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07-04-2012, 01:32 AM   #16
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QuoteOriginally posted by JimJohnson Quote
dlacouture - I'm waiting for a friend to find his old K1000 kit and lend me a couple Pentax-M lens. It is not that I doubt what you say, but you aren't providing specifics as to the level or consistency of overexposure. I will run some tests and report here.
Well, I simply cannot provide such exact figures! They are lens and aperture dependent!
And as I said : on some lenses you won't notice anything (my Porst 55/1.2 was perfect until you hit f/16, where it started overexposing a little), on others you'll have overexposure right from the start as the camera won't close the lens sufficiently.

And to counter my own proposal : stop-down metering has its own limitations, and hassle-free use can only be obtained by swapping the focus screen with a Canon EE-S screen (labeled "S-Type" over at FocusingScreen), the only perfectly linear screen I know.
Other screens will all induce metering variations depending on the selected aperture.

07-04-2012, 10:18 AM   #17
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QuoteOriginally posted by dlacouture Quote
Well, I simply cannot provide such exact figures! They are lens and aperture dependent!
And as I said : on some lenses you won't notice anything (my Porst 55/1.2 was perfect until you hit f/16, where it started overexposing a little), on others you'll have overexposure right from the start as the camera won't close the lens sufficiently.

And to counter my own proposal : stop-down metering has its own limitations, and hassle-free use can only be obtained by swapping the focus screen with a Canon EE-S screen (labeled "S-Type" over at FocusingScreen), the only perfectly linear screen I know.
Other screens will all induce metering variations depending on the selected aperture.
So do you suggest replacing the original kx focusing screem with S-type? I heard that Nikon K-3 is one of the best focusing screen ?
07-04-2012, 01:09 PM   #18
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QuoteOriginally posted by baby007 Quote
So do you suggest replacing the original kx focusing screem with S-type? I heard that Nikon K-3 is one of the best focusing screen ?
Well, if you're mainly using fast manual lenses and/or stop-down lenses (m42 or PK-m), I would say there is no better screen than the EE-S...
With fast lenses (f/1.4-2), manual focus is IMO actually easier than with a split screen.

And with stop-down lenses, metering is perfectly linear, meaning that stopping down the lens by one stop results in a one-stop metering difference (anyone that ever tried stop-down metering with the stock screen will appreciate this!).
07-05-2012, 08:51 AM   #19
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The kit was found. I'll have the lenses next week and run some tests.

My theory (if others haven't guessed) is that the amount of overexposure at mid-range stops (typical stops used for flash photography) will be both consistent (within a half-stop) and manageable within the camera's exposure compensation controls. I will post some test shots here with no post processing other than size reduction. I tend to shoot RAW+jpeg, and will use the jpeg's as created by my K-r body. I also intend to use the K-r's built-in flash -- although I may also use my Sigma EF610 DG Super in P-TTL mode mounted to the hotshoe.

Any other parameters I should consider when testing?

07-10-2012, 06:35 PM   #20
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Just so you don't think I've forgotten...
I received the borrowed M series lens only a few minutes ago. I ran a quick series of shots between my Pentax - A 50mm/1.7 and the borrowed Pentax - M 50mm/1.2. I used my K-r and its built-in flash in P-TTL mode. The A pin is shorted on my camera. The A-series lens already has f/stop contact insulators built-in. As it turns out, the M-series lens has an aperture range of 1.2 - 22 and needs no tape insulators installed. In both cases the camera e-dial recognized the same range of f/stops as the lens.

My subject was my office coffee pot and canisters at a distance of 5 feet/1.5 meters. I used a fixed ISO of 400, the shutter was at 1/180 and I shot at 4, 5.6, 8, 11, 16. The M-series lens was set to match the f/stop indicated on the camera body. The A-series lens was set the same as the M-series lens, and then set to the 'A' setting. In other words, I took 3 shots at each aperture. I shot in jpeg+RAW.

This is a report of my preliminary results based solely on a visual inspection of the jpeg images on my reasonably well tuned 22" LCD display. I will follow this with a post of more objective results and photos.

I will consider the A-series lens set at the 'A' position to be the correct exposure. That said, exposure was not consistent across the aperture range. It was correct +/- between f/5.6 and f/11. It was slightly over at f/4 and nearly a half-stop under at f/16. Comparing the 'A' position to the two other shots:

The M-series lens seemed to be about 2 stops overexposed at f/4, about 0.5 to 1-stop over at f/5.6, almost dead on at f/8 and f/11, and 0.5 stop under at f/16. Assuming the over/under at a specific f/stop is consistent between shots (to be tested), this is well within the exposure compensation capabilities of the K-r body. The lens would be usable with only a couple on-site test shots ... and with experience, the photographer would likely know how much compensation was needed for a given f/stop without doing any test shots - - any final tweak of exposure could easily be done in post processing.

The A-series lens surprised me. Again, using the 'A' position on the lens as the 'correct' exposure, I expected using a manual aperture on the lens matching that set on the camera body (exactly how I used the M-series lens) to be very close to the same exposure as the 'A' setting. Not so. I saw the same pattern of over to under exposure as on the M-series lens - only to far less extent. For example, at f/4 the overexposure was about 0.75 stops.

While I am far from finished testing, my preliminary findings seem to support my theory that with just a little experimentation, a photographer could take good P-TTL flash photographs with an M-series lens by fooling the camera body into thinking a 'A' series lens is attached. At least with the Pentax-M 50mm/1.2 lens I have at hand, I wouldn't hesitate to just set the aperture to f/8 and shoot away.

Stay tuned.... it will be a few days before my next report.
07-11-2012, 07:30 PM   #21
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The attached .pdf file shows a comparison of the Pentax-M 50mm/1.2 and Pentax-A 50mm/1.7. Let me begin by apologizing for the fuzzy rendition caused by conversion from MS Word to pdf. The originals are in focus.

The subject was my favorite office accessory - my coffee pot and related items. The wall behind the coffee pot is a light sand color and the door trim to the right side is white. All the photos are screen shots of the DNG files as displayed within Adobe Lightroom 4.1 with no adjustments. I did screen shots so you could also compare the histograms.

My Pentax K-r was modified to recognized each lens as an 'A' series lens, which is required in order to do P-TTL flash. M-series lenses should be modified to insulate the appropriate aperture contact pins on the camera body. My camera body correctly recognized the M lens as having an aperture range from f/1.2 to f/22, and the A lens as having an aperture range from f/1.7 to f/22. Aperture priority mode was used, the built-in flash set the shutter speed at 1/180, and the ISO was fixed at 400. Subject distance was approximately 5 feet or 1.5 meters.

The first 5 pages of the document compares each lens at f/4, 5.6, 8, 11 and 16. First the M lens with the same aperture set on the K-r's LCD display and on the lens aperture ring., followed by the A lens with identical settings, and finally the same aperture on the K-r body, but the lens in the 'A' position.

The last page shows the M lens again using f/4. The first shot is with exposure compensation at zero, the second shot is with exposure compensation at -2 stops.

In my previous post, I had only examined the jpeg images as produced within my camera. Subsequent examination of the RAW (DNG) images shows less exposure variability.

My conclusion:
Yes, a M series lens turned into a fictitious A series lens will overexpose at the wider apertures (but should not when wide open). However three points need to be made. First the overexposure declines in a linear fashion as you approach the mid-apertures. Second, the overexposure is within the camera's exposure compensation control range. Third, the overexposure can be corrected in post processing, although there may be a slight loss of highlights. In short, so long as the photographer takes a test shot or two and uses exposure compensation as indicated by those shots, there is no reason a M series lens cannot be used with great success in P-TTL flash photography. And even if there is no time to do test shots, try for a mid-range f/stop and plan on doing some post processing.

Finally, if at all possible, shoot in RAW or jpeg+RAW.
Attached Images
File Type: pdf Pentax M versus A using P-TTL.pdf (246.2 KB, 310 views)
07-11-2012, 08:28 PM   #22
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Jim. Your tests are similar to what I tried converting a Ricoh 50mm F2 lens to an a lens. What I fund was the following. The k series lenses, due to their aperture mechanism only gave correct exposure at minimum and maximum aperture, with an increasing over exposure towards the mid aperture. This is due to the difference in mechanism and the camera interfering in the mid range in blocking the lens from stopping down to the aperture ring setting. The reason the A lens mdid not behave well on your camera with the A pin shorted is even though the mechanism is correct in movement, the camera has compensation on exposure due to the non linear behavior of the metering, as a function of aperture .
07-12-2012, 06:12 PM   #23
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I'm having trouble understanding your explanation Lowell. With the A pin shorted the camera should have no idea whether the lens is in the A position or on a manually set f/stop. If the body and the lens have the same f/stop using aperture priority, how would the camera know to provide any kind of different exposure compensation?

Looking at the actual exposures, my guess is the lens' manual f/stop settings are simply more accurate than the camera's ability to know how far to move the aperture lever to reach a specific f/stop. The 'A' setting shots, at least to me, seem to be slightly underexposed.


Last edited by JimJohnson; 07-12-2012 at 06:18 PM.
07-13-2012, 05:36 AM   #24
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QuoteOriginally posted by JimJohnson Quote
I'm having trouble understanding your explanation Lowell. With the A pin shorted the camera should have no idea whether the lens is in the A position or on a manually set f/stop. If the body and the lens have the same f/stop using aperture priority, how would the camera know to provide any kind of different exposure compensation?

Looking at the actual exposures, my guess is the lens' manual f/stop settings are simply more accurate than the camera's ability to know how far to move the aperture lever to reach a specific f/stop. The 'A' setting shots, at least to me, seem to be slightly underexposed.
Jim, My bad, I was thinking K lens with a pin shorted. What i found in this case was max and min aperture correct, but in the middle it over exposed due to the difference (limiting by the camera) of aperture before the mechanical stop of the lens. with an A lens as you state, clearly it should be correct across the whole range regardless.

As for the way in which an M ( or K ) lens over exposes, i found it to be quite non linear. However it depends on exactly how you are measuring exposure. I do not use a scene, but a uniformly lit block wall, and measure exposure by greyscale value. It is quite accurate, and shows the exposure increasing from wide open to about the 1/2 way point on the aperture range, and then decreasing. it is relitively uniform at about +1 1/2 stops between the middle 3 stops on the aperture scale, so one could propose to set EV comp to -.5 or -.7 and as you suggest fix the errors in post. Its a hard call to make, with the obvious benefit that you do get some measure of automation, plus P-TTL flash.

Flash is to me the more important thing. I don;t really understand why they cut TTL support after the *istD series.

Last edited by Lowell Goudge; 07-13-2012 at 05:42 AM.
07-13-2012, 06:13 AM   #25
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Yeah, based on other posts I expected the overexposure issue on the M series lens ... fortunately the exposure seems to be consistent at each manual stop, and the worst overexposure well within the camera's exposure compensation controls. It wouldn't take much to write up a little cheat sheet. And at least with the lens I used for testing, the overexposure was worst at wider apertures and disappeared between f/8 & 11.

It was the variation between manual and 'A' setting on the A series lens that surprised me. When using aperture priority metering it shouldn't matter whether I used the 'A' setting on the lens or the same f/stop I have set on the camera body. The exposure should be identical. But it's not. And as I said, it looks to me like manually setting the aperture on the lens gives more accurate exposure than letting the camera body alone do the work. And if that's the case, what does it say for my DA lenses as they are effectively always in the 'A' position?

The other surprise was the greater variability in exposure when I looked at the jpeg images versus the DNG images (I shot jpeg + RAW). I've read several complaints about P-TTL exposure inconsistency here in this Forum. It makes me suspect that many of those complaints may have been based on how the camera renders jpegs. Time for some more firmware tweaks Pentax?
07-13-2012, 01:51 PM   #26
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QuoteOriginally posted by JimJohnson Quote
Yeah, based on other posts I expected the overexposure issue on the M series lens ... fortunately the exposure seems to be consistent at each manual stop, and the worst overexposure well within the camera's exposure compensation controls. It wouldn't take much to write up a little cheat sheet. And at least with the lens I used for testing, the overexposure was worst at wider apertures and disappeared between f/8 & 11.

It was the variation between manual and 'A' setting on the A series lens that surprised me. When using aperture priority metering it shouldn't matter whether I used the 'A' setting on the lens or the same f/stop I have set on the camera body. The exposure should be identical. But it's not. And as I said, it looks to me like manually setting the aperture on the lens gives more accurate exposure than letting the camera body alone do the work. And if that's the case, what does it say for my DA lenses as they are effectively always in the 'A' position?

The other surprise was the greater variability in exposure when I looked at the jpeg images versus the DNG images (I shot jpeg + RAW). I've read several complaints about P-TTL exposure inconsistency here in this Forum. It makes me suspect that many of those complaints may have been based on how the camera renders jpegs. Time for some more firmware tweaks Pentax?
Jim

What you may be seeing is some form of bounce in the aperture mechanism, within the lens. Remember normally there is nothing to stop the lens except the lever when in true A mode, with the pin shorted and the lever limiting the movement, as opposed to falling all the way back. It would in a true non A mode, there could be some mechanical resonances. With respect to body controlled exposure, while sigma is not Pentax, and much maligned over quality, my APO 70-200/2.8 is +/-2 on average greyscale value when testing exposure using a uniformly lit lock wall. This represents about +/-1/20 of a stop and this is from F2.8 all the way to F32. IMO the body control is bloody accurate
07-21-2012, 09:34 PM   #27
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one question, will shorting the A pin and insulate the appropriate aperture contact make Full P-TTL work on m42 lens?
07-22-2012, 04:53 AM   #28
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Yes, but you don't have to bother with the aperture value. You just have to always select the widest aperture available on the camera, for whatever aperture you actually use on the lens!
This is because, with a m42 lens, the metering flash always occurs at the same aperture than the actual photo (there is no aperture coupling between the camera and the lens!), just like with any PK-A lens used wide open.

But for this trick to work in a satisfactory manner, you'll have to change your focusing screen (especially with a K10/K20, as they are pretty bad in stop-down metering).
07-22-2012, 05:44 AM   #29
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baby007... the answer to your original question is YES..... my test just conducted with my K7+Pentax A50 f1.7(without mods)+ Sigma EF-610 DG Super. ZOOM manually (50mm=70mm) and press the '+' button until 'FP' appears for HSS.

If I shoot 180/f1.7/iso100 I get a white-out, change setting to 250/f1.7/iso100 and I get good exposure.
Just for the fun of it, switched camera to rapid fire. settings 500/f1.7/iso100 and got 3 shots off(with flash each time) before the flash cycle lagged. then I got ridiculous and did 2000/f1.7/iso100 and got 2 shots off before lag.

none of this may have been what you were looking for, but it proves you CAN use a manual 'A' lens in HSS....
07-22-2012, 05:59 AM   #30
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HUIH????

Shooting P-TTL should be even easier with a fully manual lens versus a M-series type lens. Short the A pin, set the body to wide open, set the lens to wide open to focus then stop down to your desired aperture. Shoot.

You might lose some range, but the exposure should be good. Your camera will meter the scene through the stopped down lens, but because it thinks you are shooting wide open won't adjust the flash duration. You won't even need to pre-calculate exposure compensation like you do with a M-series lens.

You can see my test results of P-TTL flash photography using a M-series lens a few postings above this one in this thread. Good grief! How far do we have to take this to prove you can make all this work? I fully admit that I prefer the convenience of using an A-series lens to begin with, but if I truly want to use a wonderful old lens for a specific task, I CAN do it.
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