Question about an SB-700 in Manual Mode

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Revet

Senior Member
I have this sometimes bad habit of having to know why things are like they are. Anyhow, when I put my SB-700 speedlight on manual mode I notice that as I change ISO and/or the f stop, it changes the little bar graph showing distance on the flash LCD. For example, as I increase the f stop or lower the ISO, the distance bar-graph on the LCD decreases.

Since I am in manual mode, I assume the intensity of the flash will not change (ie. 1/1). Saying that, I assume that as I increase the f stop or reduce the ISO, less light is getting to the sensor (or the sensor is less sensitive with Iso). Also, that would be true of both ambient and flash. So now I assume the camera's sensor is saying the exposure can only be exposed correctly at a shorter distance when lowering ISO and/or increasing the f stop. Is this reasonable thinking??

If this is reasonable, why then is this all occurring without any metering going on (lens cap over lens and no buttons on the camera pushed)? I guess I will take a stab at this and say that this is pre-programmed into the SB-700 (ie. at a given ISO, f-stop and flash intensity, the distance the flash is effective is a standard distance which can be calculated by tables.

So, I think I answered my own question as I worked through it on the thread but I just want to check the logic here.
 
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WayneF

Senior Member
Revet said:
when I put my SB-700 speedlight on manual mode I notice that as I change ISO and/or the f stop, it changes the little bar graph showing distance on the flash LCD. For example, as I increase the f stop or lower the ISO, the distance bar-graph on the LCD decreases.
Click to expand...

Right

Since I am in manual mode, I assume the intensity of the flash will not change (ie. 1/1).
Click to expand...

Right, full power is full power.

Saying that, I assume that as I increase the f stop or reduce the ISO, less light is getting to the sensor (or the sensor is less sensitive with Iso). Also, that would be true of both ambient and flash. So now I assume the camera's sensor is saying the exposure can only be exposed correctly at a shorter distance when lowering ISO and/or increasing the f stop. Is this reasonable thinking??
Click to expand...

This part needs work. The really big deal is that flash intensity falls off with distance - the Inverse Square Law

Four Flash Photography Basics we must know - Inverse Square Law

Flash exposure can only be precisely correct at only one specific distance from the flash.

The bar graph is showing you this proper distance for the current settings (power level, f/stop, zoom, ISO).

There is a concept called Guide Number Four Flash Photography Basics we must know - Guide Numbers
which simplifies the square law concept (is only applicable to bare direct flash).

The flash does have its guide number chart in its firmware, and this is what the bar graph is computing and showing.
The GN chart is also in the manual (at the back for SB-700)

GN depends on subject distance and lens f/stop (and zoom and ISO).

In use, GN = distance * fstop (for the correct flash exposure - meaning proper full exposure, not partial fill flash exposure). If you know GN, then GN/distance tells you the proper fstop. Or GN/fstop tells you distance. Since there can be many fstops, then this tells the corresponding many distances (for proper exposure). The big point is that it handles the Inverse Square Law for us.

Some models (including the SB-700) have a GN mode, which is Manual flash, but you enter the distance, and it knows the rest (from camera hot shoe) to automatically set the flash power level correctly for that specified distance. Only applicable for bare direct flash (not for bounce).

The distinction is, GN mode sets the correct power level for the distance you enter.
Otherwise, M mode bar graph does not know distance, and is advising about the proper distance, the distance where the subject better be, at the current settings (you could of course change settings until it shows the correct subject distance).
 
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