How the object's position effects the image

From Lumenlab

The Three Ways That A Positive Lens Can Be Used:

Now that we have identified each path we can have a closer look at them. The object can be beyond the focal length, at the focal length or before the focal length and its position will effect how or if an image is formed.

Object beyond the Focal Point: Fig 3a shows the ray paths using the letter ‘A’ as the object. Because the object is placed beyond the focal length the lens creates a real image that can be projected onto a surface. In this figure two different points on the letter “A” are being traced, the very tip in blue and the middle in green. In reality there are an infinite number of points on the object so for simplicity only the tip of the object is all that is needed to find its image position and size. Some things to observe in this figure are the position of the object in relation to the focal point and that the image is inverted.

Fig 3b shows what happens if the object is move nearer to the focal length. The image becomes larger and further away from the lens but still inverted. Fig 3c shows the object moved further from the focal length. This time the image becomes smaller and closer to the lens.

Object at the Focal Point: Fig 3d shows the object exactly at the focal length. All of the rays leaving the lens will be travelling parallel to each other so they will never converge to a point to form an image. This is basically what happens with the rear fresnel in our projectors before the rays pass through the front fresnel.

Object before the Focal Length: Fig 3e shows the object positioned before the focal length. When the object is in this position a non-inverted virtual image will be created on the same side of the lens as the object. Because the rays leaving the lens will never converge to a point the virtual image can never be projected onto a surface but it can be used as an object for another lens. This is what happens when the field fresnal is placed just after LCD, it will create an enlarged virtual image of the LCD and the triplet lens will use the virtual image instead of the LCD. Another example of a virtual image is when you use a lens to magnify newsprint. The lens is creating an enlarged virtual image of the newsprint and the lenses of you eyes refract the rays to form an image on you retina.