Please give me a simple explanation in this.

Started Feb 8, 2013 | Discussions thread
Kaj E Veteran Member • Posts: 9,316
Re: The easy explanation

As I have not heard from you since my last post below, I take it that you did not bother to read the manual as i advised. It was an attempt to let you truly learn how to use the flash. "Teach a man to fish..."

So for you and others who do not make an effort to learn, a clear text explanation to your "issue":

When you use rear sync in slow mode, the camera exposes for the background and just uses the flash as a fill (to brighten the foreground).

You have set your camera to use auto-ISO and a min shutter speed of 1/60 sec. As you can see from your examples the camera refuses to reduce the shutter speed and up ISO.

Why is that?

First to correctly expose the background as rear sync slow and (and also slow sync) the camera selects a combination of shutter speed and ISO (I assume you use aperture exposure mode).

It then selects flash output that allows you to get correct exposure.

The camera refuses to use a higher ISO because the minimum flash output would then be too strong for the high ISO at the short distance in your image. (See p.427 in your D700 manual). If the camera would follow your advice on increasing the ISO the image foreground would be grossly overexposed. Therefore it refuses to follow your "stupid" advice. The camera is smarter than I thought.

You need to understand the limitations on flash output strength. At full output the flash can not reach farther than determined by the aperture , and flash guide number. The guide number is distance divided by f-stop. At close distances it can only reduce its output strength to a certain degree. If you are too close to the subject the foreground will be overexposed (depends on ISO and aperture)

The background exposure is always determined by shutter speed, aperture and ISO. As the electronic flash duration always is faster then the shutter speed. The flash exposure depends only on aperture and ISO, not on shutter speed.

Hope this helps. I still recommend you read the manual and understand how all the factors (metering mode, flash mode etc.) affect flash exposure. If you are new to flash photography it would also be advisable to read up on flash photography from a basic photography book.

This explanation became by necessity quite long. Perhaps my first advice; "read the manual" was the easy explanation after all.

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Kind regards
WSSA member #13
It's about time we started to take photography seriously and treat it as a hobby.- Elliott Erwitt

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