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How do constant aperture zooms work?

Jernej Burkeljca , Jan 13, 2005; 04:40 a.m.

I've been trying to figure this one out for a while with no clear answer so if anyone has a better idea please let me know.

So my logical process goes something like this... variable aperture zooms actually have a constant diameter of the aperture (iris? can't remember the proper english name for it at the moment) and as the focal lenght is changed so does the ratio (the F number)

With a constant aperture zoom the diameter has to grow with the increasing focal lenght so that the ratio can stay the same but here's where I bump into a wall... if there's "room" for a larger diameter on the tele end of the range, why isn't it used on the wide end? Essentially what you'd get from this is a variable aperture zoom but with a larger F number (lets say a F1-2.8 from a 2.8 fixed compared to the usual 3.5-5.6 or whatever)

or am I missing something?

help?!

Responses


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Yakim Peled , Jan 13, 2005; 05:55 a.m.

f-stop or f-number .... is equivalent to the focal length of the lens divided by the size of the lens aperture. Hence, is the aperture is constant and the focal length changes, one must assume that the size of the lens aperture changes as well. HTH.

Happy shooting,
Yakim.

Jernej Burkeljca , Jan 13, 2005; 06:09 a.m.

of course, however it doesn't explain why the larger diameter (that is obviously possible) isn't used at wideangle

E Brennan , Jan 13, 2005; 06:49 a.m.

the larger diameter is used at the wide angle end of the zoom. the smaller the f-stop number, the larger the aperture.

Miha Steinb├╝cher , Jan 13, 2005; 07:03 a.m.

Jernej,

Definicija, da je opticna moc objektiva enaka kvocientu premera odprtine v zaslonki in zariscnice, je kar precejsnja poenostavitev. Tocna definicija govori o premeru snopa zarkov in ne o premeru zaslonke. Iz tega izhaja, da opticna moc objektiva ni pogojena z zaslonko, pac pa njegovo sposobnostjo zbiranja svetlobe. ce poenostavim, ce svetlobe ni dovolj, je tudi zaslonka ne more zapreti. Vsak zoom objektiv, pa naj bo konstantne ali spremenljive opticne moci, je precej zapletena opticna konstrukcija, ki jo je na hitro veliko tezje razloziti kot na primer derivate tripleta (vse od osnovnih tripletov, prek tessarjev do sonnarjev in izpeljank) ali simetricne objektive (vecinoma variacije Gaussove konstrukcije) fiksnih zariscnic. V splosnem pa velja, da je vsak zoom objektiv sestavljen iz dveh komponent (ki pa se lahko med seboj tudi prekrivata, racunalnisko nacrtovanje objektivov dela cudeze). Prva skrbi za spreminjanje zariscnice, torej zumiranje, druga pa predstavlja osnovni objektiv. Zaslonka je praviloma izvedena v tem drugem delu objektiva. Od tega, kako je (opticno in ne mehansko!) izveden prvi del, torej tisti, ki skrbi za spreminjanje zariscnice, pa je odvisno, ali se kolicina svetlobe, ki pride do drugega dela, spreminja. Seveda se v procesu konstrukcije z optimiranjem razlicnih elementov in skupin lahko katera od funkcij, ki sem jih omenil, prekriva ali celo prenese z enega na drugi element, prav tako pa se lahko konstruktorji odlocijo tudi za resitev, ki si jo verjetno imel v mislih, ko si zastavil vprasanje, ti, za zapiranje zaslonke. Toda taksna resitev ni nikoli namenjena zmanjsevanju opticne moci objektiva zaradi zelje po nekaksni uniformiranosti, temvec poskrbi za to, da je slika tudi pri deklarirani najvecji opticni jakosti ustrezna, torej sprejemljivo ostra in predvsem dovolj velika. Objektivi imajo namrec lastnost, da se s spreminjanjem velikosti zaslonke spreminja tudi velikost projecirane slike. Pomisli, objektiv ki vinjetira pri f/2,8, te napake ne kaze, ko ga zapremo za kaksno vrednost zaslonke ali dve. Na splosno pa je velikost slike pri krajsih zariscicah (tam, kjer bi ti zelel vecjo opticno jakost) manjsa kot pri daljsih.

Upam, da sem ti bil v pomoc, v prehude podrobnosti pa se nisem zelel spuscati.

Pa dobro luc!

Miha

Gavin Wims , Jan 13, 2005; 07:22 a.m.

Yeah, what he said.

Aaron B , Jan 13, 2005; 10:43 a.m.

Did Miha answer the question? If so, could somebody translate it into english? I've been curious about this, too.

David Vatovec , Jan 13, 2005; 01:01 p.m.

In short it states that the aperture could be grater at the wide end but the companies keep it more closed because of quality questions ie minimizing aberrations.

Aaron B , Jan 13, 2005; 01:19 p.m.

Thank you.

Stephen H , Jan 13, 2005; 01:36 p.m.

It's possible that trying to use the wider aperture at the short end would just give bad vignetting, too.


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