The estimate of upper bound for the resolution of historic photographs is essential for setting up the parameters of digitization. Today we finally have technology to digitize practically all the picture information contained in the photograph and make them widely available to public. Digitization is laborious and dangerous to the original and thus it is desirable to do it just once. The key question thus is what resolution of scan is enough so we will never need to repeat it again? I asked this question in 2004 when our project of digitizing the archive of Šechtl & Voseček started and here I try to summarize my observations about one particular image.
Update: See also part 2.
The microscopic strike in Bosnia
|Ignác Šechtl: Žižka Square in Tábor, 1880-1885, wet process negative 11x13cm,|
We had to wait another 10 years to read more of the text, because you need incredible 5588DPI scan which is presented here in full resolution. In 2010 we purchased Eversmart Select scanner capable of doing that (this king of the flatbed scanning used to cost like a Ferrari in 2000s and we still paid $6000 for that monster). The text itself is 0.5x0.2mm in the original size. Click on the image to see the full resolution
|Image inverted to positive and tiny crop (4.5x5.5mm) shows people posing for the long exposure but also well readable texts in the news stands.|
We however were able to read more of the text. You can see "Národní Kalendář"/"National Calendar" and even smaller type "výročí"/"anniversary". With help of Google I was able to identify the calendar itself it is "Pečírkův Národní Kalendář". Here you can read a thesis about this famous callendar. Clearly it is one of issues published for anniversary of the strike in Bosnia. By other attributes seen in the photo we know it was taken in 1880-1885. It is thus matter of the visit in the national library to work out which one it was.
So what is the resolution of this photograph?
|Crop of the advertisement scanned at 5588DPI using Eversmart Select (left), after digital sharpening (middle) and after downscalling to 2000DPI and resizing back|
Of course the photograph is not on par with today 90 megapixel digital camera across the whole plate. Historic lenses was sharp and contrasty in the center and the resolving power degrades around the corners. Still it is impressive that in 1880s photographers was able to meet resolution of expensive professional cameras available today. 80 megapixel Mamiya sells for 28000USD and that is one of cheaper options.
How can I estimate the resolution of historic negative?Is my 5588DPI scan enough or can I read more of the advertisement? Well, in this case I can answer easily by comparing the sharpness of the crack in emulsion with the image details. Clearly the crack is sharper than rest of the image and thus my great-great-grandfather's camera did not capture sharp details at 5588DPI.
Given large archive of negatives one however can not explore each in this detail. Fortunately there are measurements available which let us to give reasonable upper bound on the image. They are well summaries in work of Timothy Vitale: Estimating the Resolution of Historic Film Images: Using the Resolving Power Equation (RPE) and Estimates of Lens Quality.
Since 1940s some film manufacturer publish data on number of line pairs per millimeter which can be captured by a given film. By summarizing data from published measurements T.J. Vitale prepared the following table on B&W film resolution:
|Native resolution of film.|
Timothy Vitale: Estimating the Resolution of Historic Film Images: Using the Resolving Power Equation (RPE) and Estimates of Lens Quality, Figure 18a
The resolution of negative is however just very crude upper bound on resolution of the actual photograph. Every photograph is taken through lens and it depends on the quality of lens how fine details they can capture. The following image nicely summarizes the effect:
|Graphics by Norman Koren explaning loss of resolving power by combination of film and lens.|
To estimate the resolving power of the photograph one thus needs to estimate the resolving power of the lens and combine the results using resolving power equation or use the following simplified table predicting the loss of resolution:
|No.||% loss||Description of Historic Era, Film Format and Lens Quality|
|1||25||modern medium resolution film in 35-mm & 2¼”x2¼”, thru an excellent lens (100 lp/mm)|
|2||40||modern small format film thru an average good lens (80 lp/mm) with good processing|
|3||40||average small-format 1940-70 film exposed through excellent lenses|
|4||40-60||large format film (1890-1970) thru average quality (40-lp/mm) lens with fair processing|
|5||50-60||modern large format film exposed through a good quality (60-lp/mm) large format lens|
|6||50-70||very high resolution film thru an excellent lens (100 lp/mm)|
|7||55-70||modern high resolution (5000-7000 ppi) film exposed through a good lens (80 lp/mm)|
|8||60-70||all common modern film through an average (40 lp/mm) quality lens|
|9||60||large format film (including early roll film) from 1890-1930 through average quality lens|
|10||60-70||very early film (1890-1930) thru all possible lenses, assuming good alignment and focus|
|11||60-80||large format film and glass plates 1875-1900 through average LF lenses (10-20 lp/mm)|
|12||60-90||modern large format film exposed through older lenses or average large format lenses|
|Twelve Guidelines for predicting percent film resolution loss due to lens era and film format.|
Timothy Vitale: Estimating the Resolution of Historic Film Images: Using the Resolving Power Equation (RPE) and Estimates of Lens Quality.Table 5
So what scan I need for my great-great-grandfather's photo? Well it is large format negative taken in 1880's. According to the first table the resolution of the "film" should be 1500PPI. I apply guideline 9 to an estimated 60% loss. The photograph thous should be at most 600PPI. In fact T. J. Vitale would rank it even lower:
1889 Film - The native resolution of film from 1885 would be about 1460 ppi (28-lp/mm). This is a low-resolution film exposed through an average lens of the era, about 20-30-lp/mm. Using Guideline 10 in Table 6, the film would produce images with resolution in the range of 483 ppi, as shown in column 7, in Table 7. However, if the image was made with a hand-held camera such as a Kodak #3 Folding camera or the Kodak #2 Box camera, it could be even as low as 292 ppi, following Guideline
11, as shown in column 8 in Table 7.
Timothy Vitale: Estimating the Resolution of Historic Film Images: Using the Resolving Power Equation (RPE) and Estimates of Lens Quality, page 22
This is what I should see in the news stand if resolving power was 600DPI:
|The same crop downscaled to 600DPI and sclaed up.|
First of all I believe there is a mistake in Timothy's Vitale's table. 1940 is about the time when enlargers became mainstream (with advent of 35mm cameras) and photographers moved away from contact copies. For the first time the grain of negative material started to matter. I thus believe that the use of Moore's law is wrong. At the time black and white negatives was invented, they started with some resolution which was easy to produce and the improvements before 1940s was probably much less dramatic than improvements after 1940s till present day.
We can thus assume that the film resolution remained constant from the time film was invented to 1940. Lets repeat the estimate using data from 1940. 1940s film is rated to 3000PPI, with 60% loss I get 1200PPI.
|Crop scaled to 1200DPI and back to 5588DPI|
There is another mistake in my estimate - the photograph is taken by very laborious wet collodion process (used by my great-great-grandfather till early 1890s), where the glass plate had to be prepared on the spot, exposed and developed while still wet (so photographer had to carry a tent with mobile darkroom). Wet process was well received for its low grain and used for reproduction long after the dry negatives emerged.
Lets thus assume that the negative has infinite resolution and look into what lenses was available in 1880s.
|Date||Cause of Improvement||Professional Large Format in lp/mm||Amateur - Box Folding & PnS in lp/mm||Professional Medium Format in lp/mm||Pro & Amateur Small Format lp/mm|
|1893||Goerz Dagor Achromat||40-60||20-40||NA||NA|
|1925||Leica RF/FPS Elmar||40-60||20-40||NA||50-70|
|1929||Rolleiflex MF Zeiss||40-60||20-40||40-60||50-70|
|1948||Hasselblad MF Ektar||40-70||20-40||50-80||50-60|
|1949-59||first SLRs - C, N & Z||40-70||20-40||60-100||40-80|
|1960-70||adv lens coatings||40-80||20-40||70-100||40-100|
|1975-88||LF lens coating||40-90||20-40||70-100||40-100|
|Lens Resolution Estimator.|
Timothy Vitale: Estimating the Resolution of Historic Film Images: Using the Resolving Power Equation (RPE) and Estimates of Lens Quality.
In every case, the resolving power of lens should be at least 25% more than the actual resolution of the image. It still seem unlikely Ignác was able to reach this resolution with contemporary camera, bcause lenses resolving over 50lp/mm became available only in 1890s after the houses photographed has been demolished.
By family legends Ignác traveled to England and Hungary to obtain quality lenses. Perhaps he also traveled in time for his shopping?
|My great-great grandfather visiting London.|
Digitization guidelinesWhat are the official guidelines for digitizing glass plate negatives? The answer varies but mostly it is well bellow my observations:
- U.S. National Archives and Records Administration (NARA) Technical Guidelines for Digitizing Archival Materials for Electronic Access: Creation of Production Master Files – Raster Images recommends scan of 6000 pixels over the long edge, about 135MB of RGB data, that is 45 megapixels.
- Library of congress digitized civil war photographs at about 40 megapixels each negative (resolution depends on the size of the negative).
- Federal Agencies Digitization Guidelines Initiative: Technical Guidelines for Digitizing Cultural Heritage Materials recommends to digitize Photographic Still Film to 4"x5" at 1000PPI (1 star) to 4000PPI (4 star). The 4 star recommendation is fine, but if the glass plate was an inch bigger, it would be scanned at 500PPI (1 star) to 2000 PPI (4 star)
- Digitization Standards for the Canadian Museum of Civilization Corporation recommends 300 ppi, with a minimum of 3000 pixels for the longest measurement (8x10, or 1:1 for formats larger than 8 x 10)
- DFG Practical Guidelines on Digitisation recommends:
Medium format: 4000 dpi x 0.03/0.05 = 2400 dpi
9 x 12: 4000 dpi x 0.03/0.1 = 1,200 dpi
18 x 24: 4000 dpi x 0.03/0.2 = 600 dpi
Digitization of historic photographs should be done in a way that all details of the originals are captured in the digital file. Here I focused on the black and white negatives which are among the finest and also show the largest variety. The resolution of photograph can be estimated based on year it was produced and the size of negative using data presented by T. J. Vitale's work.
I can show a real example of photograph taken 1880-1885 which rivals best digital pictures we can take today (at least in the very center). Image details reaches resolution over 2000PPI and needs scan at over 90 megapixels. I would say that really fully satisfactory scan of the presented photograph (in the terms of resolution) should be at least 4000PPI, or 360 megapixel. Doubling actual image resolution is useful to make photograph damage easily distinguishable from original image (in fact, T.J. Vitale recommends to multiply by 4 for perfect capture, this is supported by Nyquist theorem). Quite a monster which exceeds most of the recommendations of the digitization guidelines with exception of FADGI.
Following guideline of T.J. Vitale, one gets estimate of 600PPI. The prediction of film resolution before 1940s, presented by T.J. Vitale, should thus be taken with a care and probably one should stick to the published data (that is, all dry plate negatives before 1940s should be believed to resolve approximately 3000PPI) and treat wet process negatives even more carefully. Negatives predating 1940 are rare and precious and the extra disk storage for a good scan is worth it.
Next I will give examples of high resolution dry process negatives from our collection and move on to the question of dynamic range, bit depths and other aspects of a good scan.
Update: This blog post describes very similar experience.