Focus, DOF, Lenses Flashcards
Why is focusing different for forensic use vs. personal use?
Personal usually focuses on one or a few individuals or objects
What do crime scenes contain?
layers, foreground, background, and items between all areas should be in focus
What can the defense argue with a blurred picture?
Defense can argue the image is not “fair and accurate” intentionally blurred to hide certain items, photographer is incompetent
3 sections of focus
resolution
acutance
sharpness
what is the goal of taking a picture of a scene
entire scene should be in focus
what is resolution?
camera system’s ability to distinguish “resolved” lined pairs
example of resolution
as a black and white line comes closer to each other, at some point the lines will become less distinct and blends into gray (line pair per mm)
pixels per inch: more pixels = ?
higher resolution = more detail
What are the resolution standards?
None for crime scene photography
AFIT, electronic transmission of 10-print cards, 1000 ppi
Too much resolution could interfere
what is acutance?
camera’s ability to render sharp edges
2 principles of acutance
-clear and precise distinction between edge of one object and another
-a white and black line would not have overlap
what is sharpness?
-being in focus
-perfect sharp focus when light makes that cone shape
when light enters lens and converge at sensor?
in focus
when light converges before or after sensor, remains in circle?
out of focus
“confused”
circles of confusion
how to use manual focusing?
-Rotate focus ring until object in viewfinder comes into focus
-good for ‘filling the frame’ with one item AND ‘film plane parallel’
what is auto focus?
-camera focuses automatically on selected object in focus point indicator (designed by small rectangles in circle) beep beep
-must be aware what camera is focusing on before taking image
what is auto focus failure?
-when there is not enough contrast
-when there is not enough light or too much light
-when near and far objects are close together
how to auto focus?
-focus on area on same plane with more contrast
-add light, take away light
-switch to manual
what exposure variable determines DOF?
aperture: designated in f stops
how can we determine what will be in focus in our photo?
-using a wide aperture gives terrible DOF = bad focus (f 2)
-using a narrow aperture gives great DOF = everything in focus (f 22)
what is the DOF scale?
the area that will appear to be in focus
what are the circles of confusion?
-Eye can detect light as small circles, 0.025” to 0.033” (not an exact point)= resolution limit of the eye
-Area before and after the point of exact focus appears to the viewer to be in focus
camera focused on 12’
f/22: 6’ to infinity
f/16: 7’ to 30’
f/11: 8’ to 25’
f/8: 9’ to 20’
f/4: 10’ to 14’
wider apertures result in ? DOF ranges
narrower
smaller apertures result in ? DOF ranges
longer
shouldn’t use wider apertures than ?
f/8
If using an f/22…
If using an f/16…
If using an f/11…
If using an f/8…
-focus the lens at 12’
-focus the lens at 16’
-focus the lens at 24’
-focus the lens at 30’
DOF extends ? of the point of exact focus
one third in front
DOF extends ? the point of exact focus
two thirds behind
how do we do ‘rule of thirds’
midway top to bottom
what is rule of thumb?
to maximize DOF when photographing crime scenes ranging between 5’ to 30’, focus midway between top and bottom of the composed image
what to do if camera is still out of focus?
use diopter dial
what is the diopter dial?
adjusts viewfinder for eye quality
varying strength of reading glasses
what does the depth of field include?
- f stop
- lens choice
- camera to subject distance
what is f/4? what is f/22?
-wide aperture
-small aperture
what does FLL stand for?
Focal Length of Lens
what is Focal Length of Lens?
distance between the optical center of the lens and the sensor of the camera when focused on infinity (in mm)
how does lens choice (FLL) affect DOF?
a closer or wider angle
3 types of lens choices
-wide angle lens < 50 mm
-normal lens = 50 mm
-telephoto lens > 50 mm
which lens choice has the best DOF? poor DOF?
-wide angle
-telephoto angle
what does DOD stand for?
diameter of the diaphragm
expressions relating to DOD, FLL, f-stop
f-stop # = FLL/DOD
DOD = FLL/f-stop#
FLL = DOD (f-stop)
close-up photos, DOF is much ?
less
what does a normal lens look like? (50 mm)
-Near objects look the same as viewed in the viewfinder and as is directly
-Far objects look the same as viewed in the viewfinder and as is directly
-Will show 46 degrees field of view
rule of thumb for normal lens
wide angle will elongate distances, telephoto will compress distances. Only 50mm will not have distortion
major parts of the telephoto lenses
-magnification
-narrower field of view
-compression of foreground to background distances
-narrower DOF ranges
rule of thumb for magnification
-To magnify, you should use a lens that has a focal length of 2mm of lens per foot of distance between the camera and the subject
-Ex. 50’ x 2mm= 100mm lens
-Reminder: Telephoto can distort perspective between objects, not a “fair and accurate representation”
