Prior to the serious introduction of autofocus in the 1980s (1986 for Nikon with the F-501 / N2020 and 1988 with the F4) cameras were designed with manual-focus in mind. They included fine-grain ground-glass focus screens, split-prism and microprism rangefinders, and electronic rangefinders.
Naturally these features have been removed over time as autofocus has been implemented throughout the product range. For autofocus users this isn't a problem, but if you want to (or have to) use manual-focus lenses with your D40-series camera they'd be a great help if they were there.
Thankfully you do have some options if you want technological help with using manual-focus lenses:
- Use a Magnifying Eyepiece
The Nikon DK-21M Magnifying Eyepiece enlarges the viewfinder view by a factor of 1.17, allowing you to check focus more precisely on the ground-glass. You can also use the DG-2 Magnifying Eyepiece, but you'll need a DK-22 Eyepiece Adaptor to fit it to the D40-series bodies.
However, as these devices magnify the whole viewfinder, you won't be able to see the whole picture without moving your eye (the DG-2 can be flipped out of the way, though).
The Right-Angle Viewfinder DR-6 also includes a 2x magnifier.
- Use a Split-Prism Focus Screen