Hi,
I like to understand more on how the iTTL mode work with camera Manual Mode.
I always use Manual Mode on my D600 because at times, I will shoot with my manual lens. When I wish to do fill-flash, be it external flash (Metz AF58-2), or the in camera flash, how shall I adjust my exposure? Based the exposure suggested by the camera? What is the highest flash sync speed I can go? Or must I set the shutter speed to be less than 1/200?
Please advise, as I always has a underexpose shot when using flash.
Regards.
Hi PhotoTime,
You have to understand that with flash photography there are two exposures going on when you fire your shutter.
One is the ambient exposure and one is the flash exposure.
Your manual exposure settings are for the ambient exposure. So this is the exposure of everything in your scene that either won't be illuminated by your flash or the flash illumination will be insignificant enough to ignore. Think of it as your normal exposure from whatever natural or artificial sources of light that exists in the scene already.
The other is your flash exposure. Whether it is from your on-board flash or off camera, this is whatever that's in your scene that will also be illuminated by your flash source.
Depending on your scene and flash placement, sometimes the flash exposure will interact with your ambient exposure, sometimes it won't.
When you pick your ambient exposure settings, it will influence how the flash exposure affects the scene.
Firstly, your question regarding shutter speeds. All cameras will have a max flash sync speed. This is the maximum shutter speed that the camera can time a full flash exposure when the shutter is fully opened.
Imagine if you had a shutter speed of 1s which means the shutter will be opened for 1 whole seconds. It makes sense that it is very easy for the camera to time the flash to fire within that full 1s. You can even tell the camera to fire it at the beginning (1st curtain) or end (2nd curtain) of that shutter cycle.
Now if you had a faster shutter speed eg. 1/80s, now the camera must time the flash to fire within that 1/80 of a second.
So the max sync speed is the highest shutter speed that the camera can sync the flash. Beyond this, the shutter opens for too short a time and what you end up with is a dark band which is the shadow left behind by the closing shutter curtain.
Now you can go beyond the max sync speed by using Auto FP which is Nikon's high speed sync mode. But instead of 1 full burst of the flash (at whatever power you've set it), it pulses many times and lights the scene in small slits. This avoids the dark band when you go beyond the max shutter sync but by doing lots of small pulses, the flash power is greatly reduced so if your subject is not close enough to you, you may not get enough flash illumination on your subject.
So in general, shutter speed only affects your ambient exposure and not flash exposure unless you exceed your max sync speed.
Your aperture affects both your ambient and flash exposure.
ISO affects the resultant brightness in your image from both your ambient and flash exposure.
For flash exposure, the flash will only light object at one distance correctly. Objects in front of it will be overexposed, and objects behind it will be underexposed. The amount of over/underexposure follows the inverse square law from the flash source. So the illumination from the flash very quickly falls off for subjects in the distance whilst accidental object that appear in front of your subject (eg. a stray hand or something) will often appear incredibly bright.
I-TTL calculates the flash exposure based on a bunch of proprietary information. Eg. all Nikon lenses from the AF-D era onwards adds distance information. I don't know what manual lens you're using but it is possible that if it is very old, the lens could be providing limited information for the camera to calculate it's flash exposure.
Nikon's I-TTL works by firing a pre-flash just before the main flash. The preflash is very quick and you won't even notice it when you shoot flash normally. But if you select second curtain (main flash goes off at end of shutter curtain cycle) and a long shutter speed, you will notice the pre-flash pulse at the start (indicating start of shutter cycle) followed by the main flash at the end of the shutter cycle. The I-TTL flash exposure is calculated from the pre-flash flash exposure among other things eg. subject distance information provided by the lens so basically the flash doesn't even need line of sight to your subject to still work.
So your problems may be lens related if it doesn't communicate with the camera.
But if it does communicate properly, then you should set you manual exposure for the ambient light condition and let I-TTL light your subject properly for you. You use flash exposure compensation +/- to dial in the amount of flash exposure you want on your subject. Just bear in mind that your ambient light exposure settings will affect how well the flash lights your subject.