28/10/2012. Onderwerpen: Scherpstelling - Focusering Sluitersnelheid en framerate. Sluitersnelheid en belichting

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1 8/0/0 Onderwerpen: Sluitersnelheid en framerate Sluitersnelheid en belichting Driedimensionale Arthrokinematische Mobilisatie Cursus Klinische Video/Foto-Analyse Avond 3: Scherpte en Scherptediepte Chris Riezebos Aad Lagerberg Object dichtbij. Scherpstellen(focuseren) is het instellenvan de afstandvan de lens tot aande film c.q. de chip. Object ver af Brandpunts-afstandafstand (mm) Gezichtsveld ( 0 ) Bijonjuistefocus wordthet beeld onscherp. Een standaard 50 mm lens geeft geen vergroting of verkleining(dus identiek aan menselijk oog).

2 8/0/0 Sluitersnelheid Sluitersnelheid en framerate Foto / /4 /8 /5 /30 /60 /5 /50 /500 /000 sec Video Beschikbare sluitersnelheden afhankelijk van de frame rate. Sluitersnelheid altijd korter dan de frame duur. Voorbeeld: Frame rate = 5 fps Duur frame: 0.04 sec = /5 sec Langste sluitersnelheid: /5 sec, c.q. /30 sec Voorbeeld: Framerate = 0 fps Duur frame: sec = /0 sec Langste sluitersnelheid: /50 sec) Bij hoge framerates wordt de sluitertijd dus automatisch korter. Daarom meer licht nodig. /50 sec /50 sec /00 sec /00 sec

3 8/0/0 /300 sec /300 sec /600 sec /600 sec /000 sec /000 sec 3

4 8/0/0 /50 sec /50 sec /00 sec /00 sec /5 sec /5 sec 4

5 8/0/0 Diafragma f = brandpuntsafstand van de lens (mm) D = diameter van de diafragmaopening (mm) f/stop of f-getal= f D 0 (s) (s) Iederestop is eenfactor in hoeveelheid oppervlak = hoeveelheid licht (s) 3 (s) (s) (s) =.44 Klein: hooggetal, b.v. f/6 Groot: laaggetal, b.v. f/ Per stap halvering van de hoeveelheid licht. /4 /8 /5 /30 /60 /5 /50 /500 /000 Diafragma opening Sluiter tijd a b r r A = π r Hoe groot moet r zijn om oppervlak b twee keer zo groot te laten zijn als oppervlak A? π r = π r r r = r = π r π r = s r r = s s r r = s r f/stops are a bit more confusing because the numbers appear so arbitrary. This is the standard sequence of f/stops from f/.4 to f/. Although it may not seem intuitive at first, in this sequence the f/.4 setting lets in the most light while the f/ setting lets in the least. Also, each of these f/stops has precisely the same halving/doubling relationship as the shutter speed sequence 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/.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/ is saying that the diameter of the aperture is 5mm. The ratio is: 50/5 =. 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 * radius. For π I'll use On our 50mm lens, the aperture at f/ has a diameter of 5mm which is a radius of.5mm. The area of the aperture is thus π X.5, or X 56.5, or 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/.8? Well, because the f/stop is a ratio of the focal length to diameter, our 50mm lens at f/.8 would have a diameter of 50/.8 = 7.86mm. Remember, we have to divide that by to get the radius of 8.93mm, so the area of the circle thus formed would be π X 8.93, or 50.5 square mm. Rounding off a bit, that's about 50 sq. mm at f/.8 and 500 sq. mm at f/, 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. f/stop Diameter of aperture (mm) Radius of aperture (mm) Area of Aperture (sq. mm) f/ ,963 f/ ,00 f/ f/ f/ f/ f/ f/ f/ f/.3. 4 (As shown on lens) f lens = 50 mm (50mm divided by f/stop) (/ the diameter) (pi X the radius squared) Diafragma groter: Diafragmakleiner:

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7 8/0/0 5 frames per second (fps) 5 sec Frameduur = 0.04 s = /5 s Sluitersnelheid: b.v. 0.0 = /00 s CCD Charged Coupled Device CMOS Complimentary Metal-Oxide Semiconductor 7