Constant Aperture Zooms?

Ok, I now have a Sigma 18-50 2.8 EX which has a constant aperture of 2.8 across the range of the zoom, but why?

The effective aperture of a lens is "The diameter of the beam of light incident upin a lens parallel to the lens axis, which completely fills the aperture of the diaphragm, is termed the effective diameter of the aperture, or the effective aperture of the lens". (Ilford Manual of Photography 5th Edition, Page 61)

Also, "The number N obtained by dividing the focal length f of a lens byt the diameter d of the effective aperture is referred to as the relative aperture of the diaphragm. Expressed mathematically: N = f/d. Thus, a lens with an effective aperture 1 inch in diameter and a focal length of 8 inches, has a relative aperture of 8/1, i.e., 8." (Ilford Manual of Photography 5th Edition, Page 62)

So, the 50mm f1.8 lens has an effective aperture of 27.8mm.

With my 18-50mm lens, this means that the maximum the lens opens up is 17.9mm. If that's the case, then that means it could have a relative aperture of f1.0 at 18mm - why doesn't it?

The attitude seems to be that if a zoom doesn't have a constant maximum relative aperture that you're losing out on the long end, but in fact, by not having a constant aperture, you're losing out massively on the wide end.

Please can someone explain this to me.

It seems to me that you would need the end of the lens to be very wide in order to open the aperture up that wide. If the glass isn't large enough then you have a aperture reduction created by the edge of the lens.

It's like when you look through a paper tube, you can only see a small opening at the end so it doesn't matter how wide you pupil opens up, the amount of light getting in isn't going to change. You need to make a paper cone or make the tube shorter - I suspect a lens is a bit of both - that's why the zoom slides in and out. In order to make a cone you need a lot of glass at the end and in order to make it shorter you need to refract the light a lot (which could sacrifice some quality or add cost for special glass).

This is just a guess though, but it makes sense in my brain.

The reason that zooms with a constant aperture were designed is so that the exposure won't change when zooming.

Assume that you are using manual exposure mode with a lens such as a (mythical, for illustration) 24-70mm f4.0-f5.6. You set the shot up and meter the scene using the camera's meter with the lens zoomed to 24mm. The exposure settings you choose are 1/125 second shutter speed and f8. Now, you zoom to 70mm. Because this example lens has a 1 f-stop change because of zooming, you now are going to underexpose by a full f-stop because the light coming through the lens has dimmed by a stop. You would have to adjust either shutter speed or aperture to keep a proper exposure.

With zoom lenses having a constant aperture, you can set your exposure and forget it. Zoom away, touch up focus at each focal length, and shoot.

Hope this makes sense. It's why I have purchased only zooms with constant apertures. They are more expensive, but they will pay for themselves when the chips are down and I am working fast.

Ah ok, thanks for the thoughts Poco, I'm not 100% sure I follow your thoughts.

Skip, that seems like a reasonable explanation - thanks.

Could you try that again?

If the lens is f/4-5.6 and you set aperture to f/8 then why would the exposure change? The viewfinder might be 1 stop dimmer, but not the exposure.

Now if you had set the aperture to f/4 at 24mm you would most certainly encounter the situation you describe. Thats why I typically set my aperture at the smallest maximum for whatever zoom I'm using if I expect to be up and down the range alot.

goatee

I think your original question - certainly one I picked up from your OP - was HOW is a constant aperture lens made when the effective aperture will change as you zoom the lens? I accept skip's explanation as to WHY they are made - but I am now equally confused as to how they are designed to work as such!

That's right Al - either do they use some exotic way of making the lenses, or do they artificially stop the wider end of the zoom getting a higher relative aperture.

This has been asked before. What actually changes is the apparent aperture, or the entry pupil of the lens. As the magnifying front element/group moves relative to the physical aperture at the diaphragm when you zoom, it, not surprisingly, magnifies the aperture as seen from the front of the lens (the "entry pupil"). Constant aperture lenses are designed so that, in addition to all the other corrections that are made to the lens, the entry pupil varies in direct 1:1 ratio with the focal length of the lens as you zoom.

Jon,

I think I understand that - thanks!

D@mn! Let me try again!

In my previous post, I was expressing my understanding of why the constant-aperture lens was invented. I was referring to a camera with manual settings, and was really thinking of a lens with a manual aperture setting ring on the lens. The point is that a zoom lens that does not have a constant aperture design will change the effective aperture as you zoom - assuming the iris opening does not get changed by the user. I suppose today's automated cameras could actually control the iris in the lens to produce the same effective f-stop regardless of the zoom (except at the widest aperture setting), but that's the camera automation taking over.

Taking my comment about the manually adjust lens a bit further - my old Pentax, for example has a non-constant aperture zoom lens and fully manual aperture settings. The camera body cannot change the iris adjustment. If you shoot a series of shots at the same exposure settings and zoom through the range of focal lengths, you will get a series of varying exposures.

But, this is the EF and EF-S lens forum, not something like you describe.

---Bob Gross---

Bob, I don't think the automation was available when the constant-aperture lenses were invented, though, and that's what I was trying to bring out in my original post.

After thinking about my original post, I came to realize that the exposure thing is essentially no longer a problem for the users of most non-constant aperture lenses, as today's camera bodies will take care of the exposure thing. However, I still like my big, fat, and fast L zooms.

Because they have trouble building prime lenses in 17mm faster than 3.5

A 17mm f/1.0 is:
1) impossible to build for SLR
2) nobody needs it
3) It would weigh about 20 kilos and cost half a million

Building lenses there are laws of physics which you have to obey as well as keeping the weight and cost reasonable.

Hollywood producers use panavision lenses which are expensive and can only be rented.

Those lenses are HEAVY and BIG, but give you lots of options. For example 11:1 lens, is a 25-275 f/2.8 lens which covers full frame! But it's also 7-8 inches in diameter and weighs about 25 kilos.

These discussions do lead me to conclude that 99% of people here (and definitely including me) dont have an idea of how lens are designed and what factors goes into deciding the limits.

Does Canon offer factory visits anywhere that covers lens making?