Aperture Q

Discussion in 'Photography Beginners' Forum' started by mykill, Apr 18, 2007.

  1. mykill

    mykill TPF Noob!

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    I have a nikon d40 with kit lens...18-55mm 3.5-5.6 i believe are the specs...

    ok well i can adjust the focal length? (zooming) but how does this effect the aperture and doesnt it change when i change the focal length?

    I know my camera has an aperture of its own but doesnt the lens aperture matter as well?

    I hope this is some what clear...?...
     
  2. dwarrick

    dwarrick TPF Noob!

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    you can control the amount of light that come into the camera 2 ways: adjusting how long the shutter stays open (shutter speed) and changing the aperture. the aperture in the hole in the adjustable diaphragm set between the lens and the shutter. light coming thou the lens is funneled thou this hole and to the shutter then the sensor. so if you want more light you make the aperture bigger, if you want less make it smaller.

    Hope this helps in some way
     
  3. Big Mike

    Big Mike I am Big, I am Mike Staff Member Supporting Member

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    Only the lens has the diaphragm (aperture)...although it can be controlled via the camera.

    The F number used to represent the aperture is actually a ratio of the focal length and the diameter of the aperture opening.

    So when you zoom the lens, you are changing the ratio...and therefore changing the F number. In the case of your lens...the actual aperture probably doesn't change size from 18mm to 55mm...but the focal length does, so the max F stop changes from F3.5 to F5.6.

    High end zoom lenses will often have constant max apertures (F2.8). In this case, the ratio stays the same...which means that the aperture opening has to get bigger as the lens zooms out. This means that these lenses are bigger, heavier and more expensive.
     
  4. mykill

    mykill TPF Noob!

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    ok...
    so when using a zoom lens (like mine) what is the point of shooting in aperture mode if i can change the aperture with my lens?

    or do the aperture for the camera and the lens function seperately?

    or am i just misunderstanding things becuase I think I have confused myself
     
  5. Dylan

    Dylan TPF Noob!

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    When you have a lot of light on the subject you can change the aperture so the subject isn't washed out. So say you're shooting something outside and it's high noon. The subject is very well lit so you change the aperture to f/13 (that's just an arbitrary number and not to be considered absolute) to limit the amount of light in your exposure. Maybe one of the senior members can explain it better but that's my best.
     
  6. EOS_JD

    EOS_JD TPF Noob!

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    If you select an aperture of f5.6 or smaller (higher fnumber), the aperture value will not change when you zoom out or in. Try selecting f8 in Aperture priority mode. Zoom out and it will remain at f8.

    The only time zooming will affect on your aperure is when you open up to the widest setting of 18mm. At the long end (55mm) the largest aperture you can attain is f5.6 and at 18mm you can make this f3.5. The aperture value of f5.6 is still selectable at 18mm but you can get that ectra speed wide open if needed.

    I'm sure it can be explained more technically but focal length will not affect the aperture at anything other than wide open (largest aperture) and zooming at between 18mm and 55mm the lens will move from f3.5 - f5.6 as you zoom out. Try it and see that happen. Select Av and set camera to smallest number f3.5 @ 18mm. Now zoom out slowly and watch the aperture value change till it reaches f5.6 and then it stays there - you cannot change it to a smaller number but you can select other aperture falues all the way down to f22 I believe.

    Now set the aperture to f5.6 or f8 or any other fstop and do the same....
    The camera should now have a constant value all the way from 18-55

    Hope that helps.

    Jim
     
  7. mykill

    mykill TPF Noob!

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    actually your answer was perfect for me...

    thanks everyone who replied they all helped
     

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