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Zoom lens apertures

Discussion in 'Help Team' started by 0, Apr 15, 2003.

  1. 0

    0 Guest

    Can anyone clear my confusion, please? If I have a zoom lens with a variable aperture, for example f3.5-4.5, if I set it at the long end, will all the marked apertures increase (decrease?) in value, i.e. marked aperture 3.5, actual aperture 4.5, marked 5.6, actual 8, etc? It's obviously not a problem with TTL metering, but if I wanted to use manual flash indoors ....

    Thanks,
    Steve
     
  2. RichardHardwick

    RichardHardwick Well-Known Member

    Hi Steve, it's just the maximum aperture which varies as you zoom. The other apertures will be constant.

    Richard
    <a href=http://www.richardhardwick.com>www.richardhardwick.com</a>
     
  3. 0

    0 Guest

    Thanks Richard. That was quick!

    Steve
     
  4. huwevans

    huwevans Not Really Here

    Err, well, not exactly, no. If you are talking about a modern camera where the exchange of information between lens and body is all electronic and the aperture is set by the camera, then what you set on the camera is what you get. The whole aperture range is shifted - i.e. you will lose something at one end, but gain at the other, but the key point for using manual flash is that the camera is intelligent enough to know that the aperture is shifted, and tells you the real value, not some nominal value that you then have to adjust for.

    If, OTOH, you are talking about an older style lens/camera where the aperture is set by a ring on the lens then you will have to judge by where you are in the zooming range as to how much the marked aperture is shifted, but it is shifted by the same amount throughout the range.

    Huw Evans.
    www.huwevans.freeuk.com
     
  5. RichardHardwick

    RichardHardwick Well-Known Member

    The physical size of the aperture varies with focal length. E.g.

    A 50mm lens with12.5mm diameter of aperture is 50/12.5 = 4 = f4
    A 100mm lens with 25mm diameter of aperture is 100/25 = 4 = f4

    Some zoom lenses vary the physical aperture as you zoom through the focal lengths in order to keep the selected f-number and exposure constant. If you look through the front element and zoom you can usually see this happening.

    Most consumer zoom lenses have a variable maximum aperture due to cost constraints e.g. a 28-80mm f3.5-4.5. To achieve an aperture of f3.5 at the longest focal length, the physical aperture size would need to be greater than the size of the lens barrel would allow. By making the barrel larger, the lens would need larger glass with all the extra costs and problems that brings.

    Therfore some of these lenses are variable maximum aperture only; the f-numbers of the other apertures remain constant and don’t shift when you zoom. The mechanics inside the lens take care of the necessary adjustment.

    Huw is right, there are some lenses which retain the same physical size of aperture whilst zooming, these will need some manual aperture adjustment.

    Richard
    <a href=http://www.richardhardwick.com>www.richardhardwick.com</a>
     
  6. huwevans

    huwevans Not Really Here

    Well, as I said the modern lenses do it by varying the control from the camera. If there are some which do it by purely mechanical means within the lens then I've never heard of them. Can you cite some examples?

    There could have been a point to doing this before the days of camera bodies controlling apertures, but nowadays it would be an entirely superfluous mechanism, since the zoom position is communicated to the camera body and a shift by the control system in the camera is then simpler, cheaper, more reliable, and imposes no additional design constraints on the lens.

    there are some lenses which retain the same physical size of aperture whilst zooming

    This has been the case with every zoom lens I have ever seen - bar none. I guess I must have missed something.

    Huw Evans.
    www.huwevans.freeuk.com
     
  7. RichardHardwick

    RichardHardwick Well-Known Member

    You're right, I'm referring to older lenses, such as Canon FD, Vivitar Series 1, but they definately would alter aperture size as you zoomed in order to compensate for the focal length. Maybe you have missed something!/img/wwwthreads/wink.gif



    Richard
    <a href=http://www.richardhardwick.com>www.richardhardwick.com</a>
     
  8. 0

    0 Guest

    Now I'm more confused! So how does this affect the Nikon 18-35mm f3.5-4.5, with a manual body, please?

    Steve
     
  9. huwevans

    huwevans Not Really Here

    Maybe you have missed something!

    Wouldn't be the first time! But I'm happy to be enlightened. :)

    Huw Evans.
    www.huwevans.freeuk.com
     
  10. huwevans

    huwevans Not Really Here

    Ah, well, now you're asking! I guess the only way to know for sure is to check and see. Set the lens to something like f/8 and zoom it through the range and see if you can see actual movement of the iris blades. This might not be easy to see, since with many zoom designs the iris itself moves within the body of the lens while zooming, so you will have to be careful that any perceived change in the aperture size is actually a resetting of the blades, rather than simply the iris moving closer and appearing bigger (for instance). Perhaps the most straightforward way to check is to use the TTL meter. Point the camera at an evenly illuminated plain wall, and see if the indicated exposure changes from one end of the zoom range to the other. If so, then you will need to compensate manually. If not, then you have no problem.

    Huw Evans.
    www.huwevans.freeuk.com
     
  11. 0

    0 Guest

    Thanks Huw, that makes sense (even to me!). I haven't actually got the lens yet - hopefully tomorrow. :)

    Steve
     
  12. Col. Hogan

    Col. Hogan Well-Known Member

    I have one, but haven't seen anything like that. Of course, I suppose I never paid much attention.

    Diane
     
  13. Larry Shone

    Larry Shone Well-Known Member

    Re: Zoom lens apertures/not exaclty constant!

    well, on my 75-300 zoom I get a shift of from f4.5 to f5.6 wide open and stopped down it goes from f22 to f45!

    I will be rich and famous one day, just not today!
     
  14. RichardHardwick

    RichardHardwick Well-Known Member

    Re: Zoom lens apertures/not exaclty constant!

    I think my comments must only apply to higher-end zoom lenses. I've been to the Canon website and the 'L' zoom lenses have a fixed minimum aperture whereas the consumer lenses have variable minimum apertures. The consumer lenses do shift their f-number values throughout the zoom range but the 'L' lenses don't.

    Sorry for the confusion I've caused, but I knew from my experience that my zoom lenses didn't shift f-number values. It appears as though the 'L' lenses still use a cam mechanism to adjust the aperture diameter as the focal length is changed.

    The Nikon website doesn't give the minimum aperture values for their lenses. From what I can gather, some Nikon zoom lenses do retain f-number values throughout the range of focal lengths but only when attached to certain bodies (F5 and F100).

    Richard
     
  15. huwevans

    huwevans Not Really Here

    Re: Zoom lens apertures/not exaclty constant!

    But the high-end zoom lenses are not variable aperture zooms, surely? Certainly none of the ones I am familiar with are. There are several fundamentally different optical zoom designs (three, IIRC, although I couldn't give you details of them all right now) but not all of them have variable apertures.

    The key quantity is the size of the entrance pupil, which is not the same thing as the physical aperture - it is basically an optical entity, which may vary in size despite the physical iris not changing. This happens in some designs by the changing relationship of the optics causing a magnification of the entrance pupil, so that the same opening may be, say, an f/8 aperture at the short end of the zoom as well as at the long end. In other designs the change in magnification does not precisely correspond to the change that would be necessary for the aperture to be constant throughout the range, and thus we have a variable aperture.

    Incidentally, as I recall in the early days of zooms in still photography back in the 70s, all the zooms were constant aperture zooms. It seemed to be only later that the variable apertures came in. I imagine this is because the new design enabled a better optical quality at consumer prices, and also it would appear, less weight and bulk. The only zoom I have from the early period has a constant aperture, but its image quality was pretty poor, and it is much bigger than the variable aperture zoom of similar range that I bought a few years later. Modern high-end or 'pro' zooms are of the former type, although vastly better quality (which of course you have to pay for).

    Huw Evans.
    www.huwevans.freeuk.com
     
  16. huwevans

    huwevans Not Really Here

    Re: Zoom lens apertures/not exaclty constant!

    some Nikon zoom lenses do retain f-number values throughout the range of focal lengths but only when attached to certain bodies (F5 and F100)

    With these cameras the aperture is normally controlled from the body, so the situation is as I described earlier. I am not aware of any mechanism internal to the lens that changes the relationship between the position of the aperture ring and the physical size of the iris opening through the zooming range.

    As a rule, if you have a lens which is a variable aperture zoom then every designated f-stop is shifted by the same amount. Whilst it could serve a purpose to have a cam mechanism maintaining the relationship between the stop set on an aperture ring and the effective f-number, I can't see any sensible reason why the same mechanism should prevent access to the smaller effective aperture that would otherwise be available at the bottom end of the range.

    I don't have any modern Nikon zooms, and the only older Nikon zoom I have certainly does not have any such mechanism. However, I do have the 60mm Micro-Nikkor, which might be instructive.

    Although obviously not a zoom, this, being a micro lens which focusses right down to 1:1 reproduction, does in effect have a variable aperture. At 1:1 the aperture is about 1 2/3 stops smaller than it is at infinity focus. The camera knows this, or probably to be more accurate, the chip in the lens knows it, and controls the setting of the aperture by the camera body so as to make the whole thing seamless. If I set f/2.8 at infinity focus, and then focus right the way down to 1:1, I find the camera now thinks (correctly) that the aperture is f/5. How ever, if I set a small enough aperture, say f/5.6, it is maintained right the way through the focussing range. But that is because of the electronic communication going on, not because of any mechanical device.

    Conversely if I take the aperture ring off its minimum setting to control the iris from the lens, then setting f/5.6 leaves me with a shifted aperture at 1:1 which I still need to compensate for. The actual opening in the iris does not change throughout the focussing range. If the zoom lenses really did have a cam arrangement changing the aperture then I would have expected the designers to have used the same sort of thing in the micro lenses.


    Huw Evans.
    www.huwevans.freeuk.com
     
  17. Larry Shone

    Larry Shone Well-Known Member

    Re: Zoom lens apertures/funny things lenses

    lenses are funny things, the marked aperture is only really that aperture at infinity, when you focus closer you change the focal length of the lens very slightly( have you noticed how when using an enlarger and you change the focus the image grows and shrinks!) and also the aperture changes too, but of copurse it's only a fractional amount, hardly worth bothering with in most cases and if it is then the TTL metering will take care of this.

    I will be rich and famous one day, just not today!
     
  18. RichardHardwick

    RichardHardwick Well-Known Member

    Re: Zoom lens apertures/funny things lenses

    Yes, as far as I am aware, all lenses lose light transmission as you focus closer. This more noticable with extension tubes and bellows. My RB has a compensation scale on the side to indicate how much extra exposure is required for the amount of bellows extension.

    Richard
     
  19. Larry Shone

    Larry Shone Well-Known Member

    Re: Zoom lens apertures/funny things lenses

    it gets drastic when using extension tubes or bellows as it follows the inverse square law that states that light falls off in intensity with the square of the distance, and as the lens is moved away from the film a factor of multiples of the focal length, it gets pretty dark in there!

    I will be rich and famous one day, just not today!
     
  20. Burgy

    Burgy In the Stop Bath

    Re: Zoom lens apertures/not exaclty constant!

    But the high-end zoom lenses are not variable aperture zooms, surely

    Canon 35-350 F4.5-5.6, 100-400 F4.5-5.6 to name but two

    Burgy

    Its not what you've got, its who you do it to. http://www.pressfotos.co.uk
     

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