depth of focus question

[DonWeston]

Steve - I would be glad to try, since I seem to be failing to get my perspective across. When one focuses a large format camera for example or in a dark room, there is a range of focus in the emulsion, i.e. different parts of the image are in focus at the plane of focus at different times as you run the focus. Part of this, is of course due to the curvature how ever minimum of the film itself, but some of it is also due to the thickness of the emulsion, and the captured elements of the image being at different points within the emulsion. I do not know for sure  what the distance from one part of the emulsion is from back to front, but there is a depth both to the emulsion and also to the in focus elements at each point.
  Emulsion
B           F   B= back of emulsion, F=front of emulsion
l             l
l      *     l     *=point in sharp focus, say at F/2.8
l           *l
l*           l
l             l

  Sensor
      l  l
      l*l
      l  l    O
 O   l  l              O= point of sharp focus not with in the
      l  l                   sensor range, due to thinness of sensor
                                  say at F/2.8

now by changing to a larger f stop, say f/22 this becomes
l    l  l     l
l    l*l     l
l    l*l     l
l    l*l     l
l    l  l     l
I hope this is alittle more understandable?.......

[DonWeston]

I still don't understand how a neighboring pixel can register something behind itself or it's neighboring pixel, can you explain this in a different way, as it makes no sense to me. This is a back to front thing, not a next pixel thing, note back to front on image above. And I have read his responses a few times......this is very frustrating, can you explain how this would be possible??? I do not think we are talking about the same thing....at this point.

[DonWeston]

I understand that the sensor records or captures "all" the photons hitting it, the issue, and I will admit it is mine, is that it, of course, doesn't record bits of the image that are out of focus at the plane, but are in focus 0.1mm behind or in front of the plane, as being in "sharp focus". They are recorded as out of focus elements only. This range is a optical property of the lense, not the sensor. It is also this point that relates to different wavelengths recording either slightly in front of, or behind the sensor plane which is the drive to make digital lenses with better convergence qualities. That is my understanding from what I have read and makes sense in an optical point of view. The image projected on to what ever media film or digital sensor is not perfectly 2 dimensional either, as far as I have read. If light was focused by the lens in such a 2 dimensional way, then no improvement would be needed in lens design.
It also seems that I am not the only one confused, if someone thinks that the next pixel over can record info from  under adjacent  pixels[disregarding bloom effects]. I think to understand what I am saying you have had to have worked in 4x5 or darkroom for a while and critically looked at focusing through the emulsion to experience such depth. Also in the strict sense of it, depth of field, if you use the term properly is what is seen in front of the camera as depth of the scene that is recorded in focus, and is not an black or white  thing, but is a gradual process from going in and out of focus with in the range. If you have used only an 35mm slr and never looked at the ground glass or used an enlarger, you may never have experienced this depth, esp. in an af world. I guess I will just live with the phenomenon and compensate by increasing f stop, which is the cure for the issue up to a point...

[DonWeston]

Jonathan - first thanks for the better explanation, and also for the lack of sarcasm, "DUHH", totally uncalled for in a professional forum. At least I thought it was one,  until your response. BTW - I NEVER said I thought film was better or had an advantage per se, only trying to understand the difference.  BTW in traditional sense, the term depth of field is used to describe the area in front of the camera and what appears in focus, not what happens inside at the film or sensor plane. IN large format, that was called depth of focus in all the photography books I have read. So most from what I have seen here, many have misused the term totally. If you increase the depth of field by increasing the fstop as shown in my diagram above, it generally is thought of as increasing the depth of the image sharpness range from say 10 ft to infinity to 2 ft to infinity. No where is it used to describe what happens at the image plane in the camera.

In fact from what you have better described it is the flatness that indeed causes the difference that I noted. The digital sensors' 2 dimensionality makes it as good as it is, but also makes it impossible to record the image sharpness in the same way as film. It is a difference, not a plus or minus per se. You may never see this, so be it. IF you never shot LF and looked at the depth I describe working in a darkroom, then you have little to no basis for commenting on what I have attempted to describe. I accept and trust your knowledge of digital sensor design and operation, it would be nice if you responded to me with the same respect, with out the sarcasm.

From your explanation, I know understand if fact, that it is the planar nature of the sensor that does limit some of the depth, if you disagree so be it.

Lastly the discussion is OVER, when I understand the effect or when no one further responds or when a moderator closes the thread, not when you say it is. Geez, what an ego....

[joedevico]

FIRST -

I have no idea about advanced optical theories and practice

SECOND -

It seems to me that if all the information in this thread is correct then what Don is saying is that there are multiple planes of focus in film. Once put on an enlarger, you can adjust the focus of the enlarger's lens and "focus through these depths". Similar to looking at a slide in a microscope. This also seems to be in my mind how one can get some more detail from a RAW digital exposure. The information has a little bit of wiggle room to still be acceptable.

THIRD -

I am under the impression that there is only on true plane of focus and therefore if you get that plane in alignment with a digital sensor you'll have the best focus one could hope for. This would support Jonathan's points. This means that the area of "sharp focus" that Don used in his diagram isn't real, just perceived.

If I am way off base on this than you can ignore the post, but I do hope that it helps clear some of the confusion for Don.

[Steven M Anthony]

Don:

I liked you re-statement of the issue.  It helped me understand what you are talking about.  

While I've thought about this from time to time, I never gave it much attention--the notion that with a wet darkroon, you are putting the image through a second lens, with its own optical properties and focal plane.

I've only worked with 35mm film, and know what you mean about adjusting for film curvature.  And if LF film is thicker than 35mm, I can see (with the help of your example) how there can be a depth of focus.  I guess it also exists in 35mm film, too.  I do remember developing film and having it feel thinner if I did something wrong--it's been to long to remember what error lead to "thin" negs...

And I understood your discussion of "layers" to mean layers of active elements of the emulsion--not color-sensitive layers.  Is that what you were thinking?  Because the chemical compounds in the emulsion have depth.  The photo-sensor elements (each pixel) just count photons striking it.  The film emulsion crystals change in different ways depending on how much light hits them.  And those crystals are 3D.  I guess the sesor also has thickness, but it is just counting photons, it's molecular structure isn't changing.

I'm curious, though--is your experience that the depth of focus is an extension of the depth of field?  That is, does the depth of focus extend the depth of field on either end, or one of the ends?  Given your description and graphic example I would suspect so.

But I'm also drawn back to the notion of sharpness.  I wonder if the second lens (in the enlarger) acts to bring these additional elements of depth into "focus."  I keep trying to think of how the depth you refer to would look on a contact print--where the neg has no lens between it and the photo paper.

But maybe that's a whole other topic and can of worms!

[AJSJones]

Regardless of the discussion of where the image in an emulsion actually is, I had been living with the following definitions :

Depth of field : the range of distances from the lens where the image elements are perceived as "sharp" or "in focus", while image elements at distances outside this range were considered "unsharp" or "out of focus".  How these are determined/calculated requires some variables to be standardized : degree of enlargement, print size, viewing distance etc.

Depth of focus : the analogous range on the other side of the lens where the FOCUSED image is maximally sharp - as long as the film or sensor is within this range, the recording will be as sharp as possible; if the sensor or film is in the "wrong" position, the  image intended to be captured (i.e. what was focused on) is not maximally sharp.

I never spent much time thinking about depth of focus because I assumed that for quite some time, lenses have been good enough to ensure it was adequate.  Once this is the case, then the two parameters DoF and DoF) must be related in a predictable way - for example, if the sensor is not positioned correctly, all shots will front or back focus with respect to what is in focus on the viewfinder screen - and it needs to be sent in for calibration.
 
These definitions had a pleasing symmetry to them which made them easier to remember.  Anyone else have a similar set of definitions?? Or are they out of date? In the long-forgotten equation containing 1/f , 1/v and 1/u,  v and u refer to the distances from the lens to subject and the other to lens to film and DoF's refer to the ranges of u and v "considered sharp".

For digital, it seems like much of this discussion has left out a relevant item : the AA filter.  I believe that this is substantially thicker than the sensor element on the chip is deep.  All photons will go through this and some will be shifted to land up to a pixel or two away from where they would have been in its absence.  Thus , there will be different amounts of the image still in focus at different depths in the AA filter, but the sensor elements will still only record a composite of those - and this controlled blurring is a good thing to prevent Moire effects.

Don, I have not experienced what describe in terms of focusing through a 4x5 onto an enlarger bed (since I only scan them) but even with my 10x loupe, I only see the transition between "all in focus" and "all not in focus" as I adjust the height of the loupe above the light table. For thick B&W emulsions, your "layers" might be detectable, but with layered colored emulsions, I think Jonathan has made a good case why you shouldn't be able to detect it as you describe (at least not without color shifts).  

In your "TTY" figure, if you were to draw lines from the X's and O's to all parts of the exit pupil of the lens, you would see that the lines cross the sensor plane where adjacent or nearby sensor elements would catch them, that's why the X behind a given pixel must be captured by a nearby pixel, and points get blurred.  Here the relationship of the two DoFs becomes a bit easier to discuss - the smaller the exit pupil (i.e. aperture) the more closely the rays converge on the same focus plane and the out of focus objects will "focus" either in front of or behind the AA filter, and point objects will be recorded by several pixels.

A couple of cents' worth of rambling

Andy

[DonWeston]

AJS and Steven - thanks, I think I understand better now, somewhat. I appreciate your patience and kindness, and simplicity of explanation or analogy. One last question, does this mean that as new lenses become more precisely made with improved convergence properties etc, that you will not have to stop the apertures down as much, or will that purely remain the only adjustment to compensate for the digital sensors' flatness. And I am still not 100% sure that if the lenses are the same basically  and DOF is DOF, why any adjustment is needed with digital compared to film. Was it the circle of confusion with film that compensated for better by having any film curvature alone. Sort of like there being two areas of imperfection that kind of complemented each other and thus resulted in a image of greater sharpness with a relatively wider aperture setting???