Macro Lens won't go down to f/2.8?

[RCAnderson]

Recently decided to get myself a Macro lens for my D7000, and I ended up deciding on the Tamron 90mm f/2.8 Macro.

Problem is, when I've been trying it, it's not been able to reach f/2.8 when focusing close up (it will do when focusing at infinity though)

Is this normal, or is this due to going for a (reasonably) budget Macro lens?

I seem to get max f/3.5 when focusing at macro distances. Does this actually make that much difference to the depth of field?

So, mainly just wanted to know if my lens is working as it should, or if I should be looking at getting it replaced?

 

[Dogfish_magnet]

They all do that !

All macro lens do this , but not all bodies report the correct aperture, as you focus closer the actual focal length of the lens also changes - when shooting around 1:1 you want to be around f11-f16 to maximise DOF

 

[RCAnderson]

They all do that !

All macro lens do this , but not all bodies report the correct aperture, as you focus closer the actual focal length of the lens also changes - when shooting around 1:1 you want to be around f11-f16 to maximise DOF

Ah, ok, cheers, I was hoping that would be the case!

 

[GHP]

The Nikon system computes and displays the effective aperture. If you set the aperture to the maximum aperture of f/2.8 and turn the focussing ring to the minimum distance, the camera will display f/5 (or whatever) and use that number when determining exposure. This is intended and correct behaviour and certainly not a malfunction.
As said above though, you don't really want to be shooting too wide open up close, as the dof will be so shallow you will struggle to see anything in focus in the image !!

 

[HoppyUK]

In theory, the lens' f/number is only true at infinity, and reduces by two stops at 1:1 - ie f/2.8 becomes f/5.6 for exposure purposes, but remains at f/2.8 for depth of field calcs.

Nikon reports this change, but Canon does not. They are both equally right and wrong, but the result is the same either way.

 

[modchild]

I wouldn't worry too much about using f/2.8 with a macro lens anyway as you'd hardly get anything in focus. I've got a canon 100 2.8 L and it never gets used wider than f/5.6 anyway. Most of the time in between f/8 and f/11 to get the DOF I require to get a good shot. Might be helpful when focus stacking but I've never tried this either.