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11-08-2010, 02:33 AM   #1
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Flash usage, and TaV or manual mode.

I'm having trouble with either the built in or my older Vivitar flash.
Shooting a K-7,35LTD in TaV mode, I find if I set the shutter speed and aperture to what it demands for iso 200(my preference) it wants 10/f3, as though it is metering for no flash.
If I set it to what my best guess is, I get a lot better results(200/f3)
Is there any way to get it to allow me to set the things based on the fact that I will be using a flash since it is activated at the time?
it doesn't expose either way, but doesn't seem to benefit any from the slower shutter, what am I missing here?

11-08-2010, 12:09 PM   #2
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Please repost your question(s), separating out question(s) for the built-in flash and question(s) for the external Vivitar flash, also indicate the model of the Vivitar flash.

The clearer you ask your questions, and the more details you provide, the more meaningful replies you will get.

The first thing to remember is that Pentax DSLR does NOT trigger flash, any flash, if the shutter speed is set above sync speed (1/180 sec for the K-7).
11-09-2010, 12:49 AM   #3
JGB
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Sorry, my question is. Why does the camera not seem to meter the scene as though I would have flash when the picture is taken? In TaV with built in flash up it is compensating by cranking the ISO to 800 or higher for my setting the speed to the same as the camera selects for flash on when I set the ISO to 200and have it do the other functions automatically.(shutter speed 1/80 F stop of 3) To get my ISO back to 200 in that case, I have to shoot at 1/10 f3, and the picture looks about the same as when I shoot at 1/80th.
Closing or opening the flash does not change the ISO in TaV, despite the fact that when the shot is taken it will have the brightness it needs.
If I try it in M mode, it tells me I will be underexposed several stops, so I have no way to determine the correct shutter speed and aperture for use with the flash except by trial and error.
Sorry about the confusion, I'm in the middle of working on my bike and type from time to time so my messages can get confusing!


The problem is the same with my external flash in manual mode maybe a little worse, but I am fine with setting everything manually when using that, I just use the F stop suggested on the back of the flash for the range of my shot and ISO 1/160(it jumps from that to 200, no 180 available for some reason) and it works tolerably well, I don't see any increase in exposure level by dropping the speed any.
11-09-2010, 02:14 AM   #4
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The jump from 1/160 to 1/200 is because you have exposure steps set to 1/3EV. Change it to 1/2EV steps and 1/180 will be available again. The practical difference between 1/160 and 1/180 is not worth worrying about. If your flash unit has an auto sensor that is your best bet for hassle-free flash photography.

11-09-2010, 04:21 AM   #5
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Personally I would stay away from auto-ISO when using flash.

QuoteOriginally posted by JGB Quote
If I try it in M mode, it tells me I will be underexposed several stops, so I have no way to determine the correct shutter speed and aperture for use with the flash except by trial and error.
This one really struck me as it sounds like you don't have any idea about flash; I don't say that I know everything but maybe the following helps.

If your camera indicates that the image will be severely underexposed, the flash will contribute mostly to the exposure and the ambient light hardly does. Therefore you no longer care about exposure like you do normally. Flash durations are short (between 1/200sec and 1/20000sec or so) and the shutter speed will not have much of an influence on the end result.

You set the shutter speed to 1/180sec or slower and the correct exposure is determined by the combination of guide number (at a given ISO), aperture and distance to the subject.

Assuming GN 14 (for ISO 100) and aperture 2.8, a subject at 5 meters will be correctly exposed. Anything in front of that will be overexposed and anything behind that will be underexposed. If the subject was closer, you need to use a narrower aperture (e.g. f/5.6); if the subject was further away, you need to use a wider aperture (e.g. f/2).

That is the basic rule.

When camera and flash support p-ttl (like the built-in flash of the K7), the camera has a means to stop the flash at the moment that the camera thinks the exposure is correct. So a subject at 2.5 meters can be correctly exposed in that case with the f/2.8 aperture; again, in front of that and behind that are over- and under exposed respectively.

When the flash does not support p-ttl, it might have an automatic mode. With that, you set the aperture on the flash to match the aperture on the camera (or vice versa) based on the basic rule. When the flash determines that the exposure was correct (by measuring the reflected light) it will cut off the flash light (similar to p-ttl where the camera does it).

When your flash does not have an automatic mode, you basically have to rely on the basic rule. Some flashes have power settings to reduce the power (duration of the flash) so you can use a wider aperture when needed (e.g with a subject close and you want shallow DOF; max power would wash out the image).

The chosen shutter speed depends on how much ambient light you still like to be 'visible' in the image; the longer the shutter speed, the more it will be visible.

Note 1:
The guide number is usually defined at ISO100 for a 50mm lens on a FF camera. Doubling the ISO will result in a 1.4x higher guide number. Fancy flashes nowadays have zoom reflectors so at e.g. 70mm the light is more concentrated and therefore the guide number is higher.

Note 2:
The above text does not apply directly when using a flash as a fill-in flash (enough ambient light available).
11-09-2010, 09:26 AM - 1 Like   #6
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Replace ETTL with PTTL if you're using a Pentax PTTL flash, and note that the sync speed of your camera is 1/180s, not 1/250s. This was originally written for Canon, but light is light so most of the principles still apply.



Basically, with flash, the FLASH exposure is solely determined by flash power (actually duration, how long the bulb is actually firing for), aperture and ISO. Ambient exposure is determined by ISO, shutter speed, and aperture (just like without any flash), so the trick is balancing the two. If I'm indoors in a smallish room (such as in someone's house), I usually just forget about ambient since the flash is powerful enough to light up the entire room (hence the 1/200s below, if the flash didn't fire, I'd have a more or less black picture) Now although you're shooting MANUAL Mode, that's only for the ambient exposure (the exposure needle in the viewfinder will blink warning you about underexposure, but ignore that). The camera's E-TTL metering will determine the needed flash output for a proper exposure.

Here's something I wrote on another forum -
"Easy" recipe for great E-TTL flash shots -
1)Point flash at ceiling/wall (to the side or behind you, experimentation is the key!)
2)Put camera in MANUAL mode on the mode dial
3)Set FEC to +1 on the flash head

4)Shoot RAW (this allows you to recover some highlights that might get blown as a result of #3 above)

5)Set ISO to 200 (to start)
6)Set shutter speed to 1/200s
7)Set f-stop to whatever DOF you want


Now if the flash runs out of "power" because of high ceilings, you can raise the ISO or open up the f-stop to compensate. Or you can slow down the shutter to bring more ambient light into the exposure (in addition to adjusting ISO/f-stop) If the ceiling is REALLY high (like in a church), you may need a reflector to throw some of the light forward (I use the Joe Demb Flip-it).

Quick and dirty outdoor fill flash tutorial -
Basically, if your subject is in shade and the background is bright (ie under a tree) or majorly backlit, fill flash is your friend. Think of those times when you got a properly exposed background, but the subject was almost pitch black.

Put camera into Av mode, metering will set the shutter speed to expose the overall shot (which in the situations that call for fill-flash will generally be the background) based on your selected aperture/ISO.
Make sure flash is set to HSS (in case your shutter speed go faster than 1/200s) and E-TTL. Fire away! The shutter speed/f-stop/ISO will expose the background, and the flash should output enough power to light up the foreground.

Now to control the background exposure, you use exposure compensation on the camera body (which would adjust the shutter speed), to adjust how much fill for the flash exposure, you use Flash exposure compensation. The trick is balancing the two (as it is with indoor work), and that comes with experience/experimentation.

IF the flash is providing all the illumination (which it generally is in a small-ish room with you bouncing it off the ceiling), the shutter speed AND how dark it is do NOT matter AT ALL.

Try this - Pick a room in your house at night. Have a bunch of lights on. Set the shutter speed to 1/200s, aperture to f/5.6, ISO400. Point the flash straight up towards the ceiling (make sure the flash is in ETTL). Shoot.

Then turn off ALL THE LIGHTS, so it's pitch black. Do not change any settings. Take a picture. The picture should turn out the same, AND the flash wouldn't even have to work any harder. Basically, the flash is hitting the ceiling, and turning the ceiling into a large light source. THIS light source is providing all the illumination to the picture. How much flash power you need depends on the aperture, the ISO, and the distance from the light source to the subject. How dark the room is has NO affect on how much flash power is needed.

Now if you went with ISO400, 1/200,s f/5.6, and did NOT turn the flash on, the shot should be pretty dark, even with the lights on.

Now turn the flash back on, but adjust the shutter speed to 1/100s. The shot will probably look VERY close to the first two shots (you can turn the room lights back on now ) Then try 1/50s, 1/25s....Eventually you'll see the room lights "creeping" into the picture. This leads into the next paragraph...

A "flash picture" is made up of TWO distinct exposures. The "ambient" exposure if comprised of shutter speed, ISO, and f/stop. The "flash" exposure is comprised of ISO, f/stop, and flash power (and of course the distance from the light source to the subject) In the above example, the ambient exposure is essentially nil, so the picture is completely made up by the flash "components".

Once you nail using the flash to provide ALL the illumination, you can move onto more advanced topics such as balancing flash and ambient exposures.


You need to decide the CAMERA mode (Av or M, forget about using Tv/P auto modes) and the FLASH mode (Manual or E-TTL).

Indoors if the ambient light is fairly low and I'm using the flash to provide all of the illumination, I'll use M mode on the camera (and generally set the shutter speed to 1/250s to just get an ambient exposure) Outdoors where I'm using flash as fill (or indoors if it's bright, but this happens rarely) I'll use Av as I can rely on the camera to set a good general exposure WITHOUT flash, and then the flash can fill-in.

Now as for the FLASH mode, E-TTL works great if the flash is ON camera and you are constantly changing the distance between the light source and the subject. Now keep in mind what when you're bouncing, the bounce surface (ceiling or wall) actually becomes the light source. If you try to go Manual flash, you'll be adjusting the flash power for pretty much every shot, and this just isn't practical. E-TTL will get your flash power "in the ballpark"

Now once you get the flash OFF-CAMERA (on a light stand and shooting into/through an umbrella), Manual flash makes sense because although YOU can change the camera position, the light source is NOT moving (unless you move the stand of course), and as long as the subject(s) stay in the same general area, the subject-light source distance is constant. I'm talking portait/formals setups here.

Hope that helps!
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