iTTL Flash in camera Manual mode


PhotoTime

Senior Member
Jun 9, 2004
551
0
16
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.
 

Many will not bother to reply because even if they say you are doing it wrong so do or set this in the camera will not give a full understanding of how a camera determine exposure much less flash exposure which plenty of books have been written about flash photography but I will try.:)

1. A digital camera will work as designed on the assumption that you will always use an autofocus lens..even if you select manual mode which simply means you select aperture and shutter speed, of course ISO sensivity too as the camera computes exposure via settings on camera and aperture info relayed via the electrical contacts on the lens.

Here's the catch, if you use a manual lens the aperture info is missing because a manual lens does not work like an autofocus lens and it's different for mirrorless cameras as there are expensive lens adapters that supply or have electrical contacts to communicate with camera.

Unless the particular digital model can also do manual lens metering..nikon cripple such feature on entry level cameras or you use nikon df which has backward compatibility with older manual lenses.

2. Understanding of camera metering systems has it's origins from the offset printing industry where there is only one colour which is black and white or black ink for newspaper, book and magazine. The way an image can be represented is by gradation of grey levels that can be printed of the highlights and shadows of a photograph so a camera's exposure meter is calibrated in 18% grey or middle grey as it's in middle values so you will not get an over or underexpose photo but that's only in theory.Somewhere in middle..it's not perfect but workable to this day!

The common knowledge is metering is at best a form of guess work as one professional portrait photographer said," Have you seen people with 18% grey skin?"..haha.

3. For flash ttl or autoexposure via camera's meter when you use a manual lens the camera does not know what aperture or distance of subject via autofocus mechanism.The manufacturer of flash or camera "conveniently" underexpose at least you see an image and if overexpose it gives a faulty impression of the equipment which I think is to be avoided at all costs.

4. Fortunately Metz 58AF2 has it's own light sensor like the previous generation of auto flash in the film era so you set aperture and guestimate subject distance and iso provided you set it to A mode other than ttl. The meter will cut off power to flash tube once it calculated exposure is reached almost instaneously ( it's not computed on the fly but tested via trial and error during product development as a voltage or current level is sensed and detected so trigger a cutoff of power to flash tube).It was known as thyristor autoflash back then in film days.

5. If your built-in flash give good exposures then assume flash metering is working, if underexpose then check if flash compensation is at zero marking which means no compensation that is you deliberately under or overexpose photo depending on how it looks on lcd screen review..negative number = underexpose in 1/3 or 1/2 steps.

6. I do not know if metz 58af2 communicates with camera or not.Nonetheless you can adjust exposure via it's power levels.For outdoor fill flash if you want to capture the ambient scene or lighting try 1/64 power and estimate via lcd review.

Hope this answer makes clearer understanding for you .
 

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.
 

  • Like
Reactions: thoongeng