# Working out Manual Flash Exposure with Macro



## weepete (Sep 5, 2013)

Hi all,

I have a few e-bay goodies on order (ocf clamp, some radiotriggers and a pc sync cable) so I can use it with my extension tubes, reversing rings and flash (yn-460) to enable me to get some macro shots. My question is how do I work out manual flash exposure using the cameras meter? I'm scratching my head a bit with this...


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## GDHLEWIS (Sep 5, 2013)

I had a similar issue a few weeks back. In the end I found it best to take multiple shots of the same thing but with different flash settings, it is honestly the best way to learn but it will take time and a bit of effort as you will also need to document each setting so you can reference it to the corresponding photo. I have read a bunch of different books about flashes, how they work and how to get creative and the best is just trial and error in the end. 
Good luck and enjoy


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## SCraig (Sep 5, 2013)

Use the standard formula:  F  (f-stop) = G (guide number) / D (distance to subject).  This is normally at ISO 100.  That will give you an f-stop for proper exposure, however shooting macro you will probably need to lower the flash power several EV from that.


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## Dao (Sep 5, 2013)

Trial and error works for me.

It seems like you are taking macro shots.   I will set the aperture to f/8 to f/11 and then shutter to or close to max sync speed of your camera (ISO 100).   Take a test shot and then adjust the flash power to obtain a properly exposed photo.   Of course, if you are able to use a external flash meter on the subject you are planning to take a photo, you can use that instead of guessing.

The amount of light from the flash depends on few factors.  The power output, the distance and the diffusion material if any.  But the test shot should give you a hint on whether you need to increase or decrease the flash power (or the distance)


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## cgipson1 (Sep 5, 2013)

Depending on your subject, and the distance to the subject... and the amount of DOF you want for that subject (which as I am sure you know, DOF is very shallow when shooting close macro subjects)... I would set your aperture first... and then calculate the flash output to match the distance using the formula Scott listed above. Depending on how you are modifying the flash... you may need to increase / decrease flash output to get the exposure you want.


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## tirediron (Sep 5, 2013)

You can't - your camera's meter cannot calculate manual flash exposure.  As Scott suggested, you can use the GN calculation method, but at macro distances that may not be terribly reliable.  The most accurate and reliable method is with a hand-held flash meter.  (That's just one example - there are LOTS of options).


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## weepete (Sep 5, 2013)

Thanks guys.

I didn't mean I would be trying to measure the flash output with the meter in the camera, more use it as a starting point to work out the exposure. For example say I meter off the subject at 1/250th at F2.8 and get a correct exposure (0 EV), then I stop down say 3 stops to cut out the ambient light so I get 1/250th f8 I then need to add in 3 stops of light to get the exposure of the subject back. So how could I calculate what the flash power I need to do this?

Or am I completley on the wrong track....


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## amolitor (Sep 5, 2013)

Are you trying to replace N stops of ambient light with Y stops of flash? You can't, really. A "stop" is not an amount of light, like a cup of water or a pound of lead, it's a relative measure like "twice as much water" or "half as much lead".

Still, you can do some calculations. First calculate the right exposure for 100% flash using the GN. For this you need the effective aperture, which is:

Effective Aperture = Lens Aperture x (1 + %Magnification)

More or less. Large format guys call this "bellows factor" I don't know what macro guys call it.

So now you know how to expose the thing right with ambient light, OR with flash. You have some power level for the flash that squirts out the same amount of light as the ambient light.

If you cut out 1 stop of ambient, you've cut the ambient in half. You need to put that back. Use half power on the flash, or back off 1 stop on it.
If you cut out 2 stops of ambient, you've cut the ambient to 1/4 of itself. You need 3/4 of the total light back from the flash. Use 3/4 power, or back off about 1/2 a stop.
Past that, just use the full power of the flash.

These are not 100% accurate, but they should get you within a 1/2 stop or so. Shoot RAW and fix it in post


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## Derrel (Sep 5, 2013)

Flash exposures are based on Distance to the subject, and the "Guide Number" (should have been called the Power Level, but oh wellll...).

Here's a scan I made of a Guide Number mechanical flash calculator, in the classical rotary calculator form. You could easily print this out and cut it apart, and using two paper plates and a single brad, make yourself a nifty Flash Exposure Calculator.




As shown, the calculator was set to a Guide Number (common abbreviation GN) of 110, using the American system of GN specified in FEET, and at ISO/ASA of 100. This is about the power of the Vivitar 285HV flash unit. Look at the 10 foot mark; multiply those 10 feet by the f/stop number of 11, and we get 110 as the GN. Taking test exposures at different aperture values, of a subject at 10 feet distant, can tell a fellow the GN of a Brand X flash, or of a flash shot at 1/2 or 1/4 or 1/8 power. Let's say with a flash unit dialed down to one-quarter power, at 10 feet, if test exposures made at ISO 100 look best at f/2.5, we'd multiply 10 feet times f/2.5, and come up with 25; meaning with 100 ISO film, at one-quarter power, TAHT specific flash unit has a quarter-power Guide Number of 25.

On a dedicated macro lens that has an elaborate magnification scale engraved on the barrel, at specific flash power levels with the flash mounted in a specific location, it's easy to figure out the exact magnification and the exact f/stop needed for each specific flash output level.


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## amolitor (Sep 5, 2013)

SCraig said:


> Use the standard formula:  F  (f-stop) = G (guide number) / D (distance to subject).  This is normally at ISO 100.  That will give you an f-stop for proper exposure, however shooting macro you will probably need to lower the flash power several EV from that.



Why would you need to lower it when shooting macro?


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## Derrel (Sep 5, 2013)

amolitor said:


> SCraig said:
> 
> 
> > Use the standard formula:  F  (f-stop) = G (guide number) / D (distance to subject).  This is normally at ISO 100.  That will give you an f-stop for proper exposure, however shooting macro you will probably need to lower the flash power several EV from that.
> ...



I used to do the majority of my macro shooting with my flash dialed down to 1/8 or 1/16 power. Many flashes are wayyyyyyy too powerful when used at flash-to-subject distances of under one foot. At times, a fractional flash power output of 1/16 or 1/32 is plenty, such as with the Base ISO 200 Nikon models, for example. With longer macro lenses, like 200mm or 180mm models, the 1:1 distance is around 18 inches, so a foot and a half; with shorter macro lenses, the 1:1 distance can be as close as say, three inches from the filter threads of the lens, so even if the flash is on-camera in the hotshoe, it's still only around five inches from a subject that's close to the filter threads.


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## amolitor (Sep 5, 2013)

Derrel said:


> amolitor said:
> 
> 
> > SCraig said:
> ...



Well, sure, but doesn't that come out in the GN calculation? I'm reading the quoted post as "calculate what the GN says to do, and then dial back", but I think you're saying "the GN will give you absurdly high numbers at very small distances, so the answer it provides, while correct, will indicate very low flash power."

Does the GN calculation fall apart at very low flash/subject distances?


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## Derrel (Sep 5, 2013)

I think you got tripped up by semantics, or by your interpretation of SCraig's words, or by SCraig's words. There...plausible deniability for everybody!

The problem is that the full-power GN will result in macro-range f/stops that are beyond the range of any macro lens. Are you not following along, deliberately? Let's just say the user has a GN 110 flash. Working at 1 foot distant. 110 divided by 1 is 110. Meaning f/110. Typical macro lens stops down to f/22, perhaps f/32. Diffraction is bad. Overexposure is HUGE. Or, let's say 1/2 a foot or 0.5 foot. 110 divided by 0.5 is... f/220.

Or how about a full-power, GN 110 flash used at 0.25 foot distance? 110 divided by 0.25...the calculation holds, and produces f/440.

See how this works??? As SCraig wrote, "..you probably will need to* lower the flash power several EV* from that."


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## amolitor (Sep 5, 2013)

Oh, ok. I see. I am following along, and maybe I was misreading SCraig. I do get that you're going to use the flash at very low powers, honestly. I get that when the flash is very close, it's gonna illuminate the crap out the object, and you'll need unrealistic apertures if you don't dial back.

So the GN calculation does NOT fall apart close up, it continues to produce correct answers. They just will turn out to be unworkable at higher powers, necessitating lower powers (which I tend to think of as a reduction in GN, for convenience sake).


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## weepete (Sep 5, 2013)

Ok, so I think I am getting this and the GN calculation.

So, say I want to expose for a subject that's 200mm away from the flash head, I use the GN number to calculate the apeture. The GN number of my flash is 28. Using the formula f=gn/d my apeture would be 28/0.2 = 140. So I need to use f140, but that does not exist on my lenses, meaning I need to stop the flash down to something managable. 1 stop basicaly = a factor of 2 if I understand it correctly so dialing down the flash to 1/2 power should give me f70, 1/4 power = f35, 1/8 power = 17.5, 1/16 power = 8.75, 1/32 = 4.375 and 1/64 = 2.1875. 

So my best choice would probably be aroung f8 and 1/16 flash power at a subject distance of 200mm. Does that make sense?


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## SCraig (Sep 5, 2013)

Derrel said:


> I think you got tripped up by semantics, or by your interpretation of SCraig's words, or by SCraig's words. There...plausible deniability for everybody!
> 
> The problem is that the full-power GN will result in macro-range f/stops that are beyond the range of any macro lens. Are you not following along, deliberately? Let's just say the user has a GN 110 flash. Working at 1 foot distant. 110 divided by 1 is 110. Meaning f/110. Typical macro lens stops down to f/22, perhaps f/32. Diffraction is bad. Overexposure is HUGE. Or, let's say 1/2 a foot or 0.5 foot. 110 divided by 0.5 is... f/220.
> 
> ...



Thanks, Derrel.  Not enough coffee when I wrote that and I was still half asleep.



weepete said:


> Ok, so I think I am getting this and the GN calculation.
> 
> So, say I want to expose for a subject that's 200mm away from the flash head, I use the GN number to calculate the apeture. The GN number of my flash is 28. Using the formula f=gn/d my apeture would be 28/0.2 = 140. So I need to use f140, but that does not exist on my lenses, meaning I need to stop the flash down to something managable. 1 stop basicaly = a factor of 2 if I understand it correctly so dialing down the flash to 1/2 power should give me f70, 1/4 power = f35, 1/8 power = 17.5, 1/16 power = 8.75, 1/32 = 4.375 and 1/64 = 2.1875.
> 
> So my best choice would probably be aroung f8 and 1/16 flash power at a subject distance of 200mm. Does that make sense?



You have the IDEA correct however apertures don't halve like shutter speeds or ISO does since the numbers are the ratio of the aperture diameter to the focal length.  In reality halving or doubling the number changes TWO f-stops.  For example, f/2.8 to f/5.6 is 2 stops (with f/4 in the middle).  F/4 to 5/8 is 2 stops (with f/5.6 in the middle).


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## weepete (Sep 6, 2013)

Grand, my new toys came today so I've been playing about with them, I'll need to get a ball head from somwhere at somepoint.

I'll probably end up winging it but could I work out the apeture using sqrt2^n then? I read in wikipediea that's how apetures are calculated to go up or down one stop...


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## JacaRanda (Sep 6, 2013)

Or you can be a true amatuer like myself.  Find a spider, grab the tripod, dial in F16 on camera, dial in 1/16 power on speedlight (with 6x6 diffuser), hook up remote triggers, yell help from wifey, give her the speedlight and tell to act like a mannequin while holding the flash, take a shot, tell her to back up or move forward etc. etc.

A true trial and error manner of doing things.  Works well until she gets tired of being my human flash bracket.

Charlie,  I have not purchased one yet.  Dang me!


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