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

[guyburns]

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?

 

[nasu]

Your eye introduces an extra lens in the path of the light rays.

 

[rcgldr]

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

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.

 

[sophiecentaur]

Your eye introduces an extra lens in the path of the light rays.

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.

 

[pixel]

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.

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?

 

[Drakkith]

See the following image:

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.

 

[guyburns]

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.

 

[pixel]

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.

 

[sophiecentaur]

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.

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.

there are two inversions going on here

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

To focus the image I have to move the lens all the way in.

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.

The slide is inserted upside down (according to sophiecentaur)

Did you never look at the back of a plate camera, with the focussing screen in place? You poor deprived boy. 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)

 

[Drakkith]

I really have a problem with introducing the Eye Optics into this explanation.

Let's just say the eye optics were included for "completeness".

 

[ThomasTheTiny]

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?

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.

 

[Drakkith]

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.

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.

 

[ThomasTheTiny]

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.

Ah, okay that makes sence. Sorry for bothering anyone

 

[pixel]

Here we go again. Both modes of viewing have the same inversion in the eye.)

I was merely trying to elucidate on Drakkith's introduction of retina images into the discussion, and you are scolding me.

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.

 

[sophiecentaur]

I was merely trying to elucidate on Drakkith's introduction of retina images into the discussion, and you are scolding me.

Sorry. I was too grumpy about that!

 

[pixel]

Sorry. I was too grumpy about that!

Peace.

 

[Khashishi]

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?

 

[sophiecentaur]

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?

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.

 

[sophiecentaur]

4. Looking through the lens:

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?

 

[rcgldr]

If you look from behind (frosted screen) the image is laterally inverted as in a mirror.

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.

 

[sophiecentaur]

Off topic, a mirror reverses an image front to back, not laterally.

The expression used in textbooks is "lateral Inversion". I was only using the accepted term. When you look in a mirror, your right hand appears to be the left hand of the person you see in the mirror. That's what lateral inversion means. A better way of describing the effect could be to say that clockwise becomes anticlockwise, of rotational inversion.
PS. "Front to back" could be a canditate for the term, except that you never see the "back" of the object that's in the mirror so it is a bit confusing.

 

[rcgldr]

The expression used in textbooks is "lateral Inversion". I was only using the accepted term. When you look in a mirror, your right hand appears to be the left hand of the person you see in the mirror.

That's because the image of the person has been reversed front to back.

"Front to back" could be a canditate for the term, except that you never see the "back" of the object that's in the mirror so it is a bit confusing.

Hold up a clear plastic plate with a word written on the plastic, so that you can read the word written on the clear plastic plate left to right, and you'll also be able to read the reflected image of the word from left to right. Another example would be a neon sign in a window to a room with a mirror on the opposite side of the room. From outside or from inside looking at the reflected image, you can read the neon sign normally from left to right.

Consider the case of a clear plastic sphere with a single dot painted on the sphere, and the sphere rotating about a vertical axis. If you view the sphere and the reflected image of the sphere from a bit above both, you'll be able to see the "back" of the sphere, and be able to see that the dot and it's reflected image move left / right in sync, but that forward / backward motion is reversed in the reflected image.

 

[sophiecentaur]

I really don't feel strongly enough this. It's just a good idea to use recognised terms if you want to be understood and avoid exchanges like this one.
Call it what you like and deal with any possible misunderstandings. You are about two hundred years late to get things altered, I'm afraid.

 

[rcgldr]

The expression used in textbooks is "lateral Inversion".

That is the term used as seen in this wiki article:

https://en.wikipedia.org/wiki/Plane_mirror

but that article links to "mirror image", where it states " A mirror image (in a plane mirror) is a reflected duplication of an object that appears almost identical, but is reversed in the direction perpendicular to the mirror surface. "

https://en.wikipedia.org/wiki/Mirror_image

In my opinion, "lateral inversion" doesn't seem to do a good job of explaining what is actually happening, but it it used.

 

[sophiecentaur]

That is the term used as seen in this wiki article:

https://en.wikipedia.org/wiki/Plane_mirror

but that article links to "mirror image", where it states " A mirror image (in a plane mirror) is a reflected duplication of an object that appears almost identical, but is reversed in the direction perpendicular to the mirror surface. "

https://en.wikipedia.org/wiki/Mirror_image

In my opinion, "lateral inversion" doesn't seem to do a good job of explaining what is actually happening, but it it used.

'Actually happening' can often be translated into 'the way I think of it'.
There is far more to the lateral inversion process than where an image happens to appear.
Your terminology ignores the Rotational Inversion of a mirror image which is highly relevant. Make a screw (or a sugar molecule) that matches the image in a mirror and it does not function in the real world. That applies however you orientate that laterally inverted copy, back to front or not.
[Edit: I can use two mirrors to produce an image that fits your description (Back to front) but in which there is no lateral inversion.]

 

[guyburns]

Thanks for all the responses. After going through them and doing some experiments, sophiecentaur seems to have the most satisfying answer, backed up by pixel (see Quotes 1 and 2, below).

Now that I understand why the image is upside down when I look through the lens (because my eye is inside the focal length), I've still got the problem that the image is upside down. Some of the films are actually the right way up – but they run backwards as sophiecentaur has picked up (Quote 3, below).

It's rather complicated, trying to turn a projector into a viewer. Why do I want to do it? Because viewers for 9.5mm films are very rare.

Experiment 1
I removed the lens from the projector and used it to look at printed words. Sure enough, at close focus the words were the right way round, but if I carefully moved the lens away from the words, and still kept looking through it, at some point the image turned upside down. It was hard to do, and see, because everything was back-to-front and quite blurry.

Experiment 2
I set up a piece of A4 paper as a screen about 30 cm from the lens, and focused the image. It was about 5cm wide. Then I set the light source (a Zebralight LED torch) to its lowest setting and put my eye in the plane of the paper – but all I could see was the bright LED chip. Moving my head, I could sometimes almost make out part of the image – maybe. This leads to a final question:

QUES
Why is it so difficult – if not impossible – to see a projected image if your eye is in the plane of the screen?

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.

… 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.

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?

 

[Drakkith]

Hmmm. I think it's because placing your eye in the focal plane (the plane of the screen) means that the image is formed at/near your eye and is too close to be brought into focus. The original image forms a 'virtual object' that your eye then images onto your retina. If this virtual object is too close, then it's like trying to bring your hand into focus when it's held an inch in front of your face.

If you move your head back a bit you should be able to see part of the image.

 

[sophiecentaur]

Why is it so difficult – if not impossible – to see a projected image if your eye is in the plane of the screen?

If you could put the lens closer to the film then that would be easier. As it is, you are probably only just inside the focal length, at which point the image 'explodes'. The rays from the lens are almost parallel and the apparent position is near infinity. Just a bit further away from the film and the image switches to minus infinity. (Hence "explodes". To see what I mean, take the lens out and play with it as a magnifying glass; when you put it really close to an object, you get less magnification but everything is much more stable. You could improve matters with a longer focal length lens. Projectors used to have different focal length lenses available to accommodate different screen sizes and 'throw'. But there won't be many 9.5mm projector lenses available, I guess. Failing that, you could try adding a concave lens to the front of the proj lens (specs from a person with severe short sight perhaps) That would increase the focal length and put the film more within the focal distance. You may find a friendly optician who could give you a strong circular blank. (I once got a +0.5 dioptre lens by chatting up a guy in town)

 

[Drakkith]

If you could put the lens closer to the film then that would be easier. As it is, you are probably only just inside the focal length, at which point the image 'explodes'. The rays from the lens are almost parallel and the apparent position is near infinity. Just a bit further away from the film and the image switches to minus infinity.

That happens in "experiment 1" from the OP's previous post, but that isn't the case when you're projecting the image onto a screen and then removing the screen and placing your eye at the focal plane.

 

[sophiecentaur]

Why is it so difficult – if not impossible – to see a projected image if your eye is in the plane of the screen?

It is pretty well impossible to see anything that's placed a couple of mm in front of your eye. That's where the image produced by the projector ends up. You would need to be say 30cm away from the screen (behind or in front of) to see that image. Without the screen, you would not see much of the image - only the bits that are in line with your eye and the lens aperture. Moving your head from side to side you could see different parts of the film image. (Not pleasant to watch, I would think) The screen catches light in all directions and scatters it so you can see the whole image.

 

[guyburns]

Thanks for all the additional responses. I've got a lot of thinking, and a few more experiments to do.