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Imaging Technology: Theory, Alternatives, Practice and Advances. This is a brand independent discussion of theory, process or device. Ignore this forum unless this matters to you!

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Old March 14th, 2017, 12:19 PM
Doug Kerr Doug Kerr is offline
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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.

************
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
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Old March 14th, 2017, 12:37 PM
Tom Robbins Tom Robbins is offline
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You might want to move those paper copies of Popular Photography out of harm's way, Doug. Mike Johnston, the Online Photographer blogger, mentioned last week that the March/April 2017 issue will be the last. So, any unstained copies might become collector's items in 40 to 50 years.
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  #3  
Old March 14th, 2017, 03:21 PM
Doug Kerr Doug Kerr is offline
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Hi, Tom,

Quote:
Originally Posted by Tom Robbins View Post
You might want to move those paper copies of Popular Photography out of harm's way, Doug. Mike Johnston, the Online Photographer blogger, mentioned last week that the March/April 2017 issue will be the last.
Well, I hadn't heard that. But the publication schedule has recently gone from monthly to every two months.

Quote:
So, any unstained copies might become collector's items in 40 to 50 years.
Well. I'm not sure we have any here that would qualify!

But I'll alert my granddaughter just in case!

Thanks.

Best regards,

Doug
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Old March 14th, 2017, 04:46 PM
Maris Rusis Maris Rusis is offline
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Thanks Doug for your entertaining dissertation. As a user of large format view cameras I look at the "upside down"or "back to front" or "flipped" or "rotated" or "mirror reversed" ground-glass image every day and think nothing of it. In fact it's been years since I noticed anything untoward about the view I see when under the focussing cloth. But that doesn't apply to kibbitzers or curious spectators when they get to look at the ground-glass image. Typical conversations:

Spectator: "Yikes, it's upside down!"
Photographer: "Sorry, it's the damn lens. Been meanin' to fix it for years but I've kinda got used to it."

Spectator: "Yikes, it's upside down!"
Photographer: "Ah, that's photography's deepest secret. All photographs come out upside down. We just flip 'em before we show 'em."

Spectator: "Yikes, it's in colour. I didn't think cameras this old did colour."
Photographer: "You're lucky today I'm using colour film. If I'd been using black and white the view wouldn't look anywhere near as pretty."

And here's one from when I taught view camera work:
Student: "I've broken the ground-glass on my Toyo-View. What do I do?"
Photographer: "Order a new one and make sure to fit it the right orientation. Double check this. If you go to the back of the camera and the picture is upside down you are good to go!"
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  #5  
Old March 14th, 2017, 07:33 PM
Doug Kerr Doug Kerr is offline
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Hi, Maris,

Quote:
Originally Posted by Maris Rusis View Post
Thanks Doug for your entertaining dissertation. As a user of large format view cameras I look at the "upside down"or "back to front" or "flipped" or "rotated" or "mirror reversed" ground-glass image every day and think nothing of it. In fact it's been years since I noticed anything untoward about the view I see when under the focussing cloth. But that doesn't apply to kibbitzers or curious spectators when they get to look at the ground-glass image. Typical conversations:

Spectator: "Yikes, it's upside down!"
Photographer: "Sorry, it's the damn lens. Been meanin' to fix it for years but I've kinda got used to it."

Spectator: "Yikes, it's upside down!"
Photographer: "Ah, that's photography's deepest secret. All photographs come out upside down. We just flip 'em before we show 'em."

Spectator: "Yikes, it's in colour. I didn't think cameras this old did colour."
Photographer: "You're lucky today I'm using colour film. If I'd been using black and white the view wouldn't look anywhere near as pretty."

And here's one from when I taught view camera work:
Student: "I've broken the ground-glass on my Toyo-View. What do I do?"
Photographer: "Order a new one and make sure to fit it the right orientation. Double check this. If you go to the back of the camera and the picture is upside down you are good to go!"
Great stories!

As I'm sure you know, in a normal view camera, the only accurate description of the image on the ground glass compared to the image we see when we look directly at the scene is "rotated 180".

"Upside down" isn't at all precise but usually conveys the proper notion. "Mirror image" definitely does not. The usual use of "flipped" in this field doesn't either.

Then of course for a TLR with the usual waist-level finder, or a Graflex (not to be confused with a camera made by Graflex) . . .

Thanks.

Best regards,

Doug
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Old March 14th, 2017, 09:53 PM
Asher Kelman Asher Kelman is offline
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Doug,

When you write such long and fascinating essays, it would be easier on the eye of you inserted some sketches to illustrate everything!

Asher
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Old March 15th, 2017, 05:59 AM
Doug Kerr Doug Kerr is offline
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Hi, Asher,
Quote:
Originally Posted by Asher Kelman View Post
Doug,

When you write such long and fascinating essays, it would be easier on the eye of you inserted some sketches to illustrate everything!
You are quite right, and in fact as I wrote that essay the thought crossed my mind that some illustrations might be helpful.

But then I wanted to finish it before breakfast!

Thanks for your recommendation.

Best regards,

Doug
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Old March 15th, 2017, 09:01 AM
Asher Kelman Asher Kelman is offline
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Doug and Maris,

I wonder if anyone sought to make corrections for film thickness or curvature. For the latter, a vacuum back, but what about for thicker film?

Asher
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  #9  
Old March 15th, 2017, 05:47 PM
Doug Kerr Doug Kerr is offline
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Hi, Asher,
Quote:
Originally Posted by Asher Kelman View Post
Doug and Maris,

I wonder if anyone sought to make corrections for film thickness . . .
To correct for the effect of film thickness on what, and in what context?

In a conventional sheet film holder for a view camera, the emulsion face of (the edges of) the film is located against the back of what we can think of as the "internal front window" of the film holder, so ideally variations in the thickness of of the film stock have no effect on the consistency of focus between the image on the ground glass and the image on the film itself.

Now in fact in most such film holders there is not a spring-loaded backing plate to press the film against the "window" (as there may be in glass plate holders); rather, the backing plate is located a fixed distance behind the "window". That distance is just a little more than the standard thickness of sheet film (a little over 0.0075" for not-yet-developed film).
Developed film is a little thinner as the antihalation layer on the back face is removed during development. The standard thickness for developed sheet film is 185 micrometers, or approximately 0.00728".
If the film were much thicker than that, its edges would not go into the "track" in a "normal" sheet-film holder.

If it were much thinner than that, there would be a small uncertainty as to its exact position (which of course could vary across its surface). We could not compensate for that in any optical sense.

Best regards,

Doug
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