Question of F-Stop and clearest shot

I have a non high quality lens for my canon 400D. Since I can't upgrade due to the economy etc, perhaps someone here can clarify this for me. I read elsewhere that for the clearest shot possible, there is a "sweet spot" F-stop that will give you the clearest picture as possible.

I have a mid to low quality lens and it is a 28-70mm lens. F4.5 - f22.

is there a calculation to figuring out what F-stop would be the sweet spot for this lens? out in the field, I DARE NOT use my laptop out in the field so I'll have to calculate what that setting will be.

any advice? pro/con on this issue?

thanks,
Chris

Lenses are generally at their sharpest about 2 stops below their widest apertures. So you'd want to be at around f/9. Much wider apertures and aberrations begin to be more visible; much smaller aperture and you'll begin to see diffraction.
The "sweet spot" isn't the same as Depth of Field. You don't need to recalculate DoF all the time either though. Just, especially with smaller apertures like your lens has, calculate the "hyperfocal distance" at the preferred aperture.

You can find the sweet spot for any lens with a simple experiment.

Set the camera on a tripod or on a table top, anyplace it won't move.

Set the exposure mode to Av, so you choose the aperture and the camera chooses the shutter speed.

Focus on something (anything) as carefully as you can.

Take a series of shots starting at the widest aperture (f/4.5 for your lens), and decreasing the aperture 1 click for each shot all the way down to the smallest aperture (f/22 for your lens).

You now have a series for shots of the exact same subject for each aperture of your lens.

Download these shots to your computer and examine them to find the image that is sharpest. The exposure used for that shot is your lens' sweet spot. Use that aperture as much as possible for general shooting. But don't be afraid to use other apertures when the circumstances so dictate.

The only thing 2D left out of above, is that when you examine them, you need to use a viewing software that you understand and can set up to display the shots "at 100%" size. I normally just use my browzer, and open two windows, resizing them side by side to occupy half the screen. I can then just open up the shots, starting with wide open, comparing down each aperture and it will be obvious to you which is sharper.

I appreciate the help. I go on a shoot Thursday morning early. I'll set my Fstop to F9.

What FL setting for your use of this lens? photozone.de shows best MTF is at f/5.6 for shortest and middle FL, but f/8 at longest FL

I use usually 28mm. its a 28 to 70 mm. 28 allows for the best wide angle shot