With an older manual Nikon lens, the camera usually has no electronic or mechanical communication with the lens. That means the Canon body can’t know what aperture the lens is set to, and it can’t stop the lens down itself.

Because of that, the fully automatic modes won’t work properly—they expect the camera to control the aperture. But metering can still work in limited ways:

• M mode: you set aperture and shutter yourself, but the exposure meter in the viewfinder can still guide you.
• Av mode: on many Canon DSLRs, the camera will show no aperture value, but it can still meter the light coming through the stopped-down lens and choose a shutter speed.

So the issue isn’t that exposure is impossible; it’s that the camera loses lens information and aperture control. Set the aperture manually on the lens first, then meter and shoot using M or sometimes Av. Compatibility can vary with the lens age and adapter.