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01-03-2019, 05:36 PM   #1
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Program vs Manual - Flash Compensation

Greetings,

As a former K20D and now K3II user I have learned the ins and outs of shooting manual and am quite good but by no means a master/pro etc. As a new father I find many great opportunities to use my camera, and most of the time it is in manual.
Indoors to supplement the lack of decent lighting I'll bounce a flash off a wall attached to the hot shoe, again most of the time using manual program line on the K3II. Today I used the program mode and something confused me I wanted to ask this great group about...

One shot - 1/80 F2.8 ISO 800
Other shot - 1/100 F6.3 ISO 800
Both shot in program line mode with the flash on P-TTL Digital, auto zoom. Flash is a Pentax AF-540FGZ from around 8 years ago or so.

So my question is: does the flash perform its own calculations and fire at the power it is required based on the lighting and camera settings for each shot (when in a more auto mode like Program Line)? Because with a faster shutter and smaller aperture on the second shot the exposure is super close (slightly different scene).

ie: First shot, sensor/camera saw more light from the slower shutter and faster aperture so the flash didn't shoot as bright, but on the second it saw a faster shutter, more narrow aperture and (the flash) increased its output on its own?

I think I was expecting the camera to adjust the aperture if I moved the shutter speed, and the shutter speed if I adjusted the aperture. However it wasn't the case I adjusted both and the ISO remained static.

Thanks in advance all. Appreciate any thoughts or guidance.

Shane

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01-03-2019, 05:55 PM   #2
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The short answer is yes. There are others much more adept at answering your question in depth, but, with the flash working PTTL instead of Manual, it reads the camera settings and ambient light and adjusts the flash output accordingly.
01-03-2019, 06:10 PM   #3
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You were controlling the ambient exposure with shutter speed, ISI and aperture.

The flash in P-TTL varies its power to match the scene.

You use FEC to adjust it to taste, or Manual to set the power with more granularity and consistency.

01-03-2019, 06:23 PM   #4
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Great thanks guys. I guess one thing I didn't consider is that when I shoot in manual I leave the flash in auto, but the results aren't as consistent or easy to control.
So there is another curiosity. When the camera is in manual but the flash is in Digital P-TTL is the flash still reading the ambient light and camera setting and adjusting?
I ask because as I said above, in manual it is just not as consistent, which I understand is the case likely do to the photographer mostly.

---------- Post added 01-03-19 at 08:33 PM ----------

Just doesn't seem to give as good a result is what I'm saying.

QuoteOriginally posted by Irishmanshaner Quote
Great thanks guys. I guess one thing I didn't consider is that when I shoot in manual I leave the flash in auto, but the results aren't as consistent or easy to control.
So there is another curiosity. When the camera is in manual but the flash is in Digital P-TTL is the flash still reading the ambient light and camera setting and adjusting?
I ask because as I said above, in manual it is just not as consistent, which I understand is the case likely do to the photographer mostly.


01-03-2019, 07:29 PM   #5
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QuoteOriginally posted by Irishmanshaner Quote
Great thanks guys. I guess one thing I didn't consider is that when I shoot in manual I leave the flash in auto, but the results aren't as consistent or easy to control.
So there is another curiosity. When the camera is in manual but the flash is in Digital P-TTL is the flash still reading the ambient light and camera setting and adjusting?
I ask because as I said above, in manual it is just not as consistent, which I understand is the case likely do to the photographer mostly.

---------- Post added 01-03-19 at 08:33 PM ----------

Just doesn't seem to give as good a result is what I'm saying.
The flash takes into account your camera settings. If you underexpose the scene by -1eV it will use flash to light normally, not by -1, unless you've chosen -1 FEC.

Your two photographs above present a dilemma. Should the flash light the whole scene (the baby will be overexposed) or just the subject (you can then lose the surroundings into darkness).

You're a photographer, take charge, that's what the controls are for.

Forget the flash at first, set the ambient on your camera so you're happy with how the background looks.

Then set the flash to say, 1/8, and iterate from there until you're happy with the flash/ambient balance in your photo.

Try not to go above ISO800 or shutter speed drop below 1/250s for a human subject , let the flash do the work. A bonus of that is that the shutter speed can end up irrelevant, the flash duration might only be 1/1000s and you get quite sharp pictures.

If you genuinely don't care about the ambient lighting, your bounce off the wall/roof is going to do it all, go for quality with 1/180s, f8, ISO100. Effectively that's a studio situation.

Last edited by clackers; 01-03-2019 at 10:30 PM.
01-04-2019, 12:38 AM - 1 Like   #6
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A picture with flash is basically two exposures in one.

First is ambient light. This is controlled directly by the camera's three settings shutter speed/aperture/ISO. Test this first with the flash turned off.

The second exposure is the flash. In P-TTL mode the flash fires a pre-flash, reads the exposure and calculates a power level for the actual flash. This happens so quickly you wont even notice it.

As Clackers mentioned, the shutter speed is pretty much irrelevant as far as flash exposure is concerned because the flash duration is so quick so it is aperture and ISO that only determine how much power the flash needs. One thing about shutter speed however is your camera wont let you shoot above the sync speed which will be 1/180 on your K3 I believe. So 1/180 or slower will work fine. (There is a mode called HSS which does allow faster shutter speeds but I won't go into that here.)

If you are getting inconsistent results in manual mode on camera and P-TTL on the flash that can be because you are bouncing the flash. The light has a lot further to travel and it may be depending on your aperture/ISO, that the flash cannot produce enough light. When the flash head is in its normal straight ahead position, it will display a range in meters or feet. That is a minimum/maximum range that you can shoot with the flash pointed direct. As you change the aperture/ISO that range will increase/decrease. Bounced flash drops off in intensity much quicker and it may be that your bounced flash distance is not enough to light your subject even at full power. You can test this by changing the flash to full manual flash output an look at the results. Also make sure your camera is set to matrix metering, not spot.
01-04-2019, 07:46 AM   #7
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QuoteOriginally posted by clackers Quote
The flash takes into account your camera settings. If you underexpose the scene by -1eV it will use flash to light normally, not by -1, unless you've chosen -1 FEC.

Your two photographs above present a dilemma. Should the flash light the whole scene (the baby will be overexposed) or just the subject (you can then lose the surroundings into darkness).

You're a photographer, take charge, that's what the controls are for.

Forget the flash at first, set the ambient on your camera so you're happy with how the background looks.

Then set the flash to say, 1/8, and iterate from there until you're happy with the flash/ambient balance in your photo.

Try not to go above ISO800 or shutter speed drop below 1/250s for a human subject , let the flash do the work. A bonus of that is that the shutter speed can end up irrelevant, the flash duration might only be 1/1000s and you get quite sharp pictures.

If you genuinely don't care about the ambient lighting, your bounce off the wall/roof is going to do it all, go for quality with 1/180s, f8, ISO100. Effectively that's a studio situation.
Thank you Clackers, great info!
The shutter speed is 1/180 maximum for leading curtain and contrast control, and 1/90 for trailing curtain. I believe a limitation of the specific camera?! And I agree I'd much rather stay faster especially with a son who loves to move without warning That is why I have the ISO as high as I do.

One item I'd like to learn is how to light just the subject, and have the background just be dark or dim.
01-04-2019, 07:49 AM   #8
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QuoteOriginally posted by Irishmanshaner Quote
Thank you Clackers, great info!
The shutter speed is 1/180 maximum for leading curtain and contrast control, and 1/90 for trailing curtain. I believe a limitation of the specific camera?! And I agree I'd much rather stay faster especially with a son who loves to move without warning That is why I have the ISO as high as I do.

One item I'd like to learn is how to light just the subject, and have the background just be dark or dim.
Just put your flash into HSS mode to get above 1/180s.

See the last paragraph of my above post about what to do when there's no ambient light.



01-04-2019, 07:53 AM - 1 Like   #9
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QuoteOriginally posted by pschlute Quote
A picture with flash is basically two exposures in one.

First is ambient light. This is controlled directly by the camera's three settings shutter speed/aperture/ISO. Test this first with the flash turned off.

The second exposure is the flash. In P-TTL mode the flash fires a pre-flash, reads the exposure and calculates a power level for the actual flash. This happens so quickly you wont even notice it.

As Clackers mentioned, the shutter speed is pretty much irrelevant as far as flash exposure is concerned because the flash duration is so quick so it is aperture and ISO that only determine how much power the flash needs. One thing about shutter speed however is your camera wont let you shoot above the sync speed which will be 1/180 on your K3 I believe. So 1/180 or slower will work fine. (There is a mode called HSS which does allow faster shutter speeds but I won't go into that here.)

If you are getting inconsistent results in manual mode on camera and P-TTL on the flash that can be because you are bouncing the flash. The light has a lot further to travel and it may be depending on your aperture/ISO, that the flash cannot produce enough light. When the flash head is in its normal straight ahead position, it will display a range in meters or feet. That is a minimum/maximum range that you can shoot with the flash pointed direct. As you change the aperture/ISO that range will increase/decrease. Bounced flash drops off in intensity much quicker and it may be that your bounced flash distance is not enough to light your subject even at full power. You can test this by changing the flash to full manual flash output an look at the results. Also make sure your camera is set to matrix metering, not spot.
Hi Peter,

Thanks for your guidance as well. Great points, you are correct on the shutter speed maximum on the K3II.
Makes sense on the distance to travel for the light when bouncing. I have gone that route because I found the direct flash to be very harsh, creating some shadows and blown out subjects, but in that I realize that is why you and Clackers are suggesting lower power (1/8) on the flash in manual to dial back how impactful the flash will be (I think).

Thanks again. Cheers.
01-04-2019, 08:05 AM - 1 Like   #10
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QuoteOriginally posted by Irishmanshaner Quote
Hi Peter,

Thanks for your guidance as well. Great points, you are correct on the shutter speed maximum on the K3II.
Makes sense on the distance to travel for the light when bouncing. I have gone that route because I found the direct flash to be very harsh, creating some shadows and blown out subjects, but in that I realize that is why you and Clackers are suggesting lower power (1/8) on the flash in manual to dial back how impactful the flash will be (I think).

Thanks again. Cheers.
Going off the wall will make for softer light because it's bigger than the little filament area of your speedlight, so it's the right way to go.

It's like playing billiards, work out whether the best shadows are created firing at an angle into a sidewall or at 45 degrees into the ceiling or into a corner, even.

You're very close to the subject so you should try to 'flag' off any spilled direct flash by putting your hand in front of the speedlight or attach a black card with a rubber band or hair tie.
01-04-2019, 08:55 AM   #11
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QuoteOriginally posted by Irishmanshaner Quote
And I agree I'd much rather stay faster especially with a son who loves to move without warning
If the flash is providing the majority of the light that won't matter. A camera at 1/180 and a flashgun will freeze action just as if you had used 1/2000 in ambient light.

Experiment with HSS by all means but you lose a lot of power in doing so which may mean your bounce distance is too far. And HSS can lead to more motion blur as you are now dealing with a light that is more akin to continuous light rather than single flash burst.
01-04-2019, 11:35 AM   #12
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In addition to all the good points already made we should bear in mind that the P-TTL flash metering takes into account the level of the ambient exposure being recorded, including in Manual camera mode. So, where there is a good amount of ambient light registering then the flash exposure iis reduced somewhat, to blend and mix with the natural light....(this is commonly termed "fill flash").

Alternatively when there is no or very little ambient light registering, then the flash exposure will be pitched higher in order to fully light the scene with flash. This is an important feature because the flash and ambient exposures combine and accumulate on a subject, and it there is already enough ambient light being recorded (say on a face) then adding flash on top as well will give over-exposure.

This feature is automatic with our Pentax DSLRs, activated by an undocumented EV level being detected during the metering phase. This will influence our flash compensation settings according to the situation.
01-11-2019, 07:17 PM   #13
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When shooting manual on camera while using flash keep the following in mind as well...
Aperture controls the amount of light the camera picks up on the subject from the flash. Higher F/Stop, less flash lighting will hit the sensor.
Shutter Speed controls the background/overall scene ambiance. Slower shutter speed, more of the background will come through, faster shutter speed, darker the background.
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