Flash and decreasing shutter speed to expose background.

[clarence_l]

Hello,

 

I have some questions about using hand-held flash:

 

Assuming that one is holding the camera by hand normally, why is it

that when one decreases the shutter speed when using flash, in order

to let the background become better exposed, the background doesn't

bleed into the sharp foreground due to camera shake?

 

Why is it that the foreground doesn't blur either from the exposure

time after the flash has shut off?

 

What are the lowest limits of shutter speed when you do want to

overexpose the background behind a flashed subject, without blurring

everything in the picture from camera shake? (I presume it's dependent

on focal length)

 

Thank you.

 

Clarence

[xenomer]

What kind of flash do you use? What is the setting of the flash? Do you use it in manual mode or in one of the automatic modes?

[steve_levine]

Both of your "why doesn't it" questions are correct. But of course the subject, unless it's moving, will block the background from bleeding through where say a person is standing. Normal flash brightness will over power ambient, and the subject is usually much more exposed that the background. But in theory, a moving subject could appear transparent, if the right conditions were created.

 

 

Every flash picture is indeed a double exposure. The shutter opens and "paints" the overall image onto the film (sensor?). When the shutter is open all the way, the flash fires for a very short duration. The shutter then closes, and continues "painting" the film with light.

 

If the background were bright enough ,say within a stop or two of the flash lit areas, the background would indeed blur. At weddings we tend to exagerate this slightly, and use it to our advantage.

 

Since flash will always freeze the subjects, far away backgrounds can be made to blurr intentionally. This works great if there are small candles, or tiny lights in the background.

 

 

Remember too that light (from your flash)falls off inversely square to the distance it travels. This means that if you keep subjects far enough away from backgrounds, the background won't be able to gain much exposure, (providing the distance is large enough).

 

 

You can also control background brightness intentionally this way. A higher shutter speed, will assure that a distracting background goes black.

 

 

As for the lowest speed and focal length question. There probably is an advanatge for the wider lenses, less magnification, less apparent shake.

 

 

Some wedding shooters use mono or tri-pods, this solves the camera shake ,blurred background issues.

[clarence_l]

Hello,

 

Thanks for the response.

 

I recently processed some photos that I had taken using my Vivitar 283 to provide fill flash. I noticed that on the shots with my 135mm lens, both foreground and background were much blurrer than on the shots using the 35mm lens. I believe the maximum flash sync of 1/60 and the ambient light were responsible for this.

 

So I'll probably have to start using a monopod, like you said, when I'm using my long lenses.

 

Once again, thanks.

 

 

Clarence

[picturesque]

The focal length of the lens technically has nothing to do with how "frozen" your subject is and whether your background bleeds into the foreground. That effect is dependent on the relationship of the flash exposure to the ambient (or background) exposure. The reason you noticed that your 135mm image seemed to be blurrier is that you were using the flash as fill, meaning the ambient light was the main source of illumination. When this is the case, the guideline for handholding is still in effect--for example, you should have been using at least 1/125th for your shot with the 135mm, assuming a non-crop camera.

 

If you were using the flash as the main source of illumination, and the difference between the flash exposure and ambient exposure was greater than about 1.5 to 2 stops, your subject would have been frozen/sharp, although some background elements or any moving parts may still have some blur.

 

On your first question, the reason the background doesn't bleed into the foreground is because the background exposure is still less than the subject's exposure. On your second question, the reason the foreground/subject doesn't blur is because the flash duration is very short, so in effect, it's as if you were using a shutter speed (on the subject/foreground) of 1/200th and much shorter, normally. The longest flash durations for the more powerful flashes are about that, and shoe mount flashes have a much, much shorter duration. On your third question, you can't keep your subject/foreground sharp by flash duration if you overexpose the background. You want to underexpose the background in relation to the flash exposure. You use aperture to control the flash exposure, and shutter speed to control the background exposure. If you want to overexpose the background, again, the relationship between shutter speed and focal length apply "as normal", as well as the "normal" guidelines for stopping motion.

[twmeyer]

Man, I know a pretty good bit about this, but after reading this thread, I'm confused. I'm not sure how anything is "painted" on the film after the shutter closes.<p>Conventional Photo Wisdom says your slowest hand holdable shutter speed should be the reciprocal of the len's focal length, ie: 50mm = 1/50th, a 85mm lens = 1/85th, a 200mm = 1/200th, etc. And you should round up to your closest actual shutter setting. These rules assume that the subject isn't moving, and that you aren't a heavy coffee drinker or a meth head. A second tier of Conventional Photo Wisdom says that this rule breaks down with shutter speeds under 1/60th or lenses over 200mm. I break all of these rules alot for some types of photos and believe them to be too conservative with other types of photos. <p> Flash only effects the parts of an image that it illuminates, ambient light effects the rest. You extend reach and amplify the effect of your flash by increasing it's power or opening your aperture, you increase the effect of ambient light with your shutter speed. The trick is to balance these influences depending on the nature of your subject, and the type of photo you want to make. For crisply defined photographs of a moving subject, both a high shutter speed and a powerful flash may be required.<p>You may be fooling yourself, when you say "the foreground doesn't blur either from the exposure time after the flash has shut off?" in that you may have a static subject and a steady hand and just aren't noticing what blur may actually exist in a small enlargement. There are no definative settings that work with all subjects, equipment, iso's and ambient light levels. Each of these parameters influences the creative decisions that must be made.<p>My suggestion is to get a sharpie and a stack of notecards, write the camera settings (focal length, shutter speed, iso, ambient exposure, flash settings) and photograph (using those settings) a person holding the card. Do this with a wide range of settings in various ambiennt light levels. Use all manual settings to assure what you get is what you asked for (no auto!). Of course the other variables are how much movement is in the person holding the camera and how much in the person holding the note cards. <p>If you do this alot, you'll begin to understand what's happening, way better than asking this kind of question on photo.net and getting lots of wordy confusing answers of dubious value (like this one)... t<div></div>

[clarence_l]

Hello,

 

Thank you very much, Nadine and Tom for your answers. I understood both of you perfectly, so don't worry. I had conducted handheld lighting tests before, but I had never done one with ambient light exposure. It has taught me a valuable lesson, I suppose.

 

I eventually decided on getting a monopod because SLR bodies with flash sync speeds of 1/125 and above are generally too expensive for me. I suppose it should give me about two stops more of shutter speed, which would make holding a 135mm lens still enough on 1/60 possible.

 

Once again, thanks.

 

Clarence