[fb_penpho]

My knowledge of using a system flash is limited. But I take it that REAR allows the light then fires the flash at the end to top up the light to the correct exposure. Therefore is it correct the normal flash would fire for the correct exposure, then any ambient light would add the the exposure.

Does that mean that rear curtain flash is likely to give better exposure value ?

[LensBeginner]

No.

Available light contributes to thr exposure in both cases.

It is desirable to use 2nd curtain flash when shooting sports, because you'll get a fuzzy movement streak and at the end of that a sharp image. With 1st curtain you get a sharp image and then a movement streak, and the effect will be the one of a car/athelete driving/running backwards.

[photoptimist]

The idea that rear-curtain flash would only use enough flash to top up the exposure may have been true for film cameras that had true TTL meters which could monitor the light reflected off the film during the exposure. But today's digital cameras can't do that because the sensor surface does not reflect light in a way that can be metered. Instead, the camera controls the balance of ambient and flash exposure by measuring the ambient and then also doing a controlled preflash that does a test-illumination of the scene. In any case, off-the-film metering and rear curtain flash would only give a better exposure if the ambient light were changing extremely rapidly between the time the user hits the shutter button and the camera completes the exposure.

The primary rationale for rear-curtain flash is creating a more pleasing appearance of rapidly moving subjects in ambient light. With a rear curtain flash, the blurred ghost of the moving object lit by ambient light trails behind the sharp flash-lit image of the object. That looks much better than the front-curtain version in which the ghost comes out of or leads the object.

[honey bo bo]

An example of rear curtain Flash. I'm experimenting with some of my pictures of the Grand Babies to get a sense of movement with them. AND MOVEMENT THERE IS ! LOTS OF IT ! Note the top hand (desirable) the bottom hand not so much,

---------- Post added 03-28-18 at 11:25 AM ----------

By the Way I think he's singing "When Irish Eyes are Smiling Sure Tis.........."

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Last edited by honey bo bo; 08-07-2018 at 04:29 AM.

[mcgregni]

The ambient and flash exposure values are no different for 1st or 2nd curtain sync modes. This is because the measuring pre-flash for both occurs at the start before any exposure has taken place ..... The pre-calculated flash exposure is just delivered at different points during the exposure time. During the preflash the ambient light levels are measured also and the flash output level is adjusted accordingly depending on the scenario. (subject brightness, back light condition, distance, reflectivity ). The resulting flash output is the same for both 1st and 2nd curtain modes.

[fb_penpho]

The ambient and flash exposure values are no different for 1st or 2nd curtain sync modes. This is because the measuring pre-flash for both occurs at the start before any exposure has taken place ..... The pre-calculated flash exposure is just delivered at different points during the exposure time. During the preflash the ambient light levels are measured also and the flash output level is adjusted accordingly depending on the scenario. (subject brightness, back light condition, distance, reflectivity ). The resulting flash output is the same for both 1st and 2nd curtain modes.

I did not know there was a pre flash, never seen one I suppose because it is so fast it just looks like one flash. Makes sense of how it has to operate, as the camera does not know what flashgun you have, what diffuser you are using, etc. So doing a sample first makes sense, thanks for the reply.

[amoringello]

Rear Curtain sync also limits your maximum shutter speed.
For the K-1 it appears to be 1/100s rather than 1/200s. Up slightly from 1/90s with older models.
So that might limit usability in some cases.

[honey bo bo]

Rear Curtain sync also limits your maximum shutter speed.
For the K-1 it appears to be 1/100s rather than 1/200s. Up slightly from 1/90s with older models.
So that might limit usability in some cases.

Doesn't appear to with the K-3. Still @ 1/180s see Exif

[mcgregni]

There wouldn't be much purpose to taking a 2nd curtain sync shot at 1/180th, as the difference in effect would be almost nothing as there is so little time for movement to record. The most effective uses are the "dragging the shutter" type scenarios in fairly low light

[BrianR]

There wouldn't be much purpose to taking a 2nd curtain sync shot at 1/180th, as the difference in effect would be almost nothing as there is so little time for movement to recordt

That depends on how fast things are moving. I've tried combined flash & ambient photos of flowers with 'rain' (from a garden hose) and with front curtain the droplets look like little comets flying up up and away. 1/180th can still have a good deal of motion blur.

[mcgregni]

Sure, you're right, with very fast moving things. I was thinking about the typical sort of result, with the "dragging" shutter allowing low ambient light to record and give the movement steaks around the subject, while the flash catches the main part of the subject sharply.

I have always understood that 1/90th was the limit for 2nd Curtain sync on Pentax DLSRs ......

[honey bo bo]

There wouldn't be much purpose to taking a 2nd curtain sync shot at 1/180th, as the difference in effect would be almost nothing as there is so little time for movement to record. The most effective uses are the "dragging the shutter" type scenarios in fairly low light

Did you even look at the above photo and the Exif. And yes the slower the movement the lower the shutter speed that will catch it.

[BrianR]

I have always understood that 1/90th was the limit for 2nd Curtain sync on Pentax DLSRs ......

Likewise, but I only have the manuals to go by. It's hard to tell from the photo above if it's rear or front curtain (which way was the child moving?) and I can't see in the exif if it is claiming to be rear curtain. I have no rear-curtain capable equipment to check what happens if you try to go faster than 1/90th with rear-curtain settings (does it stop you from passing 1/90th or default to front curtain?).

If I want the rear curtain look, I put the car in reverse

[mcgregni]

Did you even look at the above photo and the Exif .

Yes of course I did ..... The point is there is nothing in the image or the available EXIF that proves it was taken in 2nd Curtain Sync mode, as Brian has also said.

Do you have more detailed Exif available that shows the sync mode?

The point I was making before was that in shots such as your one with the little boy, it probably makes little difference.

Brian, I haven't tested this, but I suspect that if 2nd curtain is set but with a time value beyond 1/90th then the flash will just flick out of 2nd curtain mode ....I think the image would still be taken

[honey bo bo]

Yes of course I did ..... The point is there is nothing in the image or the available EXIF that proves it was taken in 2nd Curtain Sync mode, as Brian has also said

Other than I said it was!