Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

B Image is inverted when I look into a projector lens -- Why?

  1. May 31, 2017 #1
    I'm using a film projector as an editor, meaning I run the film slowly and use a low power bulb. I want to minimise any possibility of damaging the films.

    To view a film I simply look straight into the lens – and I see an inverted image.

    What I can't explain is why the projected image appears the right way up. So, sitting in front of the projector:

    • I can see the frames entering the gate are upside down
    • Looking through the lens the image is upside down
    • But replace my eye with paper, and the image is right-way up.

    Why does light that strikes paper cause the image to appear one way, yet when it strikes my eye it appears the other way?
     
  2. jcsd
  3. May 31, 2017 #2
    Your eye introduces an extra lens in the path of the light rays.
     
  4. May 31, 2017 #3

    rcgldr

    User Avatar
    Homework Helper

    Is your eye the same distance from the lens as the paper?

    I'm wondering about the case of a non-inverted image projected onto an opaque screen, viewed from the back side of the opaque screen: the image is reversed right / left since you're looking at the image from the back side of the screen, but the image is not inverted.
     
  5. May 31, 2017 #4

    sophiecentaur

    User Avatar
    Science Advisor
    Gold Member

    I don't think that's relevant. Your eye lens is always there and it inverts the image you see on the paper and the image formed by another lens (and the rest of the world too)
    This link shows the standard ray diagrams for showing the images formed in a convex lens. You are looking at a virtual image when looking into the projector - using the projector lens as a magnifying glass. Using a magnifying glass involves placing the eye nearer to the lens than the focal distance. This gives a magnified non-inverted image of the film image at a greater apparent distance than the film, which allows you to view it comfortably. (But the film is always loaded upside down in the gate so it looks upside down - which it is). When that same lens is used at a projector, the rays that your eye was intercepting, will cross the axis and form a real inverted image. Inverting the inverted film gives an upright image on the screen at a great distance from the projector. (A few double negatives there so you may need to run it though a few times.
     
  6. May 31, 2017 #5
    Using a magnifying glass involves placing the object a focal length or nearer to the lens. That produces a virtual image. The position of the eye is not critical other than closer to the lens would increase the field of view.

    Perhaps OP can provide a little more detail - focal length of lens, distance of paper to lens, distance of eye to lens when "looking through the lens." When looking through the lens, is the image sharp?
     
    Last edited: May 31, 2017
  7. May 31, 2017 #6

    Drakkith

    User Avatar

    Staff: Mentor

    See the following image:

    sc00b4ddd9.jpg

    Notice that the light rays from the bottom of the slide are heading upwards. Once they hit the screen the rays reflect in all directions and the portion of the reflected light entering your eye head down (Just reverse the arrows and imagine that the projection lens is your eye). These rays then come to focus on your retina, with the rays from the top of the image being focused at the bottom of the retina and the rays from the bottom of the image being focused at the top (imagine the slide is your retina).

    So the light from the bottom of the slide ends up being focused at the bottom of the retina and the light from the top of the slide is focused at the top of the retina. The slide is inserted upside down (according to sophiecentaur) so the image appears "normal" when viewed.

    If you put your eye directly in the light path, the light rays exiting the projection lens don't reflect off of the screen first, so the rays from the bottom of the slide are focused at the top of the retina. In other words, the slide (or film, or whatever) appears the same orientation as it does when viewed without the lens but since it is loaded upside down it looks upside down.
     
  8. May 31, 2017 #7
    Thanks for the responses. Here are some more details and three images in a zip file (http://www.mediafire.com/file/mdss54izyypvx4t/Projector_Lens.zip).

    I'm not using the original optics – too much heat on these old films for my liking. So I experimented with my Zebralight head torch (see http://www.zebralight.com/H502d-L2-High-CRI-Daylight-AA-Flood-Headlamp_p_142.html) which gives out 190 lumens at maximum output. Amazingly, I can get it to fit in place of the existing bulb without modification. It just slides in. Two problems: the torch is a 120º beam type ('flood'); and I would have to keep changing batteries.

    To save changing batteries, I had the idea of using the Zebralight on its lowest setting (several weeks run time) and looking through the lens.

    1. The lens is on a Pathescope GEM 9.5mm projector.
    2. The manual says the lens will throw an image 3 feet wide at a distance of 12 feet.
    3. The glass part of the lens is ~15mm wide and 20 mm long. The minimum distance from glass to film is ~20mm; maximum ~40mm. There is a gap of ~30mm from the glass to the outside of the lens cover.
    4. Looking through the lens: Eye to front part of glass is 30mm. Eye to film is ~60mm.
    To focus the image I have to move the lens all the way in. The rear end of the glass is then ~20mm from the film.
    5. Projected on paper: Film to paper distance is ~300mm. Very blurred unless I refocus the lens about 20mm outwards.
     
  9. May 31, 2017 #8
    Just to elaborate on Drakkith's post, there are two inversions going on here. First, the lens forms an inverted image of the slide on the screen. Since the slide is upside down, the image on the slide appears right side up. When viewing the paper, the eye actually forms an inverted image of it on the retina. That's how we normally view things, but our brain adjusts so things appear right side up to us. So the slide image projected on the paper appears right side up.

    When looking directly into the lens, you're putting your eye in converging beams of light so that's why I asked if what the OP sees is in focus.

    Edit: Just saw OP's new post. Too late at night to think about it now. :smile:
     
  10. Jun 1, 2017 #9

    sophiecentaur

    User Avatar
    Science Advisor
    Gold Member

    I really have a problem with introducing the Eye Optics into this explanation. The eye 'sees' everything in the same way and an alternative imaging system (a crude system of tubes, pointing in different directions, for instance - like a fly) would never involve an inverted image. Never mind the relative acuity of the two systems; we assume they would both be perfect and the same thing applies 'outside' the eye.
    Here we go again. Both modes of viewing have the same inversion in the eye. There is absolutely no significance in this to the brain. It (evolution) has learned that stuff in the top right of the scene turns up at the bottom left of the retina. It never sees it any other way.* Forget about it; it just adds complexity. It just involves 'double negatives', which are best avoided.
    The well known diagram of a projector optics is correct, of course but it's made far too complicated because it includes the mechanism which maximises the illumination from the lamp. We basically have a single convex lens to do the imaging. The only relevant difference will be the use of a 'telephoto' arrangement to achieve a shorter projector lens for the wanted Focal Length. Refractor telescopes do not use this method because there is a price to pay in quality / money.
    Read the OP's description of his system. In particular, he says
    This answers the earlier question about my 'reasoning' applied to the problem. A projector is normally focussed not far from 'infinity' (around 20mm compared with '12 feet') and the film would normally be not far from the focal plane of the lens. He screws the lens in a bit - to less than the focal distance from the film and he then has a Magnifying Glass, with a virtual upright image.

    Did you never look at the back of a plate camera, with the focussing screen in place? You poor deprived boy. :wink: My dad had a half frame Voighlander and I played for hours. (Was never allowed to use an actual plate exposure, though.) The image is inverted (same as in your eye - AAAArghhh!) so, to produce the same scene when projecting, you have to invert (fully) the slide / film / plate.

    * Pressing the right side of your eyeball with your fingertip. right near the orbit, will produce a black spot on the left of your vision. Newton did the same with needles - apparently. (EEjit)
     
  11. Jun 1, 2017 #10

    Drakkith

    User Avatar

    Staff: Mentor

    Let's just say the eye optics were included for "completeness". :-p
     
  12. Jun 1, 2017 #11
    YOur eyes are different from the paper, I took biology andI was told about how we actually see things upside down but our brain turns them the right way up, evidently the lens messes with your eyes and makes you see it upside down, due to there being another difference, Hope this helped you.
     
  13. Jun 1, 2017 #12

    Drakkith

    User Avatar

    Staff: Mentor

    It does not because it is wrong. The projector lens does not make you see the film upside down. The film is already placed into the projector upside down, so you're seeing the correct orientation when looking directly into the lens.
     
  14. Jun 1, 2017 #13
    Ah, okay that makes sence. Sorry for bothering anyone
     
  15. Jun 1, 2017 #14
    I was merely trying to elucidate on Drakkith's introduction of retina images into the discussion, and you are scolding me. :wink:

    OP: That was a key piece of information in answer to my question about how focused the image is when looking through the lens, namely that you have to move the lens all the way in when looking through it. As sophiecentaur points out, you have then a magnifying glass which gives an erect image of the slide (hence the image on the slide is upside down). When the lens is used as designed to project an an image of the slide, it gives an inverted image (hence the image on the slide is right side up). I think we can put this to rest now.
     
  16. Jun 1, 2017 #15

    sophiecentaur

    User Avatar
    Science Advisor
    Gold Member

    Sorry. I was too grumpy about that!
     
  17. Jun 1, 2017 #16
    Peace.
     
  18. Jun 1, 2017 #17
    When you project on paper, do you look at the paper on the side it is projected, or do you look on the backside for the light that shines through?
     
  19. Jun 1, 2017 #18

    sophiecentaur

    User Avatar
    Science Advisor
    Gold Member

    Looking at the front of the screen, you get the same 'handedness' as the image on the film (total inversion). If you look from behind (frosted screen) the image is laterally inverted as in a mirror. The cheap seats in early cinemas were behind the screen which was thin and made translucent with oil or fat. The (poor) audience couldn't all read so the backwards writing didn't matter.
     
  20. Jun 1, 2017 #19

    sophiecentaur

    User Avatar
    Science Advisor
    Gold Member

    It just struck me that the picture, looking into the lens will be inverted if the film is laced the right way. Do you run the film through backwards with the images the right way up?
     
  21. Jun 1, 2017 #20

    rcgldr

    User Avatar
    Homework Helper

    Off topic, a mirror reverses an image front to back, not laterally. Stand in front of a mirror, and raise your left arm, in the image you see the left arm image rising. Say the mirror is located north of you, move north towards the mirror, and the image moves south towards the plane of the mirror and you.
     
Know someone interested in this topic? Share this thread via Reddit, Google+, Twitter, or Facebook

Have something to add?
Draft saved Draft deleted