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Forensic Photography > Electronic Flash > Flashcards

Electronic Flash Flashcards

1
Q

why use electronic flash?

A

-ambient light is not enough
-produces the proper quality of light, color balance (tints)
-narrow apertures can be used

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2
Q

when is ambient light not enough?

A

early morning, late afternoon, nighttime, indoors

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3
Q

what are guide numbers?

A

flash unit’s relative output power

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4
Q

what is the relative output power?

A

how much light that particular flash can throw out onto a scene

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5
Q

what is the exposure triangle?

A

the use of a flash eliminates shutter speed as a variable (sync speed)

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6
Q

ISO 400 or digital equivalent will be in dim conditions or ISO ? for comparison photos

A

ISO 100

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7
Q

4 exposure variables

A

f/stop (aperture)
film speed (ISO)
Shutter speed
light (ambient/flash)

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8
Q

how to determine f/stop?

A

the guide number of the flash allows the f/stop to be determined, depending on the distance between the flash the subject

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9
Q

GN / Distance =

A

f/stop

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10
Q

what is the distance?

A

between the guide number and the subject

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11
Q

most electronic flashes have a ?

A

calculator dial
most have a dial or display so that you will not have to do any actual math

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12
Q

each flash has a specific ?, not all flashes are the same

A

guide number

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13
Q

GN 160:
GN 120:
GN 80:
GN 56:
GN 36:

A

very strong and expensive flash units
normal
less powerful
weak flash
built-in-flash unit

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14
Q

GN 120 and 10 ft away from subject. what is the f-stop?

A

GN/D = f/stop
120/10 = 12
f-12 is closest to f-11

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15
Q

GN 160: f-stop =
GN 120: f-stop =
GN 80: f-stop =
GN 56: f-stop =
GN 36: f-stop

A

f-16
f-11
f-8
f-5.6
f-3.5

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16
Q

smaller guide numbers puts out ? light and needs ? aperture

A

-puts out less light
-needs wider aperture

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17
Q

what is sync speed?

A

shutter is completely open when the flash fires

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18
Q

what does sync speed do?

A

the entire scene receives light from the flash and the scene reflects this light into the camera before the shutter closes

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19
Q

what is the designated sync speed usually?

A

1/60th or faster

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20
Q

how does faster-than-sync shutter speed happen?

A

faster-than-sync shutter speed leads to the shutter closing when the flash unit fires

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21
Q

how does faster-than-sync control motion? is it a fair and accurate “representation of the scene”?

A

freezes motion
no, because there is still a black bar in the image

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22
Q

what happens if shutter speed is slower than sync? -what if it is very slow, below 1/60th?

A

-typically nothing
-blurred image, ghost images

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23
Q

how to set the flash; old and and newer flashes

A

old flashes: must manually set the flash to the correct ISO
newer flashes: flash automatically detects (syncs) to the ISO

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24
Q

in the manual flash mode, the calculator dial has ? rings. what are they?

A

3
f-stops - outer ring
distance in feet - middle ring
distance in meters - inner ring

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25
Q

f-stop for the distance in feet

A

f-22 to 10’
f-16 to 15’
f-11 to 20’
f-8 to 30’

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26
Q

what happens if object is closer?

A

vari-power increments (1 stop)

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27
Q

as you change f/stop (aperture on camera), what will happen with the flash?

A

the flash will correspond the correct distance

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28
Q

what is a “normal” room for manual flash?

A

“normal” 10’ by 12’, light color walls, with 8’ or 9’ ceilings

29
Q

do not ? manual flash’s exposure recommendation for non-normal rooms

A

trust

30
Q

what are non-normal rooms?

A

-smaller than “normal” room
-larger than “normal” room
-outside
-dark color walls
-reflective surfaces

31
Q

-smaller than “normal” room
-larger than “normal” room
-outside
-dark color walls
-reflective surfaces

A

-needs 1/2 to 1 stop less light
-needs 1/2 to 1 stop more light
-needs at least 1 stop more light
-needs more light
-needs less light

32
Q

how do you change stops?

A

reduce light - use 1/2 vari -power
more light - change f-stop i.e. f-22 to f-16

33
Q

what is a hot shoe?

A

used to mount flash on camera

34
Q

rule of thumb: when you are 5’ or closer to your subject…

A

remove the flash from the hot-shoe, so it can accurately be aimed at your target

35
Q

remove the flash from the hot-shoe, so it can accurately be aimed at your target. why?

A

-flash is above subject/underexposed near bottom
-hot spots
-red-eye

36
Q

some on-camera flash units allow the flash to be aimed down a bit, but off-cameras are still ?

A

recommended

37
Q

what allows the flash to be off-camera?

A

PC or remote cord

38
Q

what does the PC or remote cord do for the off-camera?

A

allows camera to get close to subject while flash can be farther away
should be at least 2’ away to avoid overexposure
arm fully extended to the side should have you two feet

39
Q

when are ring-lites used?

A

closer than 2’
uniformly lights a small object without harsh shadows

40
Q

why use a flash bracket?

A

-separates the flash from the axis of the lens
-Red is produced when the light of the flash reflects off of the blood vessels in the eyes and then travels directly back into the camera​
-It’s not going to have that direct reflection and so that will help reduce some of that red eye effect

41
Q

flash and camera must be ? correctly

A

oriented

42
Q

flash heads must ? be set for the ?

A

focal length of the lens used

43
Q

external zooming:
internal zooming:

A

manually extending the flash heads
automatic zooming
as you rotate your lens, you’re adjusting the focal length

44
Q

greater field of view means that in order to get our scene evenly lit, ?

A

we need to spread our flash light wider
wide, normal, tele

45
Q

what does the flash diffuser do?

A

soften light
diminish glare

46
Q

how to achieve +1 exposure change
f-ll aligned with 26’
f-16 aligned with 18’

A

-use f-11 but move to 18’ or stay at 26’ and use f-8
-use f-16 but move to 13’ or stay at 18’ and ude f-11

47
Q

how to achieve -1 exposure change

A

-½, ¼ vari-power - 1 stop​
-Move flash farther from subject (opposite of previous slide)​
-Change f-stop (opposite of previous slide)
-finger/ handkerchiefs over flash - 1 stop
change from normal to wide - 1/2 stop
wide-angle diffuser - 1/2 stop

48
Q

what is the inverse square law?

A

As light spreads outward, its intensity varies inversely by the square of the distance it travels​

As distance (d) doubles, intensity (I) is quartered (as we double distance, the light spreads out)​

2D = ¼ I

49
Q

another way to write the inverse square law

A

I = 1/d^2
-The foundation for the derivation of f/stop numbers​
-2D = ¼ I (or ½ x ½ 2 step change)

50
Q

what happens with the automatic flash exposure mode?

A

-Relies on reflected light (18%) being sensed by the flash sensor eye​

-Sensor will cut off light duration to prevent overexposure

51
Q

advantages of sensor will cut off light duration to prevent overexposure

A

-saves battery life
-faster flash recycle times
-uses flash ranges, rather than just 1 f/stop for every different distance

52
Q

example of automatic flash exposure mode
f/22 at 10’ (ISO 400 and 120 GN)

A

f/22 at 2’ - 10’ (ISO 400 and 120 GN)

53
Q

how is the flash in dedicated flash exposure mode?

A

flash is controlled by the light meter in the camera body - TTL

54
Q

advantages of the dedicated flash exposure mode

A

When 18% has been obtained, the camera’s light meter signals the flash unit to cut off​

Shows the flash range for each f/stop (more accurate)​

Usually have a flash exposure compensation for bracketing i.e. +3 to -3​

Similar to Automatic, will save battery life

55
Q

why can the built-in flash be bad?

A

GN 35, bad DOF
red-eye
hot spots (direct lighting)

56
Q

when is the fill flash needed?

A

scene with strong ambient light and shadows
flash is used to supplement light for shaded areas

57
Q

what is bounce flash?

A

“Bounce” flash off of ceiling​

Light has to travel farther​

Light is diffused wider than direct flash​

58
Q

how to do bounce flash?

A

Use Manual flash​

Mentally calculate flash-to-bounce to subject distance to determine f-stop​

Set camera for +2 stops  more light

59
Q

what type of evidence is used for oblique light?

A

3D tire tracks, shoe impressions, indented writing, bite marks, etc.

60
Q

what is oblique lighting and why?

A

side lighting creates shadows, to see 3D aspects like depth and texture

61
Q

how to capture impressions? why?

A

mount camera on tripod, set with timer/remote
depressing shutter button produces shake

62
Q

why should you take a series of photos?

A

gives examiners variations to choose best view of different features

63
Q

what are the different series shots?

A

12 shot series
One photo “as-is” w/o scale​
Various directions (4)​
Various angles (3)​
Use flashlight as a guide
-​Take overall and midrange photos of impressions first, then take close-ups

64
Q

rules for impressions

A

*Must include a scale (L-scale), on the same level as impression​
No tripod legs or remote cord in view​
Fill-the-frame​
Proper vertical/horizontal orientation​
*Shade impression with self/object​
*Hold flash as far away as possible​
100 ISO best for comparison​
F-11 best for comparison​
Use manual or TTL flash

65
Q

how to take tire track impressions

A

must do in segments
take 7 1/2 in of tire track
split into 1 1/2 segments, overlapping 1””
use tape measure or yard stick at same level
take overall and midrange photos before close-ups

66
Q

why use snow print wax?

A

provides contrast
allows for casting material to set

67
Q

why does angle of light become even more critical with bite marks and indented writing?

A

-very shallow impressions
-almost parallel to surface
-may use flashlight-panning

68
Q

what is flashlight- panning?

A

when moving the camera side-to-side while taking a photo to create a sense of movement and blur the background

69
Q
A

Forensic Photography (7 decks)

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