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Nikon Flash - Two Separate Metering Systems

Russ MacDonald (Arkayem)


Keywords: nikon, speedlights, product, articles

After several years of using the Nikon SB-800 speedlight with my Nikon D200 while shooting weddings and events, and after writing numerous posts about this on the Nikonians Speedlights Forum, I decided to consolidate my thoughts in this series of articles.

Note: I use the term TTL to mean the same thing as i-TTL throughout the series.

20130610_104615_1.nikon-flash-picture_1.jpg
20130610_104615_1.nikon-flash-picture_2.jpg

 

Look at the two pictures above. Notice that the first one appears as though the subject is in a cave. The background is black. But the second one has noticeable detail in the background. This article is about being able to adjust between these two extremes at will.

Flash photography is a pretty complex subject, and you have to study it a while to understand it. It is also a very rewarding subject and leads to much better pictures in many situations.

The first concept to wrap your head around is that when you take a flash picture it is actually a combination of two exposures; one from available light (also called ambient) and the other from flash. Already you may be able to see intuitively that flash photography will be easier in the dark, because the contribution from ambient is zero! This concept is critical to understand.

And, it is also probably intuitive that if the ambient light is bright, things can get a lot more difficult as you try to balance the two contributions to the image.

So, Nikon made things much easier for us by developing their camera and flash metering systems - these are two completely separate systems that are used together or separately, depending on how we set up our camera/flash.

So let's look at the simplest situation first; a dim room where there is only a little ambient light; like what you would find in a typical indoor home setting at night. If you take a picture in such a setting without flash, in one of the camera’s auto modes, say A mode, a typical aperture/shutter would be about f/4 and 1/4th sec at ISO 100.

Now, put the camera in manual mode and increase the shutter to 1/80th sec and leave the aperture at f/4 and take another shot. The picture will be very very dark - to the point that the image is barely discernible.

Now, turn on the flash in TTL mode and shoot that same shot, leaving the shutter at 1/80th and the aperture at f/4 (still in camera manual mode). You will see that whatever is in the center of the frame will be properly exposed by the flash. This is because the flash metering system handled the exposure and added just the right amount of flash power to get a proper exposure. It still may not be exactly the right exposure for other reasons, but I'll discuss that in a future article.

Now, place a subject (a person is perfect for this) relatively close to the camera (say 5 feet) with a background behind the subject (say about 15 feet away), still in typical home indoor lighting. Make sure the subject is in the center of the frame and take a flash shot in straight TTL (not TTL-BL) leaving the camera settings at f/4 and 1/80th sec. You will find that again the subject came out properly exposed, because the flash metering system handled the exposure. But the interesting thing is that the background behind the subject is back to being very dark, just like in the shot without flash. This is because the flash power decreases very quickly from the distance of the subject to the background and it barely brightens the background at all.

Now, decrease the shutter speed to 1/10th sec and shoot again. The background will be brighter, but the subject will be the same brightness as before. Also, the subject will be sharp, but the background may also show some motion blur, because 1/10th sec is too slow to hold the camera perfectly still (unless you use a tripod). Since the flash was primarily on the subject; i.e., the ambient was overpowered by the flash, and the flash duration is normally faster than 1/1000 sec, the subject will be sharp with no motion blur.

This shows you that in a dim room, the flash exposure of the subject is controlled by the flash metering system and the background exposure is controlled by the settings on the camera. You can adjust them separately! This is the key behind using the flash in TTL mode and the camera in Manual mode.

Even when the ambient lighting is a little brighter than this, you can still use this technique if you stop down the camera aperture enough to make the flash the primary light on the subject, which takes the ambient light out of the equation. But then your background becomes dark. And if you try to brighten it by decreasing the shutter speed, then the bright ambient light will begin to affect the subject brightness too, and you begin to risk overexposure of the subject. In this situation, shutter, aperture, and flash power all affect the brightness of the subject. It's usually best to simply live with a dark background in this situation and make sure the flash is primary on the subject.

Now, if you are in bright ambient light (like outdoors in daylight), it is simply too bright to be able to take the ambient out of the equation by stopping down the camera, so now you have to balance the flash and the ambient. If the flash contribution to the shot will be less than the ambient contribution, then it is called Fill Flash. When there is a lot of ambient, the background and subject are no longer able to be adjusted independently, and camera manual mode becomes difficult to use. This is when you want to switch the flash to TTL-BL and the camera to one of the automatic modes (S, A, or P). The camera will then measure the ambient light to set its aperture and shutter and send this information to the flash, after which the flash metering system will set the power of the flash to make the subject the same brightness as the background.

If the ambient is extremely bright, like direct sunlight, then it's usually best to select P or S mode (and TTL-BL), so the aperture will automatically stop down to avoid overexposure. In A mode, the shutter will increase to reduce the exposure, but it will be limited by the flash sync speed of 1/250th (D200) and that is often not high enough for bright light if a wide aperture is selected.

One caveat for TTL-BL: the subject must be darker than the background for TTL-BL to work properly. The flash can only brighten the subject to balance it with the background; it can't make the subject darker. If the subject is brighter than the background to begin with, then you probably are best off not using flash. However, there can sometimes be slight shadows on the subject’s face that can be 'lifted' with a very slight amount of flash in TTL mode.

 

Editors’ note:
While the author’s experience was expanded on a D200 with SB-800, the concepts apply to all CLS (Creative Lighting System) capable speedlights and cameras, both film and digital. He currently uses a D800 with SB-800, SB-700 and SB-600 flashes.

About the Author:
Russ MacDonald is an active weddings, events and portrait photographer with an extensive professional practice in the use of speedlights. As an Electrical Engineer (MSEE USF, now retired from active duty) he also has an in-depth knowledge of the electronic inner works of Nikon flashes. Russ (username Arkayem) was appointed Moderator of the Nikon Speedlights forum in 2008.

 

(20 Votes)

Originally written on June 10, 2013

Last updated on August 24, 2016

Russ MacDonald Russ MacDonald (Arkayem)

Awarded for his high level skills in flash photography

Richmond Hill, GA (Savannah), USA
Moderator, 6142 posts

13 comments

Lisa Gott (lisagott) on June 26, 2013

Russ can you comment on using the camera body's variable P mode and TTL-BL when there is a desire to control depth of field?

Richard Walliker (richardd300) on June 22, 2013

Thanks Russ, this is the first time I've really started to understand what to me is a dark art! Although my results have been mostly acceptable, especially when doing weddings, I've often had occasions where some images are just not correctly lit. I now understand a lot more than I did.

Richard Luse (DaddySS) on June 20, 2013

Donor Ribbon awarded for his generous support to the Fundraising Campaign 2014 Donor Ribbon awarded for his generous support to the Fundraising Campaign 2015

Thanks Russ. A good refresher!

Venkateswarrao Garidepalli (gvrao88) on June 19, 2013

Simple and very valuable technique. Thanks Russ.

Ian Ross (Dadorian) on June 15, 2013

Really interesting and completely new ground for me, thank you!

Robert Parrish (nsolo2) on June 14, 2013

Now I understand more. Thank you. The concept of the flash and camera having tow unique metering systems, although logical, was insightful. I suppose that I always believed that the camera was the boss of the two, with the flash being the slave to its wishes. As they can work independent or with each other, I now have a better grasp.

Allen Henderson (AAHNikon) on June 13, 2013

Great article Russ. I learned more. That's what I love about this Nikon community!

Ellis Feibush (Ellis Feibush) on June 13, 2013

Great first article on Flash Russ. I learned from your blogs how to achieve great bounce flash with my SB800 and D7000. When used correctly, Bounce flash with the Nikon and SB800 produces what appears to be the very best "Available Light" images. In a perfect world bounce flash with the camera set on manual, is almost fool proof if I use your technique. Nikon's CLS system is almost fool proof. Looking forward to you coming flash articles.

Victor Rakmil (VR8) on June 12, 2013

Simple, straight forward. Thank you!

Rachel Garafola (rachelg2013) on June 12, 2013

Russ, I just signed up so that I could get access to your articles, and I'm glad I did. Thank you so much for taking some of the mystery out of the Nikon flash! This is great for people like me who are still trying to figure it out. Looking forward to more!

Peter Geran (gearsau) on June 12, 2013

Personally, I prefer the first photo. Who wants to see the back ground with such a beautiful subject ?

Fredrik Holmgren (frhol) on June 10, 2013

this balance between is ever so hard to achieve! one other way to do is to go manual, first without the flash set the exposure for the background i M mode then turn on the flash in manual mode to set the exposure for the foreground (person). you can then set exposure for the whole picture with the aperture and the exposure for the background only with the exposure time. This could well be an own article...best regards looking forward to the rest

Ole Martin Helgesen (omartin) on June 10, 2013

Thanks, great idea to compile your posts. I have just got my 2nd flash unit, so I look forward to getting some tips on how to use them together. The CLS system is great! The pre-TTL metering (on my old FM2) was also very impressive. Balancing the ambient and flash light is tricky, so I guess you just have to test and experiment a lot, but basic knowledge is very helpful! So I really look forward to this series!

G