Question about narrowest aperture for auto focus... Please help!

Started Apr 11, 2010 | Discussions thread
Brooks P Veteran Member • Posts: 9,183
Ken has it right

If a specific AF system needs at least f/5.6 in order to AF reliably then the maximum aperture must be at least f/5.6. A lens that is f/8-11 won’t be reliable; the lens may still AF but frequently you will experience “hunting”.

With your Sigma f/3.5-6.3 lens regardless of what you set the aperture to you are focusing with an aperture between f/3.5 and f/6.3. At 18mm the aperture will be f/3.5 while focusing even if you have selected f/22; the lens will stop down to f/22 when you fully press the shutter release. At some point in the zoom range the affective aperture on that lens will surpass f/5.6 and by the time you reach 200mm you are focusing with a f/6.3 aperture, but that is only a third of a stop past f/5.6 and your D5000 should be able to handle it just fine when the light is good – not so well in poor light.

Set the aperture fully open. Now remove the lens from the camera and look at the back of your lens and you will see a small lever, slightly clockwise from the contact points. If you move this lever with your finger you can watch the aperture open or close. It is this lever that the camera uses to close the aperture when you take a picture.

A TC doesn’t actually change the aperture but it does lose some light, generally one-stop on a 1.4x, one-and-a-half-stops on a 1.7x, and two-stops on a 2x. This loss of light effectively changes your f/3.5-6.3 lens to a f/5-9 lens with the addition of a 1.4x TC; f/9 will probably severely impact the AF capabilities of your D5000. A 2x TC would change your lens to a f/7.1-12.7 lens.

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While amateurs change the camera’s settings; many Pro’s prefer to change the light.


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