*istDS underexposure with onboard flash

tomaszmatys · Mar 28, 2005

Thanks to encouragement from this excellent forum I have recently purchased *istDS with the kit lens (18-55). The camera performs great outdoors, but I have serious problems getting proper exposure using onboard flash. While the photographs taken with wide zoom look decent, zooming in results in underexposure reaching -2 to -3 EV in extreme cases with the right side of the histogram being virtually empty.

I did the following experiments: I photographed an off-white wall from various distances (1-3 m) using 18, 35 and 55 mm focal length and ISO 400 in M program with constant aperture 5.6 and exposure time 1/30 without ambient light (so the exposure should only depend on the onboard flash duration).

At 18 mm all photographs are exposed OK and the peak of the histogram is more or less in the center.

At 55 mm, the closer to the wall, the more underexposed the photos are (by about -2.5 EV at 1m, -1EV at 1.5-2 m and the exposure starts to be OK and the histogram is centered at distances more than 2.5-3m).
At 35 mm the distance at which histogram gets centered is shorter (about 2m).

So, to summarize, photos taken with onboard flash are underexposed while shooting from close distance with long focal length. The underexposure gets worse if you zoom in or get closer to the subject.

At the same time photos taken with my friend’s Pentax AF 500 FTZ flash working in TTL are well exposed.

Could you try to do similar experiment and report the results? Do you think this behavior is normal or is my camera defective? If this is a normal behavior with P-TTL, why is it?

All these makes me consider getting an external TTL flash instead of AF360FGZ or Sigma 500 DG Super with P-TTL, unless they can work in both P-TTL and TTL modes.

Rick Ashford · Mar 28, 2005

I... and many others on this forum... have experienced the same underexposure issue using the DS onboard flash. There are numerous lengthy threads about this very subject. Just do a search -- you shouldn't have any trouble finding them!

Some of us made the mistake of saying we get better flash results with our P&S cameras than with the DS. That opened the floodgates -- the rationalizing began in earnest. The retort was that we have a "sophisticated DSLR and we shouldn't expect to get the same results as with a P&S camera." IMHO, this is hogwash. Your histogram tests say it all. If the flash was exposing properly, the histograms would be relatively centered. But you indicated they are peaking far to the left of center. Unless you are out of range of the subject, that's not operator error -- it is the flash metering system not working properly.

We can get around this issue by dialing in positive EV compensation. The trick is learning how much and when. Sadly.... you shouldn't have to do this with a P-TTL flash that is working the way it is supposed to.

tomaszmatys said:
Thanks to encouragement from this excellent forum I have recently
purchased *istDS with the kit lens (18-55). The camera performs
great outdoors, but I have serious problems getting proper exposure
using onboard flash. While the photographs taken with wide zoom
look decent, zooming in results in underexposure reaching -2 to -3
EV in extreme cases with the right side of the histogram being
virtually empty.

I did the following experiments: I photographed an off-white wall
from various distances (1-3 m) using 18, 35 and 55 mm focal length
and ISO 400 in M program with constant aperture 5.6 and exposure
time 1/30 without ambient light (so the exposure should only depend
on the onboard flash duration).
At 18 mm all photographs are exposed OK and the peak of the
histogram is more or less in the center.
At 55 mm, the closer to the wall, the more underexposed the photos
are (by about -2.5 EV at 1m, -1EV at 1.5-2 m and the exposure
starts to be OK and the histogram is centered at distances more
than 2.5-3m).
At 35 mm the distance at which histogram gets centered is shorter
(about 2m).
So, to summarize, photos taken with onboard flash are underexposed
while shooting from close distance with long focal length. The
underexposure gets worse if you zoom in or get closer to the
subject.

At the same time photos taken with my friend’s Pentax AF 500 FTZ
flash working in TTL are well exposed.

Could you try to do similar experiment and report the results? Do
you think this behavior is normal or is my camera defective? If
this is a normal behavior with P-TTL, why is it?

All these makes me consider getting an external TTL flash instead
of AF360FGZ or Sigma 500 DG Super with P-TTL, unless they can work
in both P-TTL and TTL modes.

--
Rick A.
Johnson City, TN
http://www.photographyimpressions.com

JensR · Mar 28, 2005

Hi Tomasz!

tomaszmatys said:
I did the following experiments: I photographed an off-white wall
from various distances (1-3 m) using 18, 35 and 55 mm focal length
and ISO 400 in M program with constant aperture 5.6 and exposure
time 1/30 without ambient light (so the exposure should only depend
on the onboard flash duration).

Umm, I am not 100%ly sure. But IIRC, when you are in M mode, the onboard flash works as a "stupid" flash, that only provides full power.
I'd reshoot the test with one of the "auto" modes.

BTW, the flash sync speed of the DS is 1/150 (or was that 1/180?), so you might want to set it to this value, too. Slowing down (as you did) is normally not a problem, but you let in more and more ambient light, which may or may not be what you want.

CU
Jens

tomaszmatys · Mar 28, 2005

I'll repeat the test this evening using of of the auto modes, but I do not think that the flash should work as stupid in M (or Av or Tv) modes. If it was the case, and the flash discharged at full power all the time, exposure should go up when I approach the wall and it works opposite in my test. So I think something is wrong with the P-TTL algorithm (or it is supposed to work this way for a reason I do not really understand).

JensR said:
JensR said:

I did the following experiments: I photographed an off-white wall
from various distances (1-3 m) using 18, 35 and 55 mm focal length
and ISO 400 in M program with constant aperture 5.6 and exposure
time 1/30 without ambient light (so the exposure should only depend
on the onboard flash duration).

Click to expand...

Umm, I am not 100%ly sure. But IIRC, when you are in M mode, the
onboard flash works as a "stupid" flash, that only provides full
power.
I'd reshoot the test with one of the "auto" modes.
BTW, the flash sync speed of the DS is 1/150 (or was that 1/180?),
so you might want to set it to this value, too. Slowing down (as
you did) is normally not a problem, but you let in more and more
ambient light, which may or may not be what you want.

CU
Jens

fvrtac · Mar 28, 2005

tomaszmatys said:
Thanks to encouragement from this excellent forum I have recently
purchased *istDS with the kit lens (18-55). The camera performs
great outdoors, but I have serious problems getting proper exposure
using onboard flash. While the photographs taken with wide zoom
look decent, zooming in results in underexposure reaching -2 to -3
EV in extreme cases with the right side of the histogram being
virtually empty.

I did the following experiments: I photographed an off-white wall
from various distances (1-3 m) using 18, 35 and 55 mm focal length
and ISO 400 in M program with constant aperture 5.6 and exposure
time 1/30 without ambient light (so the exposure should only depend
on the onboard flash duration).
At 18 mm all photographs are exposed OK and the peak of the
histogram is more or less in the center.
At 55 mm, the closer to the wall, the more underexposed the photos
are (by about -2.5 EV at 1m, -1EV at 1.5-2 m and the exposure
starts to be OK and the histogram is centered at distances more
than 2.5-3m).
At 35 mm the distance at which histogram gets centered is shorter
(about 2m).
So, to summarize, photos taken with onboard flash are underexposed
while shooting from close distance with long focal length. The
underexposure gets worse if you zoom in or get closer to the
subject.

At the same time photos taken with my friend’s Pentax AF 500 FTZ
flash working in TTL are well exposed.

Could you try to do similar experiment and report the results? Do
you think this behavior is normal or is my camera defective? If
this is a normal behavior with P-TTL, why is it?

All these makes me consider getting an external TTL flash instead
of AF360FGZ or Sigma 500 DG Super with P-TTL, unless they can work
in both P-TTL and TTL modes.

Hello there!

I have also been using ist DS since the end of last December. I have been done more tha 1100 pictures. I have found out also the underexposed pictures for about 0,5 - 1,0 using the built in flash. Avoiding this problem I have been using an old (bougt in 1986) Pentax AF280T flash in TTL mode. The results are much better.

Any how I have a questio in the stile of W. S. To buy a AF360FGZ or to buy a Sigma 500 DG? Sigma is cheaper and with better performances. Isn't it? But it is only the Sigma not the original Pentax.

Best regards

fvrtac

Rick Ashford · Mar 28, 2005

You're right... the flash does not operate as a "stupid" flash when shooting in M mode. Regardless of the Tv setting, as soon as you pop the flash up, the shutter speed defaults to the Tv you manually set, or 1/180, whichever is less. The flash still operates as P-TTL and should adjust its intensity and/or duration according to your manual settings to get the proper exposure.

tomaszmatys said:
JensR said:

tomaszmatys said:

I did the following experiments: I photographed an off-white wall
from various distances (1-3 m) using 18, 35 and 55 mm focal length
and ISO 400 in M program with constant aperture 5.6 and exposure
time 1/30 without ambient light (so the exposure should only depend
on the onboard flash duration).

Click to expand...

Umm, I am not 100%ly sure. But IIRC, when you are in M mode, the
onboard flash works as a "stupid" flash, that only provides full
power.
I'd reshoot the test with one of the "auto" modes.
BTW, the flash sync speed of the DS is 1/150 (or was that 1/180?),
so you might want to set it to this value, too. Slowing down (as
you did) is normally not a problem, but you let in more and more
ambient light, which may or may not be what you want.

CU
Jens

Click to expand...

--
Rick A.
Johnson City, TN
http://www.photographyimpressions.com

JensR · Mar 28, 2005

Hi Rick!

Rick Ashford said:
You're right... the flash does not operate as a "stupid" flash
when shooting in M mode.

Ok, thanks for the info.

I won't be able to do any tests, as with my M lenses, I cannot use the internal flash as P-TTL.

One final idea: The manual mentions a shortest distance at which the flash is fully functional. I guess this is not the problem here, but might be worth remembering.

Good luck
Jens

tomaszmatys · Mar 28, 2005

As I mentioned, the results in TTL seem indeed much better.

So now, I have a dillema - whether to buy an older TTL unit or P-TTL Flash (Pentax 360 or Sigma 500) - if external P-TTL flash has the same exposure problems as the internal one, I would rather go with TTL.
Maybe I'll be able to try one of them and will report here on the results.

It would be great if any of you could do the test with the internal flash in a similar way I did so I know my camera is not defective. Could you do this:
1. set the camera to M, with any aperture and exposure time but keep it constant
2. set the zoom to 55 mm
3. shoot a white wall from 1,2 and 3 m and look at the histogram ?

At least I would know the underexposure is "normal" and will have to learn to live with it

fvrtac said:
Hello there!

I have also been using ist DS since the end of last December. I
have been done more tha 1100 pictures. I have found out also the
underexposed pictures for about 0,5 - 1,0 using the built in flash.
Avoiding this problem I have been using an old (bougt in 1986)
Pentax AF280T flash in TTL mode. The results are much better.

Any how I have a questio in the stile of W. S. To buy a AF360FGZ or
to buy a Sigma 500 DG? Sigma is cheaper and with better
performances. Isn't it? But it is only the Sigma not the original
Pentax.

Best regards

fvrtac

Godfrey · Mar 28, 2005

The built in flash simply runs out of power when you use the DA18-55/3.5-5.6 at longer focal lengths: the lens is simply too slow. I've not seen evidence of underexposure with any of my lenses at normal "living room" distances and ISO 200-400 settings.

However, I'd suggest getting an external flash anyway. It's not the TTL that allows the Pentax FZ500 flash to do the job right, it's the fact that it has lots of power to work with. I'd recommend a flash unit that allows use of all features (P-TTL, TTL, second curtain sync, etc) ... so either the Pentax AF360 or the Sigma 500 DG Super would be the one.

Godfrey

Rick Ashford · Mar 28, 2005

Your post here reminded me to go back and check the manual. When you use pre-A lenses on the DS, the flash discharges at full power, regardless of what settings you are using. I guess in this situation, the flash does qualify as a "dumb" flash.

JensR said:
I won't be able to do any tests, as with my M lenses, I cannot use
the internal flash as P-TTL.

One final idea: The manual mentions a shortest distance at which
the flash is fully functional. I guess this is not the problem
here, but might be worth remembering.

Good luck
Jens

--
Rick A.
Johnson City, TN
http://www.photographyimpressions.com

Rick Ashford · Mar 28, 2005

Godfrey said:
The built in flash simply runs out of power when you use the
DA18-55/3.5-5.6 at longer focal lengths: the lens is simply too
slow. I've not seen evidence of underexposure with any of my lenses
at normal "living room" distances and ISO 200-400 settings.

I'm not sure this is true. In situations where I have encountered underexposure with the onboard flash (all of them living room situations), and it happens a lot, I am able to overcome this problem with +EV compensation. And this is usually a function of flash intensity, since the lens is usually fully wide open (when operating in P mode).

--
Rick A.
Johnson City, TN
http://www.photographyimpressions.com

Howard · Mar 28, 2005

Tomas,

I do not think there is anything wrong with your camera or P-TTL logic even though I have had a similar problem when I use P-TTL on the *istD instead of TTL with the 16-45DA AND I am within 2.5 meters of a light color wall.

I believe the problem is that P-TTL fires a pre-flash and when you are that close the camera thinks the subject is brighter than it really is due to the reflection back to the sensor from the pre-flash. Because of that, the actual image flash is at a lower intensity than needed to properly illuminate the subject. This does not happen in a wide angle setting because the P-TTL pre-flash coverage angle is not contrated in a small angle of view. But I have seen this occur above 35mm at a distance below 3 meters if the wall is a light color such as off-white.

Howard
_ _ _ _ _

tomaszmatys said:
Thanks to encouragement from this excellent forum I have recently
purchased *istDS with the kit lens (18-55). The camera performs
great outdoors, but I have serious problems getting proper exposure
using onboard flash. While the photographs taken with wide zoom
look decent, zooming in results in underexposure reaching -2 to -3
EV in extreme cases with the right side of the histogram being
virtually empty.

I did the following experiments: I photographed an off-white wall
from various distances (1-3 m) using 18, 35 and 55 mm focal length
and ISO 400 in M program with constant aperture 5.6 and exposure
time 1/30 without ambient light (so the exposure should only depend
on the onboard flash duration).
At 18 mm all photographs are exposed OK and the peak of the
histogram is more or less in the center.
At 55 mm, the closer to the wall, the more underexposed the photos
are (by about -2.5 EV at 1m, -1EV at 1.5-2 m and the exposure
starts to be OK and the histogram is centered at distances more
than 2.5-3m).
At 35 mm the distance at which histogram gets centered is shorter
(about 2m).
So, to summarize, photos taken with onboard flash are underexposed
while shooting from close distance with long focal length. The
underexposure gets worse if you zoom in or get closer to the
subject.

At the same time photos taken with my friend’s Pentax AF 500 FTZ
flash working in TTL are well exposed.

Could you try to do similar experiment and report the results? Do
you think this behavior is normal or is my camera defective? If
this is a normal behavior with P-TTL, why is it?

All these makes me consider getting an external TTL flash instead
of AF360FGZ or Sigma 500 DG Super with P-TTL, unless they can work
in both P-TTL and TTL modes.

racehorse in the desert · Mar 28, 2005

I think that your test may be faulty if you are trying to compare P-TTL vs. TTL flash, using a buit-in flash and an external flash. There are too many variables with the two flash units.

However, on the D (with 16-45mm lens), at 3 meters I experienced about 1/2 stop overexposure with the built-in flash at all zoom lengths (18, 28, 45mm). At closer distances I experienced about 1/2 stop underexposure with the built-in flash at the same zoom settings.

However, I did your test with a D and an AF360 and the exposures turned out almost dead on center on the histograms in every case. The results were near perfect and looked great.

Conclusion: I don't really trust the built-in flash and only use it when necessary. I use the AF360 almost always when needing flash.
Solution: Buy an external flash.

--
John Power
Racehorse in the Desert

'Life is too short to miss out on photography.'

Howard · Mar 28, 2005

John,

I agree 100 percent about using a hotshoe or handle flash as much as possible. I did the same test a few minutes ago and at 2.5 meters my *istD built-in flash with 16-45DA appeared to be about 1 stop under exposed at ISO200. What ISO did you use?

Also, as I posted in a reply to Tomas, I think the problem is the light concentration of the pre-flash from the onboard flash reflecting back off a light color wall and causing the built-in flash to fire a lower intensity image flash. Does this sound reasonable or am I over-analyzing the problem?

Howard
_ _ _ _ _

racehorse in the desert said:
I think that your test may be faulty if you are trying to compare
P-TTL vs. TTL flash, using a buit-in flash and an external flash.
There are too many variables with the two flash units.
However, on the D (with 16-45mm lens), at 3 meters I experienced
about 1/2 stop overexposure with the built-in flash at all zoom
lengths (18, 28, 45mm). At closer distances I experienced about
1/2 stop underexposure with the built-in flash at the same zoom
settings.
However, I did your test with a D and an AF360 and the exposures
turned out almost dead on center on the histograms in every case.
The results were near perfect and looked great.
Conclusion: I don't really trust the built-in flash and only use
it when necessary. I use the AF360 almost always when needing
flash.
Solution: Buy an external flash.

--
John Power
Racehorse in the Desert

'Life is too short to miss out on photography.'

racehorse in the desert · Mar 29, 2005

Howard said:
John,

I agree 100 percent about using a hotshoe or handle flash as much
as possible. I did the same test a few minutes ago and at 2.5
meters my *istD built-in flash with 16-45DA appeared to be about 1
stop under exposed at ISO200. What ISO did you use?

Also, as I posted in a reply to Tomas, I think the problem is the
light concentration of the pre-flash from the onboard flash
reflecting back off a light color wall and causing the built-in
flash to fire a lower intensity image flash. Does this sound
reasonable or am I over-analyzing the problem?

Howard

I think that you may well be right. I was at ISO 400, shooting against a wall with a large multi-colored map on it. Not super reflective though.
--
John Power
Racehorse in the Desert

'Life is too short to miss out on photography.'

Rick Ashford · Mar 29, 2005

Howard said:
I believe the problem is that P-TTL fires a pre-flash and when you
are that close the camera thinks the subject is brighter than it
really is due to the reflection back to the sensor from the
pre-flash. Because of that, the actual image flash is at a lower
intensity than needed to properly illuminate the subject.

I don't think this makes sense. As John Power said, a few posts previous to this one, he tried the "light wall" experiment with the AF360 flash and got perfectly exposed pictures every time. Obviously, the AF360 isn't fooled by light colored walls. So why would the DS onboard flash (also P-TTL) be "fooled" by the light reflected from a light-colored wall? I think this means the DS onboard flash P-TTL logic is not working right.

--
Rick A.
Johnson City, TN
http://www.photographyimpressions.com

tomaszmatys · Mar 29, 2005

I do no think this is the case here.
I used constant 5.6 aparture at all tests, so the lens should not slow down.

Moreover, the underexposure occurs when I am close to the wall, so the flash has enough power.

Godfrey said:
The built in flash simply runs out of power when you use the
DA18-55/3.5-5.6 at longer focal lengths: the lens is simply too
slow. I've not seen evidence of underexposure with any of my lenses
at normal "living room" distances and ISO 200-400 settings.

However, I'd suggest getting an external flash anyway. It's not the
TTL that allows the Pentax FZ500 flash to do the job right, it's
the fact that it has lots of power to work with. I'd recommend a
flash unit that allows use of all features (P-TTL, TTL, second
curtain sync, etc) ... so either the Pentax AF360 or the Sigma 500
DG Super would be the one.

Godfrey

tomaszmatys · Mar 29, 2005

Rick, I think you are perfectly right, there is obviously something wrong with the way the internal flash works.
I hope Pentax can do something with it in the next firmware update.
BTW, is there any way one could report such bugs to Pentax?

Now I know at least that it is a common problem and that the external P-TTL flash should work OK.

Thank you all for help, I've been reading this forum long since I decided to buy *istDS, you're all doing a great job here.

Tomasz

Rick Ashford said:
Howard said:

I believe the problem is that P-TTL fires a pre-flash and when you
are that close the camera thinks the subject is brighter than it
really is due to the reflection back to the sensor from the
pre-flash. Because of that, the actual image flash is at a lower
intensity than needed to properly illuminate the subject.

Click to expand...

I don't think this makes sense. As John Power said, a few posts
previous to this one, he tried the "light wall" experiment with the
AF360 flash and got perfectly exposed pictures every time.
Obviously, the AF360 isn't fooled by light colored walls. So why
would the DS onboard flash (also P-TTL) be "fooled" by the light
reflected from a light-colored wall? I think this means the DS
onboard flash P-TTL logic is not working right.

--
Rick A.
Johnson City, TN
http://www.photographyimpressions.com

jmacshriver · Mar 29, 2005

My wife and I have noticed that when we use the kit lens (18-55) and onboard flash shooting our bunny rabbits around the house. About half of the pictures has the back ground under exposed. But if we put on the FA 80-320, all of the pictures are properly exposed when using the flash. I haven't tried my Promaster 28-80 lens from my ZX-30 to see what it does.
John

tomaszmatys said:
Now I know at least that it is a common problem and that the
external P-TTL flash should work OK.

Thank you all for help, I've been reading this forum long since I
decided to buy *istDS, you're all doing a great job here.

Tomasz

Rick Ashford said:

Howard said:

I believe the problem is that P-TTL fires a pre-flash and when you
are that close the camera thinks the subject is brighter than it
really is due to the reflection back to the sensor from the
pre-flash. Because of that, the actual image flash is at a lower
intensity than needed to properly illuminate the subject.

Click to expand...

I don't think this makes sense. As John Power said, a few posts
previous to this one, he tried the "light wall" experiment with the
AF360 flash and got perfectly exposed pictures every time.
Obviously, the AF360 isn't fooled by light colored walls. So why
would the DS onboard flash (also P-TTL) be "fooled" by the light
reflected from a light-colored wall? I think this means the DS
onboard flash P-TTL logic is not working right.

--
Rick A.
Johnson City, TN
http://www.photographyimpressions.com

Click to expand...

NJDave · Mar 29, 2005

When I have my old M lense on the camera, I usually do a quick exposure to get an idea of where I'm at on the flash and then adjust from there. In two exposures, I can usually get something usable. Its not optimal but then again, its not too much different from what I would do with my K-1000.

When I hand the camera off to my wife, I just ensure that there's an autofocus/aperature lense on there.

Rick Ashford said:
JensR said:

I won't be able to do any tests, as with my M lenses, I cannot use
the internal flash as P-TTL.

One final idea: The manual mentions a shortest distance at which
the flash is fully functional. I guess this is not the problem
here, but might be worth remembering.

Good luck
Jens

Click to expand...

--
Rick A.
Johnson City, TN
http://www.photographyimpressions.com

*istDS underexposure with onboard flash

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