f51 from a f32 lens - How's this possible?

I just purchased a new Tamron Sp90mm Macro lense for my Nikon D70S. The lense is rated at a max of f32, but I'm getting the aperture to go down to as low as f51 in the view finder diplay. Camera is in in "M" mode and lense is locked at the f32 mark on the lense barrel (which I belive means the camera controls the aperture).

It seem also that when I go to this small aperture, I can see ALL sorts of dust and junk in my images which I assume is on the bypass filter.

What gives? I shoot a lot of macro and I'm always open to a little more DOF, but this seems wrong.

Thanks for any ideas,

Phil G.

Macro lenses change their 'effective aperture' as you focus closer. This effective aperture gets smaller as most macro lenses actually change focal length too as you focus close. Some lenses do not report this, but all Nikon brand macro lenses and some 3rd party ones like your Tamron do.

FYI, f32 would be the minimum aperture. The larger the number, the smaller the aperture.
F5.1 would be a much larger aperture than f32. Therefore, F32 would let in a lot less light, thus the need for much longer shutter speeds.

shutterbug, it's f/51, not f/5.1

I had never heard of a lens with this small of an aperture. That is tiny. The dof must be incredible.

Harrison,

Thanks, good info. Any thoughts as to why I see so much "junk" (dust, etc) in my images when the aperture gets real small? I had'nt noticed all of that stuff before I got this lens.

I'm tryin to track down a cleaning kit (sensor swabs and the e2 liquid), but had to order it through my local photo store, not available for shipment to me from the usual internet sources.

Phil G.

the smaller your aperture, the more DOF. the more DOF, the more of the scene is in focus. And of course, anything between your sensor and the scene. So the dust particles start showing up. If you're using f/2.8 they're still there, they're just blurred out so much you can't really see them.

Thanks Brent. I can always count on getting good answers here.

Phil G.

Quote:

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Originally Posted by mn shutterbug

I had never heard of a lens with this small of an aperture. That is tiny. The dof must be incredible.

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I have view a Camera lens with a F stop of F96.... and F128 is not unheard of.

Quote:

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Originally Posted by PhoebeG

I just purchased a new Tamron Sp90mm Macro lense for my Nikon D70S. The lense is rated at a max of f32, but I'm getting the aperture to go down to as low as f51 in the view finder diplay. Camera is in in "M" mode and lense is locked at the f32 mark on the lense barrel (which I belive means the camera controls the aperture).

It seem also that when I go to this small aperture, I can see ALL sorts of dust and junk in my images which I assume is on the bypass filter.

What gives? I shoot a lot of macro and I'm always open to a little more DOF, but this seems wrong.

Thanks for any ideas,

Phil G.

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First M stand for Manual, focus and aperture. With micro photography you focus then stop down for you DOF. It depends also one what your taking the micro of on how far down you can stop down. If it is moving you going to have to put up with less DOF.