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Correct Exposure with External Flashes
I have a dSLR (Pentax K10d) and currently only use the built-in flash for indoor photos, with limited success. I've read "Understanding Exposure" and have the basic idea of composing my shots and getting the exposure correct in manual mode. The thing that I have a hard time with is how the flash adjusts that exposure, with the built-in flash or an external one I'm thinking of buying..

Suppose I'm inside trying to take some shots of my son. Using the camera's meter, a "correct" exposure using f/5.6 is 1/10 sec. Since this shutter speed is too slow, I obviously need to use my flash. The question is what exposure do I now use? The meter on the camera still shows 1/10 sec shutter for f/5.6. Do I simply choose the new shutter speed I want (say 1/250) and the flash compensates it's intensity for a correct exposure?.

I've played around with different options indoors and really haven't narrowed down exactly what I should be doing...

Comments (6)

I shoot Nikon but I don't think there will be any difference. Choose Aperture Priority and your aperture. The camera should automatically choose the right shutter speed and flash duration. The flash duration is far more important than shutter speed. It will be shorter than the shutter speed and, provided you are in full flash auto mode, (Whatever Pentax call it. Nikon call it i-TTL (intelligent through the lens flash)) it will do everything for you..

Next query should be how fill flash works. I shall not try to answer that. I do not know the Pentax system well enough. But basically your flash intensity controls the foreground and the shutter controls ambient light in the background. Your flash can only be at a perfect intensity for one distance from the cam. Nikon sorts this automatically for you using i-TTL-BL which is the default flash setting when using matrix metering.



Chris Elliott.

*Nikon* D Eighty + Fifty - Other equipment in Profile.

Http://PlacidoD.Zenfolio.com/..

Comment #1

So you have 2 light sources, the 'ambient' and the flash..

If you stick w/ a 1/10th shutter speed, you'll get a well exposed shot but maybe somewhat blurry. The flash doesn't (to a point) care about shutter speed. It fires at some very high speed and it puts out alot of light. The on-board flash and most any decent external flash are automatic and readback how much light is reflected by the subject (or you can run things in manual mode if you really wanted to)..

So you can use the shutter speed to control the balance of brightness between subject and background..

Read this:http://strobist.blogspot.com/...9/lighting-102-33-balancing-flashambient.html..

Comment #2

Chris Elliott wrote:.

I shoot Nikon but I don't think there will be any difference. ChooseAperture Priority and your aperture. The camera should automaticallychoose the right shutter speed and flash duration. The flash durationis far more important than shutter speed. It will be shorter than theshutter speed and, provided you are in full flash auto mode,(Whatever Pentax call it. Nikon call it i-TTL (intelligent throughthe lens flash)) it will do everything for you..

Thanks Chris. So when shooting portraits indoors should I stay on Aperture Priority mode? Will the flash duration be adjusted in full manual mode?..

Comment #3

Thanks Howard. I hadn't seen the strobist site before. Looks like there's a lot of good information...

Comment #4

Scottyja wrote:.

The thing that I have a hardtime with is how the flash adjusts that exposure, with the built-inflash or an external one I'm thinking of buying..

The exact mechanism is going to vary with flash ond/or strobe manufacturers, but essentially, the flash (or the camera connected to the flash) uses the light being reflected back from the scene to determine what the correct exposure should be then shuts itself down when enough light has been generated..

(Ok. That is a very nutshell version, but it covers the essentials.).

... I obviously need to use my flash.The question is what exposure do I now use? The meter on the camerastill shows 1/10 sec shutter for f/5.6. Do I simply choose the newshutter speed I want (say 1/250) and the flash compensates it'sintensity for a correct exposure?.

More or less, yes. If you pop up the flash on your camera now (if you are in anything but full manual) you will see the shutter speed adjust to something like 1/60. That may vary depending on the camera and user settings, but when the on-board flash pops up the shutter speed will get back up to something reasonable..

If you want to choose a different shutter speed to control ambient light contribution, switch to manual mode. (I live there so I assume everyone else does sometimes.).

If you have a flash that works hand-in-hand with your camera (the on-board one, for example) then what will happen is this (assuming full TTL metering):.

The camera will have the flash send out a few pulses at very low power in order to meter the scene at your chosen aperture. Once it knows the reflectivity of the scene it tells the flash to expose accordingly. The flash then fires the main pulse of light and shuts that down once the requested exposure has been accomplished. The better ones will also let you know if the scene was exposed properly or if something didn't go right..

If you don't have the luxury of an intelligent connection between the camera and the flash, you'll need to meter the scene and calculate the proper exposure to fall within one of the ranges on the flash. (i.e. 6-18ft @ f5.6 ISO 200).

For those that were told there would be no math, get a light meter..

I've played around with different options indoors and really haven'tnarrowed down exactly what I should be doing..

Just playing around can be frustrating. Do you have a good handle on the relationships between aperture, shutter speed and ISO? That is almost a must in order to get a grip on flash. Well lighting in general..

Chefziggyhttp://www.pbase.com/chefziggy/lecream..

Comment #5

What the camera meter is telling you is irrelevant to what happens when the flash fires, so ignore it completely..

I find it by far the easiest to use manual mode. Choose a shutter speed that is below the flash sync speed (1/180 sec on my K100D; normally I use 1/125 sec). Choose the aperture you want. Shoot. The camera adjusts the duration of the flash burst to give the correct exposure. That's it (to a first approximation)..

If the picture ends up underexposed, it is because you have used to narrow an aperture (or too low an ISO setting) and the flash cannot put out enough light. This might happen wit a distant subject. So open up the aperture or increase the ISO..

The shutter speed is irrelevant if your subject is illuminated only by the flash: whether you use 1/15 sec or 1/180 sec, the subject will be lit for a tiny fraction of a thousandth of a second by the flash burst and is in darkness for the rest of the time. This is counter-intuituve: you are used to the shutter speed having a significant effect on exposure normally..

A handy trick if you don't want the background to look black (e.g. a person in a large room: the person will be correctly exposed by the flash, but the flash won't illuminate the distant back wall much and the background will be black). Use a slower shutter speed than usual, say 1/30 sec. The person will be correctly exposed, but now the long shutter speed gives the dim background a chance to register too and you will see more of it..

Best wishesMike..

Comment #6

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This question was taken from a support group/message board and re-posted here so others can learn from it.

 

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