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Why is my f2.8 lens a f.4?

Discussion in 'Lens Lust' started by MD2595, Aug 18, 2009.

  1. MD2595


    Jul 6, 2007
    Fort Worth, TX
    I put on the 60mm f2.8 micro tonight to take a shot of my youngest son's beautiful hazel eyes and the camera would only go to F4. I turned the camera off/on and it still would only go to f4. I turned off the camera, removed the lens, re-attached it, and it went to f2.8. I turned the camera off to eat dinner, then tried to resume shooting and it was now an f5.0 lens!

    This is only the second time I've used this lens, and the first it did nothing like this. I've probably taken less than 40 shots with this lens.

    Is it the lens, or the camera?

  2. Did you try "M" and "A" modes?
  3. goffer


    Nov 22, 2007
    San Diego
    how close were you focusing?

    EDIT: what edward said
  4. Ed's explanation is pretty spot on. Thom Hogan also does a pretty good job explaining it in his 105 f/2.8 VR review.
  5. This happens in all macro lenses. My Tamron 90/2.8 does the same. It's normal.
  6. Thanks Ed. (Disregard my post) :redface:
  7. MD2595


    Jul 6, 2007
    Fort Worth, TX
    Thanks for the explanation Ed. I am still kind of confused by the loss of light explanation, since I was using a Photoflex 500w lightbox and 4 60 watt CF bulbs in the room light (it's pretty bright). I've already put the gear up and the lens back in the original box, so I'll have to try it again tomorrow.

    What you are saying makes since, but I had a TON of light for this, so that's why I'm confused.
  8. But the lens does now know that, right? :wink:
  9. Chris101


    Feb 2, 2005
    MD2595, relax cause everybody who shoots with a 60mm micro experiences exactly the same changing f-stop as you do. Try this out: Put the lens on the camera (go ahead, take it out of the box, it ain't broke) and focus on something far away. Notice that the lens doesn't stick out much. Now set the lens to f/2.8. Cool, it's an f/2.8 lens!

    Now keep the same setting, but focus on something as close as you can get (the lens stuck way out and got further from the sensor, didn't it?) Check the aperture. Whoa! It's become an f/5 lens! I want either my money, or my aperture back!

    Ok then, now focus back on something far away again. ... Whew! Ok, it's an f/2.8 lens again. Now you know how the lens will behave when you rack the focus in and out. I could go into a bunch of tech talk about numeric aperture, angle of acceptance, inverse squares, blah, blah, blah... But I get the impression that you aren't a tech guy, you want the thing to work right. And so it does. Edward and Hogan explained it right, so rest assured. Your lens is fine. Now take some cool pictures and share them with us!
  10. MD2595


    Jul 6, 2007
    Fort Worth, TX
    Thanks again. You are totally right about being a tech guy. I have NO formal training, just a dad who got into this "hobby" to take good pics of my kids (portraits/sports). Working a full time and part time jobs and being involved with the family leaves little time for dad's interests. My youngest will be leaving the house in 7 years, and then I'll have all the time in the world.

    I'll post up a small pic of what I did after I get home from work.... But for now it's back to business.
  11. panda81


    Feb 7, 2008
    The observed phenomenon is a law of physics - you can't really get around it regardless of how much light you have :smile:

    If you're mathematically or scientifically inclined, here is an explanation (at least the way I understand it).

    F-stop # = (lens focal length)/(diameter of aperture)

    That's why, for example, when you stop down the lens, the f-stop number actually increases - because the denominator in the equation decreases. This is also why the same f-stop number at any focal length should give you the exact same exposure when all else is also equal - the focal length is relative to the aperture diameter. With a macro lens, when you focus at closer and closer distances, the optical elements inside the lens start shifting (or the barrel extends, depending on your particular lens), an effect very similar to increasing the lens focal length. The rest is just as how Ed described - less light reaches the camera sensor or film as a result of the shift. So, according to the equation above, the lens focal length increases, so your f-stop # increases as well.
  12. MD2595


    Jul 6, 2007
    Fort Worth, TX

    Physics/schmisycs! My lens says that it's a f2.8, and dang it, I wanna see 2.8 in the display panel!


    Your explanation makes perfect sense. I guess shooting a couple of shots of jewlery did not bring this to light, but getting right up to a human eye makes it rear it's ugly head.

    Thanks for the time you guys took to expain this commonality.
  13. MrSLR


    Jan 13, 2009
    I understand most of what everyone wrote in this thread, so even at F3 in normal viewing distances when the aperature is forced to be switched its still F2.8

    Like if im not at 10 feet to infinity, the aperature changes to F3 till near macro ranges, even at F3, its still 2.8 or am i mistaken??

    Very interesting stuff :) 
  14. Chris101


    Feb 2, 2005
    Josh, no. The Nikkor Micro lenses are designed to accurately communicate the value of the actual f-number to the camera as they focus closer. Other close-focusing Nikkors (such as the 35mm f/2) do not. But when the 60mm says f/3, 4 or 5, it actually is getting less light onto the sensor, and thus needs to use the higher f-number to calculate the correct exposure. With non-micro lenses, the difference between the stated f-number and the effective f-number is not great enough to effect the exposure. It can change up to a third of a stop with out a noticeable difference.

    What doesn't change is the actual diameter of the lens opening.
  15. waltuba


    Oct 14, 2005
    Windsor, Co
    All of you - KEEP IT UP! why is there always something new when you think you have a speck of it down?? That's why I like this hobby?! Also this quote -
    "As the circle of light increases, so does the circumference of darkness around it."
  16. TANG0F0XTR0T


    Jun 4, 2008
    Edward, thanks for the explanations. I don't have a macro lens so this has been very informative for when (if) I do get one.
  17. [MontyPythonYorkshireAccent*]

    "In my day wi't manual lens on extention tubes or t'bellows, you 'ad to look up aperture on table"

    "TABLE! You were lucky! ..."


    *OK "At Last the 1948 Show" for purists/pedants
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