Constant aperture zooms...Why?

[sk66]

I was involved in a small discussion on DOF in another post and I was reminded that most zooms are variable aperture.

I understand that completely, but for those that don't, an f/2 at 50mm is an f/4 at 100mm. The aperture opening remains the same size but it's size in relation to focal length changes due to zooming and thus the "f/stop" changes. That makes complete sense; the aperture opening can only get so large due to size/construction constraints.

Then it occurred to me...Why do we have constant aperture zooms? Like the venerable 70-200mm f/2.8. It's an f/2.8 at 70mm and f/2.8 at 200mm. That means the size of the aperture INCREASES as you zoom out to keep the relationship to focal length the same.
That was something I never actually registered before....

So why isn't it an f/1.x-2.8? If the aperture can get larger, and it must, then it makes sense it could be an f/1.x at 70mm.... Is it simply physics? I know it is much harder to focus light thru a large aperture than thru a "pin hole"...but it's being done at 200mm so why not at 70mm?

Am I completely misunderstanding something like "the aperture doesn't really increase in size"?

[rediguana]

Might be stepping beyond my knowledge level here, but the physics aside, I would guess that it is a hangover to shooting manual - the last thing you'd want once you'd dialed your exposure in would be to have the aperture changing depending on your zoom level. Hence fixed aperture is better for manual shooting than a variable aperture where you would need to be constantly fiddling with your exposure based on what focal length you were shooting at.

From an engineering perspective, it is probably harder to design a constant aperture zoom as the physical aperture in the lens would need to adjust as the lens is zoomed. More mechanics, more moving parts etc. It is probably easier to construct a variable aperture zoom.

[sk66]

Originally Posted by rediguana

Might be stepping beyond my knowledge level here, but the physics aside, I would guess that it is a hangover to shooting manual - the last thing you'd want once you'd dialed your exposure in would be to have the aperture changing depending on your zoom level. Hence fixed aperture is better for manual shooting than a variable aperture where you would need to be constantly fiddling with your exposure based on what focal length you were shooting at.

Hadn't considered that.
Pretty sorry reason these days though...I'd much rather have it variable starting at f/1.x....

[FrankLamont]

But DOF will only change, whereas the aperture (in exposure terms) should remain the same.

[OsmosisStudios]

size constraints. You could, in theory, have that f/1.4-2.8 lens if you didnt mind jhaving a HUGE lens.

Keep in mind where the aperture ring on a lens is: its usually around the rear 2/3 mark. What this means is that at 70mm, the light is focused that deep into the lens, which means that the opening can only be SO big. otherwise your front element (and the diameter of the lens) would have to be HUGE.

The same goes for at 200mm: f/2.8 at this focal length means an aperture of about 71mm. These lenses generally take 77mm filters, so we can assume that this is the diameter of the front element. 3mm around the edges isnt very much space to squeeze focus and zoom helicoids, electronics, etc.

It's all about the actual SIZE of the lens, not necessarily the physics involved.

[fletch]

Originally Posted by OsmosisStudios

The same goes for at 200mm: f/2.8 at this focal length means an aperture of about 71mm. These lenses generally take 77mm filters, so we can assume that this is the diameter of the front element. 3mm around the edges isnt very much space to squeeze focus and zoom helicoids, electronics, etc.

Whilst what you say is correct that would mean that for exactly the same size hole but at a focal length of 70mm the aperture would actually be f/0.9ish (I can't be bothered with the actuall maths).

So Sk66's conudrum is valid. If a lens has a max aperture diameter of 71 mm and a min focal lenght of 70mm why don't they let us use both at the same time?

The constant aperture thing for manual doesn't really wash either as all lenses are constant aperture once you get past the min. For example my kit lens is a 14-42 f/4-5.6. It can therefore only hit f/4 at 14 mm. However if I set f/5.6 or more I can zoom though the whole range without the aperture changing at all, it stays at f/5.6.

I suspect it is actually a factor of the optics, focusing issues, sharpness etc. It probably isn't possible to engineer a lens which is able to focus at f0.9, be sharp edge to edge, not suffer for vignetting and other issues and also be able to focus to infinity at 200mm. (Whilst still being able to be handheld and not purchased without a mortgage.)

[BCampbell]

As I understand it, once you get into artifical lighting, constant aperture zooms are invaluable time savers.

[jdepould]

Originally Posted by BCampbell

As I understand it, once you get into artifical lighting, constant aperture zooms are invaluable time savers.

It's not necessarily artificial light, so much as it is low-light situations, natural or artificial. Studio setups are all artificial, and shooting wide open would generally result in overexposure.

[fletch]

Originally Posted by BCampbell

As I understand it, once you get into artifical lighting, constant aperture zooms are invaluable time savers.

That still doesn't answer the question. If your lens was a f/0.9-2.8 rather than a f/2.8 you could still save time in the studio.

Just set you lens to f/2.8 (or f/8 or f/16 etc) and it will be a constant aperture across the whole zoom range. The real benefits of constant aperture zooms is that they have larger apertures at long focal lenghts. A f/2.8 or f/4 constant aperture is much wider at the long end that the equivalent variable max aperture lens which is often f/5.6 or more at the long end. (The varibale lens is obviously much cheaper though!)

So does anyone actually know why the physical maximum diameter of the lens is restricted when at the shorter focal lenghts?

[fletch]

Brain wave - Its got to be something about optical quality.

Why?

Even your most basic of kit lenses is wasting some of its maximum aperture. A bog standard kit lens is usually 18-55 f/3.5-5.6. Therefore at 55mm and f/5.6 the size of the hole is 9.8mm. At 18mm and f/3.5 the hole is only 5.1mm. Why not let the hole be 9.8mm at 18mm as you would have a f/1.8mm lens.

For a superzoom its even more of a waste. An 18-200 f/3.5-5.6 lens will have a 36mm whole and could in theory give f/0.5 at 18 mm.

No manufacturer is going to give away this headline limit for no reason as an f/1.8 kit lens would be a huge sales advantage. Therefore they must be artificially limited as the optics are not up to providing even a satisfactory image at such apertures.