Question about proper exposure using a flash.

[jwbryson1]

I put this question out there and am curious if I am just missing the boat. My DSLR is a Nikon D90 and I have the separate SB-400 flash. I understand how to shoot in manual mode and how to check if my exposure is correct using the exposure meter on the camera (looks like a small ruler with grid lines). For example, when the flash is set to "off" I can adjust to proper exposure by adjusting the shutter speed or the aperture depending on the effect I wish to achieve.

The question is this--does the exposure meter in the camera function when I am shooting with the flash turned on? I believe the answer is no, that the meter only works with ambient light and does not meter flash. But, I still wonder if it serves ANY purpose when I am shooting with the flash turned on?

Thoughts? Thanks.

 

[o hey tyler]

The light meter does serve a purpose with your flash on. You need to establish what you're exposing for. If you're exposing for just flash, turn your shutter speed to 1/200 (should work in most cases), and that should be enough to kill most of the ambient light. However, if you want to expose for ambient and flash, you need to meter the ambient, have your camera set up to properly expose for it, and then the flash will fire the TTL pulse and establish what the flash needs to fire at to expose your subject.

 

[Big Mike]

The 'meter' on your camera reads the ambient light. The flash being on or off has no affect on it.

When you do use the flash (and use the TTL auto flash metering), the flash uses a preflash, which is then read by the camera, which decides how much flash power to use, then the flash fires again when the shutter is open. This all happens very fast, you probably only notice one flash.

The meter does server a purpose, even when shooting with flash...because every flash photo is actually two exposures...the flash and the ambient. They usually overlap, or one overpowers the other...but they are both there. So if you are trying to balance the flash exposure with the ambient (or have one more prominent than the other) then it would certainly be handy to know where the ambient exposure is at.

Also, when using TTL flash, you can set FEC to adjust the flash exposure (while still based on the pre-flash metering).

 

[jwbryson1]

@Mike--thanks. Very helpful. I looked at the other threads on this topic and something struck me. If there are 2 different light sources to meter, and the light meter in the camera ONLY meters the ambient light (and not the light coming from the flash), why does the camera's built in light meter (at least on my D90) show the exposure is way over exposed or way underexposed when the external flash is connected and turned ON? If it is turned to the OFF position, I can expose for the ambient light using the meter. When I flip the external flash to the ON position, the meter changes and no amount of adjustments on my camera affect the displayed meter reading.

Is the answer that I leave the external flash in the OFF position while I meter the ambient light to my taste, and THEN turn the external flash to the ON position?

 

[o hey tyler]

My Canon doesn't work that way. What metering mode are you in?

 

[jwbryson1]

My Canon doesn't work that way. What metering mode are you in?

I believe it is the same in every mode. P, A, S or M. Flash off -- I meter for -2/3 stop exposure in bright daylight conditions. Flash on for fill light, the light meter shows exposure all the way to the right which I believe is overexposure. No amount of adjustment to the dials affects the reading. I don't have my D90 in front of me--I'll need to confirm when I get home tonight.

 

[KmH]

There are 2 kinds of ambient light: incident light and reflected light.

The in-camera meter only meters the reflected light.

However, in i-TTL mode, pre-flashes from the SB-400 are used by the camera to adjust the speedlight power output, so the camera is metering the strobed pre-flashes, but faster than you can see.

From the SB-400 spec's page at nikonusa.com - "i-TTL Balanced Fill-Flash with CLS compatible cameras".

Proper flash exposure can only be attained by using a had held flash meter at the point(s) you want to control the exposure.

The in camera i-TTL system makes several assumptions that likely don't mesh with your artistic vision.

For most amateur uses the i-TTL will get the exposure close enough.

An issue with the SB-400 is it doesn't have much more power or versatility than the built-in flash has.

 

[jwbryson1]

KmH, but the photographer can "override" this by shooting the camera in fully manual mode? Is that what Bryan Peterson means when he uses an external flash in "fully manual" mode--that he can override the camera sensors and get as much flash from the external strobe as he wants?

 

[KmH]

The SB-400 doesn't have a manual mode.

Putting the camera and the flash in manual mode is when having a hand-held flash meter and a wat to remotely trigger the strobe (flash) can be very useful.