0

A convex thin lens produces a real inverted image if the subject distance from the lens is higher than the focal lens.

If it is lower, the image will be virtual. Of course a virtual image appears on the subject side, so it could be photographed on that side, not on the side a camera is supposed to take a picture.

Let's consider a lens with 100mm focal length. Let's take a picture of a subject at 50mm from the lens (suppose it is a macro lens or anyway a lens which can focus the subject at such a close distance). Why doesn't the image become virtual (i.e. goes on the other side of the lens)?

3 Answers 3

1

The camera lens acts much like a projection lens it that it projects an image of the outside world onto the surface of a light sensitive surface such as photo film or digital sensor.

The power of the camera lens is expressed by its focal length. This is a measurement taken when the lens is imaging an object at an infinite (as far as the eye can see in Latin) distance, such as a star. This object is seen because it outputs light rays. The rays from an object at infinity arrive at the camera lens as a bundle of parallel rays. For objects closer than infinity, its light rays arrive as bundle of diverging rays.

As these rays traverse the lens their direction of trave is altered. This action is called refraction from the Latin to bend backwards. These light waves pass on from the lens as a bundle of converging rays. A ray trace reveals that their shape conical. Focus is achieved and an image is seen when an interpreting screen is located at the apex of this cone of light. In photography this will be the light sensitive surface of film or digital sensor.

Now the lens has limited ability to refract light. If the object is closer than infinity, the distance from lens to apex will be elongated. In other words, to obtain sharp focus for object closer than infinity we must readjust the film or sensor distance from the lens. This is the act of focusing. Should we desire to image at life-size (unity), the object will be positioned 2X focal lengths forward of the lens and the light sensitive surface will be 2X focal lengths downstream of the lens. The distance object to focused image will measure 4X the published focal length of the lens.

Another way to put this factorial: The focal length is only valid if the object being imaged is at infinity. When we image objects closer than infinity, we must adjust lens to image distance to accommodate the revised distance lens to apex of the image forming rays. This adjustment is called focus travel. We can obtain life-size or even greater magnification provided the lens and camera body provide a way to accommodate the needed focus travel.

We accomplish this ordinary lenses by dismounting the lens and inserting a spacer called “rings” or “tubes” or bellows. A macro lens is specially designed to provide the needed extra focus travel needed when imaging super close. Additionally, this act results in a loss of image brilliance that will result in under exposure unless compensation is applied. The macro lens is designed to auto offset to some degree this loss of image brilliance called “bellows factor”.

While virtual and real image phenomenon exists in the optical world, camera imaging only deals with real image, those that can be focused on film or sensor.

If you place the object super close to the camera, same distance as lens to apex, the light rays arrive at the image plane as parallel rays (afocal). These will not form an image. If you look backwards through the open camera, you will see the lens acting as a magnifier showing you a magnified view of the object.

2

Because "virtual" is not "real". The "virtual" image is just something used to allow reasoning and computation, it has no physical existence.

7
  • That's clear. But If I see it correctly, if there is virtual image, there isn't real image. So, we should not be able to take picture with subject distance less than focal length... How can macro exist?
    – Kinka-Byo
    Sep 29 at 8:42
  • 1
    A real lens isn't one of these ideally thin lenses you see in equations. On my Canon 100mm macro, the closest focus distance is 150mm from the front lens, and probably over 200mm from the equivalent thin lens (front lens is 155mm from sensor plane)... On my 35mm macro, the closes focus distance is 25mm from the front lens, which is itself 100mm from the sensor plane. so certainly much farther than 35mm from the equivalent thin lens.
    – xenoid
    Sep 29 at 9:13
  • Clear. So is the situation of virctual image practically unusual and hence never occurs?
    – Kinka-Byo
    Sep 29 at 9:23
  • 1
    As light is always going to hit the sensor whether or not there's some mysterious virtual image or not. When the image would be focussed at this 'virtual' point what you get on the sensor is simply … out of focus. Light doesn't stop when it reaches the 'virtual' plane.
    – Tetsujin
    Sep 29 at 9:38
  • 1
    @Tetsujin Beat me to it...
    – xenoid
    Sep 29 at 9:40
2

A camera takes a picture from the rays received at a given sensor plane. If this plane coincides with the plane where a real image gets projected by the lens, the sensor will pick up that image.

As a virtual image is (virtually) located on the subject side, you'd have to place the sensor there, but of course, that attempt would be complete nonsense, as the rays you'd pick up had not yet passed through the lens, so you'd get something completely unfocussed.

But, if you stay behind the lens, you can treat the virtual image like a new (virtual) subject, and project that onto your sensor using an additional lens. A close-up lens is a good example. It projects a subject at e.g. 50mm distance into a virtual image at e.g. 0.5m, to bring it into the focus range of the primary lens. Of course, you still need a primary lens to project the 0.5m virtual image onto the sensor.

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.