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Forensic Photography > Focus, DOF, Lenses > Flashcards

Focus, DOF, Lenses Flashcards

1
Q

Why is focusing different for forensic use vs. personal use?

A

Personal usually focuses on one or a few individuals or objects

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2
Q

What do crime scenes contain?

A

layers, foreground, background, and items between  all areas should be in focus

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3
Q

What can the defense argue with a blurred picture?

A

​

Defense can argue the image is not “fair and accurate” intentionally blurred to hide certain items, photographer is incompetent

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4
Q

3 sections of focus

A

resolution
acutance
sharpness

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5
Q

what is the goal of taking a picture of a scene

A

entire scene should be in focus

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6
Q

what is resolution?

A

camera system’s ability to distinguish “resolved” lined pairs

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7
Q

example of resolution

A

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)

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8
Q

pixels per inch: more pixels = ?

A

higher resolution = more detail

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9
Q

What are the resolution standards?

A

None for crime scene photography​
AFIT, electronic transmission of 10-print cards, 1000 ppi​
Too much resolution could interfere

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10
Q

what is acutance?

A

camera’s ability to render sharp edges

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11
Q

2 principles of acutance

A

-clear and precise distinction between edge of one object and another
-a white and black line would not have overlap

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12
Q

what is sharpness?

A

-being in focus
-perfect sharp focus when light makes that cone shape

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13
Q

when light enters lens and converge at sensor?

A

in focus

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14
Q

when light converges before or after sensor, remains in circle?

A

out of focus
“confused”
circles of confusion

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15
Q

how to use manual focusing?

A

-Rotate focus ring until object in viewfinder comes into focus
-good for ‘filling the frame’ with one item AND ‘film plane parallel’

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16
Q

what is auto focus?

A

-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

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17
Q

what is auto focus failure?

A

-when there is not enough contrast
-when there is not enough light or too much light
-when near and far objects are close together

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18
Q

how to auto focus?

A

-focus on area on same plane with more contrast
-add light, take away light
-switch to manual

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19
Q

what exposure variable determines DOF?

A

aperture: designated in f stops

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20
Q

how can we determine what will be in focus in our photo?

A

-using a wide aperture gives terrible DOF = bad focus (f 2)
-using a narrow aperture gives great DOF = everything in focus (f 22)

21
Q

what is the DOF scale?

A

the area that will appear to be in focus

22
Q

what are the circles of confusion?

A

-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

23
Q

camera focused on 12’

A

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’

24
Q

wider apertures result in ? DOF ranges

A

narrower

25
Q

smaller apertures result in ? DOF ranges

A

longer

26
Q

shouldn’t use wider apertures than ?

A

f/8

27
Q

If using an f/22…

If using an f/16…

If using an f/11…

If using an f/8…

A

-focus the lens at 12’​

-focus the lens at 16’​

-focus the lens at 24’​

-focus the lens at 30’

28
Q

DOF extends ? of the point of exact focus

A

one third in front

29
Q

DOF extends ? the point of exact focus

A

two thirds behind

30
Q

how do we do ‘rule of thirds’

A

midway top to bottom

31
Q

what is rule of thumb?

A

to maximize DOF when photographing crime scenes ranging between 5’ to 30’, focus midway between top and bottom of the composed image

32
Q

what to do if camera is still out of focus?

A

use diopter dial

33
Q

what is the diopter dial?

A

adjusts viewfinder for eye quality
varying strength of reading glasses

34
Q

what does the depth of field include?

A
  1. f stop
  2. lens choice
  3. camera to subject distance
35
Q

what is f/4? what is f/22?

A

-wide aperture
-small aperture

36
Q

what does FLL stand for?

A

Focal Length of Lens

37
Q

what is Focal Length of Lens?

A

distance between the optical center of the lens and the sensor of the camera when focused on infinity (in mm)

38
Q

how does lens choice (FLL) affect DOF?

A

a closer or wider angle

39
Q

3 types of lens choices

A

-wide angle lens < 50 mm
-normal lens = 50 mm
-telephoto lens > 50 mm

40
Q

which lens choice has the best DOF? poor DOF?

A

-wide angle
-telephoto angle

41
Q

what does DOD stand for?

A

diameter of the diaphragm

42
Q

expressions relating to DOD, FLL, f-stop

A

f-stop # = FLL/DOD
DOD = FLL/f-stop#
FLL = DOD (f-stop)

43
Q

close-up photos, DOF is much ?

A

less

44
Q

what does a normal lens look like? (50 mm)

A

-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

45
Q

rule of thumb for normal lens

A

wide angle will elongate distances, telephoto will compress distances. Only 50mm will not have distortion​

46
Q

major parts of the telephoto lenses

A

-magnification
-narrower field of view
-compression of foreground to background distances
-narrower DOF ranges

46
Q

rule of thumb for magnification

A

-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”

47
Q
A

Forensic Photography (7 decks)

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