rjlittlefield wrote: ↑Fri Jan 17, 2025 8:01 pm
IMAGOμM wrote: ↑Fri Jan 17, 2025 10:07 am
I guess that with the previous statement RiK means to say that, by decreasing the M factor, the effective NA increases - therefore the DOF decreases - but it is applied to a smaller image - due to the lower magnification - and the two factors compensate each other (more or less). The same, but in reverse, I suppose happens in the case of pushing-up.
Sorry to say, it's the reverse of all that. If you decrease the magnification by reducing the bellows extension, then you'll have to
increase the distance from objective to target in order to maintain focus. That increased front distance will
decrease the effective NA, which
increases the DOF. At the same time, you're increasing the information density on the sensor, which makes it more likely that sensor resolution will come into play, which also increases the DOF. All effects push in the same direction.
If you decrease the magnification by swapping in a shorter tube lens, leaving the tube lens focused at infinity, then there will be no change to the front focus distance or the effective NA. Only the effects at sensor will be in play, if you've pushed far enough for that to happen...
First of all, thank you for finding the time, desire and patience to write this long clarification.
1. Tube lens.
Let's see if this time I have correctly understood the dynamics of the objective/tube lens combination.
To summarize:
a. Push up/down of the tube lens.
Pushing down the lens the working distance increases, the NA decreases and the DOF increases (and consequently the stacking step). The opposite for pushing-up.
b. Variations of the focal length of the tube lens.
The NA remains unchanged and so does the stacking step.
I have to say that this surprises me: it seems like the optical equivalent of perpetual motion. Not very nice in the case of a short lens but very convenient in that of a long lens.
I would really like to delve into these topics. Are there any bibliographical references?
2. Step size.
Maybe there is a reason why people don't ask what is the best aperture for a specific step. In Photomicrography.net, as in general worldwide, there are many professional members but also many hobbyists. For the former, who often need to maximize working time, it is important to reduce the image creation time and consequently the duration of a stacking and the number of shots generated. For hobbyists this aspect is not relevant and they can afford the luxury of dedicating themselves to the search for maximum quality, whatever the meaning of this term. Otherwise, there would be no explanation for the large amount of threads on this site that discuss the differences between various lenses even when their visualization requires an eagle eye and a lot of experience. Not to mention the randomness due to ancillary but not negligible factors such as production tolerances, stability and precision of the rigs used, lighting, vibrations in the environment and so on.
Perhaps the most useful question is: what is the best step and the best aperture (for lenses with a diaphragm) to obtain the most balanced result? Ultimately, this is the principle that the late Robert O'Toole adheres to in his countless lens tests.
As for the best step for stacking, you also have to take into account the real world. For the virtual world, your suggestion of the λ/4 condition is certainly very useful in maximizing the time needed to create the image, even if in the 2014 white paper "What's the depth o field..." you suggest adopting the λ/8 criterion. However, rails and stepper motors are not perfect, so - as you know perfectly well - most people stick to the λ/12 criterion and many prudently choose λ/16. It is certainly not by chance that in the DOF Calculator that you inserted in Zerene Stacker the user can set the overlap of the steps as he likes more.
However, there is no more confusing topic than this. Over the years I have accumulated many books dedicated to macro/micrography written by photographers in the field, even with academic references (few are those dedicated to scientific photography, a little-frequented topic, which are reduced to the usual Morton and Lawson - very dated - and the more recent Savazzi, a highly appreciated member of this forum). Well, in almost all of them there is a DOF table depending on the magnification and aperture. There are no two tables that report data that are not identical but at least consistent with each other. Indeed, the differences are often wild. Evidently everyone has their own ideas on the matter. This says a lot about the harshness of the subject.
3. Pixel pitch.
This term normally refers to the distance between the pixels of the sensor measured in rows and columns. It is also the data provided by the manufacturer. However, it is not unusual to find a different measurement criterion, namely PP=PP*√2. In fact, it seems right to consider the minimum resolution of the sensor (theoretical) as that calculated along the diagonal, rather than that according to rows and columns. I would like to know your opinion on this.
4. Transfer of the maximum resolution from lens to sensor.
In all the discussions and papers I know of, it is assumed that the MTF of the lens (often, unfortunately, only theoretical: the equipment to measure the practical one is expensive and not very widespread) can be transferred as is to the sensor. In the "pioneering" times of film, when practice was very important and theory was little (not for nothing Ansel Adams has become a legend) there were some VERY approximate formulas that intended to define the relationship between the l/mm of the lens, the film and the resulting image. However, one thing was known for sure: even using the best film (personally I often used Technical Pan) you could hope to transfer to the film a maximum of 80/90% of the resolving power of the lens. I believe that something similar also happens in the digital world, and therefore we overestimate - and not by a little - the sensor's ability to offer adequate image transfer of the lens. (Something different here:
https://photomacrography.net/forum/view ... =8&t=47956
I have in front of me, as I write, a print of an ootheca (egg sac deposited by some insects) extraordinarily sharp, taken at the beginning of the century with a Canon 100/2.8 lens (not extraordinary in itself) on Technical Pan developed in Technidol. The resolution of this combination, which exceeded 200 l/mm, could be considered completely superfluous for a glass that, closed to nominal f/16, exhibited much lower performance. Yet I can assure you that there was no history in the comparison, in the same conditions, with the excellent Velvia 50 or Tmax 100 or Pan F. So, we have to be careful when talking about "empty magnification". May be that the lack of enhancement depends on other factors that interfere with image formation. I think it would be interesting to discuss this topic with other members of this forum. Note that for M>10 the situation could be substantially different due to diffraction.