How to calculate exposure for manual external flash

[goldberg1138]

<p>I got a second-hand external flash for my camera. The flash has a guide number of 36. It's a rather old flash unit, so I can't use ETTL mode with it. I have read numerous articles trying to figure out how to calculate what power setting to use, and also what aperture to set my camera at. I usually leave the camera on aperture priority mode, as this seems easiest for me. The flash has full, 1/2, 1/4, 1/8, and 1/16 power settings. If, for example, I have the camera set at 1/500th of a second shutter speed, f/5.6, and 100 ISO, and I'm standing 10 feet away from my subject, and I want to use the flash for fill light outdoors, what power setting would I need on the flash. I haven't been able to find a succinct answer to this. So far, I've found explanations on how to calculate power settings for taking photos nearly in the dark, and explanations on reasons why I should be using fill flash, but I'm having trouble understanding how to calculate the settings to mix the right amounts of ambient light and light from the flash. Is the dial on the back of the flash there to aid in calculating fill light, or for using the flash as the primary light source? Can someone help me to understand this?</p>

[PatrickMP]

<p>David, can you post an image of the back of the flash unit?<br>

Presumably your camera settings are leaving the image underexposed. You want to dial in the manual flash such that the settings on the flash will properly expose the subject for your aperture/ISO combination and distance. The shutter speed doesn't really come into play with a flash, except to dictate whether or not you'll actually capture the image. (If your shutter speed is higher than the flash sync speed, your image won't be captured properly).<br>

So assuming you are using f/5.6, and your distance is 10': the dials on the back of the flash should indicate what distances at f/5.6 and the given power rating will expose properly. I.e., at full power the flash may expose properly between 20' and 30' at f/5.6. At 1/2 power it may expose properly between 15' and 20', and at 1/4 power between 10' and 15'. In this example, you would set the flash to 1/4 power.<br>

Take a look at this table:<br>

http://sphotos.ak.fbcdn.net/hphotos-ak-snc6/hs018.snc6/166850_10150119821647853_690722852_7836327_6854876_n.jpg<br>

If you look at the column for ISO 100, there are a number of f-stops. At ISO 100, under f/5.6, the image is correctly exposed at 8.6' (2.6m). This is shy of your 10' distance, so you can either bring your subject closer <strong>or</strong> open up a 1/2 stop to 5.6-4.0 (since at f/4.0 the subject is exposed correctly at 12' (3.7m)).<br>

Hope that helps.</p>

[simon_crofts]

<blockquote>

<p>If, for example, I have the camera set at 1/500th of a second shutter speed, f/5.6, and 100 ISO, and I'm standing 10 feet away from my subject, and I want to use the flash for fill light outdoors, what power setting would I need on the flash.</p>

</blockquote>

<p>First, check that your camera can synchronise with flash at a shutter speed as fast as 1/500th sec. Assuming that it can, then:</p>

<p>The guide number of your flash is expressed in metres rather than feet. So it helps if you think of your subject distance in metres. Or alternatively, use the guide number of your flash expressed in feet (it will be around 125).</p>

<p>Sticking to metres for the moment, with a guide number of 36 at full power, your subject is 3 metres away, you need to divide 36 by three to get the f-stop to use at full power in order to fully expose the subject. The answer is f11 (near enough).</p>

<p>But you want to use f 5.6. You therefore need to reduce the power of the flash by two stops so that you can open the aperture up to f5.6, in other words, quarter power. But since you only want fill flash, not a full exposure, you need to reduce the power of the flash an extra stop. In other words, one eighth power.</p>

<p>You may also find that the guide number of the flash is a little optimistic, designed for a room that bounces light around. So you might not want to err on the side of overexposing the flash slightly (eg. assume a slightly weaker guide number).</p>

[lornesunley]

<p>First, your shutter speed has to be set to the flash sync speed for the camera (usually 1/200 or 1/250 - check your camera manual). With a guide number of 36 (if that is the guide number in feet) the f stop for using the flash at full power as the only source of light would be f/3.6 at 10 feet - you divide the distance from the flash into the guide number to get the f stop. The guide number is always given with the flash at full power - it may be expressed as feet or meters, so use the appropriate value for the distance.<br>

Calculating fill requires that you know the exposure for ambient light ... use the result of the camera's meter to determine this. The determination of fill light also depends on whether or not the subject is back lit (the background is brighter that the front of the subject or if the subject is brighter than the background. If the subject is back lit, and spot metering the subject gives you an aperture of f/2.8 and the background requires an exposure of f/5.6 your subject will be under exposed by two stops when the background is properly exposed. You need to add two stops worth of light to get a properly exposed subject. The flash would fully expose your subject with the flash set of full power and an aperture of f/3.6 with the aperture set at f/5.6 your subject will receive a fill light of 1 stop below ambient. You can fiddle with the flash power to determine the best ratio for the exposure.....</p>

[goldberg1138]

<p>Okay, I might have picked a bad example. I just pulled out the camera settings to use for the example. The fastest the shutter can sync with a flash is 1/250th of a second. I can use a larger aperture to accommodate. I didn't think of this limitation when I posted. The question was more about metering, calculating f-stop, and deciding on flash power setting. Thanks for the suggestions, though.</p>

[simon_crofts]

<blockquote>

<p>With a guide number of 36 (if that is the guide number in feet)</p>

</blockquote>

<p>Lorne is assuming that the 36 is the guide number in feet, while I was assuming that it is the guide number in metres, hence the difference in result. You need to check which one it is in the flash specifications. A guide number of 36 in metres would typically be fairly powerful for an on camera flash, a fairly largish top of the range model. A guide number of 36 in feet would typically be a weak small flash.</p>

 

[goldberg1138]

<p>The manual says that it has a guide number of 36. It doesn't say if it's feet or meters. How do I tell? It's a Minolta Auto 360PX. It's rather old, but it works fine with a Safe Sync.</p>

[goldberg1138]

<p>Here's the back of the flash:</p><div></div>

[simon_crofts]

<p>That's a fairly powerful flash for its size, the guide number will be for metres. If you prefer to think in feet, the guide number in feet should be approx. 118 (not 125 as I said earlier).<br>

On the other hand, the guide number is probably a bit optimistic, maybe somewhere closer to 105 might be more realistic.</p>

 

[Mattman944]

<p>The guide number is in meters, this was Minolta's top of the line at the time. </p>

[goldberg1138]

<p>For some reason, this topic seems to confuse me to no end. Thanks for the input.</p>

[simon_crofts]

<p>The simple way to look at it is:</p>

<p>Just divide the distance to the subject by the guide number to find the aperture that you need to use for full exposure. For fill flash, just reduce that aperture by one stop. Then set whatever shutter speed the ambient light requires you to use for that aperture.</p>

<p>That is assuming you're working at ISO 100, and that the flash is on full power. And it assumes that the the aperture that it gives you doesn't mean that the ambient light will force you to use a shutter speed too fast for the flash to sync with.</p>

[picturesque]

<p>While the answer above are good, there are two things I'd like to point out.</p>

<p>1. The calculator on the back of the flash should tell you what subject distance you need for what f stop, ISO and partial power setting. Mode switch to M, as shown. Set the ISO(ASA). Set the partial power--bottom switch. You may also need to set the small black dial that is shown in your photo as set to W (for Winder, which is actually 1/16th power)--don't know. Then, read off the subject distance opposite the f stop you want. If you set the partial power switch (and black dial accordingly) to a different setting, the dial should change, and your f stop and subject distances change in relation to each other.</p>

<p>2. You have auto thyristor settings (three lines under the M on the mode switch), which should make it easier to use in aperture priority, without figuring anything. I'd bet that if you set one of those, the f stop will change to show you what 'automatic' f stop you have for that ISO. Then, the green range will tell you the range in which the flash can properly expose in.</p>

<p>Download the manual from the following place.</p>

<p><a href="http://www.vikenk.com/minolta_manual.htm">http://www.vikenk.com/minolta_manual.htm</a></p>

[picturesque]

<p>Forgot about compensation. With manual--it is as described by many above. With auto thyristor, you 'lie' to the flash, and say that the ISO is slower than it actually is, for plus compensation, and faster than it actually is, for minus compensation. So for one stop less flash for fill purposes, if using ISO 400, you set the flash's ISO for 800. Or, you can lie by way of f stop.</p>

<p>The other thing to worry about would be sync voltage and the pins in the foot clashing with the contacts on your Canon camera's hotshoe. You might want to verify that the only pin on the foot is the middle one--or tape off the other pins on the foot. Apparently, you need not worry about sync voltage with this model of flash.</p>

[goldberg1138]

<p>The ASA range on the dial on the flash only goes up to 400. If I'm using ISO 800, I would set the aperture one stop lower and then do everything else the same, right? I just figured I would adjust the f-stop accordingly. I went ahead and got a Wein Safe Sync. I didn't want to take any chances with the camera. I'll look up the auto thyrstor setting to figure out how to use it.</p>

[luis_g]

<p>First, before doing anything else, I'd check to make sure the trip voltage isn't going to damage your camera's electronics. Check here:</p>

<p>http://www.botzilla.com/photo/strobeVolts.html</p>

<p> </p>

[luis_g]

<p>First, before doing anything else, I'd check to make sure the trip voltage isn't going to damage your camera's electronics. Check here:</p>

<p>http://www.botzilla.com/photo/strobeVolts.html</p>

<p> </p>

[jim_doty]

<p>David,</p>

<p>I wrote this article several years ago, but the basics still hold true:</p>

<p>http://jimdoty.com/learn/Tips/Flash_Basics/flash_basics.html</p>

<p>This question: "If I'm using ISO 800, I would set the aperture one stop lower and then do everything else the same, right?"</p>

<p>If by "lower" you mean smaller, you are correct. If the right aperture at a given distance and ISO 400 is f/8, change the ISO to 800 and the aperture would be f/11 at the same distance.</p>

<p> </p>

[goldberg1138]

<p>The manual for the flash is possibly the least comprehensive owner's manual I've ever seen. It doesn't really explain how to use the flash, other than in TTL mode. So basically - I make sure the camera's metering says that the photo will be properly exposed at the given settings - whatever those happen to be. Obviously, I make sure the shutter speed isn't over the maximum sync speed. The guide number is about 118 feet for this flash. So, at 20 feet from the subject, with a guide number of 118, I would need the aperture to be set at roughly 6 (assuming the flash is on full power and I'm using ISO 100). However, if I'm using the flash for fill, I set the power at 1/2, which is one stop lower than the full exposure. The dial on the flash only goes up to ISO 400. If I want to use ISO 800, for example, I would set the flash to whatever power I need for 400, then stop down one stop on the camera or choose a lower power setting on the flash. Correct?</p>

[goldberg1138]

<p>Thank you for the article, by the way. Everything else I've read about has made perfect sense. I guess this is the next step for me in furthering my photography skills. </p>

[pete_s.]

<p>If you shoot the flash in manual mode the only thing you need to set on the flash is flash power, nothing else.</p>

<p>If you have a dSLR you could just guess at the flash power and then take a test shot and adjust as needed.</p>

<p> </p>

[simon_crofts]

<blockquote>

<p>Obviously, I make sure the shutter speed isn't over the maximum sync speed. The guide number is about 118 feet for this flash. So, at 20 feet from the subject, with a guide number of 118, I would need the aperture to be set at roughly 6 (assuming the flash is on full power and I'm using ISO 100). However, if I'm using the flash for fill, I set the power at 1/2, which is one stop lower than the full exposure.</p>

</blockquote>

<p>All correct so far.</p>

<blockquote>

<p>The dial on the flash only goes up to ISO 400. If I want to use ISO 800, for example, I would set the flash to whatever power I need for 400, then stop down one stop on the camera or choose a lower power setting on the flash. Correct?</p>

</blockquote>

<p>Nope - as Pete intimates, in manual mode, the dials on the back of the flash don't actually do anything. They are just a kind of calculator to help you make the guide number calculation. Personally I find the calculators a bit dazzling and find it easier just to make the guide number calculation myself, as you just did, but others may find it easier to use the calculator. See what works best for you.</p>

<p>In other words, if the flash is in manual and is set to full power, then it is set to full power, and the calculator won't change the flash's output.</p>

<p>So, if you're doing the guide number calculation route, then all you need to do to adjust for an ISO of 800 is either (you have a choice) 1. reduce the power of the flash by 3 stops, or 2. reduce the aperture by three stops.</p>

<p>If you do reduce the aperture three stops, then to record ambient light correctly you will also need to adjust the shutter speed accordingly. Or if your camera is in aperture priority mode, the camera will do this automatically.</p>

<p>Once you've got used to these calculations, they get a lot easier. So, if you always have the flash at full power, and tend to stick to a certain film speed, then you will always have to use the same aperture for a subject at a given distance, and you will know the aperture without having to make the calculation. From there is gets easier to make any fine adjustments, eg. to reduce flash power a bit to make fill flash, to make an adjustment to ISO etc. You will also get to know the maximum distance that you can use the flash outside in bright sunlight. The limiting factor outside is the sync speed of the camera - a faster sync speed allows you to use a wider aperture in bright conditions, which considerably extends the working distance of the flash.</p>

[picturesque]

<p>As everyone said, the calculator does not actually do anything but give you information. If using manual flash mode, the only setting to use, as Pete said, is the actual partial power setting control. Rather than figure in your head, or carry around tables of pre-calculated information, the calculator can be used to get you close, and then you can fine tune, with DSLRs, on the spot.</p>

<p>However, over time, you get to know what power settings you need for the typical situations you run across. Or, as I do, you can make a cheat sheet which is taped to the back of the flash, for those typical situations. I have my info in a way which is faster to consult than the calculator.</p>

<p>I think you will need the calculator to help you with using auto thyristor, though, if for no other reason than knowing what auto f stop you are using, and the range.</p>

<p>Re your question about ISO 800--your calculator can be used in the way you describe, to get you the 'right' answer for ISO 800, but of course, know that the calculator isn't changing the flash setting.</p>

[subarus_subarus]

<p>I want to try to answer. Pls let me know if I am correct.<br>

The dial is to provide information, it does not do anything to change the intensity of the light<br>

To find the appropriate flash/camera settings to achieve the shadow fill, it is easier if we start the calculation for the second simplest scenario, namely the 50% light from ambient, 50% light from Flash. ( the simplest being 100% light from flash)<br>

1) Measure the Ambient light, and setup to attain 50% amount of light need for correct exposure.<br />Iso=100<br />Speed=1/200<br />Aperture= F4 gives perfect exposure according to the build-in exposure meter in camera<br>

If you dial the aperture down to F5.6, everything else the same, you will get 50% the amount of light hitting your ccd sensor. so you will be 1-stop under exposed<br>

2) Calculate and setup Flash to attain 50% amount of light to attain correct exposure<br>

X-sync is 1/200, <br />Assume the Guide Number, GN=100 in feet at Iso100 at the focal length (zoom) you are using.<br>

To achieve correct exposure in total darknesss, at F4, you need the flash to be 25 feet away from subject at full power (if flash at half power, you need to be at 25/1.4=18feet away, at quarter power, 13 feet away)<br>

to achieve 50% amount of light from above, you simply dail the aperture to F5.6<br>

3) Based on the above setttings. if you set aperture at F5.6, flash at 1/4 power 13 feet from subject, iso100, speed=1/200, you will get 50% light from ambient, 50% light from flash and you should get the correct exposure.<br>

4) For shadow fill calculation.<br>

I need to set the exposure back to 100% ambient light , ie aperture @ F4<br>

Flash at 1/32 power 13 feet away from subject, at F4, should give me 1/8 the amount of light needed for correct exposure. your subject will be 12% over exposed. You can't change the aperture to compensate for this because it's in the increment of 1 stop (100%). so your best bet is to use the exposure compensation, dial it to -0.3 ev if u like but u will be 20.5% under exposed. so u see, the best thing to do if to leave compensation at 0ev.<br>

Check the histogram, most of the time it should still be ok, even if your calculator is telling you that you have over exposed by a bit</p>

<p> </p>

[simon_crofts]

<p>Subarus, that all more or less sounds OK, except for a couple of observations.</p>

<p>The first is the bit:</p>

 

<blockquote>

<p>Assume the Guide Number, GN=100 in feet at Iso100 at the focal length (zoom) you are using.</p>

</blockquote>

<p>Happily the guide number doesn't change according to the focal length you are using. The focal length doesn't affect the calculation at all. It's just the aperture that matters.</p>

<p>The other general observation is that it seems to me that you're making it a bit more complicated than it needs to be. You don't really need to take separate approaches depending on whether you want to do a flash only exposure, or flash + ambient and so on. That can just be done by a small adjustment to shutter speed at the end once you've got everything else (in particular, aperture) set up.</p>