With a good quality camera lens, the sharpest image is usually obtained by stopping the lens down two stops from wide open. Beyond that, resolution will go down as the lens is stopped down. This "inverted U-shaped curve" is due to two competing effects: the residual aberrations present in even the most expensive camera lenses become less important as the focal ratio increases/lens shrinks but at the same time the effects of diffraction start to reduce resolution as the lens is stopped down.
Think in terms of a 6" f/10 newtonian telescope with a spherical mirror. AT F/10 it hardly matters optically that the mirror is not parabolic, the sphere is a good enough approximation of a paraboloid and images will be good. At F/5 the 6" sphere is a poor approximation of a paraboloid optically and the images would be very poor, with lots of spherical aberration. The principle is the almost the same with camera lenses used wide open--they are imperfect and need to be stopped down to get past these imperfections.
But, as you further stop down on a camera lens, the lens becomes very small, and as you know, a big lens has higher resolution than small lens (all other things being equal). As a result, the star images will start to bloat and resolution will go down (depends on pixel size, focal length too). This is all caused by diffraction.
This is shown remarkably well at this link:
Lens Reviews: Digital Photography Review
Lets choose to view the data on the Canon 100 mm F2.8 older (non-L series) macro lens, which is a great wide field astro lens:
dpreview.com - Lens Review - Fullscreen
You will see a chart that shows the resolution of the lens (in lines 50% resolved across the narrow side of the sensor, the metric is not important at this point) across the field of view, starting at the center on the left side and moving to the corners of the sensor on the right. The higher the line is, the higher the resolution.
Notice the F/stop "wheel" at the bottom of the page--move your cursor there, click and hold and you can "stop down" the lens by moving the mouse to the left. Watch what happens to the resolution line--first, when stopping down from F/2.8, the resolution rises, until about F/8, at which point the resolution will start to fall off with each increase in F/stop.
Then go to
Canon 100mm F2.8 L IS USM Macro Lens Review: 3. Test results (APS-C): Digital Photography Review which is the new L-series 100 mm macro lens. Right off the bat is is clear that this lens has higher resolution wide open at F/2.8, resolution peaks at astro? >$1K to find out!
Just to further entertain yourself, go here
Canon EF 50mm F1.8 II Lens Review: 4. Test results (Full Frame): Digital Photography Review and look at the data for the cheap Canon 50 mm F1.8 lens on a full frame camera. Note that resolution is very uneven at 1.8, very good in the center but worthless in the corners--this is indeed a cheap lens. Maximum corner resolution rises until >F/8, but by that point center resolution is falling quickly.
Here is another way to look at it. Use the tool at
Diffraction Limited Photography: Pixel Size, Aperture and Airy Disks which allows you to see how many pixels a "perfect" star (Airy disk) would cover on different sensors and different f/stops. Click on a small sensor camera (Canon Powershot G6 is good) and then scroll down the F/stops in the left column. You can see the star bloat as you move to higher f/stops. It starts with all the light on one pixel and bloats up to dozens of pixels as you stop down.
Increasing the f/stop in a telescope via a barlow lens will also "bloat" the star size at the sensor, but the scale will increase proportionately, so unlike a camera lens, no resolution is lost.
The star test is the most rigorous test you can subject a lens to. No camera lens I have tested (up to $2000) works well wide open. All have to be stopped down to provide good astro images. If you start with a fast, high quality lens, you can usually get by with a 2 stop reduction, e.g. from F 2.0 to F 4. A poor lens will likely be slower to start with and will require 3 stops reduction, and may still produce inferior star images.
This is where high quality astrographs excel, they are designed to be used wide open and provide excellent images.