USAF 1951 test chart in SVG format

[Jim Jones]

Ramon -- Google for NBS 1010a. There seems to be lots of information, although much of it will require interpretation.

[Emmanuel BIGLER]

How can I translate this number to dpi?

Simply check on the actual target what is the actual dimension labeled '150 mm' ; but you can derive it as explaines below.

It might be a true 150 mm ( I hope _not_ ! since it would mean that your scanners resolves only 1.4 cycle / mm )or anything else, this is not very important.

In the jpeg image you've posted, '150 mm' corresponds to 469 pixels, 469 samples. For example, if the image was sampled at 2400 samples per inch = 94.5 samples per mm, the '150 mm' dimension is actually 469/94.5 mm = about 5 mm. So if "150 mm" are in fact 5 mm, one mm is in fact demagnified by 150/5 = 30x.
When you have the actual dimension of this '150 mm' size, you can easily derive what 1.4 means.
The white bars labeled '1.0' feature 1 cycle / mm when the '150 mm' bar measures actually 150 mm. In the image they correspond to 469/150 = 3.1 pixel for one period.
1 cycle/mm means that the (bar+space) size is 1 mm
the 1.4 pattern corresponds to 1.4 cy/mm when the '1.0' pattern corresponds to 1 cy/mm.
in the image 3.1/1.4 yields 2 pixels per perdiod, i.e. the ultimate sampling limit for any digital sampling of a periodic object.
If you know the theoretical number of samples per inch in your scanner, you could easily derive the actual limit.
If this targer has been scanned at 2400 samples per inch, it means 94.5 samples per mm. the optical limit, @2 samples per period, being 94.5/2 = 47.2 cycles/mm, about 1.4 x 30 where "30" is the de-magnification ratio in use when the microfiche was made.

Summary in 3 lines
- find the actual size of the '150 mm' bar and derive the de-magnification ratio of the microfiche
- multiply the number : 1.0, 1.25, 1.4 etc that you read on the target by this ratio, e.g. 30, and you -
get the resolution limit in cycles/mm.
- to convert cycles/mm into samples per inch : 1 cycle/mm = 25.4 cycles per inch ; 2 samples are needed per cycle. hence 47.2 cycles/mm are 47.2 x 25.4 x 2 samples per inch = about 2400.

[ramon]

Hello Emmanuel,

Thanks for your detailed explanation.

The image I previously posted was scanned at 12800 dpi.

But size was also scaled to keep the file under 1 Mb, so pixels do not correspond with original size.

I have made some numbers and your calculus is very close to the actual enlargement (32.4x).

I obtained that the resolution of my scanner is near 2308 dpi.

This also proves why I cannot get any improvement scanning at more that 2400 dpi.

Scanner interpolation does a good job, but this also could be done postprocessing the image.

Next step I want to do to improve image is trying to find the best focus position.
I want to attach the film test target to a flat acryllic sheet rised in each corner with nylon washers.

Thanks

[Emmanuel BIGLER]

Hello Ramon
I'm was not sure that my explanations were clear, so I'm glad that you could work out the experiment.
Actually resolving above 2400 ppp (1200 cycles per inch) is something not so easy to reach for amateur-grade affordable flatbed scanners, but using a microfiche with a test target is a neat idea that will not cost and arm and leg !

[Leigh]

Please understand that the test target was already degraded by the process used to create the microfiche.

An accurate evaluation of the scanner would require scanning an original target, preferably chrome on glass.

- Leigh

[ic-racer]

Good news !

I have found a test resolution target in the last frame of a microfiche ($3 USD at the auction site)



With my scanner's (Epson V700) resolution is near 1.4.

How can I translate this number to dpi?

Thanks

?
Looks like you have someone else's resolution test. You need the chart they photographed with the microfilm, not the microfilm.

[Bill Burk]

Please understand that the test target was already degraded by the process used to create the microfiche.

An accurate evaluation of the scanner would require scanning an original target, preferably chrome on glass.

- Leigh

Yes, the target is already degraded once it is photographed (as this example photograph on microfiche). And the numbers can probably never be truly meaningful. But as a low-cost target to find the approximate resolution of a system, this seems like a neat trick.

There probably aren't many film targets that are higher resolution or finer grain than microfiche.

[Leigh]

Hi Bill,

Certainly, as a quick check, such as for focus, and an inexpensive target, the microfiche would be quite satisfactory.

There are much finer targets available, but they're expensive.

- Leigh

[Struan Gray]

One option for hi-res, high-contrast targets would be a semiconductor lithography mask. They tend to be chrome on glass (quartz if you pay more) and 5" square is a standard size that would work well with most photographic applications. They are robust enough to be used as a master for contact printing or placing on a scanner platten directly.

There are online companies who will allow you to upload a file and they will send you the mask by mail soon thereafter. But you'll need to use compatible design software to create the design file, and they won't hand hold you much, if at all. But mask-making is often an in-house service at university engineering departments and high-tech startup incubators and you may be able to find someone local who will help you with the design and produce the actual mask for a cheaper price. It's easy to over-specify things like the line quality and end up paying too much, so, again, it's good if you can find a real person you can talk to - you'll get what you want, rather than what you ask for.

I used to get 5" masks for around €100, but that was an internal price and I did the design work. It's a commodity job though, so commercial suppliers were not much more expensive.

[Jim Jones]

I've used an ultramicrofiche Bible on a 2" square chip https://patriotpostshop.com/products/166 as inexpensive target for quick projection testing of 35mm camera lenses. The detail is much finer than a consumer flatbed can resolve. These chips can be placed on a flatbed scanner with one edge raised a few mm to determine optimum height of film.