For parallel rays passing through a simple lens one finds that the image is produced at the focal distance of the lens and that the angular size of the image is rd = θS f.
In a camera obscura one can use a lens to focus the image and increase the aperture size so that the image is brighter and has more lines of resolution. The drawback is that the focal length of the lens is fixed. Using a 50mm, f = 675mm telescope lens one gets the following results for a camera obscura.
The aperture is relatively large and so one gets a much larger number of lines of resolution for the image. The problem is the small angular diameter of the Sun which makes the image size a little too small. One can try a lens with a longer focal length such as one from a pair of 0.25D computer glasses instead. The image is focused a 4 meters from the aperture and has an angular diameter of 37 mm or about 1.5 inches.
The small width of the computer glasses limits the diameter of the aperture to about 20mm so the number of lines of resolution will be reduced. The problem is finding a suitable low power lens that has a rather long focal length.