need help calculating aperture with speedlights

splash_fr · Nov 2, 2013

Hi all.

I did some test shots with my Metz 44 on manual on my Bessa R4A with Polypanf 50 ISO.
I shot everything at 1/125, at approx 1,2 meters, flash bounced from white ceiling.

I did test shots of a rather dark scene at apertures 2.8, 4, 5.6 and 8 with flash power set at 1, 1/2, 1/8 and 1/16.
Guide number of flash should be 44 at 100 ISO, so that would be 44/1.4=31 at 50 ISO, correct?

If I remember correctly, the correct aperture is guide number divided by distance to subject.
The only good negatives are f/2.8 and 4 at 100% flash power an f/2.8 at 50%, the others are too thin.

But these apertures are much bigger than my simple calculation suggested (i.e. GN31/1.2m = f/25). Where did I go wrong? How do you take into account the bounce flash when calculating aperture?

Need input...

Rgds,
Gerd.

Jim Noel · Nov 2, 2013

when attempting to calculate exposure using bounce lighting there are two major factors only one of which can be calculated.
1. total distance from flash to reflecting surface to subject
2. % of light reflected by the reflecting surface - this may be very low

Chan Tran · Nov 2, 2013

The reflectance can be assume to around 80% with white ceiling but as in previous post you must consider distance between flash and bounce surface and then to subject and also consider now the light is no longer straight ahead. The amount of illumination fall off no longer follow the inverse square law as it's no longer a point light source but a fairly large light source. But your result is what I would expected in such a situation.

LunoLuno · Nov 2, 2013

This is the procedure I usually follow when bouncing flashlight off the ceiling.

1. Focus at the bouncing point on the ceiling to measure the 'flash to bouncing point' distance. (i.e. 1.2m)
2. Stoop a little to be the same height as the photographing subject.
3. Focus at the bouncing point again to measure the 'bouncing point to subject' distance. (i.e. 1.6m)
4. Now I get the total length of the path between the flash and the subject (1+3) (i.e. 2.8m)
5. Estimate the reflective ratio of the ceiling. (i.e. 25% : 0.5stops blighter than the 18% reflective card)
6. Aperture calculating

(example)
film ISO100
flash GN 28m (ISO100)
total travel of light: 2.8m
-> 28/2.8 = F10 (provided that the bouncing surface is 100% reflective)
estimated reflevtive ratio: 25% (2 stops of light loss)
-> F5.0
safety margin (only for print films): half a stop
-> F4.0 (2.5 stops from F10 in total)

This procedure is only applicable when the bouncing point is located just between the flash and the subject (the incident and reflective angles should be the same), and the ceiling should not be too low. but it does work well for that use. You may feel somewhat apprehensive to estimate the reflective ratio, but most people usually have good experiece of estimating the ratio already. If the color of the bouncing surface is as blight as the 18% grey card, that means 18% of your flashlight will reach the subject, which is equivalent to 2.5stops of a light loss. What you have to do is compensating the aperture from there. And this is not so different from compensating your exposure setting when shooting under the available light depending on the in-camera reflective light meter.

emanuesfr · Nov 2, 2013

The traditional GN calculations are only good for direct flash. If you're bouncing it, a good rule of thumb is to overexpose by roughly 2 stops (if direct flash calls for f/11, then use f/5.6 for bounced flash). However, there are many variables involved like the color or reflectance of the ceiling and its distance to the subject.

removed account4 · Nov 2, 2013

hi gerd.

i am kind of a luddite when it comes to these sorts of things but whenever i use a flash and bounce it ...
i need to add to the distance from the thing i am lighting when i figure out my fstop. for example ..
if the "thing" is 8 feet away, and i bounce to it, well it is xfeet from the ceiling and the ceiling is yfeet from me
s0 it will be x+y feet ...as others have mentioned the total amount of light doesn't make it to the subject, so you have to
take that into account too ...
sorry to suggest this, you might think of picking up a cheap flash meter ... years ago i picked up one and it makes it easy to
make sure my i am exposing things correctly ... some, even the cheap ones, allow for 3 readings, so you can do ambient readings
as well as flash readings at the same time ...

have fun!
john

trythis · Nov 2, 2013

I will second the flash meter. Luna Pro F or Luna Six F, same thing I think.

wiltw · Nov 2, 2013

Guide Number arithmetic only applies to direct flash...a Guide Number is flash-to-subject distance ordinarily, but by bouncing that calculation is useless.
The ceiling-to-subject distance might seem to have a factor, but when a flash is bounced up to the ceiling, the ceiling is a large virtual source which is NOT governed by the Inverse Square Rule of small source falloff in intensity.

To illustrate, I just did this with my Metz 54MZ, which has GN106 with 'normal lens' coverage angle. With my subject exactly 7' from where I stand, GN arithmetic says f/15.
Most every speedlight is overrated by the manufacturer (virtually every brand is guilty of this!), and most will actually flash meter LOWER than rated by about -1EV! Direct flash from Metz 54MZ standing 7' away indicates f/11 +0.0EV with Minolta flash meter (GN77) vs. manufacturer rated aperture says f/15 (GN106)
Standing 7' away from subject, and using ceiling bounce which is 7' above the subject (subject to center of bounce) meters f/4 + 0.6EV, or GN35.

Measured direct flash = GN77, while ceiling bounce measured GN35...about -2 + 0.3 EV loss due to bounce from measured GN for direct flash.

splash_fr · Nov 4, 2013

Thanks all for the help!
So an new gadget it is!
I like gadgets

rgds,
Gred.

Tom1956 · Nov 4, 2013

This is why they invented flash meters--to introduce another variable and earn some money out of your pocket. In the end, your guess is about as good as anybody else's. Then, that's why they invented digital cameras, so you could shot it, look on the back of your camera while everybody in the picture was still there, and either delete it or shoot another. When it's all done and you get a "good print" you see for yourself bounce light off a ceiling is a bad idea. A better idea is to drag your shutter as much as you can hand-hold, and bring up the direct flash as little as you can get away with. Generally for that, the little dial on the back of the flash works out pretty well, on the Japanese flashes I've ever used. Sunpak 611 user for 37 years speaking here.

SafetyBob · Nov 4, 2013

I got a cheap Polaris flash meter many, many years ago and hardly used it until I got back into photography.

It was after I started getting into the studio flash stuff I finally put the cash down for a "real" Seconic meter. The old ones are great and very, very useful, however, I like my Seconic's features better. I have not compared actual photos using both meters, but I can imagine they would be similar......it's just that the Seconic is maybe easier at setting up multiple strobes to exacting levels. It would have been beyond my abilities (at the time) to figure that out with the old Polaris....

Bob E.

jstraw · Nov 4, 2013

The idea of using studio lights and not investing in a flash meter is beyond my comprehension. Sure, it can be done...but why?

David Allen · Nov 5, 2013

trythis said:
I will second the flash meter. Luna Pro F or Luna Six F, same thing I think.

If you consider getting a flash meter please be aware that virtually all will NOT give you an accurate reading with Hand-held types of flash. They are designed for studio units whose flash duration and power are quite different to to a Metz (except for when you use a hand-held flash on manual at full power). Also the Metz's inbuilt auto exposure settings generally work by curtailing the burst of flash to a duration that is too short for most flash meters to accurately respond to.

The best solutions are:

Calculate the distance from flash to ceiling and ceiling to subject and use this as you base distance when manually calculating the effective GN at the position of the subject. For a typical UK living room with white ceiling and light walls you can factor in (as a rule of thumb) a reflectance of 80%. For larger rooms and/or darker walls (such as dark wood panelling in a pub) you can factor in (as a rule of thumb) a reflectance of 50%.
Purchase an on camera sensor accessory for the Metz. I used to have ones for my Metz 45 and Matz 60 that controlled the automatic settings of both flashes but, and very importantly, the sensor is attached to the camera so it can work effectively with bounce or off camera flash.
Use the two flash method whereby you bounce a Metz 60 on full power off the ceiling and combine this with a Metz 45 at full power pointed directly at the subject. You only calculate the Metz 45 to subject distance to get your exposure and ignore the Metz 60 as this will simply act to soften the harshness of the Metz 45 on it's own.

Best of luck with your experiments.

David
www.dsallen.de

E. von Hoegh · Nov 5, 2013

Tom1956 said:
This is why they invented flash meters--to introduce another variable and earn some money out of your pocket. In the end, your guess is about as good as anybody else's. Then, that's why they invented digital cameras, so you could shot it, look on the back of your camera while everybody in the picture was still there, and either delete it or shoot another. When it's all done and you get a "good print" you see for yourself bounce light off a ceiling is a bad idea. A better idea is to drag your shutter as much as you can hand-hold, and bring up the direct flash as little as you can get away with. Generally for that, the little dial on the back of the flash works out pretty well, on the Japanese flashes I've ever used. Sunpak 611 user for 37 years speaking here.

When I was doing a fair amount of product photography, using 4x5 and 120 transparency film, I found a flashmeter to be indespensable, and that my best guess was worth it's weight in gold.

removed account4 · Nov 5, 2013

SNIP

Tom1956 said:
This is why they invented flash meters--to introduce another variable and earn some money out of your pocket. In the end, your guess is about as good as anybody else's.

never heard that one before

SNIP

David Allen said:
If you consider getting a flash meter please be aware that virtually all will NOT give you an accurate reading with Hand-held types of flash.

or that either ...

my old battery eating minolta has never given me a wrong reading .. (monoblocks, lumdeyne, and a variety of on camera sunpaks, speed lights ).

i guess YMMV ??

David Allen · Nov 5, 2013

jnanian said:
SNIP

never heard that one before

SNIP

or that either ...

my old battery eating minolta has never given me a wrong reading .. (monoblocks, lumdeyne, and a variety of on camera sunpaks, speed lights ).

i guess YMMV ??

When I first got a Sekonic flash meter, I found it very unpredictable when testing the auto settings on my Metz with transparency film but accurate when using the Metz on full power. I went to the Hasselblad offices (they were the importer of Metz flash guns in those days) to ask about the phenomenon. The Metz engineer explained that the peak in the output curve of a hand-held flash - when set to one of it's auto settings - was curtailed by the sensor quenching the output when sufficient illumination had been delivered and, therefore, varied greatly from the output curves of all of the professional flash systems (Broncolor, Multiblitz, Bowens, etc). As flash meters were designed to suit the output characteristics of said studio flashes, that is why they gave inaccurate readings with hand-held flashes set to an automatic setting but correct readings when the hand-held meter was set to manual.

Perhaps more recent flash meters are better at this? - I do not know as I rarely use flash these days and, when I do, I always use them on manual setting.

Bests,

David
www.dsallen.de

removed account4 · Nov 5, 2013

hi david

thanks for the explanation !
i had never heard of this before ..
maybe i have just been lucky ?

i never use flash these days either,
unless ... i have to use it.

john

wiltw · Nov 5, 2013

David Allen said:
When I first got a Sekonic flash meter, I found it very unpredictable when testing the auto settings on my Metz with transparency film but accurate when using the Metz on full power. I went to the Hasselblad offices (they were the importer of Metz flash guns in those days) to ask about the phenomenon. The Metz engineer explained that the peak in the output curve of a hand-held flash - when set to one of it's auto settings - was curtailed by the sensor quenching the output when sufficient illumination had been delivered and, therefore, varied greatly from the output curves of all of the professional flash systems (Broncolor, Multiblitz, Bowens, etc). As flash meters were designed to suit the output characteristics of said studio flashes, that is why they gave inaccurate readings with hand-held flashes set to an automatic setting but correct readings when the hand-held meter was set to manual.

Perhaps more recent flash meters are better at this? - I do not know as I rarely use flash these days and, when I do, I always use them on manual setting.

Bests,

David
www.dsallen.de

David,
I just conducted this test with my Minolta Autometer Vf, with Metz 54MZ flash. I first took a photo with ambient light to establish density of 18% grey card under ambient metering, to compare against flash (you can see my ambient light was quite uneven in intensity and white balance). I then put the Metz 54MZ into Manual flash mode, and set power levels to 1/256, 1/64, 1/16 and 1/4, metering with the flashmeter and then taking a shot with the meter suggested settings. Lastly, I put the flash into ETTL mode on my Canon dSLR, as another density reference of flash under ETTL control. Here is the result.

You can see that the 1/256 power level meter suggestion resulted in total underexposure. But the 1/64, 1/16, and 1/4 power levels resulted in the meter's suggestion being about 1EV bright. Finally, the ETTL flash control's density was close to that of the ambient metering. I do not have flash duration information for 1/256, 1/64, or 1/16, but Metz user manual says 1/4 power has 1/1400 flash duration. I surmise that 1/16 has 1/2800 duration, 1/64 has 1/5600 duration, 1/256 has 1/10000 duration. Metz specifications state the 54MZ has 1/200 - 1/20000 duration.

Manual power durations are no different than Auto quench in that both are simply shorter duration than full power, one is preset (Manual) while the other is curtailed at time of exposure (Auto). dSLR ETTL is similar in concept to Manual... the camera commands a predetermined amount of light from the flash, after the preflash is metered before the shutter is open. Here is the result of ETTL (left) vs. Auto mode, lens and flash set for f/8 (right) and flash meter says to use f/5.6 +0.5EV

Tom1956 · Nov 5, 2013

I've experimented with my Sekonic. Flash meters seem to have an accuracy cutoff point with regard to flash duration. Anybody who decides to get a flash meter should experiment to find out where that point is. Until you do know this point, stick with full power (duration) on the flash. Because once you go past that accuracy point, a flash meter is a big fat nothing. Above that line and they're pretty in good shape for the most part.

E. von Hoegh · Nov 5, 2013

jstraw said:
The idea of using studio lights and not investing in a flash meter is beyond my comprehension. Sure, it can be done...but why?

Why? Because if nobody bothered with a flashmeter, everybody would have to bracket - and that means Kodak might still be making E6 films! Even if not, the incredible film consumption of the bracketers would likely have prevented some or all of Kodak's financial woes.

BrianShaw · Nov 5, 2013

David Allen said:
... As flash meters were designed to suit the output characteristics of said studio flashes, that is why they gave inaccurate readings with hand-held flashes set to an automatic setting but correct readings when the hand-held meter was set to manual. ...

Maybe Im misunderstanding David's comments, but I never thought it had to do with any kind output characteristic compatability between stobe and meter. My experience with flashes is somewhat limtied but use one of these three options:

1. TTL flash metering (35mm) - let the camera meter work witin the range of the flash output to meter the flash light correctly.
2. Thyristor (on the flash unit) control - let the flash unit work within the range of the flash output to meter the flash light correctly.
3. Flash meter - use a flash meter to determine the appropriate aperture to set on the camera that will correctly expose film for the amount of flash light being used.

and there always is 4. Calculation of aperture using guide number and distance.

I can't even understand how one would use a flash meter in situations 1 and 2 where there is either kind of internal flash metering system being used.

jstraw · Nov 5, 2013

BrianShaw said:
Maybe Im misunderstanding David's comments, but I never thought it had to do with any kind output characteristic compatability between stobe and meter. My experience with flashes is somewhat limtied but use one of these three options:

1. TTL flash metering (35mm) - let the camera meter work witin the range of the flash output to meter the flash light correctly.
2. Thyristor (on the flash unit) control - let the flash unit work within the range of the flash output to meter the flash light correctly.
3. Flash meter - use a flash meter to determine the appropriate aperture to set on the camera that will correctly expose film for the amount of flash light being used.

and there always is 4. Calculation of aperture using guide number and distance.

I can't even understand how one would use a flash meter in situations 1 and 2 where there is either kind of internal flash metering system being used.

One wouldn't. One would set the body to "M" and set the aperture and shutter speed manually, per the flash meter. I don't know anyone that uses studio lights that uses the camera's meter or thyristor control with them.

wiltw · Nov 5, 2013

BrianShaw said:
Maybe Im misunderstanding David's comments, but I never thought it had to do with any kind output characteristic compatability between stobe and meter.

Second guessing David's underlying understandings, but he did say "when testing the auto settings on my Metz with transparency film but accurate when using the Metz on full power."
Studio flashes tend to be somewhat longer duration light output, while speedlights tend to be shorter duration light output. In the days before today's higher output flash units, most speedlights might be 1/1000-1/10000 or thereabouts, while studio flashes might be about 1/1000. As my experiment showed, when the flash duration gets very short (in my example, 1/10000) my flashmeter is totally fooled and its suggested exposure to totally wrong. That fits David's experience.

The Hassy guy apparently made comments about speedlight output being 'different' than studio flash. Paul Buff's charts might help explain that: http://www.paulcbuff.com/sfe-flashduration.php

You can see that reduced power of (some) studio flash lowers the max voltage, resulting in a lower peak, but fairly uniform duration of light output. OTOH, the IGBT output reduces duration, not the peak amplitude...but that is how speedlights work. Modern studio flash use an approach like IGBT, for example my Dynalite flash duration drops as power is lowered.

My tests showed that the flashmeter could reasonably meter even the abbreviated flash duration of speedlights -- to a point. As I said earlier in this post, when the flash duration gets very short (in my example, 1/10000) my flashmeter is totally fooled and its suggested exposure to totally wrong.

What I can't explain is why my flashmeter's suggested exposure when reading my Metz on Manual power results in a brighter result than one really would want.

removed account4 · Nov 5, 2013

jstraw said:
One wouldn't. One would set the body to "M" and set the aperture and shutter speed manually, per the flash meter. I don't know anyone that uses studio lights that uses the camera's meter or thyristor control with them.

i have one i use with my lumedyne 244 ...
output can be full 400WS or 2ws or trimmed in between.
its pretty useful ... and i wish everything i used had this feature ..

Chan Tran · Nov 5, 2013

I have 2 flashmeters and they are both accurate with all kind of flashes. I do use them when the flash is in auto mode just to check if the auto feature is working correctly.

need help calculating aperture with speedlights

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