jimtrue.com : school : CJT1221 : 2003-05-23: Exposure Methods
Posted by Jim True on May 23, 2003 6:09 AM. Last Updated October 22, 2006 9:23 PM
Disclaimer for all material noted here is at the bottom of this web page.
Negative (opposite)
Contrast is the difference in Intensity, Shade, Black & White Must be able to differentiate the print from the background.
To Add Contrast:
Red Background, Red Filter
Allows more red to get into the film, makes it more a uniform color or white.
Film - Dark
Print - Light
Temperature is a big factor in processing film; one degree in temperature changes one minute in developing time.
Order of Developing Film and Print:
At least an hour to process a roll of film from start to finish.
Photograph, get into the darkroom; has to be done in total darkness.
Load the reel, average 20-40 minutes.
Camera Meters - in the camera we use are - Reflective Meters work off the fact that light reflects off a subject and that's what they read.
All reflective meters, and incident meters, 18% Grey. Want to adjust themselves so that the final photo will give you an 18% grey averaging throughout the entire photo.
If you don't meter correctly, and balance with a grey card, camera is trying to average the black and the white to 18% gray. Hold the card in the same light falling on the card, take a meter reading within the camera.
Needs to have the same amount of light falling as the light falling on your subject. Better to get all the way up to your subject, hold the card close until the average light reading. You've performed an exposure reading prior to actually taking the photograph.
How to develop and print film. How to take a correctly exposed picture. 18% grey is what the camera meter wants to see.
Copy stand, take a meter reading off the copy stand base prior to taking photograph to get the correct exposure.
As long as you don't change your light; your exposure's will all be to the same 18% grey.
Three things control Correct Exposure in the camera and the darkroom:
Depth of field - amount of area in front of the subject and behind the subject that is in focus. 1/3 in front and 2/3 and back.
In the field of forensics, more concerned with capturing the image. Only have one chance to capture the image. Pushes the limits of photography; asking camera and film that they weren't really prepared to do. Adverse weather conditions, physical conditions, clean lenses.
We choose film based on best compromise of image quality and lighting situations. 200 speed in daytime; 400 speed at night.
Only time we shoot film now is when they need to provide a negative for comparison purposes.
Everything in Pinellas today is done with digital. You are bringing a representation of what you saw at the scene to court. It's not the medium that is important.
Shoeprints, tiretracks, toolmarks, blood spatter, latent prints - still work for film. Comparison prints still need the detail offered by film.
You can not understand how to take a correctly exposed digital photograph unless you know how to take a correctly exposed film photograph.
Pinellas is hiring 2 people a year; if you want to work in the field of forensics, you need to look outside of pinellas. Pinellas is more advanced than most forensic agencies.
Everything in this class if about 4 inches from the camera; depth of field incredibly important, lighting.
Diffused lighting because of the glare.
Transmitted lighting because of transparent surfaces.
In court, a lawyer never asks you a question that they don't already know the answer to.
Exposure:
| F Stop | Shutter Speed |
| 3.5 | 500 |
| 5.6 | 250 |
| 8 | 125 |
| 11 | 60 |
| 16 | 30 |
| All identical balanced exposures | |
10 rolls of film, 36 exposures, 360 photographs at a crime scene. Plan everything. Difference between a photographer and a picture-taker.
186000 miles per second speed of light; stop motion with tripod or with the flash.
Aerial photographs, no tripod.
Composition - trying to compose and capture an image on that film that will identify the tool, the person the tire track, the fingerprint. How are we going to get a fingerprint on a coke can? Curved surfaces create problems with depth of field when photographing up close. 400 speed Black & White film.
Consistency - Every picture needs to be at the same exposure, balanced to 18% grey, or you won't easily be able to control the contact sheet. Negatives must be perfectly balanced.
Images should take about 8-10 seconds of enlarging time at F8 for every photograph per print.
If your light changes, you need to re-meter.
Need to understand how to get same light metering with FlashDisclaimer: These are MY notes taken from classroom lectures while I'm in the classroom. While I'm perfectly happy to share my notes with my classmates and I know I take very good notes, you should still make every effort to attend the class and TAKE YOUR OWN NOTES. I will not transcribe everything the instructor says in the classroom, and I will NEVER post pre-exam reviews. My notes will not replace the value of actually attending class and taking your own class notes.I also cannot attest to their accuracy, other than they are what was provided in the lecture; you should not reference my notes as "expert opionion" by any means, and if you notice an error or omission, please do me the favor of e-mailing me with the correction and I will re-post my notes. End of Disclaimer.