aperature question re: Tamron 90mm macro...

Discussion in 'Beyond the Basics' started by rob1116, Feb 5, 2005.

  1. rob1116

    rob1116 TPF Noob!

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    Hello hello,

    Just for the record:

    <------ "New" Thanks ;)


    Anywho, I have a question regarding max. aperature labelling on some lenses. Example: The "Tamron 90mm f/2.8 DI macro" . I understand the basic concept of f/numbers, (ie. aperature diameter is the focal length over the f/number... ie. 100mm f/4 lens should have a max. aperature diamter of about 25mm, right?)

    So I'm in the local camera store the other day trying this lens out on my D70. The D70 powers up and I smile as I see the aperature listed at 2.8 . (what can I say, I have only ever used my 18-70mm f3.5-4.5 nikon, and my Tamron 70-300mm 3.5-5.6, so 2.8 seems so fast to me!)

    Then, to my horror, after focusing on something moderately close the aperature reading starts to drop as if I'm zooming in and losing stops!?!? I'm guessing this is because of the large focus barrel required for proper 1:1 macro, but WTF? Are all macro lens like this? What about other lenses that are labelled with constant max. ap?

    Is this normal? If so, then this lens can't really shoot 1:1 from 90mm at 2.8 can it?

    Okay, this is the part when half a dozen of you gurus chime in and enlighten me to the one piece of the puzzle I'm obviously missing... please!? :hail:
     
  2. Rob

    Rob TPF Noob!

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    Was it in Auto, Program or Shutter Priority mode(s) and the camera was adjusting the d-o-f to compensate for less light on a closer object? Stick it in aperture priority mode at f2.8 to check this.

    Rob
     
  3. rob1116

    rob1116 TPF Noob!

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    sorry, I should have said that... I tried it in Ap Priority and Manual, and it would only allow me to use f/2.8 when I wasn't focusing on something relatively close, like maybe 1:2 mag max, and then it start stopping down, or at least the camera read that it was.

    So this isn't normal?
     
  4. Bob_McBob

    Bob_McBob TPF Noob!

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    1:1 macro lenses close focus by extension, which is essentially the same as using extension tubes. You lose some light because the lens barrel is extended. Be happy your camera auto-adjusts for the aperture change, it's a real pain in the ass on manual cameras!
     
  5. rob1116

    rob1116 TPF Noob!

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    bob : So you're saying this is normal with macro lenses... hmm.

    I kinda wondered how the aperature could remain constant when the lens extends so far out.

    So, my next question: What about other lenses listed as having a constant max. aperature? Ex. AF-S VR Zoom-Nikkor 70-200mm f/2.8G IF-ED . If it really is constant, then the aperature would have to open up as you zoomed in right? So if that's what it's doing, and the aperature can already open that far, why not leave that larger aperature setting available at shorter focal lengths... turning it into a 70-200 f/1.4 - 2.8 or something... ???

    I'm definitely missing something here. I'm gonna have a look for some detailed lens construction diagrams. If anyone knows of any decent sites explaining/showing these things I'd really appreciate it!
     
  6. deepakram

    deepakram TPF Noob!

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    maybe i'm going to sound stupid saying this, but i'll say it anyway!
    the max-aperture listed on a lens pertains to the min-zoom of that lens, ie., the min. focal length. so, if i have a tamron 28-200 f3.8-5.6, then it means that the max aperture at 28 mm is f/3.5. right? now, if you keep your lens at its min focal length and focus on anything, no matter how far or near, the aperture should remain the same! while typing this reply out, i tested my statements on my camera, and it works as expected, atleast at my end!!
     
  7. rob1116

    rob1116 TPF Noob!

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    yes, I understand and agree with that deepakram... but how, when the length of this macro lens doubles when focusing on something very close (because the focus barrel [correct terminology?] protudes waaaay further than a regular lens to allow focusing from 1:1 to &#8734; ) can it physically be the same f/number?
     
  8. Digital Matt

    Digital Matt alter ego: Analog Matt

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    I don't own a true prime macro lens. I use extension tubes. There is a light loss, but I'm still able to open up to my max aperture.

    You won't be able to use f/2.8 for anything when you are focusing in the macro range anyway. Your DOF will be way too shallow. Most macro shots are done at f/11 and below, to have anything in focus. The 2.8 comes in handy if you are using that as a portrait lens, for which it will work very well also. You will not be focusing in the macro range for portraiture, so the 2.8 will be available.
     
  9. rob1116

    rob1116 TPF Noob!

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    Excellent point Matt; the DOF is extremely shallow shooting macro at max. aperature... but I'm more interested in getting an idea of the mechanical workings of the lens.
     
  10. Bob_McBob

    Bob_McBob TPF Noob!

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    Yes, you can open the lens to its maximum marked f/stop, but the reason you lose light with extension tubes is because the effective aperture size decreases as the length of the barrel increases. You get the same effect with bellows extension in large format cameras. It's called the bellows effect.
     
  11. rob1116

    rob1116 TPF Noob!

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    :( hmm... tried the Sigma 105mm macro on my D70 today. It allows me to choose f/2.8 regardless of focus distance and subsequent barrel extension. Still not getting it... how is the aperature the lens length/2.8 when the length of the lens is obviously changing? This must have something to do with the focusing elements moving without effecting the focal length, but I still can't picture it, and I definitely DON'T get how you can have a constant aperature on a zoom lens, like a 70-200mm f/2.8.

    Anyone care to continue this discussion for me? The girl at Henry's looked at me funny for a second after I asked her about it, then said she was sorry but she didn't have the answer
     
  12. Kodan_Txips

    Kodan_Txips TPF Noob!

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    The iris is a small round source of light, as far as the film is concerned. The further away it is, the less light gets to the film. (A bit like the Sun and the stars. Good, or the sky would be too bright!)

    To make the calculations easy, let's pretend that it is 100 mil from the film plane, and you are able to use f2.8. By focussing on something that moves the aperture out to about 140 mmm, you have lost one stop, and your max F stop is F4. At 173 mm it is F5.6. If the focussing racks the iris mount out to double the length between the iris and the film, you will have lost a massive 3 stops, and the max aperture is F8. (Or is it 4 stops, and f11? That is even worse!)

    Awful, isn't it?

    Some zoom lenses have the same problem, as the iris is mounted within the front clump of elements. My 100-200 MD rokkor reduces the effect of this by keeping the iris in the same place, but as the iris is using the lenses at the front of a tube in front of itself, I still lose a stop or two as I lengthen the tube by zooming out and then focussing. The damned thing is so long and heavy, I intend to use it to beat up on muggers!

    However, some zooms might be about a fifth as long, for a much wider zoom range, yet still keep the same aperture throughout the range. They do this by moving the iris backwards AND forwards as you focus and zoom. (It also helps that digicams usually use smaller sensors than the 35 mil neg size, which increases the apparent length and range of the zoom) Some zooms may even open the iris up as it moves away from the film plane, to make it appear the same size to the film.

    Sorry if all this may seem like talking down to you, but it is a fault in many teachers like myself, and the only reason I am able to type such a lengthy title is that I am off work with bronchitis, which is making me even more ponderous. Also I was reasoning the whole thing out as I went along, as up till now I hadn't realised that the fixed iris position on my long zoom still lost at least one f stop at the 200 mm position.

    Does anyone want to buy a telescopic drainpipe?
     

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