Pentax *istD underexposed with AF-360FGZ


Link Posted 01/12/2003 - 05:16
I am using the af-360fgz flash with my istD and every picture I take is coming out under exposed by at least 1 stop. My normal use is P-TTL flash mode, Program(and Green)mode, iso200, raw and fine jpg. The environments are generally room-size (5-40 foot depth of field) using 28-50mm focal length on a SMC Pentax 28-80 FA AL. I was under the impression that this flash was well suited to this camera. Any suggestions about possible causes would be most helpful.

Don Schoolmaster


Link Posted 01/12/2003 - 07:39
Hi Don Schoolmaster,

I too have the *istD and AF360 and general get beautiful results.

However, occasionally, I have had underexposed shots. I have been trying to pin it down, and I think I may have had the flash mode (on the *istD) set to wireless, rather than automatic/redeye/always (ie the little remote control icon appears on the *istD top LCD).

I would suggest setting up a little test - a still-life about 6ft away - and go through the flash and camera modes. If that still doesn't work, try the same setup with the built in flash alone - double check the internal flash is correctly exposed.

In the meantime, I will see if I can get the same results as you in some way to add anything to this.

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Link Posted 04/12/2003 - 06:12
When testing with the built-in flash see if there is a difference between the 1st and subsequent shots.
I have had an occasional problem with the *ist D when taking a series of flash shots. The first is OK. Immediate and subsequent shots are underexposed as you describe. I think I have pinned this down to low battery power and a slow battery recovery. The camera thinks all is OK for the subsequent shots but the battery needs a few more seconds to fully recover before the shutter button is pressed. This all relates to the built-in flash so it might not be fully relevant. Under the Custom functions, "Release when charging" is "Off" so the camera should not work until the flash is fully powered and ready.
I would be interested to know if this is a general problem or specific to my camera.


Link Posted 04/12/2003 - 11:16
I've been trying to get the AF360 to be underexposed...

If the power switch is on the middle position, and the wireless mode (the small lower switch) is set to "C", then it will always be underexposed.

If the contacts between the flash and the *istD are not clean enough, there is a possibility of under/over exposure.

If the mode on the AF360 is not at P-TTL then is could be improperly exposed. Conversely, when in P-TTL, the flash can set the proportional power, so it should be at 0.0 to be properly exposed. (This is very useful indeed to control the flash amount and balance with ambient light.)

The metering mode also affects the flash - in spot metering, the flash will meter only for the centre, and could adversely affect the rest of the image (unless that's what you want ).

In my case, I have had a few situations where things didn't work out as expected. When I tried to work out the cause, it disappeared. If I can pin it down to anything in particular, I will, of course, let everyone (including Pentax) know

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Link Posted 07/12/2003 - 16:48
Thanks for the response about this problem. I have been trying to reproduce and experiment with this situation (just had a new grandson born so time is limited right now ). Like any good scientist I figured the best thing to do is sit back and look back at any changes that have been made recently. I don't recall the exact date that I started using the Battery grip, but it has not been that long ago. I mainly use it for better grip stability (making the camera body slightly larger). I noticed that I had the grip attached without any batteries loaded. I have a fairly new set of Lithium batteries in the camera and saw no need to add the grip batteries until these ran down. I was just able to conduct a test. I shot the same medium light scene without flash, with flash, without flash and the empty grip, with flash and the empty grip. Each shot with the empty grip was underexposed by around one stop as compared to the same shot without. These tests were conducted using the integrated flash, not the af360. I wil be conducting further tests to try to assign and relate all variables, but it looks like you cannot use the grip without the additional batteries in place.

BTW, I have also notices the subsequent shot syndrome as described in a previous answer. Thanks for the help with this.

Don Schoolmaster


Link Posted 18/12/2003 - 15:24
Well, after much fussing and fuming I hope I have this problem figured out. It turns out that the battery grip was not at fault. What was happening was inconsistent behavior of the lens (SMCP 28-80 Pentax). At times it will work fine, at others it goes way underexposed. I changed to a fixed 50mm f1.2, and a 100mm Macro and every shot comes out with perfect lighting. I had heard that this combination (*istD + AF360) was unbeatable. I guess I will be sending the lens in for repair.

Thanks again to all for feedback about this...HAPPY HOLIDAYS

Don Schoolmaster


Link Posted 19/12/2003 - 09:24

Thanks for the information. I have been trying to work out what goes on with the flash too, and I do remember having wireless probs with a Tokina 20-35mm - I took a few shots all underexposed (but corrected them in PS).

Armed with that knowledge, I'll go back and retry all lense/flash combinations to try and sort this thing out once and for all

You never know, it may not be the lense, but firmware. Hang on in there...


PS. If you haven't tried Photoshop CS - it's superb There are some reallly, really nice features for photo editing. Let me know if anyone wants a quick review - we have the full Creative Suite CS Premium here, such a killer package
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Link Posted 19/12/2003 - 13:31
Thanks for all the help Matt. I was able to find time to take several setup shots yesterday evening. I left the flash in PTTL mode, using two different lens systems (50mm f1.2, 100mm Macro f2.. These were shot in a combination of finest jpg and raw. All shots came out perfect (with respect to lighting, not my talent ). I then went back to the 28-80, shot about six frames at 50mm. Two of them came out just fine, the rest were underexposed, badly. I want to find time to test the 70-300 f4-5.6 and the 80-200 f4-5.6 that I have here. It's hard to setup an indoor shot with these long lens systems and the weather has not been good. I am of a mind to go back to all fixed focal length lenses and scrap the zooms. I do need to decide whether to send this 28-80 back to pentax for a look.

I have been using PS CS since it came out. This is really one of my favorite upgrades of this product. They finally started concentrating on the photographic aspects again and not all this web-based stuff. The camera raw is wonderful! I hope that the *istD is added to the list of supported cameras soon.

Again, Thanks for all the help -- Happy Holidays to All

Don Schoolmaster


Link Posted 20/12/2003 - 16:19

I've spent a little time this afternoon trying to test out the AF360... I have three Tokina zooms: AT-X 24-200, AT-X 28-70/f2.8, and 20-35mm (not ATX)

I tried to take the same scene each time, using P-TTL, Auto sensitivity, ISO800 (starting at), and 35mm on each lense. I found the 28-70/f2.8 produced underexposed shots (nearly 1 stop), while the 20-35mm produced the brightest. I tried this in Depth, MTF, and Normal program lines - each the same. However, the 20-35mm did only use f5.6 for depth (which is weird) while the other two used f10 and f11 (I haven't double checked the ISO choice for each shot, since it was on auto sensitivity - it's difficult with my lively kids running around )

Going back to non-flash, the 28-70/f2.8 produced the brighter image!

Soooo confusing! And I am sure it is not down to the Tokina lenses per-se. Maybe the *istD is reading something from the lense and making other judgments I am unaware of.

In addition, I did find out that putting the diffuser on and trying 20mm often produced badly underexposed images. I need to try this with and without auto-ISO, and with different subjects.

This is a really knotty problem, and I am going to have to set up a controlled experiment with a static subject.

I am not sure that it is down to any particular lense problem, but more of a lense-to-body-communication and algorithm issue. When I had the problem with the 20mm, AF360 wireless I tried to repeat the experiment the next day and all was fine! It may even involve the subject - perhaps the contrast ratio or maybe even lense flare of some kind.

Still, it's a good challenge

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Malcolm Riches

Link Posted 20/12/2003 - 16:34
Can I suggest testing the lenses with "Green Everything" - especially ISO 200. I have found that this is a good starting point and gives the best method of exposure comparison. As posted in the Lens Forum I have found that anything other than ISO 200 with my Pentax A 20mm f/2.8 causes consistent overexposure by one stop. However, having set the ISO at 200 as the default, Automatic Sensitivity works OK when needed.

Also, ensure that there are no reflections of the flash in any part of the subject. I took some test shots, using flash, of my bookcase (I'm nuts?) and the reflection of the flash in just a couple of the glossy spines of some books caused serious underexposure.


Link Posted 21/12/2003 - 22:09
(In the meantime...)

Here is an interesting link regarding the AF360 and *istD combination:
(focal length appeared wrong on flash, manually set zoom and exposure okay)

Something else to check...

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Link Posted 22/12/2003 - 16:05
Don & Malcolm,

I've managed to rattle off a few test shots using my three Tokina zooms, the AF360FGZ, and comparing in PS-CS (live histogram has been useful here)...

First interesting thing: Pentax Photo Browser reports the three lenses as follows:
A) Tokina 20-35mm = "smc PENTAX-F 35-70mmF3.5-4.5"
B) Tokina 28-70/f2.8 = "smc PENTAX-F 28-30mmF3.5-4.5"
C) Tokina 24-200/f3.5 = "-- -- --"
Now, the lense ID (I suspect) is probably just an ID number, and I don't think has too much to do with the issue.

Second big thing when comparing images is to force the White Balance. When you have AWB, the different colour characteristics of the lenses changes the way the *istD compensates for white balance. When using AWB I was getting lense C as the brightest, A as the next, and B as the darkest. However, after testing with Flash WB lenses A and C produced equivalent images (with slight colour shifts), while B was a good stop darker.

Now, I tested the following flash combinations:
(1) Straight flash, (2) Manual zoom flash, (3) Diffused, (4) Auto sensitivity and auto flash zoom. Note, 1-3 were on P-mode, ISO200, non auto sensitivity.

Why, oh why, the AF360FGZ shows the zoom size 1.5x shorter that the real zoom, I don't know (it shows 13mm when the 35mm format is 20mm - it should have shown 30mm, or just 20mm, on the AF360)! Since the EXIF information and the zoom markings are all in 35mm format, why didn't they stick to that on the AF360?? However, judging by the noises the zoom makes, it appears that this is only an incorrect display, rather than an incorrect zoom.

All sets (barring the AWB issue mentioned) produced consistent results. Lenses A and C produced good balanced images, and B was underexposed.

Now, it's interesting to note that Malcolm had the problem with a Pentax 28-80, and my lense is ID'd as a Pentax 28-80

Just to throw a real spanner in the works, when taking non-flash images (with white balance on sunny), lense B produced consistently brighter images! Lenses A and C had seriously long "tails" on their histogram - stretching about 1/3 across the histogram. (The order, from bright to dark was B, C, A, with C about 1/3 stop brighter than A, and B about 1 stop brighter than B)

Exposure information:
Available light: A=1"/f4.5 (EV4.34), B=0.7"/f2.8 (EV3.49), C=1"/f4.0 (EV4.00)
Flash: A=45/f4.5, B=45/f3.5, C=45/f4.0

Why the same scene can produce such a wide range of available-light exposures is a mystery

So, it looks like there's something a little strange going on. Whether it is down to colour shifts in the lense, or (more likely I think) a firmware issue, let's hope we and Pentax can get this sorted out soon

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Link Posted 24/12/2003 - 12:55
Hi All!

Ok, now I am more confused. I decided to go back and try a genuine Pentax-FA 28-70mm F4 and do some more comparisons and tests.

Here's what I did: take the *istD with lense on a tripod looking at an evenly lit magnolia wall. Set focus to manual and infinity. Try different ISO settings at each zoom setting of 28, 35, 50, 70. White balance set at neutral white fluorescent, and pattern metering.

The first set yielded half stop variations at the same ISO setting between each zoom. Ah, you say, but the shutter speed shown in the EXIF is not necessarily accurate. True, the EXIF only shows the nearest "standard" shutter speed. BUT, Photoshop shows histogram shifts - ISO200 not bad, ISO400 has a shift at 70mm, ISO800 shows 28&35 shifted down from 50&70, ISO1600 shows more shifts.

After resetting the camera (using the menu function), the results were identical, although the Av/Tv combinations used were slightly different, and seemed more consistent. Having laid out a composite image of zoom vs ISO with two layers (one before reset, one after) showed no significant differences.

To summarise:
It appears that genuine lenses have issues too - so it points to the body.
The differences vary greatly between different lenses and different zooms (different lenses have very different colour responses - some are more sensitive to low level light etc). Here are the results:

I am positive that this can be resolved... I would just like to pin it down. It would be interesting to know more about how the exposure is read - presumably in a similar way to the MZ-S with a separate sensor - and what the interplay is between the zoom, f-stop, lense ID, ISO and other settings.

Any comments or suggestions would be greatly appreciated

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Malcolm Riches

Link Posted 24/12/2003 - 13:51
On the other hand, we may have a random problem with the camera.

Amateur Photographer's test of the camera (04/10/03) concluded "... the camera will provide an underexposed image from time to time, for no apparent reason. It may be only one image in 20 or 30 that's affected by this underexposure, but there's no way of telling if or when it will happen, so no obvious way of preventing it."

I wonder if Pentax read the review and have a comment to make on such a publicised fault?

My own tests have been very confusing and consistently inconsistent!

At the moment I'm bracketting all important shots to guarantee a correct exposure. This should not be necessary!


Link Posted 24/12/2003 - 14:43
Thanks for that Malcolm!

In my tests, when you have AWB off, the results are much more predictable... in fact, I did the Pentax-FA 28-70mm f4 test twice with exactly the same results (once with multi-segment, once with centre weighted).

However, since the problem appears to be zoom related, as well as lense related, as well as ISO related, and the AWB compounds the problem, in normal use it would appear a 'random' event

My efforts were to isolate the Tokina lenses I have from the equation, and to at least pin it down somewhere.

The flash issue is probably partly due to the no-flash exposure issue. If the *istD is not getting the normal exposure right, how can we expect it to get the flash setting right? From what I've seen, if the ambient conditions produce an underexposed image, then the flash will be over-exposed. Conversely, if the ambient conditions produce an overexposed image (in relation to the other images), then the flash will be correspondingly under exposed.... At least that's what I think

(I'm glad I am not the only firmware coder that has his work cut out )

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