Go Back   Steve's Digicams Forums > Digital SLR and Interchangeable Lens Cameras > Pentax / Samsung dSLR, K Mount Mirrorless

Reply
 
Thread Tools Search this Thread
Old Dec 14, 2008, 9:09 PM   #1
Senior Member
 
Wingman's Avatar
 
Join Date: Dec 2003
Location: Hebron, Kentucky (northern Kentucky/Greater Cincinnati):KCVG
Posts: 4,327
Default

I use a Sigma 500DG Super which is listed as being PTTL. I understand that part of PTTL is that it fires a pre-flash to ensure the proper metering. In my use however, I have never noticed a pre-flash. Therefore, I have the following questions:

1. Is the 500DG Super lacking the pre-flash capability or could it be that myunit is defective?

2. If you use a 530 DG Super, can you tell if a pre-flash is generated?

3. If a pre-flash is fired, should it not cause a delay in the actual flash since the flash would have discharged and therefore require some time to charge? Unless the flash is of low intensity similar to the red-eye reduction flash. But I figured if it is used to meter, the flash should be at some "normal" level:?

Would appreciate any information on this subject.


Wingman is offline   Reply With Quote
Sponsored Links
Old Dec 15, 2008, 2:11 AM   #2
Senior Member
 
snostorm's Avatar
 
Join Date: Jan 2004
Location: Chicago Suburb, IL, USA
Posts: 2,770
Default

Hi jelpee,

If the camera will meter for the flash correctly using different shutter speeds and apertures when in P-TTL mode, then I'd say that it's safe to say that it preflashes. The preflash can be seen in the viewfinder JUST before the mirror flips up and blacks out the VF. If it doesn't preflash, then you might get lucky on some, but a fairly large percentage of your flash shots would probably be badly exposed.

The Preflash takes some of the charge away from the main flash, depending, I think, on the distance reading that it gets from the focused lens (longer distance would increase the preflash strength to a preset limit), so when you're using P-TTL, you don't get all the power potential from the main flash. The flashes are only milliseconds apart, so there's no time to recharge for the main flash.

I found it important to watch the preflash because P-TTL is known to be easily fooled by a reflection of the flash directly back to the camera. If I see such a hotspot at the time of the preflash, I know that the shot will be massively underexposed, and I'll have to shoot it again. I also immediately take a step to the side and tell the people to hold on a second.

This is how it works. The camera uses the same exposure meter to read light levels for P-TTL as it does normally, without the flash, so the preflash must happen before the mirror flips up for the exposure. Information from the lens and camera (aperture, shutter speed, ISO, and distance of the focus point enter into the calculation for the lenses that transfer the information. The system must have this information to meter reasonably, so that's why fully manual lenses won't work. The flash and camera body work together in the split second between the preflash and the exposure to set the flash intensity and duration. It's a very complex system, and though it can be fooled (like with the reflection), it works pretty well, IMO.

In the previous Through The Lens metered flash system (TTL), there was a separate sensor located inside the lightbox, under the mirror, and it read the amount of light reflecting off the CCD image sensor during the exposure. TTL was developed for film, and since the emulsion of the film doesn't reflect light like the surface of the sensor assembly (film is dull and the sensor is shiney, for one), the system doesn't work as well for the wide range of apertures and especially ISO speeds that we have in digital. In fact, for the DS, the practical limits were 200-800 ISO, and f4-f8 for the aperture, the way I saw it.

P-TTL would ideally work reasonably with just about any distance within reason, aperture and ISO speed, but very small and very large apertures cause some metering problems. The range of combinations of distances and settings that can be used is significantly greater than TTL. This is a work in progress, as the algorithms have obviously changed between the D series and K series, or the 3rd party flashes wouldn't have needed the firmware updates.

Scott
snostorm is offline   Reply With Quote
Old Dec 16, 2008, 12:27 PM   #3
Senior Member
 
Wingman's Avatar
 
Join Date: Dec 2003
Location: Hebron, Kentucky (northern Kentucky/Greater Cincinnati):KCVG
Posts: 4,327
Default

Thanks Scott for the posting and detailed response. My Sigma flash delivers accurate exposures even though I cannot tell if a pre-flash is being used. Guess I will need to sit with someone owning a P-TTL flash and see for myself!
Wingman is offline   Reply With Quote
Old Dec 16, 2008, 2:48 PM   #4
Senior Member
 
jachol's Avatar
 
Join Date: Aug 2006
Location: NW. England
Posts: 1,201
Default

jelpee wrote:
Quote:
Thanks Scott for the posting and detailed response. My Sigma flash delivers accurate exposures even though I cannot tell if a pre-flash is being used. Guess I will need to sit with someone owning a P-TTL flash and see for myself!
Hi,

I can see the preflash from my Metz 48 AF-1, it's quite pronounced, but using the internal flash I can't say I've ever detected it, but the exposures seem to come out OK ...Jack.
jachol is offline   Reply With Quote
Old Dec 16, 2008, 7:50 PM   #5
Senior Member
 
snostorm's Avatar
 
Join Date: Jan 2004
Location: Chicago Suburb, IL, USA
Posts: 2,770
Default

jelpee wrote:
Quote:
Thanks Scott for the posting and detailed response.* My Sigma flash delivers accurate exposures even though I cannot tell if a pre-flash is being used.* Guess I will need to sit with someone owning a P-TTL flash and see for myself!
Hi jelpee,

Try this -- go into a darkish room -- enough light to see a little, then put the camera in MF mode so you don't have to obtain a focus lock to get an exposure. The image you get is irrelevant, so if it's not in focus, it really doesn't matter for this test. Make sure that you're using an AF lens, or at least an "A" capable lens mounted, and the flash is in P-TTL. Hit the shutter button while watching through the viewfinder and closing your "off" eye. The very last thing that you should see in the VF before it blacks out is the preflash lighting the scene.

Once you get used to looking for it, it's pretty easy to watch for.

Scott
snostorm is offline   Reply With Quote
Old Dec 24, 2008, 10:09 PM   #6
Senior Member
 
Wingman's Avatar
 
Join Date: Dec 2003
Location: Hebron, Kentucky (northern Kentucky/Greater Cincinnati):KCVG
Posts: 4,327
Default

snostorm wrote:
Quote:
jelpee wrote:


Hi jelpee,

Try this -- go into a darkish room -- enough light to see a little, then put the camera in MF mode so you don't have to obtain a focus lock to get an exposure. The image you get is irrelevant, so if it's not in focus, it really doesn't matter for this test. Make sure that you're using an AF lens, or at least an "A" capable lens mounted, and the flash is in P-TTL. Hit the shutter button while watching through the viewfinder and closing your "off" eye. The very last thing that you should see in the VF before it blacks out is the preflash lighting the scene.

Once you get used to looking for it, it's pretty easy to watch for.

Scott

Scott,

I tried the experiement your recommended above, and good golly Miss Molly it works! In fact I went a step further and set both the camera and the flash on the trailing curtain mode and set a shutter speed of 3 seconds. When I pressed the shutter button, I noticed that the flash fired right before the mirror locked up (presumably the pre-flash) and then fired again 3seconds later when the curtain began its closing sequence.

So...no need to get a new flash.

Thanks again.

And since I have the floor, a Merry Christmas to All and to All a Good Night!

Wingman is offline   Reply With Quote
Old Dec 26, 2008, 8:08 AM   #7
Senior Member
 
kazuya's Avatar
 
Join Date: Sep 2008
Posts: 1,007
Default

i cant say ive noticed my pre flash either, (only had it a day though) but if its only milliseconds before the main flash then its hardly surprising. i just tried the test in the darkened room and i did see a white flash in viewfinder just a fraction of a second before the mirror flipped up, but its not easy to tell. i think its a case of when it says it does a preflash you expect it to be something like the anti redeye preflash.

pictures come out fine so it must be working even if i dont see it
kazuya is offline   Reply With Quote
 
Reply


Thread Tools Search this Thread
Search this Thread:

Advanced Search

Posting Rules
You may not post new threads
You may not post replies
You may not post attachments
You may not edit your posts

BB code is On
Smilies are On
[IMG] code is On
HTML code is Off
Trackbacks are Off
Pingbacks are Off
Refbacks are Off



All times are GMT -5. The time now is 2:19 AM.