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08-02-2009, 05:50 AM   #1
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Built-In Flash in Manual Mode

Just returned to the Pentax SLR world after years in the Point & Shoot weeds. My new K200D arrived less than a week ago, and one of the first things to do was figure how to make use of my old M50 1.4 lens. This wasn't too hard with the help of some posts from this forum: Enable aperture ring other than A, Manual mode, focus, green button, and I'm good.

Until I tried to add the built-in flash. When popped up the flash fires properly, but since the metering did not expect the added light my shot is completely washed out, no matter how much I try to compensate.

Should I stop wasting my time and just revert to the more-than acceptable kit lens when needing flash? Or is there a way combine my nice old lens with the built-in flash?

08-02-2009, 09:25 AM   #2
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Built-in Flash in Manual Mode

I believe that the only way to use the built-in flash with such a lens is to treat the flash as completely manual. When a manual lens is attached, the flash fires at full power every time.

You must do it old school-style. Look up the guide number in the manual. Focus the lens and read the distance off the focus ring. Divide the guide number by the distance, giving the proper aperture.

With an auto lens, such as the kit lens, the flash fires a low-power preflash to "meter" the scene. Then, a fraction of a second later, it sets the aperture, trips the shutter and fires flash for a variable length of time to achieve the proper exposure. None of that is possible with an M-series lens.
08-03-2009, 09:37 AM   #3
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QuoteOriginally posted by noblepa Quote
Look up the guide number in the manual.
Thanks for the reply. But I'm not familiar with the guide number and/or the manual. Not seeing anything in the K200D's supplied manual.

Could you direct me to where this could be found?
08-03-2009, 11:11 AM   #4
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Google. Guide numbers are a very old concept in photography; any book or article on flash should cover how to use them. You just need to know the specific guide number for your specific flash, and that info is almost definitely in the manual, probably in the back in a chart of technical specifications.

08-03-2009, 11:49 AM   #5
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According to the Pentax specs, the GN for the k200d built-in flash is 13:
K200D Digital SLR - Official PENTAX Imaging Web Site
08-03-2009, 11:58 AM   #6
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Guide numbers

When dealing with guide numbers, be aware that they are stated in either feet or meters.

For instance, I believe that the GN of 13 that PentHassyKon refers to is 13 in meters. Multiply by 3.3 to get the value in feet, 43. If you're used to working in feet, that's the number you want to use.

The basic equation is: GN = aperture x distance.

Solving for aperture, it becomes: aperture = GN / distance

In any equation involving distances, the units must match, so if you are an American and are used to feet, use 43 feet. If you're somewhere in the rest of the world, use 13 meters.

aperture = 43 feet / X feet = 43 / X
08-03-2009, 12:21 PM   #7
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Perfect, this is the information I needed.

The manual lens won't be used everyday, so having to do the math on occasion shouldn't be too much trouble - I hope!
08-03-2009, 12:37 PM   #8
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This is cool stuff. I wish I had known this in my manual slr days. This is the reason I love this forum.

04-21-2010, 07:01 PM   #9
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Glad I found this thread, I was about to ask the same question like the OP. Having skipped the film days and growing up with a point'n'shoot I couldn't figure out how to use my old Pentax M 50mm F1.4 on the K-x with the built in flash.

This should help. I'll have to give it a try tonight.
04-22-2010, 01:09 AM   #10
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There is also a trick to use full P-TTL with manual lenses.
It involves drilling a second locking hole into the lens so the aperture is free to move (easy to do!), and shorting the recessed A pin of the camera mount.
This way, the camera will be tricked into thinking it has an A lens. You just have to select the greatest aperture possible on the camera (regardless of what you actually use on the lens!).
04-22-2010, 11:33 AM   #11
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QuoteOriginally posted by dlacouture Quote
There is also a trick to use full P-TTL with manual lenses.
It involves drilling a second locking hole into the lens so the aperture is free to move (easy to do!), and shorting the recessed A pin of the camera mount.
This way, the camera will be tricked into thinking it has an A lens. You just have to select the greatest aperture possible on the camera (regardless of what you actually use on the lens!).
This trick may get P-TTL working, but will disable stop-down metering in the M mode with the green button. As a result, the lens can only work with aperture wide open when flash is not used.
04-22-2010, 11:45 AM   #12
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The last time I did this was 35 years ago.

So if you're shooting full manual, does this formula only work at the cam's fastest sync speed?
04-22-2010, 01:45 PM   #13
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QuoteOriginally posted by xjx Quote
This trick may get P-TTL working, but will disable stop-down metering in the M mode with the green button. As a result, the lens can only work with aperture wide open when flash is not used.
Well, you're wrong (and I already played quite a lot with this trick, believe me!) : as you're already working in stop-down mode, it will still work.
The camera will adjust the shutter speed accordingly to the actual lens aperture and current ISO. Just like a wide open A lens would do... You just have to select the widest aperture available on the camera, whatever actual aperture you choose on the lens. Moving the aperture ring will physically change the aperture, and so will change the amount of light reaching the metering cell. Using the widest 'virtual' aperture available will ensure that the camera does not compute a different Shutter speed than what is needed.

And you will gain access to Av mode in the process, too! Just like with m42 lenses.

Only drawback comes when you shoot at small apertures, as the viewfinder will become very dark!

This is so easy it's a wonder Pentax didn't include this in a firmware update...
The algorithm is pretty simple, in fact, and could be enabled/disabled in an additional custom function...

If the A pin is not shorted (meaning you have either a PK, PK-m or m42 lens):
- then the aperture actuator should be in the "no action" position, so the lens will react as a preset lens (like m42 lenses, where the aperture ring is linker to the actual aperture).
- DoF preview (or AF button) could "open up" the lens, for ease of focusing (would only work with PK lenses, not m42, for obvious reasons).
- the P-TTL metering burst will occur with the lens already stopped down, so the reading will be accurate (just try the above trick and you'll see by yourself!).

This would add tremendous value (and perfect backward compatibility) to old lenses, so we cannot really hope for Pentax to include this in their firmwares... This would undercut new lenses sales quite a bit, IMO.

Last edited by dlacouture; 04-22-2010 at 01:52 PM.
04-22-2010, 01:53 PM   #14
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QuoteOriginally posted by Ira Quote
The last time I did this was 35 years ago.

So if you're shooting full manual, does this formula only work at the cam's fastest sync speed?
No, when you're shooting with the flash, shutter speed is largely irrelevant. Only when you get down to shutter speeds so slow that ambient light has a significant effect on the exposure would you need to consider adjusting accordingly (eg, lowering ISO or stopping down a bit). Normally, though, when shooting flash, you're shooting a little stopped down and at relatively low ISO, so any shutter speed faster than half a second or so might as well be 1/180" or even 1/4000" - ambient light is not going to affect the exposure much.

Last edited by Marc Sabatella; 04-23-2010 at 08:12 PM.
04-22-2010, 02:49 PM   #15
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Your "ISO ISO" was meant to be read "Low ISO, right? If so, I understand.

I played with this today and actually got good exposures, so inside at 100 and stopped to 8, it didn't matter if I was 180 or 60.

Thanks for explaining it. It's all kind of coming back to me.

I
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