If it is second hand make suure all custom functions are off.
Also, check which mode you are shooting in, Jpeg, Raw, etc.
Raw will give you a photograph without any adjustments performed by the camera.
I have a 1DMKII and the images are softer when out of the camera, this is to limit noise, but a slight bit of sharpening in photoshop brings it up. This might be the same with your 1Ds.
........there are also several settings for Jpeg, from low to high, high will give you the best image, but takes up more card space