Sharpness vs. Resolution vs. MTF vs. Diffraction

[georgl]

I just wanted to post a reply on the "f64"-thread which seemed to raise very basic questions - so I've decided to give it it's own topic.
I hope it's well understandable, and helps to clear up certain missconcepts:

When we speak about "sharpness" we usually don't mean extinct resolution at high contrast from the lens, the film OR the scanner.

What's important is MTF - which basically means contrast at a certain resolution/ frequency. As most of us know, lenses have a MTF, but film and scanners also have MTF - they all "render" detail with a certain contrast.

Of course there is no "absolute" MTF-value for 4x5" or 8x10" - but let's play with a few values that are common:

- 20lp/mm frequency, which results in about 5000 "pixels" resolution @ 4x5", a frequency which is barely visible at 4x magnification (5lp/mm)

- 40lp/mm frequency to see what happens when we become more demanding - that is the Nyquist-limit (0% contrast, we have to scan with higher resolution to render detail at 40lp/mm) at 2000spi scanning resolution.

The lens:
Sironar-S 150mm - pretty much the best you can get for 4x5". It reaches 80% center contrast @20lp/mm at f11 and about 65% at f22 due to diffraction
There are no 40lp/mm-values so we can only guess given other MTFs:
60% @ 40lp/mm at f11 and 30% at f22?

The Film:
Ektar 100 or Portra 400 resolves 20lp/mm nearly perfect with >90% contrast and 40lp/mm with about 70% contrast (varies with the color spectrum)

The scanner:
I have no MTF-value except for the 1M$ 6000ppi ARRISCAN for the movie-industry (it's custom-made Zeiss lens propably costs as much as a drum scanner) - the MTF is worse than you would expect, because a scanner is a MTF-chain for itself (sensor, lens...).
Let's say 80% contrast @ 20lp/mm and 60% contrast @ 40lp/mm.
A very good drum scanner might be a little better, the very best Epsons are a lot worse. Let's say 50% contrast @20lp/mm and 20% contrast @ 40lp/mm

Now starts the fun:
We can calculate the final MTF of the file by multiplying the MTF-values.

Drum-scan from 4x5" at f11: 80% x 90% x 80% = ~57%
Very good rendition of detail at 20lp/mm.

Drum-scan from 4x5" at f22: 65% x 90% x 80% = ~47%
Still good rendition of detail, difference perhaps not visible below 5x magnification

Epson-scan from 4x5" at f11: 80% x 90% x 50% = 36%
The scanner resolves way more than 20lp/mm, the lens has higher contrast than at f22 but still, the file will appear "softer" than the drum-scan from the f22-image!

How far can we push resolution? So let's calculate with 40lp/mm (under ideal conditions):

Drum-scan 4x5" at f11: 60% x 70% x 60% = 25%
Well defined detail at 40lp/mm - that are 10000 x 8000 pixels of actual resolution rendered with 25% contrast - beyond a 80MP-MFDB!

But why are so many MFDB-samples so good in comparison to 4x5"?

Epson-scan 4x5" at f22: 30% x 70% x 20% = 4%
Each component renders detail @ 40lp/mm but the final image barely contains any information at this frequency! The 80MP-MFDB doesn't have contrast there, either but it holds contrast better below this frequency and therefore appears sharper without actually rendering more detail.

Finally a 8x10" negative with a different lens (let's say f22-MTF is the same) with the same file size as the 4x5" (so 20lp/mm at 8x10" equals 40lp/mm at 4x5" in the file).
Drum-scan 8x10" at f22: 65% x 90% x 80% = ~47%

47% vs. 25% - the lens resolves less contrast but due to the large negative, the 8x10" appears sharper, even below extinct resolution.

What have we learned today? ;-)

1. Extinct resolution (taking pictures of contrasty resolution charts) doesn't really matter in reality
2. Contrast over detail (MTF) is the key to sharpness - not megapixels or file sizes
3. Final MTF is the product of the whole image chain - from the lens to the scanner. One weak link can ruin it all while seeming sufficient on it's own

I hope this is helpful to some!?

[Ken Lee]

MFDB = Medium Format Digital Back

[Ken Lee]

I was able to follow your presentation. It is rather "compressed"

Could you please expand upon your conclusions: What have we learned ?

[Michael Graves]

I was able to follow your presentation. It is rather "compressed"

Could you please expand upon your conclusions: What have we learned ?

That a darkroom print from a 4x5 FP4 negative onto Oriental Fiberbase paper still renders a nicer print than a computer?

[jb7]

I've learned a few things-

Math and testing is not my thing; there are others who can do it better, and I have several opinions to choose from.
Resolution and scanning produces the most abrasive, defensive, and personal comments, of all topics posted on this forum.
Anyone who sticks his head above the parapet will likely have it shot off.
All the numbers, be they lp/mm, MP, %, they all come down to one thing-
a measure of how a print will look at a given (usually maximum) size-


A member here, bglick, who hasn't posted here in a long time, produced this graphic to show comparable print sizes for multiple dof scenarios, on multiple formats, based on a common 5 lp/mm print size.

I find it as good a guide as any- http://www.largeformatphotography.in...ad.php?t=45186

There are times when you need to make a print with less lp/mm-
and nothing is stopping you doing that.

However, if you want to claim that you can achieve many times the resolution than is considered physically possible, you might have a debate on your hands, and chances are, it will descend into personal abuse, and name calling.

I notice from your numbers, that you claim 80MP at 47% MTF for a drum scanned 8x10 negative at f/22-
although some of your parameters are estimated, I'm not going to argue with you...
I wonder what your numbers are for f/64?

[georgl]

Resolution and scanning produces the most abrasive, defensive, and personal comments, of all topics posted on this forum.

Which surprised me the most, when I stumbled over the f64-thread. This is the peaceful world of large-format-photography ;-) not a marketing platform for photographic gimmicks.
Some of the parameters I used are estimated, but realistic and the basic science behind it is still true and hopefully helps to calm down the discussion a bit.

What have we learned ?

Mostly "sharpness" ("resolution") is evaluated by shooting test charts, Whether it be the film, the lens or the scanner that is tested.
Several times (not just here) I stumbled over the missconception that the evaluated/ measured sharpness/ resolution is just one fix value, whether it be dpi, ppi, spi or lp/mm.
You use a not too-sharp lens at diffraction limit -> you don't need a sharp film
You have diffraction-limited, grainy slide at 400ASA -> you don't need anything but a Epson!
Wrong!
Just because an Epson-flatbed can resolve 2000 or 2500 spi, doesn't mean it affects "sharpness/ resolution" below these values not as well (double negative? ;-)).

You want a really "sharp" image? You have to consider the whole image chain!

[Brian Ellis]

That a darkroom print from a 4x5 FP4 negative onto Oriental Fiberbase paper still renders a nicer print than a computer?

Please tell me how I can make my computer render a print. Mine won't do that, it just sits there doing nothing until I make it do something.

[Bob Salomon]

"20 lp/mm....40lp/mm"

Just in case there is any confusion:

Those are 20 line pairs per mm and 40 line pairs per mm. They are not 20 lines or 40 lines per mm.

[Ken Lee]

You want a really "sharp" image? You have to consider the whole image chain!

That makes sense of course, and I'd be surprised if anyone would disagree.

I think you are also showing - which people do not always consider - is that a single system has different responses, depending on the level of detail that we send through it. That's why MTF charts contain more than one line.

By analogy, an automobile may get 100 kilo/liter at moderate speed, but when moving very fast or very slow, it will be less efficient. We can't say that a car has one rating. We might calculate an average rating, but that would be only a synthetic number.

Is that what you're saying ?

[Brian K]

Sharpness and resolution is a chain as others have stated.

For those of use using view cameras, this is Large format forum after all, that chain is extensive.

Film, the combination of films and various developers affects both the resolution and sharpness (acutance) of the recorded image.

Optics, you need to use the right lens i.e. a macro in a macro setting, and a non macro in a scenic setting, and the aperture that yields the required DOF while limiting the affects of diffusion and giving sufficient edge to edge resolution.

GG placement and accurate focus. You need to be certain that your ground glass is actually on the same plain of focus as the film and that your holders hold the film flat and consistently in that exact plane of focus. Sheet film will sag and move in holders. You then need to be able to focus on that point, most often relying on a loupe for assistance. And you also need to know WHERE in the scene to focus for optimum DOF as well as what camera movements can optimize that focus. You also need to be certain that your rear and front standards are plano parallel to each other and to the scene (this is assuming that no movements are required).

Vibration elimination. You need to have sufficient camera support to keep the camera steady and vibration free. If on vibration prone floors you need to be aware of footfalls and even heavy exterior traffic. When doing exposures in the 2 second to 1/15th second range, especially when using longer focal lengths or doing close up work, you need to be aware of shutter vibration, any vibrations still resonating in the camera or camera support from having inserted a film holder and any external sources of vibration to your camera or the subject.

Filters, the use of resin or other non glass filters, with the exception of kodak wratten filters behind the lens, will cause a loss of sharpness with longer focal length lenses and are to be avoided.

While all of this may sound too "perfectionist" to some of you, I can assure you that myself and every professional still life photographer I have ever known has taken all of these steps as part of their normal course of business. This is how a professional does their job.

As for scanning, the use of a prosumer scanner may be acceptable to some, but I doubt if it's output quality is considered sufficient for serious editorial or advertising work. And when one compares the image quality of an Epson 750 to that of a drum scanner or pre press flatbed scanner, one will see a world of difference.