Not sure if its the lens or the camera D300s + 40mm f/2.8 macro lens

[crawfish37]

Just got a brand new 40mm f/2.8 micro nikkor lens and it is exhibiting a very odd behavior.

It seems that the aperture is stuck based on the available lighting. I can change the aperture but if the lighting is poorish it seems I can only select 3.5 as the widest open aperture. If I aim the camera at bright light I get the f/2.8 back. I am using manual mode.

I just can't figure this one out. I have tried resetting the setting to default by holding the qual and the exp comp buttons wit the green dots for more then a few seconds and it seems to reset everything except for the custom functions.

So I thought maybe the lens was defective. Unfortunately the closest best buy where I purchased the item from did not have another copy and I wound up driving to another BB about 1 hour away in order to exchange it for the only 40mm macro in all of southeast Michigan. So I get to this Best buy and there is apparently a roped off line but no one is in it. So I walked up the counter behind a gentlemen being taken care of by a young clerk. She looks at me like I am the biggest dufus in the world and says very rudely to go stand in this line. I oblige. When I get to her I explain my problem and would like to do an exchange. I went to the camera dept and met a nice fellow who asked me if it might be a setting on my camera. To which I replied it very well could be! Could I be allowed to to bring in my camera and test it. Meanwhile he brings up the potential replacement to the rude clerk and explains what I am doing. I left both lenses on the counter and ran out to the car. I came back in tested the camera with the replacement lens and bam it seems fine f/2.8 no problem. Then when I get home the issue persists. This time though I am better able to diagnose what is actually going on which seems to be the aperture is being controlled by the available light despite being in manual mode.

Now I have a sneaking suspicion that this clerk just gave me back the defective lens just because she was having a bad day and did not like the looks of me or the way I talk, whatever it happens I understand as nice as I try to be not everyone is gonna like me and I may remind them of an abusive ex, father, etc. I dunno but the two receipts that they gave me for the original and the new do not indicate that there is any difference in the information for the lens.

Now I am

[crawfish37]

This is something I just read in the manual: If the camera is in exposure mode A or M, aperture will automatically adjusted by up to 1.2 stops to maintain the same effective f-number at different focus distances. :/

What the? So weird. I guess its behaving normally. Back it goes. The whole point of this lens was to be able to use this at f/2.8 for low light macro work. Poo.