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Exposure Program with and without Flash


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5 replies to this topic

#1
Kokomodo

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I'm using a D40 with a 35 mm 1.8G and a SB-400 speedlight.  I use some card stock as a bounce diffuser on the speedlight.  I also have the speedlight on a one foot extension so that I can orient it in any direction.  I love this setup and have used it for years for capturing family gatherings, indoor portraits, etc.  I've been using my phone more and more, and the D40 less and less, so perhaps my confusion stems from some setting that I've forgotten about.  But, I recently grabbed the D40 to take a picture of some visiting cousins.  I often use Program Mode because in most situations, the algorithm makes essentially the same decisions that I would.  However, on this shot, I noticed two things (unfortunately, I did not notice until the cousins had left.)  First, the available shutter speeds in Program Mode (and the alternate "flexible" solutions) did not utilize the full aperture range of the lens.  With the flash on, the largest available aperture is f/4, not the f/1.8 maximum aperture of the lens  With the flash off, the Program Mode will utilize the full aperture range of the lens.  I assume this is because the algorithm is assuming a minimum output of the flash, but, I'm not sure.  Second, in Program Mode with the flash in TTL mode, the algorithm chose a relatively long shutter time.  Something like 1/20 or 1/40 when 1/320 would have been more appropriate.  (Indoor, but a room well lit by ambient daylight through the windows.)  This surprised me, and I'm wondering if anyone has published the metering/exposure algorithm for the D40.  If not the whole algorithm, then perhaps just the priorities the algorithm follows between shutter, aperture, iso, and flash?  (By the way, I do use ISO Auto and in the shots in question the algorithm left the ISO at 200, but that is clearly because I had the minimum shutter time set too long.  I've since changed it to 1/125 since I shoot mostly hand-held.)  Finally, while trying to understand all this, I switched to manual mode and turned off ISO auto.  What I find quite surprising is that the exposure meter (in the viewfinder) doesn't change when I turn the flash off or on.  So, if I shoot in manual mode with a flash, I have no indication on the camera of what my actual exposure will be.  Is this correct?

 

Generally, my question is to understand the D40 exposure algorithm with and without flash.  More specifically, do I have something setup incorrectly that is limiting the f/# range of the Program Mode for this lens?  And, finally, is there any way to tell (on the camera) what my exposure will be when shooting in manual mode with a speedlight?

 

-Kokomodo

 

Oh, one more thing...in general the algorithm appears to set the exposure program _before_ sending out the probe flashes to determine the TTL flash strength.  If this is the case, then I don't understand how the algorithm is utilizing the TTL information.  Anyone have any insight into this?

 

-K



#2
Merco_61

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I wonder if you have managed to activate the slow sync setting in the D40.



#3
Kokomodo

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That's a great point.  In fact, as I initially investigated the issue, I found that the flash sync was set to "slow."  And, changing the flash sync from "slow" to "normal" did decrease the shutter time, but only from 1/30 to 1/60.  An improvement, but likely not the fundamental issue.



#4
Merco_61

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I wonder if that 1/60 is because the program mode doesn’t switch to the CCD gating and only uses the focal plane shutter? The cheap mechanical part in the hybrid shutters had quite slow sync speeds if I remember correctly. I never owned a CCD Nikon as I used Kodak digital bodies in those days.



#5
Kokomodo

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Another great question, but the flash sync speed on the D40 is 1/500.

 

And, what really gets me is the Program mode not utilizing the full aperture range of the lens.  That just seems bizarre.



#6
Merco_61

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The program mode probably doesn’t utilize the electronic shutter. This leads to a stopped down aperture to let the flash be the dominant light source. With the flash as main light and a stopped down aperture, there will be little ghosting despite the long exposure since the flash duration is short.

 

Flash with auto-anything will always be unpredictable, even if more modern bodies have better algorithms.

 

For PJ-style shooting, I usually put everything in manual unless the ambient light or the distance is constantly changing. In that case i-TTL will be your friend.