How to use a flash, NEED SOME BREAKDOWN

[ilia_isakov]

<p>Hi, I am starting to use my external flash (sb-600 on nikon d90),<br>

and I have some questions that I would appreciate being discussed<br>

First of all what is TTL, I have tried to read up about it, but quite honestly I dont grasp things too quickly, I am only good at visualizing things and testing them out. If anyone can describe the role of the flash and how it communicates with the camera's metering. TTL means through the lens, that I know, the camera has its sensor and I believe that flash itself has a sensor. How the two interplay I'm not sure, so if anyone could break it down for me chronologically and in real dummy terms, lol.<br>

Assuming the flash works with camera, then why is it when use flash on a scene with very even tones, why is it off so frequently, and by so much. I just fired a few test shots and many of them were overexposed at least two stops, that is a huge miscalculation.........</p>

<p>Secondly it is said that the flash if it is the main source of light becomes the main exposure for the photos; does this mean that when flash is main light that shutter speed is entirely irrelevant and that I can shoot as low as I want and still not get motion blur......... And what happens if I turn up the flash to bounce it from a nearby source, is it still safe to shoot at low speeds.</p>

<p>Thanks again, help is really appreciated</p>

 

[jcuknz]

<p>The basic rule is that the shutter controls the ambient light and the aperture controls the flash input. In working to this rule you probably will not use TTL and be working manually perhaps outside and using the flash to fill sunlight caused shadows.<br>

With a DSLR you also need to be aware of the sync shutter speed of the camera and if you pick a faster speed the flash will only expose part of the frame due to the second shutter blind closing off part of the sensor as the first shutter blind reaches the point where it has fully openned and triggers the flash. That is how a focal plane shutter achieves faster shutter speeds with a narrow gap crossing the sensor.<br>

If you are working in lowlight situation with flash beingthe principle light source then the shutter speed is less important but a slow shutter can lead to ambient light giving you a 'second' exposure source toperhaps show blur fromsubject or camera movement coupled with the sharp flash exposure.<br>

I don't haveTTL flash so cannot answer your other points.</p>

[cabbiinc]

<p>

/> This video should help a little.</p>

<p>Actually you should state which mode the camera is in and which mode the flash is in. What are your settings on everything?</p>

[cabbiinc]

<p>Assuming that your camera is in an Automatic or Semi-Automatic mode as well as your flash here's what happens.</p>

<ol>

<li>You press the shutter release.</li>

<li>The camera tells the flash to emit a low powered flash called a pre-flash.</li>

<li>The flash emits the pre-flash and the camera meters it though the lens. A calculation is made for the power of the flash for the actual exposure.</li>

<li>The mirror lifts up, blocking the viewfinder.</li>

<li>The first curtain starts to travel.</li>

<li>When the first curtain is completely open the flash fires at it's precalculated power.</li>

<li>The second curtain starts to travel and closes the shutter.</li>

<li>First and second curtains return to their ready position while the mirror lowers unblocking the viewfinder.</li>

</ol>

<p>Now, with that in mind the camera will meter the scene for ambient light and set the aperture and shutter speed as if the flash doesn't exist (shutter priority or aperture priority, probably a few of the full automatic modes too) or set the shutter speed to a very slow rate but fast enough to try to limit motion blur (P mode, most automatic modes). So even though you're using flash in Aperture priority mode the shutter speed is the same as if you didn't use flash. If the ambient light is contributing too much then you'll need to limit the shutter speed, as JC says (note that the aperture and ISO do also factor into the ambient light metering). The best way to do this is to switch to full Manual mode on the camera. It's not nearly as scary as it sounds. Take a few test snaps and read your histogram (which is an art form of itself).</p>

<p>The camera will meter the scene in a color blind, 18% gray, kind of way. In other words many times the camera will overexpose dark scenes and under expose intensely bright scenes (snow scenes in sunlight are common for underexposure and result in gray snow). If your even tones scene is of dark color the camera may overexpose to compensate.</p>

<p>In most of your modes other than M mode your flash is considered as fill by the camera. So the camera will always try to lengthen the shutter speed to compensate. Until you press the shutter release it can't assume anything. So ambient light will always be considered, and running with an extremely slow shutter speed can result in poor exposures, motion blur, color mismatching (your flash is daylight balanced and most artificial lights are not), etc... Again, M mode is the only way for you to tell the camera exactly what you want, and with non-run and gun scenarios or anything posed or setup it's not nearly as scary as it sounds.</p>

<p>Bouncing the light off a ceiling for example will result in softer light. It may result in less light available for the exposure, but often it's plenty of light. It doesn't affect the speed of the shutter or the speed of light, light travels plenty fast.</p>

[jcuknz]

<p>In the days before automatics to 'help' us there was a simple way of working out how to use flash. You determine the "Guide Number" of the flash unit and divide the distance of the flash to subject into the GN and the result is the aperture to use.<br>

The GN is found from the aperture needed to properly expose an average subject ten feet [ 3metres] from the flash when using 100 ISO. A GN can be in imperial or metric. Often the GN is expressed as the model number of the flash so a 560 flash could have a GN of 53<56, though this to confuse us is when the flash is in telephoto mode and concentrating its light into a narrower tube of light. Better is to do an actual test as mentioned elsewhere ... 'Pinnochio' at the factory tends to be optomistic. <br>

That GN53 is metric so actually an imperial GN would be about 160 or f/16 at ten feet. At 5ft 160/5=22 or f/22. at 20ft 160/20=8 or f/8 .... although without nice reflecting walls of a small room you need to open up a bit to say f/6.3 or f/5.6 ... whatever, within the range that you can correct in editing these days.</p>

<p>Change the ISO and the GN doesn't double when the ISO does but increases like an F/stop range.<br>

So GN56 at 100 ISO [ f/5.6 at ten feet ] becomes GN80 [f/8 ] with 200 ISO and GN110 at 400ISO. </p>

<p>It gets a bit complicated when you do syncro-sunlight work because firstly you usually need the flash at reduced power so it doesn't look like a 'flash shot' and after working out the aperture for the flash you then need to know what exposure you would give without the flash and the sun behind you. Then thinking of aperture/shutter relationship as a see-saw you work out what shutter speed you need for the ambient light to match the aperture the flash needs.<br>

Bright sun = 1/250 at f/8 but only lighting shoulders of subject from behind them. GN 110 with flash 5ft from subject means f/22 ...... f/22 at 1/30 is the equal of 1/250 at f/8 ....you probably need to reduce the flash power if you can so that you can use it at f/16 at 1/60 or f/11 at 1/125. Even further reducing the flash so it is only half to a quarter of its normal strength for a correct exposure without any ambient light. So the flash is like a 'fill' light in the studio reducing the shadows to a nice level instead of the 'black' of a single light not shining on the subject fully .. a daylight contra-jour shot at f/8 at 1/250.</p>

<p>If you work out what is happening and the effect you want it will become clear as when I worked it out for myself way back when I started syncro-sunlight. The beauty of digital is that you get instant review to see how things have worked or have not ... instead of fingers crossed it will work with film</p>

[jcuknz]

<p>Dan describes how a modern flash TTL works but older units did as you suggest have a sensor which received a reflection of the flash off the subject and cut off its power when it thought it had enough for a proper exposure. I have several of those units although I have always worked by Guide Numbers as the way I learnt. Those units have two settings f/5.6 and f/8 and depending on the power of the flash will give a correct exposure with the subject anywhere between maybe two feet and 10 or 8 feet with the lower powered one with a GN of 56 while another with a Guide Number of 80 gives a working range out to around twelve feet or so.<br>

Today 'good' units to my way of thinking have a built-in optical sensor so you can trigger them with the camera flash, or other ways, and you have a multi-light set-up for more exciting lighting. The 'strobist' approach to lighting. Except as a fill light, flash on the camera is a no-no for most people, unless it is the only way they can go. Read the recent thread where the OP asks if a unit is junk, apparently it was unsuitable, but the comments from everybody were interesting.</p>

[frank_skomial]

<p>Your camera and flash team up together and prevent you from doing many wrong things, that some were already explained.</p>

<p>The most concern would be under exposure from insufficient light, due to closing aperture too much, using ISO too low, or bouncing the light too far, or using FP mode for out of range.</p>

<p>Your flash reads your camera settings, and caclulates and displays maximum flash light distance for your actual camera setings. Just observe it, and you will have properly exposed flash pictures in flash iTTL automated mode.</p>

[Spearhead]

<p>The SB-600 has TTL, the question is why it is over-exposing. TTL is the most reliable method of automated flash usage, but you need to understand all the settings. The first thing to check is that you are in the proper TTL mode. The next thing to check is that you don't have flash exposure compensation set to +2. If you do, set it to 0 if you are using the flash as the primary source, and something below zero, depending on effect you want. The next thing to check is that you are using an aperture that matches the distance range for the flash. This can be an issue shooting close up with a wide aperture - there should be a readout on the SB-600 that tells you the proper distance. If not, try stopping down. Also, your camera's metering modes should be checked.</p>

<p>If you are shooting the flash as "fill," i.e., as secondary, maybe to add some life to a subject that is backlit for example, use aperture priority on your camera, and try dialing the flash down a little at a time until you find the look you want. If you are shooting the flash as primary, use manual mode and set the shutter speed to something fairly fast, like 1/200. Don't go beyond your sync speed.</p>

<p>You should be aware that using a low ISO (100 - 400) will drain your batteries faster than a high ISO. If you are shooting an event, this needs to be kept in mind so you aren't changing batteries constantly.</p>

<p>You should find a willing subject and try shooting with a number of different settings in different lighting conditions, it should become clear after some experimentation. Here's a few examples.</p>

<p><img src="http://spirer.com/kikisept2011/content/images/large/_57P7065.jpg" alt="" width="792" height="594" /><br /> <em>Flash as fill. Aperture priority mode, flash at -1/3 compensation.</em></p>

<p><img src="http://spirer.com/pb1/content/images/large/357P9692_Edit.jpg" alt="" width="700" height="525" /><br /> <em>Flash as primary lighting. Manual mode, flash at 0 compensation.</em><br>

<em> </em><br /> Finally, there is the use of what is called "dragging the shutter" or "slow sync." This gives equal prominence to low ambient light and flash in reduced light situations and as a result, can produce dynamic photos with a sense of motion yet retaining the background. The Wedding Forum here on photo.net has quite a bit of info on this technique.</p>

<p><img src="http://spirer.com/newdiplomatnov2011/content/images/large/_57P3226.jpg" alt="" width="792" height="528" /><br /> <em>Slow sync flash, manual exposure at 1/8 sec and flash compensation at 0.</em></p>

<p>After you have all this down, you should start looking at things like bounce and off-camera flash, both of which can enhance your images with flash.</p>

[Spearhead]

<p>I should add that flash is something that you really need to experiment with, all the theory in the world isn't going to make it easier. You need to find the look(s) that you want, typically by experimenting. I had read everything there ever was to read on flash but found that it didn't work for me until I just started shooting, examining, shooting, examining...</p>

<p>This picture of my friend Ruby Ruin shows what happens when you can use flash to maintain background ambience with a flash-lit foreground. Fairly low shutter speed to maintain the background, bounce flash with white card.</p>

<p><img src="http://spirer.com/saintsjune2011/content/images/large/_57P1211.jpg" alt="" width="792" height="594" /></p>

[lisae]

<p>Jeff, in your post with the 3 pictures, you mention bounce and off-camera flash in the last paragraph. Does that mean you shot all 3 pictures with the flash on-camera and pointed at the subject? </p>

[Spearhead]

<p>The first three were all shot with direct flash, although the flash was off-camera on a stand for the first one. However, it was shot with TTL, using PocketWizards.</p>

[dan_south]

<p>First, think about how the flash would work in manual mode. If you set it to 1/4 power it will always send the same power for every image. It won't matter if the image is coming out too light or too dark. It doesn't matter how far the subject is from the camera. The flash just does exactly what you told it to do: shoot at 1/4 power.</p>

<p>If the image doesn't suit you, you can make some adjustments. If it's too bright, you can reduce the power to something less. 1/8th power, for instance. Now there's only half as much light from the flash. But the ambient light level is the same, so the blend between flash and ambient will change. That might look fine.</p>

<p>Another option is to leave the flash at 1/4 power and close down the aperture a stop. Go from f/8 to f/11. Now the flash will have less on the image. But the ambient light level will be reduced as well. You turned them both down the same amount, so the blend will remain the same. And that might look fine.</p>

<p>There are other options, but they won't help with understanding of TTL, so I'll just mention them quickly. You can move the flash farther away (if it's detachable). You can bounce the flash off of some surface (if it has that capability). And you can modify the flash, e.g. through some sort of soft box.</p>

<p>Now to discuss TTL flash: TTL is a technology that lets the camera make decisions about how much flash to use. It's no longer limited to doing what you say explicitly (e.g. shoot at 1/4 power). The camera will determine how far away the subject is and how dark or light it is. It will combine this information with the aperture setting of your lens, and it will send an amount of flash that it thinks will make a good photo. And this works reasonably well in many cases. </p>

<p>Sometimes the TTL system will be fooled by a subject that is particularly light or dark or reflective. You can compensate for this by using your camera's flash compensation control. +1 would send twice as much flash power. -1 would send half as much. So, even though the camera is making the decisions in TTL flash mode, you can influence those decisions.</p>

<p>(Note: It's important not to confuse flash compensation, which only changes the amount of flash, with exposure compensation, which controls the lightness or darkness of the entire image. These two controls actually work together, so give them each a try. Just make sure you know which one you are adjusting at any given time, and why.)</p>

[Spearhead]

<blockquote>

<p>if it has that capability</p>

</blockquote>

<p>He says what flash he has. It does have bounce capability.</p>

[jcuknz]

<p>Every flash has bounce capability, even your on-board flash if you reflect the light upwards as apparently the Lightscoop does and I experimented with some silver paper :-) It depends of how much light reaches the subject as to if it is of any value.</p>

[dan_south]

<p>Jeff - I'm glad that they OP's flash has bounce capability. My advice was meant to be general, not specific to this one poster.</p>

<p>JC - Fair enough, but strictly speaking a piece of gear does not have capability X when it requires an accessory to enable that capability. I don't know of any built-in flashes that can bounce without accessories.</p>

[ilia_isakov]

<p>thank you guys for taking the time to answer some of my questions; I am a bit interested in how things change in proportion to each other, here is what I know or think to be true, correct me if I am wrong.....</p>

<p>flash reach and subject distance are inverse; as you go farther away, flash is less noticeable<br>

flash and ISO are proportional..........increasing sensitivity increases sensitivity to flash<br>

flahs and aperture are proportional, the bigger the hole the less hard the flash has to work to get the same exposure<br>

and flash and focal length are inverse; as you go from wide angle to telephoto flash is reduced.</p>

<p>am I right?</p>

<p>But anyway, here are the conclusions I am makking;<br>

-that I need to shoot in Aperture priority mode when using flash, to keep it from having to work and fluctuate from shot to shot.<br>

-flash is basically thought of as the camera as a fill light source and does not give you too much extra leverage with shutter speed and aperture unless you work in manual mode.</p>

<p> </p>