SamuliSchielke
Dear Forum,
I have a few fundamental questions in an attempt to do something about my extensive lack of knowledge when it comes to optics.
What exactly is the relationship between resolution and sharpness, and how does the choice of film and lens affect this?
To what extent does sharpness depend on resolution, and what other factors does it depend on?
Does a lens’s resolution depend solely on the quality of its construction, or can one generally say that a large aperture equals high resolution? To what extent does resolution depend on the aperture?
And then the practical question: what sort of resolution can one roughly expect from medium-format and enlarger lenses? And how should this influence the choice of film?
Samuli
cfb_de
Hello Samuli,
The drama begins much earlier. Define ‘resolution’ and state the contrast relative to your definition. And the aperture.
Line-pair resolutions at a contrast of 1:1000 have little to do with practical photography and, at best, yield purely academic findings.
In practice, with a good medium-format lens, you’ll end up in the region of 30–90 line pairs per millimetre, with 90 being *only* good.
Some films can do more, but even there it depends on the definition of ‘resolution’. A film with the highest acutance will never produce optimal results at 1:1000 because the grain size distribution comes into play. That is why ‘resolution records’ are set using microfilm.
And then there is the matter of enlargement. Decent V-lenses are probably the best link in the whole chain leading to the image (a rough calculation: plan against plan; perfectable to the point of nausea). But everything regarding ‘resolution’ in terms of the camera lens, film and aperture becomes format-dependent the moment the image is enlarged. It may be that 35mm lenses resolve more line pairs (which is indeed the case). To achieve this, however, the print must be enlarged further, thereby negating the primary ‘advantage’ of higher resolution. ‘Line pairs per millimetre’ apply to the negative, after all.
So, of the 135 lines from a good 35mm lens, nothing remains when, for a print of the same size, it has to compete against a 40-line fragment from a 6x9, and grey values also come into play. That is precisely the difference between theory and practice.
A modern microfilm can show optics their limits (the current Zeiss publication on their lenses and a track process is very nice), but a run-of-the-mill 35mm film like an FP4 or HP5 cannot.
What was that again about degrees of freedom and variables in terms of predictability? ...
Best regards,
Franz
Wolfgg
Hello Samuli,
and once you’ve tried it out for yourself, seen and understood that, for example, with 100-speed films, linear tonal loss sets in at enlargement factors of over 4x – meaning the image shows fewer tonal values than the eye could resolve – the sharpness and resolution of a film or lens are, given the performance available today, merely secondary factors, with the exception of a few cheap zoom lenses. The most effective way to achieve true sharpness is a rock-solid tripod anyway. With a heavy 6x9 (Mamiya Universal), I can tell from the negative – even at 1/60 sec – whether a tripod was ‘in play’ or not. And I’m not (yet) one of those with shaky hands! And as for cameras where, of all things, half the camera’s weight is rapidly accelerated and then decelerated just before exposure – well, what more can one say...
Regards, Wolfgang
Godot
And cameras where, of all things, half the camera’s mass is rapidly accelerated and then decelerated just before the exposure – well, what more can one say…
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Yes, it’s not the topic here, but I’ll never understand why almost all mid-to-high-end digital cameras are designed as SLRs. Perhaps to provide a use for expensive stabilisation technologies?
OK, let’s move on,
Regards,
Stefan..
piu58
There are three types of blur:
- Motion blur, e.g. caused by people or leaves moving in the wind -> short exposure time
- Depth of field: objects near and far are equally sharp -> small aperture
- Film resolution or sensor noise: less sensitive films are sharper, larger formats are sharper.
This is a contradiction in a given lighting situation. Either I use a short exposure and have to use a wide aperture or a sensitive film. Or I put the camera on a tripod, use a small aperture and expose for a long time. But then nothing must move. Or I use a sensitive film and gain something from it; that’s just the way it is.
A small compromise: high-speed film and a larger format. But that causes back problems :rolleyes:
cfb_de
Hi Uwe,
One more thing: ‘acutance’. This relates to resolution, but creates the impression of high sharpness. And it’s highly subjective.
And with “depth of field”, the lens’s “diffraction blur” naturally comes into play at some point; with “film resolution”, the “lens resolution” can counteract this despite the larger format.
Just so that the conclusion is at least *completely* open to exhaustive discussion :-)
After all, we are looking for the absolute maximum on an n-dimensional surface. This problem is almost solvable if n is known exactly and is less than infinity :-)
Best regards,
Franz
SamuliSchielke
First of all, thank you very much for your replies.
I’m familiar with depth of field and motion blur, but when it comes to things like acutance versus resolution, I start to get a bit confused. Am I right in thinking that acutance means that, due to fine grain (high detail contrast), the visually perceptible sharpness increases at the expense of the objectively measurable resolution?
And what exactly is ‘diffraction sharpness’?
And can anyone explain to me how the aperture affects resolution (i.e. not depth of field – that part is clear to me)? You can see the effect quite clearly with an enlarger: if you set the enlarger to maximum enlargement and look at the image through the grain sharpener, you can see that at f/16 or f/22 the resolution visibly deteriorates. But what are the exact ratios involved, and does this matter in medium format, or can it be ignored?
For me, it ultimately comes down to a practical question: matching the film and enlarger lens to the camera lenses. For medium format, I work with an 80mm Meopta from the 1940s and newer Ukrainian 120mm Vega and 65mm Mir lenses, none of which are resolution marvels, but all of which have lovely tonality. The Meopta lens has slight vignetting and images that are out of focus at the edges, which might be considered a drawback in other applications, but looks very nice in portraits. Judging by what I’ve read in this thread, I probably don’t need to worry about film and enlargers – the resolving power of ADOX & Fomapan and the 75 mm Componar C is likely far better than that of my shooting lenses. Things get tricky with 35mm film, because my Canon FD 50/1.4 really lets a lot through – so much so that I’ve now ordered a roll of the new ADOX CMS on a trial basis. It only occurred to me afterwards that the resolution might be pointless if my enlarger lens isn’t up to the job – and that’s why I started this thread.
> and once you’ve tested it yourself, seen and understood that, for example, with 100-speed films,
> tonal loss sets in at enlargement factors of over 4x, meaning the image shows fewer tonal values than
> the eye could resolve, the sharpness and resolution of a film or lens are, given the performance available today, merely > secondary parameters, with the exception of a few cheap zoom lenses.
I’ve noticed this, as I often struggle to reproduce the beautiful tonal range of contact prints in enlargements. Why is that? Is there anything that can be done about it (apart from large format)?
> Yes, it’s not the topic here, but why almost all medium-to-very-good digital cameras are designed as SLRs is something
> I’ll never understand. Perhaps to have a use for expensive stabilisation technologies?
Wouldn’t there be a gap in the market for digital cameras in the Rolleiflex format?
All the best,
Samuli
Gast
Hello,
I don’t really feel like writing anything, as there are some really good books on the subject, but you also have to take halation (yes, my favourite topic) into account.
There are two types here again:
I) Light diffusion
II) Light reflection
I) This means that the light wanders around within the emulsion; this occurs mainly with overexposure and old thick-layer emulsions.
The result is a general reduction in sharpness.
II) The light is reflected at the back of the contact plate or on the support material. This occurs when there is insufficient antihalation, particularly in areas of high light contrast, and leads to light fringes, or, for example, a bright textured surface such as a house wall or blonde hair losing all its contours.
Roland
Wolfgg
Hello Samuli,
The best way to understand the loss of tonal range caused by enlargement is as follows:
The smallest image element that the eye can resolve at a normal viewing distance is a square with a side length of 0.1 mm (in printing terms, this is referred to as 300 dpi, for example). Such a square with a side length of 0.1 mm on the film contains only a limited number of bromide silver crystals, which become silver crystals after development. The number of these crystals in the square with a side length of 0.1 mm determines how many tonal values the film can store for the eye. If there are, for example, 10,000 crystals, then 10,000 tonal values are possible. If this film is now enlarged 10-fold, these 10,000 crystals are distributed across a square with a side length of 1 mm, so that only 10,000/(10*10)=100 crystals remain in a square with a side length of 0.1 mm (the smallest image element visible to the eye). And that would then amount to just 100 visible tonal values; such an image would look ‘modest’ at best, because the eye can distinguish around 500 tonal values. Consequently, there is a maximum enlargement ratio for every film beyond which tonal loss begins. The record holder was Technical Pan with Vmax=8 linear, but unfortunately it has disappeared into a vault in Rochester. Presumably for all time. A great pity.
Here are two links on the subject of optimal aperture and diffraction:
http://www.kenrockwell.com/tech/focus.htm
http://www.largeformatphotography.info/fstop.html
Best regards, Wolfgang
AntiLynd
Thanks, Wolfgang,
This isn’t really the topic of this thread, but it was the answer to a question I’d been pondering for ages but had somehow never actually asked publicly: what’s the reasoning behind “increase the magnification factor -> tonal values get ‘worse’ (whatever that means)”?
If we take these 500 distinguishable tonal values as a basis, can we then say that the maximum possible magnification factor without noticeable loss of tonal values in our 10,000-grain example film
is 10,000/(x^2)=500, i.e. around 4.5? ...as I, being a complete maths dunce, have just worked out...
Regards,
Nils.
PS: this mysterious RMS granularity – does it have anything to do with it? Isn’t that also about the number of artists in a given area? At the very least, the abbreviation itself suggests that there’s a fair bit of squaring and root-taking going on, so that seems a likely suspicion...
Wolfgg
Hello Nils,
Of course, RMS isn’t entirely relevant to the thread’s question, but in every thread there are bound to be readers who are interested in more detailed information that goes beyond the question itself.
When we say ‘tone values are getting worse’, it simply means that the image is showing fewer and fewer tone values. And what we perceive as a tonal value is nothing more than a certain number of silver crystals that are so close together that they merge into a specific grey value in our eyes. With digital image processing, you can simulate the effect of low tonal values by, for example, reducing the brightness resolution to 6 bits (=64 tonal values) in black-and-white images.
Figuratively speaking, the RMS value of a film is nothing other than the visible ‘grain’ at a point on the film that should have a uniform density. The finer the grain of the film, the smoother the area on the film appears. The ‘RMS’ comes from the calculation of the root mean square (root of the integral over the squares). Here under “Grain” there is a clear description of the RMS value:
http://www.sw-magazin.de/swmag_wollstein_32.htm
As a rough rule of thumb: the smaller the RMS number, the finer the grain and the higher the magnification limit at which approx. 500 tonal values are still discernible. As the RMS value takes into account not only the grain size but also any irregular arrangement of the grain (uneven coating, migration during development), it is unfortunately not possible to simply state that half the RMS value allows for double the magnification scale. But it is roughly accurate. The values published by the perfectionist Andreas Weidner (max. 5x for KB, 4x for RF, 3x for PF) correspond quite well to my observations for 21–24 DIN; only with Technical Pan and similar films can one roughly double the magnification. Your calculation is correct; the 10,000 crystals are an example figure I chose that isn’t entirely far removed from practice.
One more thing: fine-grain developers by no means increase the number of crystals and thus the range of tonal values that can be stored in a film pixel, but merely ‘chop up’ the crystals so that the image appears smoother (the RMS value only decreases during development).
Regards, Wolfgang
AntiLynd
Of course, RMS isn’t entirely relevant to the thread’s question, but in every thread there are bound to be readers who are interested in more detailed information that goes beyond the question itself.
Yeah, I think so too. And anyway: we’ve now covered the topics of resolution, sheep, tonal values and RMS in a single thread of barely more than a page, in a way that’s understandable even to people who didn’t really know much about it before. That’s great. It just goes to show which of the two forums is the more suitable one if you just want to ask a question. I don’t even want to know how things would have gone ‘over there’. The hotheads would certainly have found a pretext to tear each other apart again... and the supposed troll along with them :)
Best regards
N.
PS: although, admittedly: there is, of course, a certain category of questions that practically invites this and which, assuming a certain degree of common sense, are best not asked...
