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.

After my first Dutch Photo Walk - technicalities

Previously I have shared some thoughts about my participation in the Dutch City Walk. In this post I would like to share some technical aspects of the event. Note that most of my observations refer to the other topics, like "Street photography", "Shooting people", "Post processing".

Before

Check the proposed route in advance. It will help you to identify the potentially interesting spots and to pre-visualize the route.

Check the weather forecast. There is a couple of hours walking and you need to feel comfortable, without hassle of too cold, or too warm clothing. And, if it is going to rain, don't forget to take a proper stuff for protecting your gear. What I take with me is a rain sleeve, a small towel and a lens cleaning cloth. 

Obviously, take enough memory cards and fully loaded batteries.

More tough questions to answer are: which lens (or lenses) to take, to take or not a tripod. Before the photowalk I wasn't sure so I have asked other co-walkers. So learn from each other.
Finally I took the 24-70/2.8 lens (which, by the way, became my workhorse) and as a reserve a 18-200/3.6-5.6 lens. I didn't take a tripod because the interesting indoor places (like churches) were either in renovation or closed.

During

Personally I like to work lately with a predefined theme (and for this photowalk it was Delftware.

So the pictures like these were taken:

But in practice it is a walk and interacting with other people. So the photographic theme evolved during the walk. And it is a city, so I got inspired by a lot of interesting stuff:

My advice is also to look every now and there what other co-walkers are doing. You can learn a lot, and sometimes take interesting shot of them:

On the technical side: I tried to keep the camera settings as simple as possible and as universal as possible. Each moment something interesting can happen and I didn't want to ruin my photo just by having wrong settings. Here's what worked for me: the camera in the Aperture mode with auto-adjust ISO settings and with the second command dial button set for exposition compensation.

Every now and then I have switched the bracketing on, but kept it very simple (just 3 shots, under-, normal-, and over-) exposed with the F-stop distance between them set to 1. One important note,

I learned to switch bracketing mode off practically after each shot taken. For very simple reason: I wanted to be prepared for the next shot, starting from the stable set of settings. Forgetting to switch off bracketing can cost you unexpected under- and overexposed images.

So I kept the camera settings actually quite basic and simple and was able to focus more on the composition of my photos and on looking for the interesting places.

After

Generally first thing that I wanted after the event was to share the photos with the rest of the group and see what other did. And because group was so enthusiastic about the photography, first results were published the same evening. Discussions about the photos started, +1 on G+ were given.

For me it was important to see how others see the same places I saw through their camera. Again: a great learning moment.

Project D800E -secure the shot in low light conditions

This is third post in the series of D800 project. Previous post you can find here and here.
Today I'll move the project a step further, from the general camera configuration settings to the practical shooting situations. I'll start with the low-lighting conditions.

Shooting in the low light conditions is one of the topics that are heavily discussed on Internet in photography communities in context of D800. D800 gives some interesting possibilities to achieve very good results in such situations. For me it is especially important since I take regularly shots of the different events that take place in not so great lighting conditions. If you can control it, you can boost your lighting, but if not - you need how to get the maximum out of your camera having only the ambient light.

So this post is a kind of a theoretical preparation before actual assignment I'll take next week.

When a camera calculates the exposure basically it takes three factors into account: aperture, shutter speed and the ISO sensitivity. Of course one can try to match all three factors manually to achieve the desired result. I will focus rather on the 'semi-automatic' modes that D800 (and BTW many other cameras) offer. Let's start with the most interesting mode to me, i.e. the aperture priority (A). This is the mode I use most of the time. The workflow is quite simple: when one fixes the ISO to an arbitrary value the camera will adjust the shutter speed to ensure the proper exposure. If the shutter speed is too slow, increase the ISO value to let the camera increase the speed. Nice, but as an event photographer I often don't have so much time during the assignment. Meet the Automatic-ISO mode.

Automatic ISO mode

In this mode the camera adjusts the ISO sensitivity in a situations when the two other factors (aperture and shutter speed) don't provide sufficient exposure. Here's how it works in case of D800:

Aperture priority mode:

First the shutter speed is decreased to meet the required exposure. If the shutter speed reaches the limit (more on it in a second) and the exposure is not enough, the ISO is increased (to a predefined limit). If the exposure is still not enough after reaching the limit of the ISO, the shutter speed is lowered further.

Important to note is that here one have two control options: minimal shutter speed at which the ISO starts to increase and the maximum value of the ISO. For the ones who rather prefer the graphs, the diagram below illustrates the whole process:

The two parameters that control the process are marked in red in the diagram.

The auto ISO mode can be set in the Shooting menu, option ISO sensitivity settings.

This menu opens a sub-menu where the Auto ISO sensitivity can be switched on and off and both parameters described earlier (Maximum sensitivity, Minimum shutter speed) can be setup.

The setup sequence is as follows:

1. Enable Auto-ISO:

Menu,,, -> ISO sensitivity settings,,->Auto ISO sensitivity control,,->On, Ok
2. Setup maximum sensitivity:
 -> Maximum sensitivity,, ->Choose value that suits you, Ok
3. Setup minimum shutter speed:
 -> Minimum shutter speed,, ->Choose value that suits you, Ok

The minimum shutter speed offer the Auto option. In this option the shutter speed will be set based on the value of the current lens focal length (or 1/30s for non-CPU lenses). So if for example the focal length is set to 50mm, the shutter speed will be set on 1/50 s. While it might be tempting in some situations, I would be cautious here. This rule of thumb may be not so well applicable for the D800 due to its huge sensor resolution and sensitivity to the camera body movement.

Last word about settings: Auto-ISO mode can be easily switched on and off by pressing the ISO button and turning the secondary dial button.

A final question is on how far one can crank up the ISO in case of D800. This of course depends on the scene, lighting conditions, the purpose of the photo, etc. My preliminary experiments show that setting the ISO to 2000 is a safe choice. Just one example. The photo below has been taken at ISO 1800.

Taking the 100% crop of the image reveals some noise grain in the background, but it is acceptable, provided that any noise reduction has been applied to the image. This amount of noise is easily removable in Lightroom.

For the assignment next week I'll start with the minimum shutter speed equal to 1/125s and maximum ISO 2000. We'll see how it works.

As said last time, I have the first version of the D800 cheat sheet ready. Please keep in mind that it is in a very draft status, with lots of empty (or even non-existing) sections. You can see it here.

Next time I'll look more closely at the Dynamic Lighting feature of D800 and the dynamic range of this camera in general.