External Flash + Shutter Speed

[xyrandomyx]

I originally posted my question here:


http://www.scubaboard.com/forums/sealife/443256-digital-pro-flash-shutter-speed.html


No replies after a few days and it's possible that it's not specific to Sealife, so I figured I'd post in the main underwater photography section. I'm a beginner photographer, so I apologise in advance if it's a daft question.


Basically, I started with these settings for a test shot (on land):


- external flash's brightness as low as it will go
- ISO: 100
- Aperture: F7.9
- Shutter: 1/60


I then increased the shutter speed to 1/125 and then to 1/250. The test shots (of the same subject, same distance, etc.) with the faster shutter speed were brighter/ more exposed.


How does this work? My understanding is that the shutter speed should not affect the flash. And the ambient light should decrease with faster shutter speeds. So, if there was any change at all, the shots with the faster shutter speed should be darker/ less exposed.

 

[herman]

The amount of exposure near field- a few feet from the camera- is mostly controlled by strobe power and F-stop, more power or a lower F stop number means more exposure. Near field will be a wider area on land as opposed to underwater due to the light absorbtion of water but the effects are the same. Far field, the part of the photo outside of the strobes range is mostly controlled by shutter speed and ambient light. I say "mostly" because there is always some effects of Fstop on far field and shutter speed on near field but they are minor compaired to the major players. All this assumes that there is not some automatic controls at work, either in the camera or flash itself. My guess is there is an auto feature somewhere you do not have turned off. Also, make sure it's actually your strobe lighting the shot and not your internal flash. At close ranges, the internal flash will be strong enough to make it appear that you strobe is doing the work when it is actually not. If your stobe is mistimed it may well be firing before or after the shutter has closed. For a quick check, just shoot one shot normally and then a second with the internal strobe covered- a couple of fingers over it will do fine. IF your strobe is actually lighting the shot, there should be little diffference in the lighting, if it goes a lot darker, strobe timing is your problem. Another quick timing check is to photograph yourself in a mirror, turn the strobe off to the side some but still so you can see it. Your stobe should be shown firing in the shot, it it's not, you have a timing issue. Here is a post I made several years back that you may find useful. http://www.scubaboard.com/forums/underwater-photography/214500-help-manual-camera-settings.html

 

[elan]

What is the sync speed? The flash is indeed connected with the shutter speed because with different speeds on some cameras - say DSLRs curtains either are fully open or a small window that travels across the sensor is open. There are few modes on cameras and depending on the mode and the flash you use it can force the camera to go to specific shutter setting. So the outcome is not obvious if you do not understand how each mode works.

You can read about it in general here : Flash Sync Speed

You also need to know the features of the specific camera and flash and the modes the camera can operate in.

 

[xyrandomyx]

The amount of exposure near field- a few feet from the camera- is mostly controlled by strobe power and F-stop, more power or a lower F stop number means more exposure. Near field will be a wider area on land as opposed to underwater due to the light absorbtion of water but the effects are the same. Far field, the part of the photo outside of the strobes range is mostly controlled by shutter speed and ambient light. I say "mostly" because there is always some effects of Fstop on far field and shutter speed on near field but they are minor compaired to the major players. All this assumes that there is not some automatic controls at work, either in the camera or flash itself. My guess is there is an auto feature somewhere you do not have turned off. Also, make sure it's actually your strobe lighting the shot and not your internal flash. At close ranges, the internal flash will be strong enough to make it appear that you strobe is doing the work when it is actually not. If your stobe is mistimed it may well be firing before or after the shutter has closed. For a quick check, just shoot one shot normally and then a second with the internal strobe covered- a couple of fingers over it will do fine. IF your strobe is actually lighting the shot, there should be little diffference in the lighting, if it goes a lot darker, strobe timing is your problem. Another quick timing check is to photograph yourself in a mirror, turn the strobe off to the side some but still so you can see it. Your stobe should be shown firing in the shot, it it's not, you have a timing issue. Here is a post I made several years back that you may find useful. http://www.scubaboard.com/forums/underwater-photography/214500-help-manual-camera-settings.html

I'm certain it's actually my strobe that's lighting the shot.

a) The external strobe has an optical cable connection with an adapter that covers the internal flash.
b) If I switch the external strobe off, the pics are almost completely black.
c) Taking a shot of myself in the mirror as you suggested shows the strobe firing.

So my guess is the same as your guess -- there's a manual setting somewhere that's not turned off. I can't find it. My working theory (not sure how to confirm it for sure) is that the strobe's power is still automatically adjusted to a degree even when it's manually set.

---------- Post added January 10th, 2013 at 05:22 PM ----------

What is the sync speed? The flash is indeed connected with the shutter speed because with different speeds on some cameras - say DSLRs curtains either are fully open or a small window that travels across the sensor is open. There are few modes on cameras and depending on the mode and the flash you use it can force the camera to go to specific shutter setting. So the outcome is not obvious if you do not understand how each mode works.

You can read about it in general here : Flash Sync Speed

You also need to know the features of the specific camera and flash and the modes the camera can operate in.

I can't find the flash sync speed -- it's not mentioned anywhere in the manual, spec sheet, anything I could find on Google. That said, I don't think it's the issue (could be entirely mistaken) because:

a) It's not a DSLR -- the issues seem to apply mainly to DSLRs.
b) The symptoms of exceeding the sync speed seem to be partially blacked out shots or simply that the shutter speed doesn't actually change beyond the max. It's not the former (the image is actually overexposed with the higher shutter speed). It doesn't seem to be the latter, since I'd then expect that the exposure would not change at all past a certain point.

 

[Gilligan]

1. Since your strobe is connected with a fiber optic cable it is not true TTL so the camera is not sending info to the strobe as with a hard wired strobe.
2. Does your strobe have F stop settings as well as manual ones? If using the F stop settings on the strobe they should closely correspond with the settings on the camera. There is still some guesswork involved. If using manual settings on the strobe then there is more guesswork as respects the strobe and camera settings. For example I have a strobe with manual settings only. I shoot in the M mode at F5.0 and 1/250 as a base setting for my G10 and G12. I adjust the strobe power according to the subject and background. If that won't do then I adjust the F stop and/or shutter speed. It's guesswork but I am use to it but still have to take a few shots of the same subject.
3. Are you using your camera in the M mode or Auto mode or some other mode? The M mode gives you the most control. The Av or Tv mode is better than the Auto mode. Auto mode is the poorest choice.
4. 1/60 is way too slow for use with a strobe.
5. The camera may have a maximum shutter speed that is compatible with the flash. Check for that in the manual. For example my Canon G10 had a max shutter speed of 1/500 with the flash. Any setting beyond that automatically defaulted to 1/500.

 

[xyrandomyx]

1. Since your strobe is connected with a fiber optic cable it is not true TTL so the camera is not sending info to the strobe as with a hard wired strobe.
2. Does your strobe have F stop settings as well as manual ones? If using the F stop settings on the strobe they should closely correspond with the settings on the camera. There is still some guesswork involved. If using manual settings on the strobe then there is more guesswork as respects the strobe and camera settings. For example I have a strobe with manual settings only. I shoot in the M mode at F5.0 and 1/250 as a base setting for my G10 and G12. I adjust the strobe power according to the subject and background. If that won't do then I adjust the F stop and/or shutter speed. It's guesswork but I am use to it but still have to take a few shots of the same subject.
3. Are you using your camera in the M mode or Auto mode or some other mode? The M mode gives you the most control. The Av or Tv mode is better than the Auto mode. Auto mode is the poorest choice.
4. 1/60 is way too slow for use with a strobe.
5. The camera may have a maximum shutter speed that is compatible with the flash. Check for that in the manual. For example my Canon G10 had a max shutter speed of 1/500 with the flash. Any setting beyond that automatically defaulted to 1/500.

2. No. It's just got a 'flash brightness control', with settings labelled 1--10 plus 'auto'.
3. I'm using a mode called 'ExtFlashMNL' (external flash manual). It allows the aperture and shutter speed to be set manually (although aperture values are very limited).
4. I thought so. It is, however, the value that the camera seems to select when in auto mode in low-ish light conditions.
5. Could be. I can't find any mention of it in the manual, technical specs, Google searches.

I've emailed the manufacture now. I'm hoping they can give me some more precise info on the behaviour of the modes and flash settings.

My guess, after some more more experimentation, is that the ISO is automatically adjusted even when it's set manually:

- faster shutter speeds have some pretty noticeable CCD noise
- setting a lower ISO makes no difference to either exposure or noise
- setting a higher ISO increases exposure and noise

 

[Gilligan]

I was looking at the specs on this site for your camera.

It appears you have ISO of 64 to 3200. 64 up to 200 should not result in digital noise, especially 64 and 100 which should be for strobe shots. 200 for non-strobe shots. Above 200 usually results in digital noise.

I don't know why the ISO would change if you can set it manually. Hopefully the mfg. will answer that for you.

Sorry I can't be of more help but I would need a hands on to play around with the camera.

 

[xyrandomyx]

I was looking at the specs on this site for your camera.

It appears you have ISO of 64 to 3200. 64 up to 200 should not result in digital noise, especially 64 and 100 which should be for strobe shots. 200 for non-strobe shots. Above 200 usually results in digital noise.

I don't know why the ISO would change if you can set it manually. Hopefully the mfg. will answer that for you.

Sorry I can't be of more help but I would need a hands on to play around with the camera.

Thanks for your help. I'll wait and see what the manufacturer has to say (if anything). I am almost certain it's setting the ISO automatically, even though it makes no sense.

 

[xyrandomyx]

I'm resurrecting this rather old thread of mine to say that I discovered by accident that there's a new firmware version for the Sealife DC-1400 (V2014) and that it fixes the issue I described here. I never did manage to confirm what the issue was exactly, but my guess is still that the camera was automatically adjusting the ISO when it was supposed to be in full manual mode. Anyway, it's now like I have a new toy to play with, so I'm happy.