off of increase in ISO is noisier (grainer) shots.

  13. Situations For Higher ISO Settings • Indoor Sports Events – where your subject is moving fast yet you may have limited light available. • Concerts – also low in light and often ‘no-flash’ zones • Art Galleries, Churches etc- many galleries have rules against using a flash and of course being indoors are not well lit. • Birthday Parties – blowing out the candles in a dark room can give you a nice moody shot which would be ruined by a bright flash. Increasing the ISO can help capture the scene. • Read more: http://www.digital-photography-school.com/iso-settings#ixzz1TKC5Zi00

  14. ISO – By the Numbers • AUTO ISO – digital camera automatically sets the ISO speed according the the brightness of the scene, increasing or decreasing the sensitivity. User has no control over which ISO number is used. • ISO 80 – for taking photos in bright light; excellent for close-ups, landscape, and portraits. Produces fine detail and image quality. • ISO 100 – for extra sensitivity with little, if any, reduced image quality. • ISO 200 – cloudy and overcast days. Acceptable image quality, with some visible noise. • ISO 400 – suitable for indoor photography whether or not a flash is used. Useful for “stop-action” and sports photographs. Most compact digital cameras produce high to very high image noise. • ISO 800, 1600 and above – useful for taking photos in very low light, or outside in good light when increased shutter speeds are required. Results can be disappointing when shooting at these high numbers with compact digital cameras, so take test photos before photographing an important event. http://www.digicamhelp.com/camera-features/advanced-settings/iso/

  15. MeteringMeasuring Light

  16. Metering/Illumination • Cameras need to know how much light is entering the camera, • Actual light reflected from an object being photographed. • Cameras measure the light in order to choose starting point for appropriate exposure. • Modern digital cameras use TTL (through-the-lens) metering systems • Cameras measure the amount of light reflected from object to the digital sensor.

  17. Illumination • The light that should be measured is actual light hitting the object – • Incident light. • TTL system measures reflected light • Approximation of the incident light, which is why cameras don’t always get it right. • Sometimes TTL system forces the camera into taking under or over exposed photographs. • Backlit situation • Older cameras did not have built-in metering systems, • forced the photographer to use hand-held meter to measure the incident light hitting the object.

  18. Light Source External Meter Incident Light Object Photographed Reflected Light TTL: Through-the-lens In-Camera Metering • If all objects reflected the same percentage of incident light, the TTL would work just fine. • Real-world subjects vary greatly in their reflectance.  • In-camera metering: Standardized based on the light reflected from an object appearing as middle gray.  • Camera aimed directly object lighter or darker than middle gray, • In-camera light meter will incorrectly calculate the amount of light. The result is an under or over-exposed photograph. • Hand-held light meter: calculates the same exposure for any object under the same incident lighting.

  19. Canon PowerShot Metering Modes : Evaluative, Center Weighted Average, Spot

  20. Lenses • Telephoto • Capture images at long distances • Long-range photography of objects • Wide angle • Capture panorama • Establishing photography • Macro – Close-up lenses • Allow close photography • Detailed evidence photography Lenses Focus Light http://en.wikipedia.org/wiki/Lens _%28optics%29

  21. White Balance

  22. White BalanceWhat is White

  23. WB – set on “auto” Photos from Philadelphia Flower Show WB – set on “fluorescent”

  24. White Balance (WB) • Tells camera how to “interpret” white. • Allows camera to produce accurate colors under a variety of lighting conditions. • Poor white balance signifies sloppy work … • Someone simply going through the motions of taking pictures without paying attention to the business of forensic photography. • Little or no thought about the consequences of selecting – or not selecting – the proper WB. [1] Lezano, ibid, page 42.

  25. White Balance (WB) • The camera relies on the photographer to tell it what the lighting condition is via the white balance setting. • If the photographer chooses the incorrect color temperature (WB), the colors present in the photograph can be “off,” which for forensic work is unacceptable • White balance can only be corrected using software – • If the original photograph was taken using the RAW setting.

  26. Camera Options for White Balance

  27. Using Gray & Color checker Cards Place gray card in scene oriented toward the direction of the incident light. Take reading from it using a reflected light meter. Assures photographer of consistent exposures across their photographs. This technique is similar to using an incident meter, as it depends on the illuminance but not the reflectivity of the subject. 18% Gray card A gray card is only useful for setting or correcting the balance of neutral colors. Use photograph of a Color Checker as a reference for color balance adjustments.

  28. What Are The Three Problems With This Scene Photograph? • White Balance • Light • General Illumination • Glare • Forensic Perspective • Relationship to other Evidence • Deceased? • Weapons? • Trail direction?

  29. The Perfect Forensic Photograph A Roadmap Readjust WB – DOF Choose Perspective Set WB Set Aperture Take Photo Check WB & DOF On Photo Check Exposure Set Camera To Manual Close up Establishing DOF Set Shutter Bracket Shots Observe &Smile

  30. Flash • Drawback • Creates harsh light • Washes out evidentiary detail • Critical Uses • Can Eliminate shadows • Can Minimize glare • Bounce light to provide softer illumination

  31. Flash BasicsFlash illumination is dramatically affected by distance.The inverse square law • The difference between a light at 2.8 feet & the same light at 4 feet is 1 stop. • Same for 8 feet to 11 feet. • Same for 16 feet to 22 feet. Cup @ 2.8 feet is 1 Stop Brighter than cup @ 4.0 feet Takes 16x as much light to illuminate Cup @ 11 Feet to = cup @ 2.8 feet http://photography-on-the.net/forum/showthread.php?t=171657

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