Trying to fix the blur I see when shooting via a 75-300mm telephoto lens (this one) on a tripod, I enabled mirror lock-up in my Canon EOS 500D, and set a 2-second self-timer, so that after raising the mirror the camera made actual shot after a 2-second delay.
But whatever I try — releasing shutter from LiveView, doing shots without LiveView — I invariably get blurry photos. All this despite seeing a much sharper image in LiveView.
Below is an example of the shots I get with different focus points (which I adjusted using focus single-stepping commands by gPhoto). I shot at 300 mm focal length with aperture f/5.6 and exposure of 1/25 s. Test target was at 3.8 m away from the lens (this is the test environment at home, the problem reproduces with 50 m away objects outdoors). I get basically the same results with and without MLU.
Here's the sharpest version I get in the LiveView:
Below is the comparison of a shot with exposure of 1/25 s and aperture f/8 (blurry frame) vs a shot with 1/60 s exposure with aperture f/5.6 (well-focused frame). Both were taken with MLU enabled.
What could be the reason for this inability to get a sharp image when shooting? Is there any other tool than MLU to help me make exposures of 1/25 s usable?