Unwanted Vignetting - 40D/Tamron ???

[AFABS]

I've notice on a few of my shots that there's a unwanted vignette on the bottom left and upper right corners of the frame. I have a Canon 40D and a Tamron 17-50. The instances occur when shooting at 17 and at night...so the exposures are somewhat long, half a second to about 5. I think for a few of the shots I had my lens hood on, so maybe that's the problem? However I shoot during the day....short exposures.....with my lenses hood on with no issue at 17mm. Anyone have an explanation

Examples:

This one's got it pretty bad ^

 

[Edge100]

Can you take a picture of a white wall or something so that we can see the vignetting against something a bit brighter?

I'd say this looks abnormal, esp. considering it's most visible in the lower left and maybe top right, and not so much in the other corners. Vignetting should be almost absent on a 1.6x crop body like the 40D./

 

[compuwar]

The instances occur when shooting at 17 and at night...so the exposures are somewhat long, half a second to about 5.

It could be a lens hood or filter blocking the light. Long exposures will show it more- try w/o the hood and/or filter and see if it makes a difference under the same sort of time conditions. 17mm is wide, and you have to be careful about anything blocking part of the light, especially if your illumination is off-axis.

It could just be that you're seeing the flaws of the lens in longer exposures too-

http://www.bobatkins.com/photography/reviews/tamron_17-50_review2.html

 

[toxic]

it's in the wrong place for hood vignetting, unless you have it on strangely...

 

[AFABS]

well...I noticed that the lens hood doesn't go on perfectly, it's offset a bit (the dots that mark where they should line up is off a bit) leaving the hood a little skewed. this leaves me to think that this might be the problem because heres a shot i took another night at 17mm with an exposure time of ten seconds...i'm 90% sure i had the lens hood off for the shot. maybe tomorrow ill get around do doing some shots at a white wall or something with and without the hood on and at various exposure times and focal lenghts.

Here it is

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