Lenses for video projectors are normally designed and manufactured for each projector. Although some manuafacturers such as Sanyo, Barco and NEC make large projectors with interchangeable lenses, the lenses are still made for specific models. There is no standard for lens mounts as yet and it is unlikely that there will be a standard in the near future.
since you need to show the image on a big screen so you require a diverging lens i.e. a concave lens
The object should be placed beyond the focal point of the convex lens to obtain a real image of the same size as the object. This distance is greater than the focal length of the lens, and the image will be inverted. Placing the object closer to the lens will produce a virtual image that is larger than the object.
used to focus the image
Image formed by a concave lens or a diverging lens is virtual,upright and smaller than the object.It is used by short sighted people.
A magnifying glass is actually a convex LENS. The object you are looking at is closer to the lens but pass the focal point therefore you create an imaginary image that is larger than the actual object. An imaginary image is an image that you cannot focus on a screen.
In a simple microscope, a lens magnifies the image of the sample. in a compound microscope, the objective lens magnifies the image (there may be more than one objective lens) and the eyepiece lens enlarges the virtual image (which is typically inverted by the objective lens).
The objective lens is used in optical instruments such as microscopes and telescopes to gather and focus light onto the image plane. It plays a critical role in determining the magnification and resolution of the final image.
The objective lens focuses and magnifies the object and transmits this image into the body tube of the microscope.
A convex lens is used in a microscope to magnify the image of the specimen being examined. The shape of the lens allows it to focus light rays and produce a larger, clearer image for observation.
An image produced by a lens is formed where all the light rays converge or appear to diverge. This point is known as the focal point or focal plane, depending on the type of lens used.
A magnifying glass typically uses a convex lens. This type of lens converges light rays to create a magnified image when placed close to an object.
The opposite of a magnifying lens is a reducing lens or a minifying lens. These types of lenses are used to reduce the size of an image or object being viewed.