venerdì 1 aprile 2011

Nikon Flash in P Mode Aperture Lock... EXPLAINED!

After one month of investigation, I finally discovered the logic behind the Nikon P mode, when there is a flash activated.

If you remember, the problem was to understand WHY when we use the flash in a Nikon DSLR camera in automatic P mode, the P mode is blocked and there is no way to use the P* shift, and, also the aperture is reduced at an apparently random value, and is also reduced when the ISO is gained up.

 The problem is that The Photographer logic seems not be followed by the P program....  Because,  if subject is in low light conditions, if I activate the flash, if I increase the ISO... This means I would like to catch as maximum light as possible from the scene, and, apparently,  seems completely stupid or not logic to close the aperture, as the P program does!

But, of course, there is an explanation...

The P mode with flash logic is to completely delegate the lighting to the flash and the exposure calculation to the FlashTTL system.

To do that, as the flash is activated, the camera blocks the shutter at the SYNC Speed.

SYNC Speed breaks the logic behind the P* shift program.  If the ambient light permits to set a different sync speed between the mimimum and maximum sync value it is still possible to change the sync speed, but, for this explanation, let-s consider the Shutter speed  BLOCKED at the SYNC speed. 

As the shutter time is blocked at sync speed,  the reflective ambient light meter is ignored (but it does not goes off!),  there is no way to calculate the right exposure  because the light will comes "only" from the flash, not from the scene! Again, there is no way to do a Shift between different P* modes, because the Shutter speed is blocked!

 Lot of confusion comes from the fact that Nikon P* still works when the gears are rotated, but P*  works "in the background": P* is displayed...  P* does not change Shutter and Diaphragm? Apparently. The shift does not work because the flash is on,  but if you switch  the flash off, the P* goes in the shifted program you set!

Let's go back to the main problem: Which aperture the P program choose when the flash is on?

 Remember! The logic is to delegate the exposure calculation to the FlashTTL system.... With this logic the P program sets an f/ that gives the maximum degree of freedom at the TTL system in a standard subject distance case. The best f/ number,  is influenced by the medium Guide Number of a TTL flash.

To understand that, let's take as example a faulty case:

The TTL flash can automatically change his power, from a minimum GN to a maximum GN. Supposing, with the photographer logic, we mount a wide aperture lens, because there is low ambient light, we increase the ISO, we set the aperture at f/2.0 and we shoot a subject that is very close... The Flash has to fire with these settings at a very low power in order to get a good exposure... It can happen that the minimum power of the flash is still too high, and, as result, the main subject is over-exposed.
That's because f/2.0 was too wide for a standard  close subject with a TTL flash of that power, or, the Flash, was too powerful for a so short distance... Choosing  the wider aperture of the lens,  with the flashTTL on, it could be not the best choice, because, even if the light is low, the FlashTTL system has a very little degree of freedom of reducing the Flash Power.

And remember (!!! very important point in this logic!!)  when the subject is close to the light, the power of the light becomes CRITICAL because of the very short distance  fall off, that could be of only few centimeters!! So, when the subject is close to the flash, it is extremely important to set the proper light power with a precision that increases as more as the subject get closer to the flash.

Summarizing, the Aperture is calculated depending on the medium Guide Number of the flash in order to give the maximum degrees of freedom in terms of flash power settings to the FlashTTL system. Increasing the ISO, of course, the equivalent GN increases, so, the f number goes down!

At ISO 200, the P mode with the D700 in camera flash, the aperture is reduced at a maximum value of  f/3.5, with the SB600 it is reduced at f/5.6

The SLOW mode, is apparently stupid. If there is a low light condition scene and the photographer would like to get the light from the background, Why do not open to the  widest aperture automatically? Again,  the logic is to delegate all the exposure of the main subject to the FlashTTL and give to the flash maximum range of possibility of getting the right exposure of the main subject.

So, the aperture remains blocked as discussed before depending on the GN, and SLOW mode uses only the shutter speed to get the light from the background according from the ambient light measured by the internal reflective light meter.

Amazing logic! but never documented before.

Luigi Gallerani

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