Focus, DOF, Lenses

Question 1

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

Answer 1

Personal usually focuses on one or a few individuals or objects

Question 2

What do crime scenes contain?

Answer 2

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

Question 3

What can the defense argue with a blurred picture?

Answer 3

​

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

Question 4

3 sections of focus

Answer 4

resolution
acutance
sharpness

Question 5

what is the goal of taking a picture of a scene

Answer 5

entire scene should be in focus

Question 6

what is resolution?

Answer 6

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

Question 7

example of resolution

Answer 7

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)

Question 8

pixels per inch: more pixels = ?

Answer 8

higher resolution = more detail

Question 9

What are the resolution standards?

Answer 9

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

Question 10

what is acutance?

Answer 10

camera’s ability to render sharp edges

Question 11

2 principles of acutance

Answer 11

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

Question 12

what is sharpness?

Answer 12

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

Question 13

when light enters lens and converge at sensor?

Answer 13

in focus

Question 14

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

Answer 14

out of focus
“confused”
circles of confusion

Question 15

how to use manual focusing?

Answer 15

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

Question 16

what is auto focus?

Answer 16

-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

Question 17

what is auto focus failure?

Answer 17

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

Question 18

how to auto focus?

Answer 18

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

Question 19

what exposure variable determines DOF?

Answer 19

aperture: designated in f stops

Question 20

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

Answer 20

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

Question 21

what is the DOF scale?

Answer 21

the area that will appear to be in focus

Question 22

what are the circles of confusion?

Answer 22

-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

Question 23

camera focused on 12’

Answer 23

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’

Question 24

wider apertures result in ? DOF ranges

Answer 24

narrower

Question 25

smaller apertures result in ? DOF ranges

Answer 25

longer

Question 26

shouldn't use wider apertures than ?

Answer 26

f/8

Question 27

If using an f/22... If using an f/16... If using an f/11... If using an f/8...

Answer 27

-focus the lens at 12’​ -focus the lens at 16’​ -focus the lens at 24’​ -focus the lens at 30’

Question 28

DOF extends ? of the point of exact focus

Answer 28

one third in front

Question 29

DOF extends ? the point of exact focus

Answer 29

two thirds behind

Question 30

how do we do 'rule of thirds'

Answer 30

midway top to bottom

Question 31

what is rule of thumb?

Answer 31

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

Question 32

what to do if camera is still out of focus?

Answer 32

use diopter dial

Question 33

what is the diopter dial?

Answer 33

adjusts viewfinder for eye quality varying strength of reading glasses

Question 34

what does the depth of field include?

Answer 34

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

Question 35

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

Answer 35

-wide aperture -small aperture

Question 36

what does FLL stand for?

Answer 36

Focal Length of Lens

Question 37

what is Focal Length of Lens?

Answer 37

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

Question 38

how does lens choice (FLL) affect DOF?

Answer 38

a closer or wider angle

Question 39

3 types of lens choices

Answer 39

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

Question 40

which lens choice has the best DOF? poor DOF?

Answer 40

-wide angle -telephoto angle

Question 41

what does DOD stand for?

Answer 41

diameter of the diaphragm

Question 42

expressions relating to DOD, FLL, f-stop

Answer 42

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

Question 43

close-up photos, DOF is much ?

Answer 43

less

Question 44

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

Answer 44

-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

Question 45

rule of thumb for normal lens

Answer 45

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

Question 46

major parts of the telephoto lenses

Answer 46

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

Question 46

rule of thumb for magnification

Answer 46

-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”