Shooting an event with two cameras - first observations and learnings

I have a new photographic experience behind me. Yesterday I was shooting almost the whole day with the two cameras. Before the event, I was considering shooting with just one and change the lenses if required. More on these considerations here.
During the event it turned out that switching the lenses is not an option. There were too many interesting things happening simultaneously. Just to give an example: at the certain moment there were children playing in a gym on two fields, two different sports. A very nice opportunity to capture some team movements and take a number of nice close-ups. But since the games were of short duration and I wanted to take photos of as many children as possible, I couldn't afford swapping the lenses.

I equipped my Nikon D80 with 24-70 f/2.8 lens and my D800E with 70-200 f/2.8. Next to it I put SB-900 on each of the cameras. There was not coincidence in such choice. Since my intention was to capture lots of faces I wanted to reach maximum quality for the close-up or tightly composed photos. For the scenes with a broader angle of view I could sacrifice some quality since my intention was to take pictures of groups of scenes implying more distance to the subjects and less details in the scene.

During the day I have made some observations with respect of using two cameras:

• It is easier to carry them than I thought. I was a bit worried about how to take care of two cameras. After all it was not such big issue. One camera (D800 with 70-200 lens) was hanging on my right arm (I use a very decent strap of OptechUSA which was sticking to my arm very well). The other camera was hanging on my neck.
• Keeping the settings simple helps a lot. I was trying to keep the settings of both cameras as simple as possible so I didn't have to think too much about them when switching between the cameras. So I set them both to manual mode, and when I was using flashes, I set them to the TTL mode. In the lighting conditions that I had it worked very well.
• Knowing the camera limits is very important. Different cameras have different limitations. It is very important to know the limits and not get tempted to apply settings from the camera with higher possibilities to the 'weaker' one. A good example is setting the ISO. D800E can very easy handle ISO sensitivity of 2000. When one tries to apply such ISO to D80 will get very disappointing results. I stayed with ISO of max 640 on the D80, knowing it is more or less the highest value still delivering pictures with acceptable noise level. 
• Don't forget to synchronize the clocks of both cameras. I unfortunately forgot to do so. As a result I got quite mixed set of photos when I have stored them in Lightroom. Sure, the time offset can be corrected in the software, but it cost time. So next time I will do better.
• Divide the load between two cameras is good for batteries. Especially the flash batteries. I didn't have to change the batteries of any flash. While shooting with one camera during a comparable event, I have to change them at least once. It is logical but the consequence is that I don't loose the time on battery change.

All with all it was an interesting experience that I will be practicing more during interactive, time-intensive events.

Exploring the limits of my gear - Nikon strobes and the iTTL system

Couple of months ago during the portrait shooting session I have noticed quite annoying fact that I couldn't control the light strength of one of the speedlight flashes used for the lighting setup. In this particular case it was the rim light that was causing problems. I have set the strength of this flash to be +1.7 stops above the default strength determined by the iTTL system. Since this light was too strong, I have reduced it by 1 and then 2 stops, but without any result. The exposure corrections were neglected by the system. Due to the time pressure I didn't investigate it, just moved the light stand a bit further away and obtained the required exposure in this way. But the questions remained:

- why did the system bahave like that?
- how can such behaviour be circumvented?
- how do I know such limits in advance next time?

Due to the other activities I couldn't answer all those questions till this weekend.

I had suspected that the problem with this flash was related to the distance between the flash and the subject. I thought that when the distance is too big, iTTL fires as much light as it can and gives up the control possibilities. To prove it I have tested the system with a simple setup, sketched below:

When the subject is properly exposed by the flash it looks like shown below:

The test was straightforward. I selected a number of distances between the subject and the flash. For each distance I took the series of shots, with the relative exposure of the flash set to -3, -2, -1, 0, +1, +2, +3 stops. For each shot I observed if the flash exposure correction had effect. I did the observation based on the shot but also taking the histogram into account. To illustrate what I mean look at the two tables below, showing the situations where I had full control over the exposure and where I had the control over the exposure from -3 to +1 stop.

First the situation where I had full control over whole range of exposures (distance from the subject to flash was 0.9m):

Exposure	Histogram

It is clearly visible that the flash can produce enough light for the whole range of the exposure corrections.

Then the situation when the control is no more possible above +1 stop  (distance from the subject to flash was 2.9m):.

Exposure	Histogram

So indeed, my assumption was right: if the distance between the subject and the flash grows, the iTTL narrows the range of the relative exposures that can be used. For my measurements I have got the following results (for SB910, set at ISO 100, zoom 24mm):

Distance	0.9m	1.9m	2.9m	3.9m	4.9m	5.9m	6.9m	7.9m
Control	-3..+3	-3..+2	-3..+1	-3..0	-3..0	-3..-1	-3..-3	-3..-3

So the lesson learned is that one can manipulate the distance between flash and the subject as long as loosing of the relative exposure control is acceptable. Which answers my second question: to have the control back over whole range of exposures, one has to move flash closer to the subject.

How far? I found that iTTL can give quite accurate hint about the distance (provided you have SB-800, SB-900 or SB-910 flashes). Here is the procedure:

1. Connect the flash to your camera (either directly or by using the flash release cord)
2. Setup your flash to work in TTL or TTL BL mode, depending on your needs (or planned exposure)
3. Setup the zoom of the flash for the value you intend to use (either by the flash settings or by using the lens zoom, if applicable), required ISO to be used (this one needs to be set in the camera) and the aperture (again, either in the flash or in the camera).
4. Note that the distance scale of the flash shows the range of the distances where the flash will expose the subject properly.
5. Now change the relative exposure of the flash (example for SB-910):

6. Observe that the maximal distance changes with the relative exposure (here an example for the reading for relative exposure +1.3 stops:

The maximal distance together with the exposure correction can be interpreted as a maximal distance at which the exposure can be corrected from -3 up to the value set. In the presented picture, if the subject is at 2.4m from the flash, the exposure can be corrected between -3 and 1.3 stops. Setting it above 1.3 stop will not have effect on the exposure (the exposure will be the same as for +1.3 stop).

Note that it is not exact math, rather indication. I have noticed, for instance, that while iTTL indicates that I can control exposure up to -2 stops at the distance 7.7 meters, I was able in fact to do so at the distance less than 6.9 meters. But for the values -1 to +3 it matches my experimental results quite accurately.

So for the next time I have to remember:

- Check the distance between the subject ant the flash,

- For this distance validate the ranges of exposure correction that will make sense. 

- Don't waste time on trying to go above maximal exposure correction. It will not work.