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02-26-2016, 08:00 PM   #1
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Strange exposure problem but only w/teleconvertor

Hi,

I noticed a kind of strange problem tonight when I was "playing" with a (actually 2 different ones) Takumar-A 2x teleconverters, I was testing them with my DA35mm and K-r, in my kitchen. For orientation, here's a pic of the back wall area:



I've circled the area on the left as "A" and on the right, where the orchids are, as "B".

I took a pic of "A" and it was ok:



But then, I took at pic of "B" (I was kind of sitting in the same place and just turned to point the camera towards the "B" area) and got:



Notice that the latter is way over-exposed.

I thought that there was something wrong with the TC, so I tried the same thing, but using a second identical Takumar-A 2x TC, and got the same results.

Also, if I don't use the TC, and just the DA 35mm, I don't get that over-exposure when I shoot towards the "B" area.

What is going on? And why is it only happening when I use the 2x TC?

I'm guessing it has something to do with the reflection of the light fixture that is hanging there and reflecting off of the back window?

But why is this happening just when I use the TC?

Thanks,
Jim

02-26-2016, 08:52 PM   #2
dms
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If you meter a black area--camera will try to expose so the black area is medium gray (it does not know it is supposed to be black).

Without TC you are metering a larger area and likely martix metering that has some kind of logic to anticipate this.

Also the DA35 does not have an aperture ring so how can it stop down?
02-26-2016, 09:22 PM   #3
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QuoteOriginally posted by dms Quote
If you meter a black area--camera will try to expose so the black area is medium gray (it does not know it is supposed to be black).

Without TC you are metering a larger area and likely martix metering that has some kind of logic to anticipate this.

Also the DA35 does not have an aperture ring so how can it stop down?
Hi,

I'm not clear about your last question?

The Takumar-A has the contacts for passing through from the DA 35mm to the body, so I can see, and I think, set the aperture in the K-r.

You notice that aperture for that over-exposed shot was f/5.6, so the lens should be stopped down to f/5.6. Also, I just checked, and with the TC installed, I *can* see the lens opening shutting down during the shot, i.e., so the camera is able to control the lens opening even with the TC.

I think that you may be close, that the camera is measuring mostly black/darkness, but why is that only happening when the TC is included? Why does it seem to be metering differently with the TC vs. without the TC?
02-26-2016, 09:40 PM   #4
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I have no DA lenses, but I am pretty sure it does not stop the lens down--but that you can see/check.

As I mentioned w/o the TC the area being measured is larger and whether or not larger it may be metering in a different pattern--namely matrix metering--which generally has some logical checks--so it may try to recognize/anticipate this situation.

The usual way to meter in the scenario you have is to put a gray card in the critical area and spot meter it, or use a white sheet/card and open lens 2 stops, or use a hand held incident light meter (and open two stops to account for the 2x TC)--or adjust shutter speed (slower) by 2 ev.

02-26-2016, 09:59 PM   #5
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QuoteOriginally posted by ohaya Quote
The Takumar-A has the contacts for passing through from the DA 35mm to the body
If the TC has the "A" contacts and is working properly (no F---), the body should be able to control the aperture and the iris should stop down. This result is expected based on the luminance range in the scene and the use of matrix metering. The camera is biasing to the dark background and ignoring the bright highlights at upper left. Center weighted or spot metering might yield better results, but the best solution would be a incident reading (hand-held meter) from the position of the orchids or (as noted above) a gray card reading. Lacking either, you can do a spot measurement off a middle value somewhere in the room and use the AE lock button or sustained half-press before recomposing to shoot.


Steve
02-26-2016, 10:05 PM   #6
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QuoteOriginally posted by ohaya Quote
why is that only happening when the TC is included?
Field of view is half as wide with the TC attached. If you normalized the view (stood closer with the TC off), you should get similar exposure for both.


Steve
02-26-2016, 10:17 PM   #7
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QuoteOriginally posted by stevebrot Quote
If the TC has the "A" contacts and is working properly (no F---), the body should be able to control the aperture and the iris should stop down. This result is expected based on the luminance range in the scene and the use of matrix metering. The camera is biasing to the dark background and ignoring the bright highlights at upper left. Center weighted or spot metering might yield better results, but the best solution would be a incident reading (hand-held meter) from the position of the orchids or (as noted above) a gray card reading. Lacking either, you can do a spot measurement off a middle value somewhere in the room and use the AE lock button or sustained half-press before recomposing to shoot.


Steve
Hi,

I think the contacts are working... I can see the "F..." in the viewfinder and if I look at the lens when I shoot, I can see the iris closing down.

I set the AF/AE-L button to lock exposure, then I set the epxosure on a white wall, then I shot the orchid area and it's still all washed out/white.

Again, this is only with the TC (actually tried 2 different Takumar-A TCs). I understand what you are saying but it seems like there's something else going on when the TC is in place. Is it possible that because of the design of the TC, it's causing some kind of internal flaring?

EDIT: I did the following experiment: I had the camera in Av, and originally aperture was set to f/5.6.

I tried closing aperture to smallest (22 or 32) and the white out got worse.

I tried opening all the way (2.4 or 2.5) and white out got much less. It wasn't totally gone, but it was less.

I think that "white" might be because the longer exposure is causing flaring inside the lens, but only when the TC is installed, so I just did another experiment, where I put a shade over the lens and shot the "B" area, and it was much, much better, even at f/5.6:


Last edited by ohaya; 02-26-2016 at 10:48 PM.
02-27-2016, 07:02 AM   #8
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Sorry for the bump, but still looking for input/feedback?

Based on my last testing above, it seems like the answer wasn't because of the metering, but because of flare from light and using a hood might be the answer when using the teleconverter?

02-27-2016, 07:40 AM   #9
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Does the TC have 6 pins or 7? Does it have the power zoom contacts and a screw drive? If it only has 6 pins (no data pin) and no screw drive and pz contacts then the body will treat the lens and TC as if it were an "A" series manual focus lens. This means it will use the "passive" contact system to determine the min and max aperture settings. Since it is a 2x TC you effectively lose 2 stops of speed on the lens. The camera has no way of knowing this since the TC has no data pin (or chip to adjust the effective f-stop range).

So either you have to apply 2 stops of exposure compensation or use manual stop-down metering. Since the lens does not have an aperture ring you can't use stop-down metering.

Last edited by Not a Number; 02-27-2016 at 07:47 AM.
02-27-2016, 10:23 AM   #10
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The Takumar A teleconverter or any TC except perhaps the new Pentax TC and the AF 1.7x adaptor do nothing t correct for aperture value with the TC attached. I don't know the KR metering but there has been a lot of discussion in the past about errors using non A lenses because the camera does not know the true open aperture, and this was link d back in about 2007 also to exposure errors when using teleconverters.


Check this plot from years ago



Look at the exposure shift as a function of FStop

Now imagine a 2 stop error in the knowledge of your lens considering say F2.8 to F5.6. Metering can get pretty screwed up.

Not saying this is e cause but DSLR metering is not constant over the entire aperture range, and the cameras are compensated for this using the knowledge of the native aperture. Adding the TC changes the native aperture by 2 stops so the camera's internal compensation ie not using the right value when it reads the lens aperture through the pass through contacts of the TC
02-27-2016, 11:29 AM   #11
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QuoteOriginally posted by Not a Number Quote
Does the TC have 6 pins or 7? Does it have the power zoom contacts and a screw drive? If it only has 6 pins (no data pin) and no screw drive and pz contacts then the body will treat the lens and TC as if it were an "A" series manual focus lens. This means it will use the "passive" contact system to determine the min and max aperture settings. Since it is a 2x TC you effectively lose 2 stops of speed on the lens. The camera has no way of knowing this since the TC has no data pin (or chip to adjust the effective f-stop range).

So either you have to apply 2 stops of exposure compensation or use manual stop-down metering. Since the lens does not have an aperture ring you can't use stop-down metering.


Hi,

The TC is the same as this one:

F4507 Asahi Pentax PKA Takumar A 2X Tele Converter for Film Digital Cameras | eBay

---------- Post added 02-27-16 at 11:34 AM ----------

QuoteOriginally posted by Lowell Goudge Quote
The Takumar A teleconverter or any TC except perhaps the new Pentax TC and the AF 1.7x adaptor do nothing t correct for aperture value with the TC attached. I don't know the KR metering but there has been a lot of discussion in the past about errors using non A lenses because the camera does not know the true open aperture, and this was link d back in about 2007 also to exposure errors when using teleconverters.


Check this plot from years ago



Look at the exposure shift as a function of FStop

Now imagine a 2 stop error in the knowledge of your lens considering say F2.8 to F5.6. Metering can get pretty screwed up.

Not saying this is e cause but DSLR metering is not constant over the entire aperture range, and the cameras are compensated for this using the knowledge of the native aperture. Adding the TC changes the native aperture by 2 stops so the camera's internal compensation ie not using the right value when it reads the lens aperture through the pass through contacts of the TC


Just to be clear: The problem I was seeing with shooting in that one direction (the "B" one) was that the image I got was WAY OVER-exposed, and not under-exposed. In a couple of cases, it was almost entirely white.

I understand about losing two stops with the TC, but that would explain if what I got was UNDER-exposed if I hadn't accounted for losing two stops, correct?

That was why I said above that I don't think the white-washing was because of metering... I think that it's because with the long exposure times and light was bouncing around inside the lens/TC and causing flare. Also, in the last experiment above where I simulated a hood, the over-exposure went away.

I'm think I'm getting a hood and will try again tonight with that if it fits the DA35mm and see what happens.

Thanks,
Jim
02-27-2016, 12:50 PM   #12
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I have an old film era Vivitar 2x teleconverter that has exposure issues. It has a painted rear surface (common with film era A-series kit), which may be causing electrical issues. IIRC modern Pentax cameras require the rear surface to be conductive, which the painted surface is not. I haven't taken the time to try making the rear surface conductive (e.g aluminum foil, scraping paint), as it is not a particularly good converter, and I doubt that the results would be worth the effort. Yours sound like much better ones, and might be worth a try.
02-27-2016, 12:59 PM   #13
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Jim

I think you mis understood my point. Using for example a Kseries lens a K 10 has a potential 3 stop difference between exposures depending on the aperture the lens was set to.


This error is what I will call the errors due to native aperture.

Now consider a lens mounted on a TC, the native aperture is 2 stops different than what the camera reads rom the pins . So you add a TC and the camera meters a lens plus TC and applies the correction to what it reads through the pins ( perhaps correcting for an F2.8 lens, but it is really an F5.6. Now it reads the light correctly but applies the wrong factor to the whole process.

If you look at the curve, an F2.8 lens if uncorrected would enter lower than what is needed. So the camera compensates, by increasing the exposure, but with the TC the actual open aperture is F5.6 and the camera with this native aperture should have actually reduced the sexosure because it tends to overexposed when using a native f5.6 lens.

The net result is it gets the exposure wrong.

Not saying. This is your issue. It is just an issue you ne d to be aware of.
02-27-2016, 06:18 PM   #14
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QuoteOriginally posted by Lowell Goudge Quote
Jim

I think you mis understood my point. Using for example a Kseries lens a K 10 has a potential 3 stop difference between exposures depending on the aperture the lens was set to.


This error is what I will call the errors due to native aperture.

Now consider a lens mounted on a TC, the native aperture is 2 stops different than what the camera reads rom the pins . So you add a TC and the camera meters a lens plus TC and applies the correction to what it reads through the pins ( perhaps correcting for an F2.8 lens, but it is really an F5.6. Now it reads the light correctly but applies the wrong factor to the whole process.

If you look at the curve, an F2.8 lens if uncorrected would enter lower than what is needed. So the camera compensates, by increasing the exposure, but with the TC the actual open aperture is F5.6 and the camera with this native aperture should have actually reduced the sexosure because it tends to overexposed when using a native f5.6 lens.

The net result is it gets the exposure wrong.

Not saying. This is your issue. It is just an issue you ne d to be aware of.

Thanks for the explanation/clarification.

However, I think that I've been able to confirm that the problem that *I* am seeing, when using the TC (and only when using the TC), is because of light coming through the front of the lens, from above.

I don't know if you can tell from the earlier wider shot, but the place I was shooting from was almost directly underneath the 4 light fixture that is over our kitchen table. So I theorized that light, coming from above was making it into the lens.

So, I used some dark construction-like paper to make a makeshift hood, and tried different lengths. It looks like I had to make the hood pretty long, to the point that I couldn't really avoid getting vignetting, in order to eliminate most of the flaring problem. Here's an example. You can see the vignetting, but the lighting is much better (sorry about the focus):



I guess that I'm still surprised because, if you recall from earlier, this is only happening if I have the TC, and I'm surprised that just having the TC would make the flaring so bad (without the hood).
02-29-2016, 11:50 AM   #15
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QuoteOriginally posted by ohaya Quote
I think the contacts are working... I can see the "F..." in the viewfinder
If you can see "F..." in the viewfinder, the contacts are not working.


Steve
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