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Forum: Flashes, Lighting, and Studio 12-31-2014, 08:09 PM

GN calculator.
Posted By 45 Mike

Replies: 22
Views: 4,098

sigh.

Please really think about what a set up would require using several flash units with different guide numbers.
How would you set up the ratios?
Assume your biggest flash will be your main. set that main, and fire it on the meter.
Now guess where to set a fill, for 1/4 ratio, set it, fire that flash on the meter. Is that flash set to 1/4 ratio? No? then change it and reflash the meter. Is it right yet? No? Retest until it is.
Now add a hair light, set it and test it with meter, keep adjusting and testing until it is right.
Now flash the whole setup, on the meter, and set up your camera, take a test shot. chimp.

Then, consider the problem I posed in my original post, are you certain that a meter will answer that correctly?

With the calculator, you set the main light at the distance and power calculated, set the fill light at the calculated distance and power for 3 stops down, set your hair light, test shot, chimp.

Just saved about 6 steps and numerous test flashes.

I get it, that some people have no use for this calculator. However I have seen other people request this, and I was looking for one myself, and no one had made one.
So I made it myself, and I am giving it away for people who might want it.
I am ok, with you not wanting it.

If people can afford to buy a $300 flash meter, then fine, they are welcome to it.
For those who just want a flash GN calculator, I built one.
http://45ink.com/gallery/images/demo/Flash-guide-slider.pdf

Forum: Flashes, Lighting, and Studio 12-31-2014, 06:11 PM

GN calculator.
Posted By 45 Mike

Replies: 22
Views: 4,098

This is the GN slider part, I need to build the reverse side for the log-log side, and then the cutting folding marks. but it is useable as it is if you cut out the blue area, and cut the grey area out on the FAR outside line, (giving you a way to move the slider easily under the cut out top part.
http://www.45ink.com/gallery/images/demo/Flash-guide-slider.pdf

Forum: Flashes, Lighting, and Studio 12-31-2014, 04:29 PM

GN calculator.
Posted By 45 Mike

Replies: 22
Views: 4,098

... snip...

A flash meter would not do what this does. That is set up the lights and ratios of light without actually firing the flash(s) individually and in concert. This would assist a meter to get the ratios correct, and then a reasonably accurate first set. Someone who has used a flash set up over years, will have guesstimated a set up and then refined it by trial and error, then that prior knowledge would do be the basis for setup. Without that prior knowledge, it's a setup, test, setup test setup test situation. Whether that test is with a meter or polaroid or digital, the process is setup and test and retest until the setup works. The calculator can reduce that retest cycle considerably.

I REALLY like your rings!! The only reason I am going with a slider is that allows me to use a log-log on the reverse to add up the photons.

I don't think dropping the ISO designation is a good idea at all. If I am shooting paper negs at ISO 3, then the calculator should let me designate that I am using ISO 3, not +5 EV from zero.
I looked at your rings and saw that my own setup requires an added step and conversion factor. Your rings require a rotating index. I reconfigured to allow a single slide and a static index. I had the fstop indexed with the GN, and setting the distance indexed with GN solves that extra step and conversion.

Thanks for your input!!

Forum: Flashes, Lighting, and Studio 12-16-2014, 11:01 PM

GN calculator.
Posted By 45 Mike

Replies: 22
Views: 4,098

The problem is that a guide number dial or chart that comes with a flash is only valid for that flash, or another flash with the same guide number. That is assuming that the guide number is accurate for that flash in general use, not in a lab test.

For instance, my Holga flash has a GN of 72, rated by the manufacturer. However in real use I might find that it actually has a guide number of 68. The chart they have on the back lists ASA to 800, but does not help me if I have my ISO set to 1600.
In addition, that chart does absolutely no good for my GN 125 flash. That flash has power levels, and can be rotated for bounce. Then what about using an umbrella, or softbox?
On the Holga the ISO listed to 50, and then 25, but what if I am shooting ISO32 film, or ISO 3 paper negatives?

If all I shot was ISO 100, then yes, I could use the dial, set it to the GN value of distance/aperture that gives me GN, then I would have all the distance/aperture settings for that flash, but ONLY at ISO 100.

That dial no longer works for ISO 64, on a different GN flash. That is what I am doing, building a tool that handles any flash at any ISO. The tool that I have gotten so far does require an index shift for changing GN to get accurate values for distance/aperture. A dial or slider without that index shift ability would not work.

---------- Post added 12-16-14 at 22:16 ----------

I have several different flash units. 3 of them have GN listed as meters and as feet, one has a chart listed in both meters and feet and one has a slider listed in feet only and one has a GN given in meters, with no chart or dial, or any thing other than GN in meters. One is a full auto, with no manual mode, no chart, no dial, no GN.

If I had ONLY one flash unit, or model of flash unit to work with, then yep, a chart and a couple values for different conditions, umbrella/softbox/bounce etc would likely work.

However having four totally different flash models, all with different GN to deal with would require 4 charts, and 4 sets of reminder values. Not an elegant solution.

I think many people are in a similar position, they may have multiple flash units, that have different light outputs, and want to use them in multiple lighting setups, with manual control at non listed ISO values.

If that is true, then they might want to use a tool to manage all those lights, without having six charts and six sets of reminder values, and a digital calculator to juggle.

That was my goal, to get away from multiple charts and use a single tool, that does the work, with reasonable accuracy and assures me a reasonable exposure.:)

---------- Post added 12-16-14 at 22:31 ----------

Thanks, yes, I can build a chart for each flash that I use. In fact I did build charts for not only the flash units I do have, but also flash units that I don't have and prolly never will have, for instance I have a chart built for a flash with a GN of 800 and I have a chart for a GN of 25, and several in between. I built those charts to let me design a slider that works for GN values between 3 and 6400.
Why would you need a GN of 3? Or 6400?? If your flash has a GN of 125 @ ISO 100, what is your GN @ ISO 3200? Or ISO 3??
However, I have built the slider. It works.
I just need to get it laid out in the 2D CAD program that I know nothing about yet and get it outputted as a PDF, then I can share it with you folks.
:hmm::lol:

Forum: Flashes, Lighting, and Studio 12-16-2014, 01:32 PM

GN calculator.
Posted By 45 Mike

Replies: 22
Views: 4,098

I ran into a snag. I have it worked out and am trying to figure out a 2D CAD program to lay it out from hand drawn papers to an actual slider. The snag concerned how the index mark must be adjusted as the GN changes to get correct distance/aperture values.
I worked that out. I originally thought that that i would need separate sliders, one to calculate GN/ISO and another to provide the distance/aperture values, but it appears that I can do that with one slider.
Now, I have seen flashes that only give a GN in meters, and only in feet. I can build two versions, one for meters and one for feet, or just provide a table to convert from feet to meter GN values. Not sure which would be better. Or if any conversion is actually needed.

Forum: Flashes, Lighting, and Studio 12-10-2014, 05:00 PM

GN calculator.
Posted By 45 Mike

Replies: 22
Views: 4,098

I get it. I did create spreadsheets to plot the data to use to make my tool.
I do not want charts, graphs, curves, plots to pack around. I want a simple slider that tells me what GN my flash is when I use ISO 32 film, or ISO400 film.
That slider then tells me the range of apertures and distances for correct exposure at that GN.
A slider is easier than a wheel guide for me to make, so that is what I did.
It works!
OK, at this time I have a single slide, that coverts GN for ISO range from ISO 1.5 to ISO 6400, and gives aperture/distance values for GN values ranging from GN 6 to GN 3200. Aperture ranges from f:1 to f:90, and distance from 1 foot to 208 ft.

I could leave it there and it would be adequate for much of my needs, but I also want to have a log-log slider that allows me to add up the photons from multiple flashes and derive a GN from that.

If there is no interest, that is fine, but if there is I have the GN / exposure slider working, and am working on the log-log and directions, with flash info pockets.

Forum: Flashes, Lighting, and Studio 12-10-2014, 01:03 PM

GN calculator.
Posted By 45 Mike

Replies: 22
Views: 4,098

Yes, and I did that by hand to build the scales for the ISO conversion. A spreadsheet is good for when I am sitting at a computer, not when I am working with a camera and lights.

Again, the answer is not to find or build some chart, but to build a tool, that does the job, the complete job, not just a small part of it.

For instance, your spreadsheet. Does it solve the film lighting problem I originally posted?
Look at that problem, and consider the steps, and calculations needed to answer that.
What about multiple lights? With different guide numbers? What steps are needed to solve that situation?
If we shoot digital, then chimping is obviously the mode we use, and eventually we can arrive at a working set up. With film, that is not a good option, especially with some film like 4x5 where a single negative can cost upwards of $5 per shot.
With continuous lights we can set up the lights, go for composition, ratios and then meter for exposure. With flash, we need to figure the exposure before hand, based on ratios, then go for comp.
Totally different process.
Thanks!

Forum: Flashes, Lighting, and Studio 12-10-2014, 11:45 AM

GN calculator.
Posted By 45 Mike

Replies: 22
Views: 4,098

No, unfortunately I have no zoom flashes. However, if you were to record true GN at each zoom setting, then my calculator would work well for you.
I am aiming for an accuracy of fairly close to 1/3rd stop, and I think if a zoom were recorded at full wide/normal/full tele then those three values would be sufficiently accurate for getting very close to good exposure.

Forum: Flashes, Lighting, and Studio 12-10-2014, 01:01 AM

GN calculator.
Posted By 45 Mike

Replies: 22
Views: 4,098

##UPDATE##
I have built a pdf file that does what I want.
I will update again when I have cleaned it up with the cut and fold marks.
http://45ink.com/gallery/images/demo/Flash-guide-slider.pdf

When I have it completed, with log-log scale, and full directions and assembly instructions, I will post the link to that separate.

Basic usage:
1. Set ISO/ASA 100 to your flash Guide Number (GN)
2. Find the ISO of your "film", and the GN of your flash is displayed above that ISO mark.
3. Now set ISO 100 to the GN you calculated from step 2.
4. The f:stop and distance scales below will now show valid exposure settings for your flash at the desired ISO.

Example:
flash GN = 72ft
set 100 ISO to GN 72
Film ISO=32
GN= 42~
set ISO 100 to GN 42
bottom scale indicates f:8 @ 5 feet

I have done several web searches for a circular or slider flash guide number calculator, but there just does not seem to be any out there.
On other forums, I have seen people ask about how to get one, and the responses they get generally come in three flavors:
1. Use an app for your iPhone.
2. You don't need one, do the easy math in your head.
3. Test your flash, and write your results on a paper and tape it to your flash.

Well, I don't have a smart phone, and I don't want to spend money on a phone, I want a dedicated slide or circular guide for my flash usage. I don't want to look up a web page to use an online calc.

As for # 2 and #3, quick,
You have an MX with a 35-80mm f:4.5 lens, you have a manual flash with a factory GN of 72 but testing shows it actually rates a GN of 68, you need to shoot through a white umbrella and the flash MUST be at least 5 feet from the subject, otherwise it gets in the frame. You need minimum f:11 for depth of field. Can you use that flash? What aperture? what distance? Oh wait, I forgot to mention you are using ASA 64 film that is 15 years old, so you need to rate it at about 32-40 ISO.

Yep, so the answer then becomes, well see, what you need is a flash meter, cuz all them variables just have to be measured.

Umm, well, no, I need a flash guide number calculator. Just like I said I need.

Anyways, No One makes one, so I am doing it myself.
The old Kodak circular guide is not for modern usage and does nothing for modifiers.

I just finished proof of concept on the ISO conversion scale. I know that works. The GN to dia/dis scale should be relatively trivial, and the log-log scales are well documented.

So, this is my project.

Test a flash at ISO 100 to find accurate GN, test that flash with modifiers to get modified GN values, for instance, outside in dark, as opposed to indoors in a white studio, and again in a normal "home" space. Record those base GN values for each flash used.
(I currently have two GN 125 with power level adjustments, two manual GN 72 flashes, and a GN 78 flash.)

Use the recorded "true" GN and convert to GN of ISO of actual film/sensor being used.
If multiple flash used, repeat conversion with each flash used, calculate "photons" used by the light package as a GN, set exposure of camera to match, then adjust each flash distance to match exposure aperture, using the individual GN for each.

Now I have a VERY good confidence that my lighting will give me a proper exposure, and I didn't need a phone, or a meter to do it.

Anyone else interested in trying out my calculator when it is done??

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