Orientation blur

[Suede]

I am using a D700 with some older manual focus lenses. The combination works fine ( photographs are acceptably sharp) when photographs are taken in the landscape orientation. In the portrait orientation, I find that the photographs appear distinctly blurred (soft). This does not happen when using the later-manufactured Tamron. This lens is from the 'digital' generation.

Can somebody help shed light on this peculiar phenomenon ? Or is it just that older lenses are not optimised for digital cameras ?

Thanks in advance for any inputs.

~ Pritam

[Black Pearl]

Optimised or not they wouldn't do that - stick it on a tripod, focus it landscape then flip the head 90 degrees. If its still sharp.....which it will be......its you.