A6500: the mystery of the TTL flash delay

[rtcary]

After much testing of the A6500 in an effort to identify a delay I experience with both the builtin and external flash, I have been able to isolate the circumstances but not why this happens. Here is my methodology and results;

The A6500 has the 16-55mm f/2.8 lens. Though the testing is with a Godox V860iiS external flash, I found the same results with the internal flash. All testing was done with the Meter Mode set to Multi, Red Eye Reduction set to Off and Flash Mode set to Fill-flash. The e-Front Curtain Shutter is On and the Mode is Manual. Results:

With the Focus Area set to Wide, the TTL delay is almost imperceptible; the other settings for Focus Area display a noticeable TTL delay. The Focus Area I prefer to use for events is the Flexible Spot: M and that setting causes me to miss those fleeting expressions. Using the Wide does not provide for exact focusing. The wider the f/stop, the delay appears to be somewhat shorter which is understandable.

I cannot come up with and explanation as to why the Wide setting appears to eliminate the delay other than what is due to the pre-flash. Anyone have an explanation?

\rtc

[Chrissie]

10 hours ago, rtcary said:

Anyone have an explanation?

Basically the flash only fires, once proper focus has been acquired.

The delay in releasing the flash is a direct consequence of the delayed focus acqusition.

In a "flash use" situation, the lighting conditions are probably rather low, otherwise you wouldn't be using a flash in the first place. In low light conditions, the camera cannot use contrast AF, because contrast is also too low.

So it has to use PDAF, which uses kind of a range finding mechanism, which employs the evaluation of the parallax between looking at the same object from two different positions. That's why humans and animals have two eyes, which are a little distance apart. Each eye generates a slightly different image, and the brain fuses them together for a 3D-Image, including distance estimations along the way.

For special applications, this effext is amplified by dedicated range finders, which use an increased distance between the two lenses.

Back to the camera:

The base for the range finding in the camera are the left-to right (or top-to-bottom) opposite edges of the lense aperture. The wider the aperture, the easier the range finder can operate, resulting in faster focus acqusition.

Stopped down, the base for range finding is narrowed, possibly up to the point that focus hunting occurs. Anyway, a narrow base makes precise distance computation difficult, also due to the limited precision of the computations involved. (A computer cannot distinguish infinitely miniscule differences between floating point values. Below a certain threshold ever smaller differences between two floating point values cannot be represented any more, making those values appear to be equal. In that case, a signal to move the focal plane cannot be derived any more).

[rtcary]

With the Focus Area set to Wide, the TTL delay is almost imperceptible; the other settings for Focus Area display a noticeable TTL delay. 

The detailed explanation definitely explains why smaller apertures have a longer delay than a wider one. However I am not able to use the focusing explanation to answer why the camera in Auto mode or with the Focus Area set to Wide do not have a noticeable delay other than the expected TTL delay.

Have I missed something?

With appreciation....

\rtc

[rtcary]

In browsing other forums, I found where a user did a careful testing and found that it appears to be an error in the firmware that is not in the a6000.

+++
I think I found the problem, but it's either a bug or I have a defective camera. I'll explain step by step. I started by resetting the camera to default settings :

* taking a picture with flash works fine (no delay between shutter press and flash firing)

* when I set the camera to AF-C and then take a picture with flash > delay

* when I set the camera back to AF-S, problem remains

* only way to fix it is by resetting the camera again

Is this a known issue?? I tried the same thing with my A6000 and do not notice this behaviour. Very annoying. If I can't fix this, the camera is going back.
+++

Later learned that he sold the camera. I am wondering if the same problem exists in the a6600?

\rtc

Edited December 14, 2019 by rtcary