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Possible Duplicate:
Why do zoom lenses and compact cameras have varied maximum aperture across the zoom range?

Just starting out with lacrosse pictures on my new Canon EOS 60D and zoom lens EF 70-300mm f/4.0-5.6 IS USM. Why can't I set aperture at 4.0 and it stay there? Why does the aperture get smaller as I zoom in? I just discovered that this happens. I just thought I couldn't dial to 4.0 and couldn't figure out why wouldn't go any larger. Doing all in Manual mode. I hope this isn't as stupid as it sounds, but I think when I learn the answer I will have a lightbulb moment and really grasp this. Thank you for anyone's input.

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    \$\begingroup\$ pretty much an exact duplicate of photo.stackexchange.com/questions/14492/… and photo.stackexchange.com/questions/13918/… \$\endgroup\$ Apr 16, 2012 at 18:06
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    \$\begingroup\$ Welcome to the site, Michelle. It's not stupid at all, and hopefully the links ElendilTheTall provided will answer your question. If not, or if you have others, feel free to ask. This is a question and answer site, so apologizing for asking is never necessary. \$\endgroup\$
    – mattdm
    Apr 16, 2012 at 18:12
  • \$\begingroup\$ You will need a premium lens such as the "L-series" 70-200mm to maintain a constant aperture. \$\endgroup\$ Apr 16, 2012 at 18:47
  • \$\begingroup\$ @Jakub: sorry to nit-pick, but that's not quite right. There's at least one Canon zoom that is constant aperture throughout the zoom range, but is not an L: the 17-55 F/2.8. It is true that most of the "L" zooms are constant aperture, but not all of them are (for example, the 28-300 f/3.5-5.6 L and the 100-400 f/4.5-5.6 L). \$\endgroup\$
    – djangodude
    Apr 16, 2012 at 23:42
  • \$\begingroup\$ No worries @djangodude. You are right. Thanks for correcting that. I didn't mean to imply that he must buy an L lens. Personally, I would consider the great 17-55 f1.8 a premium lens although it is not an L. \$\endgroup\$ Apr 17, 2012 at 0:01

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