Constant Aperture Zoom Lenses - Why?

[chris321]

I've been thinking (always a dangerous thing...), about lenses quite a lot really. At first, it took me quite a while to understand why some lenses the maximum aperture changes when you increase the focal length, and why is doesn't on some. Now I understand that bit, but there's something I don't quite get!

Right, thinking about for example, a 70-200mm f/2.8 lens. Now I understand why f/2.8 is desirable at the long end, and I get why it costs a lot to make this.

HOWEVER, what I don't get is that if the barrel of the lens is capable of an aperture of f/2.8 at 200mm, surely it's capable of a smaller f-number at 70mm? Technically wouldn't it be able to be something like 70-200mm f/1-2.8? And wouldn't this be more desirable? What is the advantage of keeping the aperture constant throughout the zoom range, if it's possible to make it faster at the short end?

I hope I made this clear, I'm almost confusing myself just typing this!

Chris

 

[HoppyUK]

I see where you're coming from, and optical theory would suggest that what you are suggesting might be theoretically possible, and surely desirable. So it must be that such a design would be impossible to manufacture economically, and would be massively complex, both optically and mechanically. Otherwise, there'd be plenty about

Richard.

 

[alexisonfire]

HOWEVER, what I don't get is that if the barrel of the lens is capable of an aperture of f/2.8 at 200mm, surely it's capable of a smaller f-number at 70mm?

No, as f/2.8 is its minimum aperture across the variable focal range.

Technically wouldn't it be able to be something like 70-200mm f/1-2.8? And wouldn't this be more desirable?

The aperture would be desirable, but the size of the lens would not. A 70-200 with an aperture of f/1 would need to be stupidensly huge.

What is the advantage of keeping the aperture constant throughout the zoom range, if it's possible to make it faster at the short end?

Its only possible to make it faster until you hit that brick wall, f/2.8 is just that. I think the fastest zoom lens ever made is f/2.

 

[chris321]

So it must be that such a design would be impossible to manufacture economically, and would be massively complex, both optically and mechanically. Otherwise, there'd be plenty about

Richard.

Well that's what I thought, but I don't see why!

No, as f/2.8 is its minimum aperture across the variable focal range.

The aperture would be desirable, but the size of the lens would not. A 70-200 with an aperture of f/1 would need to be stupidensly huge.

Its only possible to make it faster until you hit that brick wall, f/2.8 is just that. I think the fastest zoom lens ever made is f/2.

I think you've misunderstood me a bit (hardly surprising really!). I know f/2.8 is the minimum aperture across the focal range, but why?

A 70-200 with an aperture all across the range of f/1 would be massive I agree, because the front element would have to have a diameter of 200mm (I think ). But if f/2.8 is attainable at 200mm, I don't understand why f/1 isn't attainable at 70mm, and varies in between like most (cheaper) zooms!

 

[alexisonfire]

Well that's what I thought, but I don't see why!






I think you've misunderstood me a bit (hardly surprising really!). I know f/2.8 is the minimum aperture across the focal range, but why?

A 70-200 with an aperture all across the range of f/1 would be massive I agree, because the front element would have to have a diameter of 200mm (I think ). But if f/2.8 is attainable at 200mm, I don't understand why f/1 isn't attainable at 70mm, and varies in between like most (cheaper) zooms!

Its most probably a quality thing, think how soft some 1.4 etc prime lens are, i doubt something has been designed yet suitable for sale in a zoom format.

 

[HoppyUK]

Its most probably a quality thing, think how soft some 1.4 etc prime lens are, i doubt something has been designed yet suitable for sale in a zoom format.

I think Alex is probably right - it's a quality issue. Wide apertures are lens designers' most difficult challenges.

Just look at what's available and one common theme is, the wider the aperture, the shorter the zoom range. Is there an f/2.8 zoom with more than x3? Look at Canon's acclaimed EF-S 17-55mm f/2.8 - lovely lens, but much bigger, heavier and more costly than the significantly longer EF-S 17-85mm which is f/4-5.6.

Of course the widest aperture lenses don't zoom at all - they're primes!

Richard.

 

[allanm]

what I don't get is that if the barrel of the lens is capable of an aperture of f/2.8 at 200mm, surely it's capable of a smaller f-number at 70mm


As f2.8 is the lens maximum aperture at any focal length, which is governed solely by the physical dimensions of the lens, the trick is getting the long end to stay at f2.8.
So, 70mm ( on a 70-200 f2,8 lens) could never be any wider then f2.8 as that is its maximum to start with.
Allan

 

[photon]

Guys, don't forget f/2.8 is the MAXIMUM aperture.

The only zoom I can think of along the lines of your discussion is the superseded Tokina f/2.6 - 2.8 28-70mm. Note that the maximum aperture of f/2.6 is not a great deal larger than the max at the long end.

Also, on the theme of not looking for maximum zoom range if best IQ is required, Tokina once made an f/2.8 60-120mm for 35mm film, tailored to portrait photography. When I first heard of it I thought "They're not going to sell many of them" simply down to the zoom range.

 

[andrewc]

The f stop is a ratio between diameter of the lens at a focal length.

It requires bigger diameter glass at the long end which dramatically adds cost as well as weight and bulk. Its easier to compromise and have a lens which is faster at one end, but slightly compromised at the long end. This lens is therefore much smaller and cheaper.

 

[StewartR]

I think most people here (ie evryone apart from HoppyUK and me) are missing chris321's point.

Taking the 70-200mm f/2.8 zoom as an example, at the long end the aperture is 200/2.8=71mm. So the lens is clearly capable of opening up to an aperture which is 71mm across.

Now, the question is, why can't it open up to a 71mm aperture when it's at the other end of the zoom? That would make it f/1.0, and the lens would be 70-200mm f/1.0-2.8.

HoppyUK got it right. It seems to be theoretically possible, but the fact that it isn't done is because it's too difficult/expensive to do well.

 

[rogprov]

Could it be because most zoom lenses use retrofocus (reversed telephoto) to achieve the focal length at the wide end?

 

[pxl8]

Plenty of reading on the subject along with technical notes here:

http://photography-on-the.net/forum/showthread.php?t=471313

and here:

http://photography-on-the.net/forum/showthread.php?t=86975#post777253

 

[chris321]

I think most people here (ie evryone apart from HoppyUK and me) are missing chris321's point.

Taking the 70-200mm f/2.8 zoom as an example, at the long end the aperture is 200/2.8=71mm. So the lens is clearly capable of opening up to an aperture which is 71mm across.

Now, the question is, why can't it open up to a 71mm aperture when it's at the other end of the zoom? That would make it f/1.0, and the lens would be 70-200mm f/1.0-2.8.

HoppyUK got it right. It seems to be theoretically possible, but the fact that it isn't done is because it's too difficult/expensive to do well.

I knew someone would be able to turn my befuddled ramblings into plain speech!

So maybe f/1.0 is a bit extreme, but it could at least be f/1.4, or f/1.8 surely, without too many complications? The only advantage I can see of keeping the aperture constant is so that the exposure can remain the same if the focal length is changed. Hmmm, I still don't see why they don't do it!

Chris

 

[pxl8]

Then read the links I posted above, they explain the physics behind why it's not as simple as it appears.

 

[Blapto]

You should be able to see this yourself if you have appropriate lenses hanging around (glasses, or a magnifying glass to some degree).

As the front element moves away from the aperture it magnifies it, making it appear larger. That way the effective aperture is larger at 200mm than it is at 70mm, because you're using a lens that's further away to look at the aperture at the back of the lens.

With modern lenses, close range correction and all that it's always much more complex than us mere mortals can explain, but that's the basic physics at play.

Edit:
This is why ultrafast lenses are always designed around the 50mm-200mm mark.
Longer than that and you need a HUGE front element.
Shorter than that and you're using a wide angle lens to view the aperture, which means that the aperture (or effective aperture) looks very very small from the front of the lens.
With a 17mm lens, you need much wider than a 17mm aperture to get f/1. You need an aperture that look 17mm across when viewed through a wide angle lens.

 

[HoppyUK]

HoppyUK got it right. It seems to be theoretically possible, but the fact that it isn't done is because it's too difficult/expensive to do well.

Thank you Stewart Some of the posts above, and links, appear to me to explain why eg Canon EF 70-300mm f/4-5.6 being f/5.6 at the long end, cannot be f/4 at the short end And I don't understand "...why ultrafast lenses are always designed around the 50mm-200mm mark" when Canon makes a 24mm f/1.4 and a 16-35mm f/2.8. I've obviously misread this somehow, but this stuff sure is hard to grasp

It is true that the iris blades in both fixed f/number zooms and variable f/number zooms change as focal length is altered, but I can only assume this is a red herring in reference to the OP. Maybe it's something similar to why many zooms reduce signifcantly in actual focal length when set to macro focus, when this too is generally undesirable, ditto loads of field curvature The only thing that makes sense to me is that it is an optical/mechanical/economical compromise to produce the overall 'best' lens.

And for sure there is obviously more to it than the formula focal length/aperture=f/number. But I am not a gifted optical physicist with access to a huge computer capable of running the millions of formulae necessary to design a modern 15-20 element zoom. They are hugely complex things. Enough already.

The only thing that makes sense to me is, as Stewart says "it's too difficult/expensive to do well" and given that it is evident that apertures brighter than f/2.8 start to cause real optical difficulties in zooms, I can begin to see why 70-300mm f/4-5.6 is possible, whereas even with Canon L the best they can realistically achieve is 70-200mm f/2.8. Consider that this lens is also twice as heavy and twice the price of the 70-300mm as it is.

Look at it that way, and it makes sense. Any other way just hurts my brain

BR,

Richard.

 

[Blapto]

Thank you Stewart Some of the posts above, and links, appear to me to explain why eg Canon EF 70-300mm f/4-5.6 being f/5.6 at the long end, cannot be f/4 at the short end And I don't understand "...why ultrafast lenses are always designed around the 50mm-200mm mark" when Canon makes a 24mm f/1.4 and a 16-35mm f/2.8. I've obviously misread this somehow, but this stuff sure is hard to grasp

The 24mm f/1.4 is huge, expensive and not a great performer at f/1.4 with marked vignetting (OK, it's not awful, but it's not a perfect lens). Perhaps that should read that the good, economical ultrafast lenses for 35mm camera imaging circles are between 50-200mm, and much more often at the 50mm side of that.

 

[HoppyUK]

The 24mm f/1.4 is huge, expensive and not a great performer at f/1.4 with marked vignetting (OK, it's not awful, but it's not a perfect lens). Perhaps that should read that the good, economical ultrafast lenses for 35mm camera imaging circles are between 50-200mm, and much more often at the 50mm side of that.

Okay, I follow I think the key factor is that it is better to start with lenses which don't have their optical wings clipped by having a focal length less than the distance from lens mount to image plane, which dictates the infamous inverted-telephoto design.

<cue deluded Leica fans >

Richard.