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Vidoo @Vidoo 5. Feb 2008, 12:21

Zanima me ali gre pri svetlobnem razmerju 1:4 za 4x več vpadne svetlobe ali za 2x več. Glede na to da gre za spremembo 2 zaslonk bi rekel da 2x več?
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iztok @izo 5. Feb 2008, 12:23

http://en.wikipedia.org/wiki/F-number
pa ni mi jasno, kaj točno misliš s tem 1:4, ali blendo (f/4) ali makro povečavo (1:4)?
če misliš to, kolk je razlike v svetlobi med f/1 in f/4, je tako, da je razlika 4blende:
se pravi pr f/1 imaš 16x več svetlobe kot pr f/4.
a sm prav povedal?
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Vidoo @Vidoo 5. Feb 2008, 13:06

Govorim o svetlobnem razmerju. Kot npr tukaj



Razmerje Razlika zaslonke
1:1 Ni razlike
2:1 1 stopnja
3:1 1 1/2 stopnje
4:1 2 stopnje
8:1 4 stopnje

Se pravi ali gre pri 2:1 razmerju glavna luč/stranska luč za 2x več svetlobe ali 1x več svetlobe oziroma prenešeno na 4:1?
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iztok @izo 5. Feb 2008, 13:13

bi lahko takoj povedal, da govoriš o flash razmerjih ipd in dal primer zraven.
po mojem gre samo za to, kolk je ena luč močnejša gleda na drugo. sej je videt na primeru.
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iztok @izo 5. Feb 2008, 13:17

Really concise version...Imagine

Scenario 1: Light A and light B, both outputting same intensity of light. Place Light A on subject so that it illuminates the full area seen by lens, so it can be said to provide 1 Unit to that area. Place Light B on subject so that it illuminates part of the area seen by lens, so it can be said to provide 1 Unit to that area. Where the lights overlap, that area has received 2 Units of light. So while the ratio of intensities of the two lights is 1:1, the lighting ratio is 2:1. A ratio of 2:1 to 1:1 is generally considered to be a low contrast ratio.

Scenario 2: Light A, and light B outputs twice (+1EV) as much light as Light A. Place Light A on subject so that it illuminates the full area seen by lens, so it can be said to provide 1 Unit to that area. Place Light B on subject so that it illuminates part of the area seen by lens, so it can be said to provide 2 Units to that area. Where the lights overlap, that area has received 3 Units of light. So while the ratio of intensities of the two lights is 2:1, the lighting ratio is 3:1. A ratio of 3:1 is generally considered to be a moderate contrast ratio.

Scenario 3: Light A, and light B outputs 4x (+2EV) as much light as Light A. Place Light A on subject so that it illuminates the full area seen by lens, so it can be said to provide 1 Unit to that area. Place Light B on subject so that it illuminates part of the area seen by lens, so it can be said to provide 4 Units to that area. Where the lights overlap, that area has received 5 Units of light. So while the ratio of intensities of the two lights is 4:1, the lighting ratio is 5:1. A ratio of 5:1 is generally considered to be a high contrast ratio.

The above uses the 'school solution' definition of 'Fill' vs 'Highlight', where the 'fill' falls everwhere the lens sees.

Take all the above scenarios, and if there is NO overlap area on the subject, the lighting ratios become 1:1, 2:1, and 4:1 (same as the lighting intensity ratios). (just corrected the numbers in this last paragraph...!)

http://photography-on-the.net/forum/showthread.php?t=212220
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Alen @snela 14. Feb 2008, 18:32

F/d=f

F - gorisce
d - fizicni premer (Ce gledamo za Leica format 50mm kot osnova)
f - svetlobna jakost (enacba pove, da je brez enote)

Povrsina P = pi * (2*r)^2 / 4
Ce bi hotel imet pri 400mm zaslonko 1, bi rabil 40cm zaslonko - neke vrste teleskop potem smile In zato nekateri proizvajalci zaslonko premikajo grin


Se pravi, da je razmerje svetlobe f=2 proti f=1, je 4x manj svetlobe pri f/2.
Ce hoces 2x manj svetlobe rabis sqrt(2)=1.4. Se pravi f/1.4 ima "samo" dvakrat manj svetlobe of f/1.

2^n = X ali t.i. "blenda" ali "ena ekspozicijska enota" (zaklop/cas ali zaslonka).

Cifre gredo po formuli takole:
1 => 1x (2^0=1)
1.4 (koren 2) => 2x (2^1=2)
2 => 4x (2^2=4)
2.8 => 8x (2^3=8)
4 => 16x (2^4=16)
5.6 => 36x (2^5=36)
8 => 64x (2^6=64)
itn..

To pomeni, da je med vsemi temi stopnjami 2x manj/vec svetlobe. 8/4=2, 64/32=2, vedno je racunica 2.

Pri casih isto: med 125 in 60 je 1 blenda, kot je to med f/2.8 in f/4. Samo da pri standardnih casih ni teh cudnih decimalk pa se mal so zaokrozeni (ni 128 ampak 125, ni 32 ampak 30, ni 64 ampak je 60). To so bli zacetki fotografije, ko so te postavljal.

Vecina objektivov pa dela najbolje pri svojih srednjih vrednostih in slabo na mejnih.

Iz situacije ugotovis, da rabis 125/5.6, in ce hoces dodat 1 blendo (kreativno izrazanje grin ), 125/4 ali 60/5.6, +2 blendi: 30/5.6 ali 125/2.8 ali 60/4.

Zaslonka daje globinsko ostrino (DOF), zaklop (cas) "ustavlja akcijo" ali pa jo "magicno megli".

V novejsi dobi fotografije so pa zacel ta razmerja in lestvice (zaslonke in zaklopa/cas) cist pacit in drobit. Zdej mate na digitalcih swasta.


btw baje ma nek Minoltin objektiv z f/0.7 zaslonko - ce ga kdo najde, da potrdi rekla-kazala, bi blo fajn policeman
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iztok @izo 14. Feb 2008, 18:34

dej še enkrat preberi, gre za umetno osvetlitev ipd zadeve, o blendah pa imaš vse razloženo tudi že na tistem linku, ki sem ga dal..
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Aleš @lial 14. Feb 2008, 18:53

snela je napisal/a:
btw baje ma nek Minoltin objektiv z f/0.7 zaslonko - ce ga kdo najde, da potrdi rekla-kazala, bi blo fajn policeman


Če me spomin ne vara, to ni bil Minoltin objektiv, ampak skupni projekt Zeiss-NASA, za kaj se je tisto čudo uporabljalo, pa ne vem. So pa res pisali da je f 0.7

Dodajam:

google vrže tole slikico:


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