# How do constant aperture zooms work?

Discussion in 'Accessories' started by jernej, Jan 12, 2005.

1. ### jernej

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?!

3. ### jernej

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

4. ### eoghan

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

5. ### miha_steinb_cher

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

6. ### gavin w.

Yeah, what he said.

7. ### mendonphoto

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

8. ### vatovec

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.

Thank you.

10. ### stephen hazelton

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

11. ### miha_steinb_cher

Short translation of my reply to Jernej:

Definition of f stop as quotient between aperture diameter and focal length is gross approximation, more accurate would be one between diameter of beam of rays and focal length. Light gathering power of a lens is of prime importance, as an aperture really can't block light which doesn't take part in forming an image.
Zoom lenses are complex optical devices that can't be explained as easy as prime lenses either triplet derivatives (triplets, tessars, sonnars...) or symmetrical designs (mainly Gauss type derivatives). Generally, there are two parts of a zoom lens, a focal length changing part and an image forming lens (they may be interlinked, modern day computer design can do miracles). Aperture is usually located in second part of such lens and many times relative aperture of a lens depends solely on light gathering power of the first part, the one that varies focal length.
There are of course also cases where lens designers deliberately reduce max aperture at certain focal lengths, usually to minimize vignetting.
It's important to notice that lenses usually deliver larger picture at smaller aperture. Telephoto lenses on the other hand usually cover a wider image circle than wide angle ones. An idea of f/1-2.8 zoom is thus not very valid optically.

Best regards,

Miha

12. ### Ed_Ingold

Constant-aperture zoom lenses accomplish this optically, not by altering the physical size of the aperture. The apparent diameter of the exit pupil is increased in proportion to the focal length.

Aperture is one of many design parameters for a zoom lens. In order to optimize other parameters, like distortion and focus stability, it is necessary to add elements and/or mechanical complexity to the way elements are moved internally. Constant-aperture is particularly expensive to design with overall performance in mind.

hvala

14. ### tom_burke|3

Let's go back the other way - let's work out some actual aperture sizes. Let's take the
Canon 24-70 f2.8 as an example.

According to the formula on photonotes.org, the f number is the 'focal length of the lens
divided by the size of the aperture', and the example they give is a 50mm lens with an
aperture of 6.25mm = f8. If this is correct then dividing the focal length by the f number
should give us the size of the aperture:: 50 / (f)8 = 6.25, which is where they started.

OK, now let's move to the constant aperture zoom lens. At 24mm with a maximum
aperture of f2.8, the actual size should be 24 / (f)2.8 = 8.57mm. At 70mm, and a
maximum aperture of f2.8, the actual size ought to be: 70 / (f)2.8 = 25mm.

Hmm.... this suggests that the maximum physical size of the aperture varies between
8.5mm (approx) to 25mm. This doesn't seem right; as Jernej asked right at the beginning,
if the aperture can be 25mm at the long end, why not use 25mm at the short end?

I suspect that the formula for calculating the f-number isn't right. Does anyone know what
the formula is? Or, if the formula quoted above & borrowed from photonotes is correct,
what's wrong with my theorising?

15. ### jbq

Tom - you need to consider the *optical* side of the aperture. It only matches the physical side for "thin lenses", i.e. lenses thare are almost infinitely thin compared to all the other distances involved, which is most definitely not the case in photography.

16. ### bacsa

As above is said, it's not the physical size of the aperture that matters. It is quite a common (mis)conception to say, f-number is focal length divided by aperture diameter, but it's not true. Entrance and exit pupil are the parameters that matter, and constant-aperture zooms are designed in order to change the size of the exit pupil while changing the focal length. It's a design that usually results in bigger physical size of the lens as optical costruction.

There are very good descriptions/explanations in some books, e.g. "applied photographic optics" by sydney f. ray and others.

17. ### bacsa

Sorry - i was too lazy to read Edward's reply

18. ### bacsa

An idea: Just look into a zoom lens from the front. Without changing the f/number, when you zoom in the aperture seems to get larger. It's because you look at the magnified aperture not the real one, and the magnification varies w the focal length. And that's what matters for light entering the lens.

19. ### tom_burke|3

Yes, I think I'd got to the point of recognising that "f number = focal length / aperture
size" was wrong. Going on from that, I've often read that the f-number is a ratio. But of
what? What's the mathematical formula? (there must be one....)

20. ### bacsa

You know what, when i go home i'll take the book i mentioned and look it up for you. At latest on Monday, i'll be back with a formula )

21. ### markci

entrance pupil diameter / focal length.

The nonsense about "the manufacturers could do it but don't because of quality" is just that. 100% nonsense.

22. ### markci

Sorry, should be focal length / entrance pupil diameter, obviously.

23. ### chetan_dighe

Hello all,

Sorry for entering the thread so late, seems like it has died.

I have gone thru all the responses, but frankly, I am still confused by what is written here.

For one, I believe that f number is the ratio of aperture diameter / focal length. For instance, a 50 mm aperture with a 100 mm focal length would make it f/2. If we want a 600 mm focal length in f/2, we would need an actual opening of 300 mm. I know it would not be practical to carry around a lense so huge, but theoretically, I suppose this explains the point.

This (constant aperture) would ensure that the angle the focusing beam makes would be the same at all focal lengths. Not very sure, but I guess this 'angle' is what matters in deciding the depth of field, spherical aberration caused by paraxial rays, etc. Correct me if I am wrong.

Now, about constant aperture for the entire zoom range.

Over and above what is covered so far in this thread, I believe if we can somehow maintain the same beam angle at all focal lengths, we would achieve "constant aperture".

One way to achieve this would be, to move a fixed diameter hole back and forth in the tube, depending on the focal length (probably in a non-linear way if the optical combination so behaves), so as to maintain the curtailed angle of the light beam constant. This way, we would have a constant aperture.

Those who have played around with optics would know that this will lead to barrel or pincushion distortion (which can again be corrected by some more optical elements), as basic optics textbooks explain, but it would achieve our "constant aperture" objective.

I don't know if this approach is actually used by the manufacturers, whether the resulting image quality would be acceptable, would "professionals" pay more for "constant aperture" if the image distortion is high, would this still be called "professional" lense, and so on.

Any comments on this one?

Regards,

Chetan.