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  1. #1

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    Variable aperture lens question

    Hello everyone, this is my 2nd dumb question of the day. I have a question about variable aperture lenses, specifically a (cheap) Tamron 70-210mm F4-5.6 which came with the Nikon FM2 I just bought. I have only ever used primes and I am not sure how variable aperture works in terms of using an external meter.

    So with this lens zoomed to 210mm the largest effective aperture is F5.6. This is presumably the amount of light which is let in when the lens aperture is set to F4 on the aperture ring.

    However, if I set the lens aperture to F5.6 on the ring and then zoom to 210mm there is clearly less light coming in than when zoomed to 210mm and set to F4. This is clear when looking through the lens and is confirmed by the camera's TTL meter. So would the effective aperture actually be F8 with the lens at 210mm and the aperture ring at 5.6? Do I have to compensate a whole stop throughout the entire range of the lens?

    If using TTL metering then this obviously isn't a problem, but I use an external meter so I need to know what the actual effective aperture is. Also, how do I tell at what point in the lens's zoom range the aperture switches from F4 to F5.6?

    Any advice is appreciated!

  2. #2

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    Use the aperture numbers on the aperture ring of the lens. The maximum available aperture at 70mm is f4; at 210mm f5.6. It should couple properly with the built in meter of the camera. Any more info should be in the lens' instructions, which may be available online somewhere.

  3. #3

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    if I did that, then wouldn't I be underexposing at F4 and 210mm? The lens says its maximum aperture is F5.6 at 210mm

  4. #4

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    Quote Originally Posted by nwilkins View Post
    if I did that, then wouldn't I be underexposing at F4 and 210mm? The lens says its maximum aperture is F5.6 at 210mm
    F4 admits twice the amount of light as f5.6. If the lens' maximum aperture is f5.6 at 210mm, that will be the maximum amount of light admitted regardless of the setting of the aperture ring. Try to find a set of instructions online.

  5. #5

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    as I said in my original post, when the lens is at 210mm it lets in LESS light when the aperture ring is at F5.6 than when it is at F4. This is borne out by using the TTL meter in the camera. That is why I am confused.

    If the same amount of light was getting in regardless of the aperture ring's indicating 4 or 5.6 when zoomed to 210mm, then I would not have posted my questions.

  6. #6

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    Apparently, f4 on the ring becomes f5.6 effective, and the rest of the settings follow suit. This is why I suggested finding the instructions, or someone familiar with that lens. I do not use zoom lenses on my 35mm (or other) cameras.

  7. #7

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    You are over thinking this.

    If you set your lens to 210mm, f/5.6 is f/5.6 - just like if you have a prime lens at that focal length and aperture at that size.
    If you set your lens to 70mm, f/4 is f/4 - just like if you have a prime lens at that focal length and aperture at that size.

    You measure, you set your lens, you are good to go. There is no need to compensate.
    Develop, stop, fix.... wait.... where's my film?

  8. #8

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    Quote Originally Posted by tkamiya View Post
    You are over thinking this.

    If you set your lens to 210mm, f/5.6 is f/5.6 - just like if you have a prime lens at that focal length and aperture at that size.
    If you set your lens to 70mm, f/4 is f/4 - just like if you have a prime lens at that focal length and aperture at that size.

    You measure, you set your lens, you are good to go. There is no need to compensate.
    I've no idea the age of the OP's lens. Some of the older/cheaper zooms behaved as I described,(I guess they didn't expect them to be used with handheld meters?) the fact that the OP's meter indicates an increase in light admitted leads me to think this might be the case with his lens.

  9. #9

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    Hi E von Hoegh, thanks for trying to solve this - I have been looking all day for an instruction manual but I can't find anything online. I was hoping someone here had used the lens and would know. I can only conclude, like you, that you must need to compensate throughout the whole aperture range, but that seems crazy.

    Tkamiya according to your thinking, what would the F number be if I set the lens to 210mm and the aperture ring to F4? Because that setting definitely lets in more light than when set to F5.6 at 210mm.

  10. #10

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    most lenses of the type you have behave like this. When you set it at the shortest focal length (70mm) then the aperture is what set on the aperture ring. Since it's specified as an f4-f5.5 zoom then at 210mm it's f/5.6 when you set your aperture ring at f/4 and f/8 when you set the ring at f/5.6. I also found that the light loss is proportional to the focal length set. for example if you set the lens at f/5.6 and zoom to 140mm (midway) then the aperture is 1/2 stop less than f/5.6 (f/6.7).

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