Doug Kerr
Well-known member
Carla maintains for mew a subscription to the the print edition of Popular Photography. I keep a stack of these in a basket in my bathroom, next to the toilet. Sometimes at night when I'm barely awake, and don't turn on the light, I will accidentally pee on the latest issue. Sorry, Miriam. But I digress.
In a recent issue there was a nice article about a photographer who was working in the tintype medium. In the latest issue a reader had written in and commented that one of the images shown was clearly "reversed" (in the sense that we call a "mirror image" - more on that later), and wondered why that error had been made.
The answer given by the magazine was in part "correct" but actually gave no insight into the matter, and is in fact overall incorrect. It was said that a tintype is a "direct positive" (true), while most often we see images that were taken on a negative. And when the negative was printed, this causes a "reversal" that cancels the reversal of the image as it was on the negative. The suggestion was that this "reversal" was an artifact of "negative" vs "positive".
Here is the real situation. When an image is formed by a camera lens, if we were to look at the "aerial image" while looking in the same direction at which we would look at the scene directly (that would be looking at the aerial image "from behind", as if through the open back of the camera with no film in place), we would see the scene not "reversed". (It would be rotated by 180°, a consequence of the negative magnification of the usual lens situation, but that's not what we are speaking of here.)
But if we were to regard the aerial image from the opposite direction from that at which we would look at the scene directly (that would be looking at the aerial image "from in front"; that is, from the lens' vantage point) we would see the scene "reversed" in the "mirror image sense". And it is that situation that is captured on a camera direct positive medium, such as in the tintype process. We look at the "print" in that same direction - toward its "face" - and see the scene "reversed".
Now what if we, as is common, expose a negative medium. Is the image on it "reversed" or not? Well, if we look at the face that was toward the lens, yes; if we look at the "back" face, no.
When we make a contact print, we normally place the "front" (emulsion) face of the negative against the print paper. If we think about it, if we look at this "stack" from the negative side (and thus seeing, though the negative, the sensitive side of the paper), we are looking in the same direction as we would have looked at the scene directly. And thus the image printed on the paper is "not reversed".
Now it's a very good thing it works out that way. The image on the negative is in fact on its "front" surface, the side with the photosensitive "emulsion". By placing the negative as I described above (as we must do to get a "not-reversed" print) it is the emulsion that is intimate contact with the paper. Thus we eliminate the opportunity for the light from any point on the image on the negative to "spread" during its travel through the negative base material, which could (sightly, of course) degrade the image on the print.
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A closely-related issue is why does a mirror "reverse" the image of things left-to-right, and not top-to-bottom? Like the old joke about the Thermos bottle,"how do it know"?
Imagine that to the south of me is a house. I look at it. I notice a bay window toward my left on the front of the house - that is, toward the east. Then I turn around to look (northward) at a mirror, in which I can see the house. In the image that I see in the mirror that bay window is still toward the east. But now, that is toward my right (since I am now looking northward).
But the chimney of the house, which looks to be "up" when I look at the house directly, still looks "up" in the image in the mirror.
And so the image looks "left-to-right reversed", a form of a - yes, you got it - "mirror image".
Now why is the image reversed "left-to-right" and not "top to bottom"?
It is because, when I "turned around" to look at the mirror rather than the house itself, I did that by revolving around a generally-vertical axis. That is, the association of "to my left hand" and "to my right hand" with "east" and "west" was reversed, not the association of "toward my head" and "toward my feet" with "up" and "down".
If I had instead gone from looking at the house directly to looking at the mirror by doing a back bend to a handstand (fat chance), the association of "toward my head" and "toward my feet" with "up" and "down" would have been reversed, but not the association of "to my left hand" and "to my right hand" with "east" and "west" . Then indeed the image would seem to me to have been reversed top-to-bottom and not right-to-left.
But since the usual way in which we operate is head-up, we think of a mirror as reversing the image left-to-right and not top-to-bottom.
Well, I need to head to the bathroom now. Miriam, look out!
Best regards,
Doug
In a recent issue there was a nice article about a photographer who was working in the tintype medium. In the latest issue a reader had written in and commented that one of the images shown was clearly "reversed" (in the sense that we call a "mirror image" - more on that later), and wondered why that error had been made.
The answer given by the magazine was in part "correct" but actually gave no insight into the matter, and is in fact overall incorrect. It was said that a tintype is a "direct positive" (true), while most often we see images that were taken on a negative. And when the negative was printed, this causes a "reversal" that cancels the reversal of the image as it was on the negative. The suggestion was that this "reversal" was an artifact of "negative" vs "positive".
Here is the real situation. When an image is formed by a camera lens, if we were to look at the "aerial image" while looking in the same direction at which we would look at the scene directly (that would be looking at the aerial image "from behind", as if through the open back of the camera with no film in place), we would see the scene not "reversed". (It would be rotated by 180°, a consequence of the negative magnification of the usual lens situation, but that's not what we are speaking of here.)
But if we were to regard the aerial image from the opposite direction from that at which we would look at the scene directly (that would be looking at the aerial image "from in front"; that is, from the lens' vantage point) we would see the scene "reversed" in the "mirror image sense". And it is that situation that is captured on a camera direct positive medium, such as in the tintype process. We look at the "print" in that same direction - toward its "face" - and see the scene "reversed".
Now what if we, as is common, expose a negative medium. Is the image on it "reversed" or not? Well, if we look at the face that was toward the lens, yes; if we look at the "back" face, no.
When we make a contact print, we normally place the "front" (emulsion) face of the negative against the print paper. If we think about it, if we look at this "stack" from the negative side (and thus seeing, though the negative, the sensitive side of the paper), we are looking in the same direction as we would have looked at the scene directly. And thus the image printed on the paper is "not reversed".
Now it's a very good thing it works out that way. The image on the negative is in fact on its "front" surface, the side with the photosensitive "emulsion". By placing the negative as I described above (as we must do to get a "not-reversed" print) it is the emulsion that is intimate contact with the paper. Thus we eliminate the opportunity for the light from any point on the image on the negative to "spread" during its travel through the negative base material, which could (sightly, of course) degrade the image on the print.
************
A closely-related issue is why does a mirror "reverse" the image of things left-to-right, and not top-to-bottom? Like the old joke about the Thermos bottle,"how do it know"?
Imagine that to the south of me is a house. I look at it. I notice a bay window toward my left on the front of the house - that is, toward the east. Then I turn around to look (northward) at a mirror, in which I can see the house. In the image that I see in the mirror that bay window is still toward the east. But now, that is toward my right (since I am now looking northward).
But the chimney of the house, which looks to be "up" when I look at the house directly, still looks "up" in the image in the mirror.
And so the image looks "left-to-right reversed", a form of a - yes, you got it - "mirror image".
Now why is the image reversed "left-to-right" and not "top to bottom"?
It is because, when I "turned around" to look at the mirror rather than the house itself, I did that by revolving around a generally-vertical axis. That is, the association of "to my left hand" and "to my right hand" with "east" and "west" was reversed, not the association of "toward my head" and "toward my feet" with "up" and "down".
If I had instead gone from looking at the house directly to looking at the mirror by doing a back bend to a handstand (fat chance), the association of "toward my head" and "toward my feet" with "up" and "down" would have been reversed, but not the association of "to my left hand" and "to my right hand" with "east" and "west" . Then indeed the image would seem to me to have been reversed top-to-bottom and not right-to-left.
But since the usual way in which we operate is head-up, we think of a mirror as reversing the image left-to-right and not top-to-bottom.
Well, I need to head to the bathroom now. Miriam, look out!
Best regards,
Doug