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02-11-2008, 09:32 AM   #1
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M42 and On-Board Flash

Hi... I'm a new K10D owner. Sorry if this is a repeat question... I am sure it is... when using search I get a zillion hits and many do not contain all of my search terms.

I've been playing with M42 lenses as I have quite a few, and overall am happy with what I've found. There may be exposure issues based on what focusing screen is used, but I really don't want to go there for purposes of this discussion.

My question is simple. How does one meter correctly, if at all, with on-board flash and M42. I have used the typical 1. set M-Mode and M-Focus (also use A-ring); 2. set aperture, 3. hit green button; 4. shoot.

The only thing I added was step zero... 0. pop up flash.

I first tried this with a f/3.5 135mm lens and a subject about 12 feet away. With the lens wide open, no problem. I then tried an f/1.8 55 and it was way over-exposed. Setting it to 3.5 of course cured that.

My question is simple. Is the process one of trial and error until you get the correct exposure, or am I missing something?

Thanks in advance.

Shane
woof!

02-11-2008, 11:26 AM   #2
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Thanks for posting this question....I've been wondering the same thing with my K100D super and a pentax 35mm screw-mount lens. (Couldn't find a response by way of search either.)

Without flash in good light, it exposes as it should, but with the flash turned on it overexposes to the point of almost white-out.

Is there a flash table somewhere that we should be referring to when using the built-in flash with a completely manual lens?
02-11-2008, 11:57 AM   #3
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Good question - I suspect that you can't do it. I've tried using exposure compensation and such but was never able to get a decent result with the onboard flash. With an external flash, it's fairly straightforward; just make sure the flash and the camera (in M mode) are set to the same ISO, aperture, and shutter speed.
02-11-2008, 12:05 PM   #4
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I haven't done it with onboard flash, but I would suspect that the best way to control the exposure is to stop the lens down, as you would with a fully manual flash attached to the hotshoe. If you check your manual there should be a guide number listed for the onboard flash.
You can search for a chart that gives the required Tv..Av.. and distance.

02-11-2008, 01:12 PM   #5
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With manual lenses the K10's onboard flash fires at full power every time. As suggested above you will have to figure out what the proper F-stop/exposure is for the flash at full power at the distance you are using the flash at, either by using a table (if you can find one) or by trial and error.
02-11-2008, 01:42 PM   #6
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Right. Then it is as I suspected. I was kind of kidding about trial and error, though frankly I do work often that way and let that experience inform later scenarios.

The suggestion of putting together the grid is a good one. Given guide number we can devise the start of an ISO/Distance/f-stop grid. It looks like the the flash has a guide number 15.6 (at ISO 200/m). It looks like the K100D is the same.

I believe I saw somewhere that 15.6 at 200 means a GN circa 11 at ISO 100. That could be helpful. I think that the formula is:

GN = f-stop * distance

So, testing that, I was getting good exposure at an unknown distance with an f/3.5... so...

11 = f/3.5 * 3.14 meters

That actually seems just about right. Very close. So to use f/1.8, the forumla says...

11= f/1.8 * 6.1 meters - I'll buy that.

Given this, we have the start of our grid. The next thing is to bring in a calculation for the guide number change when we double the ISO. Our spec has already told us the GN for 200 is 15.6. If that holds true in a linear fashion then I'd predict that it would be 15.6 * 1.4 for ISO 400. 21.8? That seem to fit because as I recall doubling ISO does NOT double GN.

After all that, I was able to look it up and the multiplier IS linear, using 1.44 as the multiplier each time you get:

ISO Factor (multiplier)

ISO 100 1
ISO 200 1.44
ISO 400 2.07
ISO 800 2.99
ISO 1600 4.30
ISO 3200 6.19

So working from that, the real starting point GN is 10.83 at ISO 100/m. From all of this we can construct a proper grid for the onboard flash, which I'll undertake tonight perhaps...

Thanks!

Shane
02-11-2008, 02:01 PM   #7
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QuoteOriginally posted by woof Quote
Right. Then it is as I suspected. I was kind of kidding about trial and error, though frankly I do work often that way and let that experience inform later scenarios.

The suggestion of putting together the grid is a good one. Given guide number we can devise the start of an ISO/Distance/f-stop grid. It looks like the the flash has a guide number 15.6 (at ISO 200/m). It looks like the K100D is the same.

I believe I saw somewhere that 15.6 at 200 means a GN circa 11 at ISO 100. That could be helpful. I think that the formula is:

GN = f-stop * distance

So, testing that, I was getting good exposure at an unknown distance with an f/3.5... so...

11 = f/3.5 * 3.14 meters

That actually seems just about right. Very close. So to use f/1.8, the forumla says...

11= f/1.8 * 6.1 meters - I'll buy that.

Given this, we have the start of our grid. The next thing is to bring in a calculation for the guide number change when we double the ISO. Our spec has already told us the GN for 200 is 15.6. If that holds true in a linear fashion then I'd predict that it would be 15.6 * 1.4 for ISO 400. 21.8? That seem to fit because as I recall doubling ISO does NOT double GN.

After all that, I was able to look it up and the multiplier IS linear, using 1.44 as the multiplier each time you get:

ISO Factor (multiplier)

ISO 100 1
ISO 200 1.44
ISO 400 2.07
ISO 800 2.99
ISO 1600 4.30
ISO 3200 6.19

So working from that, the real starting point GN is 10.83 at ISO 100/m. From all of this we can construct a proper grid for the onboard flash, which I'll undertake tonight perhaps...

Thanks!

Shane
Your ISO factors are a little off each doubling of ISO is 1 F stop. Guide number is multiplied by 1.414 (square root of 2) for each doubling of ISO, starting with 100, 200 and 400 are close should be 1, 1.414, 2) , 800 the multiplier should be 2.828, 1600 shoud be 4, 3200 should be 5.656
02-11-2008, 07:16 PM   #8
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QuoteOriginally posted by Lowell Goudge Quote
Your ISO factors are a little off each doubling of ISO is 1 F stop. Guide number is multiplied by 1.414 (square root of 2) for each doubling of ISO, starting with 100, 200 and 400 are close should be 1, 1.414, 2) , 800 the multiplier should be 2.828, 1600 shoud be 4, 3200 should be 5.656
Lowell... thank you very much. I took a look at my AF160Sa and used the "slide rule" on the back to come up with the following scheme... The math is based on the calculations discussed above as further modified by Lowell's data. I did not go beyond 12 meters because I actually am not sure the flash would be effective past that distance. Perhaps someone can advise on this. I will test more. I did take one long shot in the house that seems to indicate that it may work at least that far. Perhaps the limitation really is the aperture. If so that would be expressed as:

Max Distance = GN / max aperture

Using that formula, the max distance at 3200 ASA would be 44/1.4 = 31 meters? Wow.

I did validate this chart somewhat at 400 ASA, and was pleased to find that it does seem to work. Method for validation was simple. Same as for using chart... focus on subject, look at focus window to determine distance to subject, set f-stop based on ISO and distance. Shoot.

If anyone cares to work though this and validate it a bit more, please feel free to modify this work to suggest a better tweak. I will do the same. Might take a little while, and I will also see how this hold up with wider angle versus telephoto.

Please tell me if you cannot see the following image:



If you can't see it, cut and paste the URL:

farm3.static.flickr.com/2296/2261117056_74774c80da.jpg?v=0

Respectfully,

Shane


Last edited by woof; 02-12-2008 at 11:16 AM. Reason: Fixed missing image.
02-11-2008, 08:35 PM   #9
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I'm afraid that I can't see the table you posted, but I looked around a little more and PG.176 of the K10 manual has guide numbers for the onboard flash. I'll calculate out a table based on them and post it up so you can compare results.
02-11-2008, 08:41 PM   #10
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QuoteOriginally posted by Steinback Quote
I'm afraid that I can't see the table you posted, but I looked around a little more and PG.176 of the K10 manual has guide numbers for the onboard flash. I'll calculate out a table based on them and post it up so you can compare results.
I'm working on that. Don't know why this happens to me. I will PM table URL to you. Try cutting and pasting this URL:

lh3.google.com/noonecantellimadog/R7EEiKHV-oI/AAAAAAAAAMk/LG7mZ7tQDaE/K10D-Flash.jpg
02-11-2008, 09:10 PM   #11
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Cutting and pasting the URL worked fine. We used slightly different distance numbers, but rounded to the nearest F-stop we got the same values. A few quick shots around the house in dark rooms gave me shots that were exposed to within a half stop or better of where they should be at 5 meters, which is certainly usable. I'm going to try a few longer distance shots and see how they turn out.
02-11-2008, 09:46 PM   #12
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I uploaded the table I came up with to my flickr account, but like I said our values are pretty much the same. I'll play around with my camera a little more to see how well the numbers match up at different distances, but I'll probably print off and laminate a copy of my original values rounded to the nearest decimal point to keep in my camera bag in case I need it. Next project - make a table for my Vivitar 285 covering up to 1600 ISO : )

K10d Flash Tables_beta on Flickr - Photo Sharing!

(hopefully that link works)

Oh, and if you're looking to soften the flash up a bit for closer images or just to use it for fill light, cutting a translucent white film canister to fit over the on board flash works as a (very) cheap diffuser, with decent results. Saw the trick on another forum, I've been using it for close range fill inside of cars.

Last edited by Steinback; 02-11-2008 at 09:55 PM.
02-12-2008, 06:48 AM   #13
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QuoteOriginally posted by Steinback Quote
Cutting and pasting the URL worked fine. We used slightly different distance numbers, but rounded to the nearest F-stop we got the same values. A few quick shots around the house in dark rooms gave me shots that were exposed to within a half stop or better of where they should be at 5 meters, which is certainly usable. I'm going to try a few longer distance shots and see how they turn out.
Your link works fine.

I did in fact round all my numbers to "whole" stops. I tried to stay within the normal stops as they are usually expressed in these types of charts. I noted a couple of places where the rounding was very significant, ie: where 3.5 was much closer than 4.0. It seems to work very well indeed.

I walked around the house taking pictures and had the same experience as you... everything seemed to be as at least within a half stop or better, and using the click between 2.8 and 4.0 where noted worked very well. As a rough guide this works very nicely.

Steinback, would you mind posting your results with the Vivitar... I have one too, and do not want to duplicate effort. I tend to use it with a flash meter, but it would be nice to have a card with the guide on it as well.

Thanks...

Shane
02-12-2008, 07:13 AM   #14
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I have made a flash calculator, which I use in my PDA, for all flash calculations.

It generates a table of maximum distances based upon ISO setting, flash output, and F Stop.

all the formulas are readily available, and I find this useful to have. Since I travel everywhere with my PDA, it is not like taking a PC along, and there is no need to have multiple tables as cheat sheets.

I also have a lens magnification and focus distance calculator for using extension tubes. It is quite useful to have the calculations easily available when travelling
02-12-2008, 04:27 PM   #15
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Here are links to the guide numbers for the 285hv from the flash manual (plus a bit of extrapolation, so there is a little bit of rounding error) and the flash table I put together for 28/35/50/105mm head zoom. Like the table for the onboard flash, the exposures seem to be accurate within a half stop or so at 4.5m in a dark room.

If you want the original excel file to update or edit, just send me a PM here on the forum with an email address. If you find problems with the tables, let me know and I'll post updated versions.

I realize that some of the F-stop values are unrealistic, but I used the same formulas for the whole table and just rounded the values to the nearest decimal so people can make their own call on the correct stop or partial stop to use.

Vivitar 285HV Aperature Tables on Flickr - Photo Sharing!

Vivitar 285HV Guide Numbers on Flickr - Photo Sharing!
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