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04/11/2013

Onderwerpen: Scherpstelling - Focusering Sluitersnelheid en framerate Sluitersnelheid en belichting Sluitersnelheid en bewegingsonscherpte Drie dimensionale Arthrokinematische Mobilisatie

Lensopening – Diafragma - en belichting Lensopening – Diafragma - en scherptediepte

Cursus Klinische Video/Foto-Analyse Avond 3: Scherpte en Scherptediepte

www.damcursus.nl

Scherpstelling - Focusering

Chris Riezebos Aad Lagerberg


Object dichtbij.

http://electronics.howstuffworks.com/camera1.htm

Scherpstellen (focuseren) is het instellen van de afstand van de lens tot aan de film c.q. de chip.


Object ver af

Sluitertijd

Bij onjuiste focus wordt het beeld onscherp.

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Sluitersnelheid en bewegingsonscherpte

Sluitertijd en framerate Foto 1 1/2 1/4 1/8 1/15 1/30 1/60 1/125 1/250 1/500 1/1000 sec

Sluitersnelheid: 1/50 sec = 0.02 sec

Video Beschikbare sluitertijden afhankelijk van de frame rate. Sluitertijd altijd korter dan de frame duur.

Framerate = 1/0.04 = 25 fps

Sli

de 34

Frameduur = 2 x sluitersnelheid = 2/50 = 1/25 = 0.04 sec

Voorbeeld: Frame rate = 25 fps Duur 1 frame: 0.04 sec = 1/25 sec Langste sluitertijd: 1/25 sec, c.q. 1/30 sec Voorbeeld: Framerate = 210 fps Duur 1 frame: 0.0047 sec = 1/210 sec Langste sluitertijd: 1/250 sec)

Bij hoge framerates wordt de sluitertijd dus automatisch korter. Daarom meer licht nodig. Meestal: sluitertijd = ½ frameduur

1/50 sec 25 fps


Sluitersnelheid: 1/100 sec = 0.01 sec Frameduur = 2 x sluitersnelheid = 2/100 = 1/50= 0.02 sec Framerate = 1/0.02 = 50 fps

1/100 sec 50 fps



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1/300 sec 150 fps



1/600 sec 300 fps


Sluitersnelheid: 1/1000 sec = 0.001 sec Frameduur = 2 x sluitersnelheid = 2/1000 = 1/500= 0.002 sec Framerate = 1/0.002 = 500 00 fps

1/1000 sec 500 fps


Sluitersnelheid: 1/50 sec = 0.02 sec

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1/50 sec

1/100 sec

1/215 sec




Je kunt in principe kiezen voor b.v. 25 fps, maar een sluitersnelheid van 1/500 sec. De sluitersnelheid mag immers wel korter zijn dan de frameduur, maar niet langer. Hoe meer licht wordt gebruikt, des te korter de sluitersnelheid welke de camera gebruikt.

1/100 sec

1/215 sec



Lensopening – Diafragma- en belichting

Diafragma

1.4 2.0 2.8 4 5.6 8 11 16 22

Klein: hoog getal, b.v. f/16

Groot: laag getal, b.v. f/4

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b a

f = brandpuntsafstand van de lens (mm) D = diameter van de diafragma opening (mm) f

f/stop of f-getal =

r2

r1

Iedere stop is een factor 2 in hoeveelheid oppervlak = hoeveelheid licht

D

A = π r2 0

1

(s 2) 1 1.4

2.0

2

(s 2) 1.4 2.8

3

(s 2) 2

4

5.6

(s 2) 2.8 8

11

4

(s 2) 4 16

(s 2) = 1.414

Hoe groot moet r2 zijn om oppervlak b twee keer zo groot te laten zijn als oppervlak A?

π r22 = 2 π r12 22

r22 =

2 π r12 π

Diafragma opening

Per stap halvering van de hoeveelheid licht.

1/4 1/8 1/15 1/30 1/60 1/125 1/250 1/500 1/1000

f/stops are a bit more confusing because the numbers appear so arbitrary. This is the standard sequence of f/stops from f/1.4 to f/22. Although it may not seem intuitive at first, in this sequence the f/1.4 setting lets in the most light while the f/22 setting lets in the least. Also, each of these f/stops has precisely the same halving/doubling relationship as the shutter speed sequence. 1.4 2.0 2.8 4 5.6 8 11 16 22 On the face of it, going from f/4 to f/5.6 doesn't sound like halving the amount of light. What's more, 5.6 is a larger number and sounds like it ought to be more light, not less. Neither does f/4 to f/2.8 sound like doubling the amount of light. In fact, each of the numbers in this sequence is a halving/doubling of the amount of light from its immediate neighbours, just like the shutter speed settings are. Not only that, but it makes sense, as I shall show below. The reason that both the halving and doubling and the smaller numbers mean more light things make sense is that the f/stop is a ratio. The ratio is between the diameter of the aperture in the lens and the focal length of the lens. The focal length is generally measured in millimeters, so we'll stick with those as our unit of measure. On a 50mm lens, f/2 is saying that the diameter of the aperture is 25mm. The ratio is: 50/25 = 2. That seems pretty straightforward. A good question might be, what is the area of that aperture? Well, the aperture is usually a set of five to fifteen blades which form a roughly circular hole, so we'll use the formula for the area of a circle, which as I'm sure you'll recall is π * radius2. For π I'll use 3.14159265. On our 50mm lens, the aperture at f/2 has a diameter of 25mm which is a radius of 12.5mm. The area of the aperture is thus π X 12.52, or 3.14159265 X 156.25, or 490.9 square millimetres. This fact by itself isn't all that useful. It is useful in relation to the adjacent f/stops. What is the area of the aperture at f/2.8? Well, because the f/stop is a ratio of the focal length to diameter, our 50mm lens at f/2.8 would have a diameter of 50/2.8 = 17.86mm. Remember, we have to divide that by 2 to get the radius of 8.93mm, so the area of the circle thus formed would be π X 8.932, or 250.5 square mm. Rounding off a bit, that's about 250 sq. mm at f/2.8 and 500 sq. mm at f/2, a double/half relationship. Aha! So that's it! The area of the hole doubles and halves, it's just represented by a ratio on the lens! No wonder it's so darn confusing.

Diafragma groter:

r2 = s 2 r12 r2 = s 2 s r12

Sluiter tijd

r2 = s 2 r1



r22 = 2 r12

Lensopening – Diafragma- en belichting f lens = 50 m m

f/stop

Diameter of aperture (mm)

Radius of aperture (mm)

Area of Aperture (sq. mm)

f/1.0

50.0

25.0

1963

f/1.4

35.7

17.9

1002

f/2.0

25.0

12.5

491

f/2.8

17.9

8.9

250 123

f/4

12.5

6.3

f/5.6

8.9

4.5

63

f/8 f/11 f/16 f/22

6.3 4.5 3.1 2.3

3.1 2.3 1.6 1.1

31 16 8 4

(As shown on lens)

(50mm divided by f/stop)

(1/2 the diameter)

(pi X the radius squared)

Lensopening – Diafragma- en scherptediepte

5.6 4 2.8 2.0 1.4

Diafragma kleiner: 5.6

8 11 16 22

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Invloed op de scherptediepte: Diafragma:

Groot diafragma (laag getal) : kleine scherptediepte Klein diafragma:(hoog getal): grote scherptediepte

Grote brandpunts-afstand (tele): kleine scherptediepte Brandpuntsafstand:

Kleine brandpunts-afstand (groothoek): grote scherptediepte

Kleine afstand: kleine scherptediepte Afstand camera object Grote afstand: grote scherptediepte

B.v.: scherpstelpunt op 6 m. B.v.: Scherpte diepte van 2 – 10 m

Dit is geen kleine scherptediepte, maar bewegingsonscherpte (van zowel voor- als achtergrond) door het ‘meetrekken’ van de camera.

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25 frames per second (fps)

1

1 sec

25

Fram eduur = 0.04 s = 1/25 s Sluitersnelheid: b.v. 0.01 = 1/100 s

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