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The max aperture on my Tokina 100mm at-x pro is f/2.8 but when I focus on anything closer than around 9 feet, the aperture automatically starts to close down and get progressively smaller (higher f-stop) the closer I get to my subject. Extreme macro shots result in the aperture being hard set to f/5.6 I realize the closer you get to the subject, the more shallow the depth of field, and that 2.8 is probably too large on macros, but I've never experienced where the camera deems the appropriate aperture (IN MANUAL MODE).
what's it doing? How is it determining this ideal f-stop? Thanks!

Nikon D610 by the way...

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  • Strange. I've never seen that happen on any macro lens but I have not used any third-party ones either (only Canon, Nikon, Pentax, Sony), so I'm very curious what is going on. – Itai Apr 23 '16 at 2:05
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As the others have alluded to here, the aperture number (aka. f-stop) indicated on the physical aperture ring only indicates the effective aperture when focused in the non-macro range - i.e. at distances greater than perhaps 1 meter or so. The reason why the camera reports a narrower aperture than the lens setting is because of the effective lengthening of the lens when focusing in the macro range, making the effective aperture smaller than you might otherwise expect.

In other words, your 100mm lens effectively becomes a 100~150mm lens when focused up close, and the area of the aperture is therefore smaller in proportion to the length of the fully extended lens.

You can visualise this effect simply by looking down a cardboard tube. Cut the tube in half and look down it again; the diameter of the tube hasn't changed but hole at the other end appears effectively much larger simply because the tube is shorter.

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Like most macro lenses, the f/2.8 aperture is only for large distances. As you get closer, the actual aperture of the lens changes.

It's just how macro lenses work.

http://www.kenrockwell.com/tokina/100mm-f28.htm under Falloff shows that this specific lens only does f/5.6 at 1:1.

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    That is not how Canon Macro lenses work. They are constant f/2.8 at any distance. The shutter speed will need to be slowed down as you get to 1:1 but the aperture reading stays at f/2.8 – Mike Sowsun Apr 22 '16 at 22:57
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    That is how Canon lenses work. It is just not how Canon lenses report aperture. – Michael C Apr 23 '16 at 2:57
  • Please see page 8 of the MP-E 65mm 1-5X Macro Manual for how this works out with Canon Macro lenses. – Michael C Jan 27 '18 at 20:46
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That sounds like normal behavior for macros (and even some telephotos I've noticed). My Tokina 100mm does the same thing. As you get closer to your subject and focus closer to the lens the aperture will change.

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