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# Article: Flash-to-Subject Distance Calculator Utility

1. ## Flash-to-Subject Distance Calculator Utility

Here is a handy little software utility that I wrote for my own use and have been using for several years. I just completed some upgrades and thought it might also be useful to a few others.

This utility is designed to create flash-to-subject distance tables for any source of flash illumination whose guide number is known, or can be measured. It does nothing that you cannot do for yourself with a hand calculator. It just does it faster and more conveniently.

I use it primarily for producing small printed tables to attach to various flash units and older antique cameras. These tables replace the often worn and unreadable, missing, broken, or poorly designed calculator dials. It can also be used for producing subject distance tables for flashbulbs where no dial exists.

But it also has several other oddball uses, such as calculating intermediate f/stop values in arbitrary fractional stops, or figuring out subject distances for pinhole cameras, or producing multi-column distance tables for certain flash units with multiple zoom settings, or even figuring out how much further back a person must stand to reduce a fixed flash illumination by, say, exactly 1-4/7 stops.

If you only ever need to do the one-off rough mental calculation of GN/fstop=distance, after rounding the numbers to make it easier to think quickly, this utility is likely not for you.

But if your needs are more complex, such as requiring multiple distance tables for differing output flashbulbs as a function of varying Guide Numbers defined by varying leaf shutter speeds, perhaps even after having also determined your own non-standard GNs for each bulb type you use, this utility might be real handy to do the calculating grunt work for you.

It's also great for doing what-if scenarios by quickly changing the variables around to help determine what source of flash illumination will work best for your anticipated situation. Especially where such situations fall into the oddball category.

Because it was originally created only for myself, it should not be considered as a polished piece of commercial software. It is a small Windows-based command line utility whose simple character-based interface is all that is required. No visual interface version exists. No smartphone version exists.

The software requires no formal installation. There is only a single self-contained executable file which is available for download at the end of this article. Once downloaded, simply place it anywhere on your Windows computer that is reachable from a command window either directly, or by the PATH environment variable. That's all that is required. To uninstall, simply delete it.

The best way to demonstrate how to use this utility is by some examples. Feel free to review only as many as you think you might need, then just start playing with it using your own examples.

After copying it to your system, start a standard Windows command window as you normally would, then replicate the following command line entries as shown.

The first example command line entry below is "flash /?" Then press the Enter key.

The result is the standard title page and usage options Help screen. It displays all of the required and optional elements and capabilities of the utility:

Of the options shown, the only required command line elements are the first three numeric values. In the above Help screen example, those are 160, 100, and 2.

The first value, the Guide Number, can be entered as either feet or meters. The second, the Exposure Index of the film, is also known by the older term ASA. And the third, a fractional f/stop value, represents the partial f/stops to be displayed in the generated table.

A fractional value of '1' means display only whole stops. A value of '2' means display to 1/2-stops. A '3' to 1/3-stops. And so on.

These required three values must always be entered in this order. Additional values beyond the first three will be ignored. If less than three are entered, the utility will prompt you for the missing ones.

All of the other entries precede by the '/' character are optional command line switches that alter the behavior of the application. Some result in visible differences. Others in internal calculation differences.

Using the three basic values from the Help screen, here is just about the simplest possible run of the utility:

The table produced is for a flash with a Guide Number of 160, used with a film speed rated at ISO (ASA) 100, and displaying the results in 1/2-stop increments.

The table header shows the film ISO being used. It also shown a NORM column identifier (more on this later, for now it just means the column represents the flash unit's normal standard output). Below the NORM it shows the overall Guide Number for the combination of film speed and flash output in use.

On the left is a column representing the applicable f/stops. And on the right are the calculated flash-to-subject distances for each fractional f/stop shown. Since the Guide Number on the command line referenced feet, then this column also represents distances in feet.

A slightly more complex run is shown below:

This table is for a Sylvania Press 25 flashbulb using the manufacturer's suggested Guide Number of 220, an ISO 400 speed film, and a table display resolution of 1/4-stops.

Additionally, the /s switch has been added which results in the calculation and display of all of the actual 1/4-stop intermediate values. The default precision for the /s option is two significant digits. To make them easier to pick out visually, whole f/stops are marked with an asterisk.

The fractional f/stop resolution (the third command line numeric value) can be almost anything you wish. If you have a pressing need to see a distance table in 1/876th stop resolution, you can do that. Same for a resolution of 1/123456789th stop. I won't try those here as the tables would be huge and time-consuming. But if you really need it, you can do it.

Another sample run also showing calculated intermediate f/stop values:

This time it's a no-name generic GN 80 flash that your flash meter tells you is only outputting at GN 68, paired with Ilford FP4+ at box speed, and displayed in half-stop increments, where the fractional f/stops are calculated out to 5 significant digits. Note that the effective Guide Number for this combination is 76.

This /s switch is a different variation that allows selectable significant digits from 2 (the default) out to a maximum of 5. This might seem like overkill. But if you decide your table needs an f/stop resolution of 1/13-stop, this switch might come in handy to distinguish between adjacent fractional values.

Practically speaking, of course, there's no way on standard cameras to set the aperture to this level of fineness. But the values can be generated for you anyway, if you want to see them.

The utility also contains switch options for several manufacturer-specific flash units. If you happen to own a Vivitar 285HV flash, here's an example run for that specific flash:

The distances here are for the manufacturer's suggested GN of 120, used with Portra 160, and displayed in 1/3-stop increments, with those increments calculated and displayed to 2 significant digits.

You are not limited to using only the manufacturer's suggested GN. You can plug any value you like into the first command line value. That means if you have actually measured your 285HV and discovered it's really outputting at a different (usually lower) level, you can use that more correct GN level to create a more correct table.

However, if you do use a custom GN value, the utility will output a small warning above the table just to remind you to be certain that's really what you wanted to do.

The above table differs from the earlier examples in the number of distance columns presented. This is because the Vivitar 285HV has both a 3-zoom mode, and an external extra-wide-angle diffusion filter. The NORM column is the standard output setting for 50mm normal lenses in the 35mm format. There are also columns for TELE (105mm), WIDE (35mm), and DIFF (28mm, when the zoom is set to NORM).

Note that each zoom setting column has its own unique calculated GN specific to its own combination of variables. This is the number you would use if manually figuring the distances in your head. (You would probably want to round them to the nearest convenient values for ease of head-based calculating on the fly.)

The utility also includes other switches for the Sunpak 622 Standard and Sunpak 622 Super flash units. These also have TELE/NORM/WIDE settings, and thus also produce multicolumn tables.

Here's a table for the Sunpak 622 Super:

This configuration uses the 622 Super at the manufacturer's rated GN 200, Kodak TX320 sheet film, and a table display of only full f/stops. There are no intermediate values since a '1' was specified in the third command line value.

The difference here is that the /e extended switch is included on the command line. This option extends the table out from the default of f/32 to a minimum value of f/1024. This option may be useful to those who may wish to calculate flash-to-subject distances when using pinhole cameras.

If your particular pinhole f/stop is not visible, you can play around with the f/stop resolution value (the third numeric value on the command line) until you find a close approximation to it.

For example, if your pinhole works out to an f/400 value, then trial-and-error should pretty quickly converge to a table showing intermediate f/stops in resolutions of 1/7-stop, where f/400 is then 2/7 of the way between the whole stops of f/360 and f/512, with the resulting displayed distances accurately calculated.

For the Sunpak 622 Super on full manual the proper distance to the subject at f/400 is a mere 0.9-feet (or about 10.8-inches) when the head is set to NORM and the film is TX320.

(It's worth noting that at these very close distances it's probably wise to test the individual flash units in advance with a trusted flash meter and use the refined GNs that it indicates with this utility.)

The entire example table is too long to include, so here is the applicable table segment for the above example, with the f/400 line highlighted. The command line "flash 200 320 7 /622s /e /s" was used:

Perhaps you have a an old lens whose maximum aperture that is an odd value you don't immediately recognize and you would like to know where that aperture value falls in fractional f/stops. A simple trial-and-error result like the following can quickly help you narrow it down without needing to look up and use the formula.

For example, recently an Olympus OM 600mm f/6.5 telephoto lens was advertised in the Classifieds forum. So just what fractional f/stop is that? Trial running the following table to display a resolution of 1/5-stops gives the answer.

An f/6.5 lens is 2/5ths of the way between f/5.6 and f/8.

Or perhaps all you would like to see is what the values of the intermediate f/stops between f/5.6 and f/11 really are at a resolution of one-fifteenth of a stop to a precision of 5 digits. Not sure why. But maybe you want to.

The command line "flash /s=5 0 0 15" will produce the following table, from which I have clipped out the following partial display:

The /s=15 option, like all of the switch options, can be placed anywhere on the command line, including between any of the required three values. Here I placed it first so the only number I might anticipate repeatedly changing, the f/stop resolution, is at the end of the command line for convenience.

And finally, if you enter the command line Guide Number value (the first of the three values) as a value in meters, then all of your distances will be presented in the generated table as meters.

I should mention that if you decide to generate a multi-column table in meters for one of the zoom flashes you should also add the /m switch to the command line.

This switch subtly modifies the internal calculations to reflect the fact that a flash's published Guide Numbers in feet and in meters are normally rounded values that do not exactly convert to each other. The rounding errors are often minor, but they do exist. Using the /m switch corrects for that discrepancy.

OK, just one more for fun...

This example shows the distance table in feet for a pair of currently-still-in-production Meggaflash PF-300 flashbulbs, fired simultaneously, using Ilford Delta 3200 film at box speed, and displayed in one-half f/stop increments out to 3 significant digits. The GN for this combination is an astounding 4400.

The PF-300 flashbulb has a manufacturer-rated Guide Number at ISO 100 of 550. Two of them fired together are therefore equal to 550 x sqrt(2) = 777.8174593052023. If used with a Graphic 4x5 press camera mounted with a standard 135mm f/4.7 Optar lens wide open, the flash-to-subject distance for a properly exposed negative is a cool 925-feet.

Not only does this example show that the Guide Numbers (and Exposure Indices) may also be entered as non-whole-integer decimal values, it also shows that two of these on-camera Meggaflash PF-300 flashbulbs would probably suffice to light up the entire Grand Coulee Dam with that particular film.

O. Winston Link would be proud...

Flash-to-Subject Distance Calculator

The utility will download as FLASH.BIN. Simply rename the file to FLASH.EXE, copy it to a reachable location, then open a command window and run it from within that window.

If you are presented with a security dialog saying "The publisher could not be verified" just go ahead and uncheck the "Always ask before opening this file" checkbox, then click the Run button. I am the unverified publisher. Doing this will prevent seeing the security dialog again. At least on my Windows 7 system.

Ken

2. ## Version 1.30

For the record, the initial release in the original article is Version 1.30. Any future updates will be documented here, and remain available from the same download link at the bottom of that original article.

Ken

3. Wow, excellent thank you!

4. of course,I have to ask: where is the Mac version?

5. Originally Posted by RalphLambrecht
of course,I have to ask: where is the Mac version?
Waiting for someone to buy me a Mac??

Seriously though, I don't have access to, or much development experience with, Macs. I do know there are a number Windows and DOS emulators available for Macs. These seem to range from full-featured and reportedly excellent (VMware Fusion, Parallels Desktop 9) down to simple freebie MS-DOS emulators (DOSBox for Mac, see: YouTube video).

Because no one wants to spend money in order to run a free utility, I might take a look at the DOSBox solution first. It's a little more techie to get going. I've used the version for Windows to run an ancient MS-DOS planetarium program I still keep around and that works pretty well. But I've never tried the Mac version. Nor have I tried running a Windows command-window-targeted executable under any DOSBox MS-DOS emulation.

Maybe I can try that tonight when I get home from work?

Ken

6. Well I did try and as it turns out the Windows DOSBox 0.74 MS-DOS emulation is actually a pretty good implementation. So good, in fact, that it correctly identifies the utility executable as being stubbed a Windows 32-bit Portable Executable (PE) format file.

That means it won't run in the Windows DOSBox environment. You (correctly) get the default stub notification "This program cannot be run in DOS mode." I would presume the same would be the case for the Mac DOSBox emulator as well.

Ken

7. Just a short notice, especially to those many who continue accessing this article from outside of APUG...

The bad news is that my ISP has recently informed me that they are permanently discontinuing their personal web space storage feature for all of their users. The cutoff is currently stated to be October 5, 2015. Unfortunately that web space is where the utility that is the subject of this article is located. So after that date the utility will no longer be available at the download link provided above.

But the good news is that it's now been a year-and-a-half since I first posted that original link, and during that period there has been a fairly large number of improvements and new features added to the utility for my own continuing needs and use, along with a few bug fixes. At this same time APUG itself is about to undergo its own major upgrade within the very near future, and I'm not yet certain how the Articles section may be affected.

So once the APUG upgrade has been completed and stabilized I intend to then upload the new version, probably also with a new updated article listing the improvements and illustrating how to use everything. As well as providing a new link for downloading it.

Ken

8. Originally Posted by Ken Nadvornick
The cutoff is currently stated to be October 5, 2015.
Oops. My mistake. The cutoff is actually stated to be October 8 2015. Meaning the final active day is October 7th.

Sorry.

Ken

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