Underexpose an image, then add flash?

[cooldude]

In low light situations, would it be ok if I were to underexpose and image and then compensate for the underexposure by adding a flash in manual mode? For example, if the proper setting for the inside of a room are 1/125 @ f/3.2 ISO 2500 and instead choose to put my ISO at 800 and put a flash in manual at 1/32 power, would that cause any problems in the images? Also, could this be accomplished using TTL? I have a Canon 60d and a Yongnuo 568.

 

[jwbryson1]

In low light situations, would it be ok if I were to underexpose and image and then compensate for the underexposure by adding a flash in manual mode? For example, if the proper setting for the inside of a room are 1/125 @ f/3.2 ISO 2500 and instead choose to put my ISO at 800 and put a flash in manual at 1/32 power, would that cause any problems in the images? Also, could this be accomplished using TTL? I have a Canon 60d and a Yongnuo 568.

Not sure I understand the question. It's already a low light situation and you want to further underexpose the image? Do you want any ambient exposure at all or just flash exposure?

 

[480sparky39 38]

The color temp of the ambient lighting will be a huge issue. Rarely is it ever the same as the flash.

 

[jwbryson1]

See this: 02 - flash + ambient light « Neil vN - tangents


and see this: 03 - dragging the shutter « Neil vN - tangents

 

[cynicaster]

There is no definitive answer to your question, because you haven’t said what you’re photographing. If it’s just an empty room, then the result will depend on a lot of things, including how far away the walls are from your flash, where you’re pointing your flash, the power of the flash, and on and on.

If you have a subject in the foreground, it will likely be exposed much brighter than the background due to the flash, but how much will depend on a number of factors, as above.

If you use ETTL, the camera will detect a foreground subject, and automatically set the flash power it thinks it needs to to properly expose the subject. The background will then be exposed according to the exposure triangle (i.e., in your case, underexposed).

 

[Big Mike]

It completely depends on what you want to accomplish.

When you take a photo with flash, it's actually two exposures; one from the ambient light and another from the flash. If you know how to do it, you can control the exposure of each of them independently, altering the ratio between them, in order to get the results that you want.

If you want to underexpose the ambient, and then add flash...sure, no problem. But it may or may not give you want you want.

 

[KmH]

2500 ISO is about 1 1/2 stops faster than ISO 800.

A flash unit turned down to 1/32 power is 5 stops less light than full power.
Then you need to allow for the inverse square law which is about the distance the flash unit is from the subject.

TTL uses one or more pre-flashes reflected from the subject to adjust the flash unit power.

 

[rambler]

That is a very effective way to shoot in sunlight. Remember, shutter speed has effect on ambient light, aperture has effect on light of the flash. In other words, changing the shutter speed will either darken or lighten the ambient light. Keeping the aperture constant will render the same flash light on your subject. Let's say you have a person in some shade standing in front of a bright background. When you take the shot with the correct exposure of the background without using the flash, you will get the perfect background exposure with the person appearing as a silhouette. When using your flash with the aperture at f/11, if you want the background darker, underexpose it, if you want it lighter, overexpose it, in both cases using a change in shutter speed either up or down. Keeping your aperture constant, the light from the flash on your subject will keep the look exactly the same, but the background will be different depending on the shutter speed.

Therefore, suppose the same bright light is on both subject and background. Underexpose the scene by decreasing your shutter speed 2 or 3 stops, but keep the aperture constant. Get the right power on your flash for the correct distance of your subject from the flash, power it down if you are close. Hold the flash out to the side. You will get a nicely side lit subject with a nice dark background! If the subject is close to the background, look for some nice shadows in your image, too, which will give it a nice depth.

Recommended reading: Understanding Flash Photography by Byran Peterson (especially for the above info, pp. 28-31)

PS Cooldude, in your example when you change the ISO you are changing the shutter speed. Let's say the correct exposure at ISO 100 is 2 sec. at f/8. Change the ISO to 800 and the correct exposure is now 1/3 sec. at f/8. If I had left the ISO at 100 and changed the shutter speed from the correct exposure of 2 sec. down to 1/3, I would have a very underexposed subject. So, if you wanted to you could keep the ISO constant and just change the shutter speeds.

 

[CaptainNapalm]

In low light situations, would it be ok if I were to underexpose and image and then compensate for the underexposure by adding a flash in manual mode? For example, if the proper setting for the inside of a room are 1/125 @ f/3.2 ISO 2500 and instead choose to put my ISO at 800 and put a flash in manual at 1/32 power, would that cause any problems in the images? Also, could this be accomplished using TTL? I have a Canon 60d and a Yongnuo 568.

Not sure I understand the question. It's already a low light situation and you want to further underexpose the image? Do you want any ambient exposure at all or just flash exposure?

He's trying to use lower ISO to prevent noise in photos. At this expense, he wants to compensate with flash to prevent/minimize under exposure.

 

[Robin_Usagani]

If you underexpose it and use flash (manual or TTL), several things can happen (assuming the flash is pointed at the subject):

1. The room is tiny with a lot of white wall: your image will turn out OK. The flash will pretty much brighten the whole room and give you the right exposure.
2. The room is tiny with dark wall: your image will turn out OK but not as nice as #1. The shadow behind the subject will be more noticeable.
3. The room is big: your subject will be at good exposure, but the whole room will be dark (sometimes it is good, sometimes it is not what you are trying to achieve).