This is completely normal, and all lenses do it. It's just most noticeable at high magnification, and so most people ignore it for non-macro lenses..

My Canon 60mm macro lens has a data sheet that specifies the "effective aperture" at different magnifications (I don't have it here at work, so I can't tell you what the figures are)..

Http://doug.kerr.home.att.net/pumpkin/Photographic_Optics.pdf is a good primer on the theory behind it. According to this, at 1:1 magnification your effective aperture is halved, so your minimum effective aperture becomes f/5.6..

Chris..

With macro lenses it moves through very fine adjustments when focusing near the closest focus range. They do that so that during a macro photo you will be more likely to get an accurate focus. The down side is that it is necessarily slower..

As mentioned, the aperture shift is also normal. Your lens is almost certainly working properly.'Be right, fearless, faithfull, and true to others...'T.S. Elliott..

Japc wrote:.

Just bought a Tamron 90 2.8 Di Macro lens for my Nikon D80 andnoticed that it slows down a lot when focusing 1:1..

There's a lot of travel from infinity to 1:1 and half that travel is in "close range" so the lens moves as much to focus from infinity down to some number of feet as it does from that same number of feet down to 1:1 ... so in one case, n revolutions gets you hundreds of feet, up close those same n revolutions gets you a few feet..

I had the ideia that this lens had a constant aperture (well, itmentions just an aperture, if weren't constant it would mention therange) so I don't understand. Isn't the lens functioning as itshould? Shouldn't it be 2.8 when focusing 1:1 (closer, bigger lensextension)..

I'm not sure what you're asking. What makes you think it isn't f/2.8 ? Aperture has nothing to do with AF speed. Well, technically, a big aperture lets more light in so that the AF sensors have an easier time locking on a subject, but that doesn't change how fast the camera can physically focus the lens (or cause the lens to focus)..

- DennisGallery at http://kingofthebeasts.smugmug.com..

But slow I mean it has a closer aperture, I needs a slower shutter speed...

Ok, thanks a lot for everyone for the replies..

I found the following a very nice and enlightening read about this:.

Http://www.nikonians.org/...m=1927&forum=DCForumID6&viewmode=threaded.

Http://doug.kerr.home.att.net/pumpkin/Photographic_Optics.pdfhttp://www.wrotniak.net/photo/tech/dof.html..

Japc wrote:.

But slow I mean it has a closer aperture, I needs a slower shutterspeed..

Ahhh, ok, I took "slows down" as you "focus close to 1:1" as meaning it actually focusses more slowly..

I never really paid attention to the fact that you're getting less light in at close focus (despite the constant actual aperture) ... I'd just setup a shot and set my exposure without ever having the opportunity to see a change from far focus to close..

That site you provided a link for was interesting (though I got a 'not available' for the actual formula) ... I was able to put it into a context I understand better, which is extension. To get close focus with a non-macro lens, you can use extension tubes which lose light. When a macro lens focusses toward 1:1 it does so via extension, so it makes sense that you lose light. There are some macro lens designs (the Minolta 200/4 for instance) that do not extend when focussing to 1:1, instead, they sort of zoom so that by the time you reach 1:1, the 200mm lens is really a 135mm lens. With a constant actual aperture, this probably allows the effective aperture to remain the same (or close)..

So thanks for asking - I learned something !.

- DennisGallery at http://kingofthebeasts.smugmugcom..