[jarnos]

Please test, if your flash gives maximum output at shutter speed 180 by doing some test shots. Insturctions are at the end of this message. I am especially interested in test results of Metz mecablitz 58 AF-1 digital Pentax/Samsung, since I have read about rumors that tell you couldn't get full flash output in the flash X-sync speed of K10D. Of course, it is also interesting, if Pentax brand's own flashes perform any better with the camera. The feature is important when you do daylight fill flash shots.

Choose an object that is in dim light or in complete darkness. Put your camera on a tripod few meters away from the object and attach the flash. Select the manual mode in camera and flash. Set flash to full power. Choose ISO 100 and aperture so that it is the guide number of the flash divided by the distance between the flash and object in meters (or do test shots to determine usable aperture setting). Please note that the guide number depends on the flash zoom position. Then do some test shots changing only shutter speeds: 180, 125 and 60 (available when you select 1/2 stop EV steps). From these photos you should be able to see, if there is difference. Please post you test shots here.

[kurt]

Hello,

Here my quick test as requested.

Technical details:

Shooting distance: about 4.5 meters into the corner of room.
Flash in manual mode (1/1, full power), zoom set to 35mm.

K10D:
Manual mode (M)
Aperture 8.0

Lens:
16-45mm/f4 set to 29mm

Four pictures taken in shutters speeds 1/180, 1/125, 1/60, 1/30 (with same aperture)

1/180
Attachment 6970

1/125
Attachment 6971

1/60
Attachment 6972

1/30
Attachment 6973

Make your own conclusions ...

[jarnos]

I suppose you post-processed the images equally.

Brightness varies more between the lower part of the images. Logical explanation would be that shutter curtain starts to cover that part of the image first when there is still some flashlight firing.

I opened the images at Gimp and took the HSV brightness values at certain points of each image; values calculated from "sample merged":

Coordinates 180 125 60 30
20,20 (upper left part) 77% 78% 81% 80%
700,10 (upper right part) 73% 73% 76% 76%
10,494 (lower left part) 52% 64% 71% 71%
720,450 (lower right part) 35% 42% 44% 46%

[jarnos]

Tell me if I am wrong, but taking into account K10D's normal contrast tone curve the above numbers mean that by using exposure time 1/180 s you loose roughly one stop light in lower part of image and one sixth of stop in upper part of image.

[rogerstg]

[QUOTE=jarnos;138756]Logical explanation would be that shutter curtain starts to cover that part of the image first when there is still some flashlight firing.

If that were true there would be an obvious shadow line. Also, I doubt that any flash would be firing for more than 1/180th of a second.

It's more likely that there is more ambient light nearer the camera than in the corner and it is beginning to slightly affect the image.

[kurt]

Post-processing was equal (I did not do anything, Silkypix defaults). WB from camera etc.

The situation was not totally dark. The light was so dim that AF did not work without flash. There was small amount of light coming from the direction of down right corner.

[jarnos]

rogerstg, there would not be obvious shadow line, if flash light brightness dims slowly enough while shutter curtain is closing. I have also been sceptical about such a long firing time, but I have been told by Goeschel Lutz / Metz that "with a full power flash, a 58 AF-1 has a flash duration of 1/125s measured according T10 metod" and "in T10 metod, the measurement starts at that moment when the light curve is triggered plus 10% (of its maximum) and stops after reaching the maximum when the light is fall down on 10%". Flash is triggered by X-sync exactly when the shutter curtain is fully open, but the opening period is part of exposure time so you need to choose at least shutter opening duration plus flash firing duration as exposure time to get full flash input.

If the difference is caused by ambient light, how do you explain that there is significantly more difference between the images shot by 1/180 s and 1/125 s than between the images shot by 1/60 s and 1/30 s in the lower part? (Ambient light should give half stop difference to the former couple and one stop difference to the latter couple.)

Maybe Kurt can tell, if the difference is visible in the areas where the ambient light does not come directly to.

[kurt]

I did the similar shot when room was totally dark and I can see the same effect. When the exposure time is 1/180 the low part of image is liitle bit darker than upper part. So, I did not see any big effect of ambient light.

There is also a Metz mecablitz 58 AF-1 flash for Canon and Nikon. Has someone heard about a similar kind of behaviour?

Still, I am happy with Metz flash. Fits and fullfills my needs

[jarnos]

I did the similar shot when room was totally dark and I can see the same effect. When the exposure time is 1/180 the low part of image is liitle bit darker than upper part. So, I did not see any big effect of ambient light.

Thanks for making sure.

There is also a Metz mecablitz 58 AF-1 flash for Canon and Nikon. Has someone heard about a similar kind of behaviour?

I have not, but I would expect to see similar effect with other cameras since I doubt the flash models differ in manual mode. Furthermore, some Canon and Nikon cameras have X-sync time 1/250 s where the effect is more dramatical.

[jarnos]

Still, I am happy with Metz flash. Fits and fullfills my needs

Good for you. I have no information about any other flash with as high guide number that has faster flash duration. Can anybody do similar test for PENTAX AF-540 FGZ?

[Lowell Goudge]

After looking at this thread for several times, I fail to understand what you are trying to test.

if the shot being taken is done with the background many stops under exposed at 1/60th, then all frames will be identical and equally exposed. the only thing you need to look at is the grey scale value at the center of the image.

If the background changes slightly (brighter at 1/60) it is because you are getting some influence from natural exposure (without the flash)

Note that the flash sync speed is the highest shutter speed where the entire frame is exopsed at one instant and the flash duration even at full power is below the time the shutter is fully open. Above this speed, the shutter is never fully open but actually is a slit of varying width (narrower with higher shutter speeds) that moves across the sensor.

For any one who has seen a camera flash shot taken above maximum sync speed, you will see a perfectly exposed band with one or 2 dark bands (usually horizontal due to vertical moving shutter) where the shutter curtain was at the time of the flash.

You will not see this at 1/180th on the K10 unless the shutter is defective. If this is the case, you should see the problem even without flash below the max sync speed, because it would imply that the shutter curtain is late leaving and fast arriving, in that case you see a darker band or region on one side and a lighter region on the other with the middle of the frame properly exposed.



That is why high speed flash sync is a multiple burst, the flash fires repetitively to expose each full new section as the slit moves one slit width at a time.

[jarnos]

Note that the flash sync speed is the highest shutter speed where the entire frame is exopsed at one instant and the flash duration even at full power is below the time the shutter is fully open.

Frame is exposed also during the opening and closing of shutter. It seems that at least with the Metz flashgun flash duration can be longer than the time shutter is fully open even if using camera's flash sync speed as shutter speed. If the flash is emiting light even after shutter begins to close, you may get uneven exposure. Camera does not know how long it takes to get full flash output and I am not aware of any standard about such maximum time.

[Lowell Goudge]

Frame is exposed also during the opening and closing of shutter. It seems that at least with the Metz flashgun flash duration can be longer than the time shutter is fully open even if using camera's flash sync speed as shutter speed. If the flash is emiting light even after shutter begins to close, you may get uneven exposure. Camera does not know how long it takes to get full flash output and I am not aware of any standard about such maximum time.

the frame cannot be exposed before the opening of the shutter because the flash is synchronised with the leading curtain being fully open, or trailing curtain about to move.

I can believe to some extent the possibility of the flash duration being long, but not with electronic flash.

These deliver all the energy in microseconds.

Years ago I did some testing using a sunpak autozoom 3000, GN30 (meters) at ISO 100. I was actually testing the susceptibility of fibre optic systems to high pulsed ambient light, and my flash was a good reliable source, as it came with a HV power supply and recycled faster than my motor winder. Minimum intensity was just 2 microseconds, and this doubled for each power setting, there were 5 additional, hence the settings were 2, 4, 8, 16, 32, 64 micro seconds.1/180 of a second is 5.6 milliseconds or almost 1000 times longer than maximum flash duration.

It was the old flash bulbs that had flash duration longer than exposure and were essentually (as far as film was concerned), ambient light.

[jarnos]

Lowell Goudge, it it true that a frame is not exposed by flashlight before a shutter is fully open (if high speed syncronization is not used), but closing shutter does not stop flash firing in manual mode, if the flash needs so much time to discharge.

Please read above about the information I got from Metz concerning results of flash duration test done using T10 method for the Metz flashgun. Also I don't think the test results made above can be explained by ambient light; kurt reported he saw the same effect when shooting in no ambient light.

[jarnos]

I did the similar shot when room was totally dark and I can see the same effect. When the exposure time is 1/180 the low part of image is liitle bit darker than upper part. So, I did not see any big effect of ambient light.

But was the low part in images exposed 1/180 s darker than the low part in images exposed 1/60 s (and was the difference between the images there bigger than in upper part of the images)?