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04-28-2010, 06:03 PM   #1
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M42 lens metering differently in liveview vs viewfinder

Hi all,

I have attempted to research this before posting, but have not been able to found any answers. Apologies if I missed an obvious thread!

I have started noticing that my k-x seems to meter inconsistently when I use my m42 super tak 55/1.8 lens. I think in the past it has metered correctly and produced correctly exposed pictures. But lately I feel like they are all crazy underexposed or at the very least highly unpredictable.

So I did an experiment tonight to try to help see if I am crazy (answer is 'yes') or if something odd is going on.

Quick synopsis:
1. Using liveview the camera set my shutter speed to 50. Pic looks great.
2. Using the viewfinder the camera set my shutter speed to 250. Pic looks underexposed

More details on my "test":
I put the camera on a tripod and set up a boring test shot in my kitchen counter. I made sure to set camera to allow use of aperture ring. White balance set to tungsten. Camera mode set to Av. The lens itself was set so that when I move the aperture ring the aperture blades actually close. Manual focus mode. The first picture was taken using Liveview. And then all I did was turn off liveview and take another photo without changing any other settings.

Am I missing something? Why the heck do pictures get good exposure using liveview but stink using the viewfinder. Am I missing something stupid / obvious?

I could post the pics later if you think they would be helpful.

Thanks,
Sam

04-28-2010, 06:19 PM - 1 Like   #2
Ira
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Piece of cake:

First, did you actually upload images to your computer, or are you using the LCD as your guide for exposure? If the latter, bad boy. But even if the former, here's the deal that I found with ALL of my Taks:

Av mode STINKS on the kx--always underexposes.

Instead, set camera to M, lens on M (not A), turn to your desired aperture, hit the EV button at the top of the camera near the Green button, and voila--perfect exposures. The camera has selected the proper shutter speed.

With all of the documentation here about using manual m42 lenses on Pentax DSLRs and setting the Green button for Tv shift, no one seems to have updated the important fact that on a kx, all you have to do is hit the EV button, and you can assign a different function to the Green button.

You are, in effect, shooting in Av mode, although the camera is set to Manual. You've selected your aperture, but you just have to hit the Ev button to lock in the exposure, which is the way to go anyway. Once you hit that button, it won't change on you.

So, with center weighted metering, you can lock in your proper shutter speed on a neutral gray part of the scene, recompose/reframe, and you're cool. Unlike Av mode, where the slightest turn of the camera changes your settings before you pull the trigger.
04-28-2010, 06:25 PM   #3
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There's more to this but I don't want to complicate things:

In LiveView, when your eye isn't up to the viewfinder, weird things happen--like light enters there and you don't want that. So even though these shots look okay to you, they're still underexposing based on my "M42 lenses suck in Av mode on the kx" theory.
04-28-2010, 06:33 PM   #4
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QuoteOriginally posted by sameagle Quote
Am I missing something? Why the heck do pictures get good exposure using liveview but stink using the viewfinder. Am I missing something stupid / obvious?
I don't think you are missing anything.

It's been a while since I last looked at light metering with manual lenses, but I did some experiment with my K10D a year or two ago. The results were all over the map. It was one of the reasons I gave up on manual lenses. Photography is supposed to be fun. I don't want to deal with frustration. Maybe it's just me.

When you use the optical viewfinder, light metering is done by the light sensors in the prisms. They measure the light going through the lens, bended by the mirror, and having gone through the focusing screen. The focusing screen has significant effect. Let's just say that the focusing screen is not optimized for manual lenses. As an example, to have accurate light metering with manual lenses, I had to replace the stock screen in my K10D with one designed for the *ist D series.

Also, if you don't cover the viewfinder, stray light can fool the sensors too.

When you use live view, light metering is done by analyzing the light actually hitting the image sensor. It is not affected by the focusing screen. The result is thus more consistent and reliable.

That's my story, and I stick to it.

04-28-2010, 06:37 PM   #5
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How sure are you that when you have focussed and before you hit the shutter button, your head is not casting a shadow on the viewfinder ?

If you use a tripod and your eye is not going to be at the viewfinder when you trip the shutter, you should really use manual exposurel mode. Using Av (or other auto mode) will give you innacurate meter readings as light will be able to find its way in through the viewfinder and affect the metering.

Unless you are using live view to focus you need to cover up the viewfinder with the little plastic cover, especially if you have a strong kitchen light shining down on the camera.
04-28-2010, 06:51 PM   #6
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Another factor, the lens is designed for 24mmx36mm 35mm film; the lens projects an image circle larger than a lens designed for crop sensor. There is more total light in the larger image circle projected into the mirror box, so the meter reading is too high, which results in underexposure. When using live view, only the portion of the image circle striking the sensor is metered, thus correct exposure.

Light entering the uncovered eyepiece also inflates meter reading, causing underexposure; not a problem with liveview as the mirror blocks it.
04-28-2010, 07:32 PM   #7
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QuoteOriginally posted by Ira Quote
Piece of cake:

First, did you actually upload images to your computer, or are you using the LCD as your guide for exposure? If the latter, bad boy. But even if the former, here's the deal that I found with ALL of my Taks:

Instead, set camera to M, lens on M (not A), turn to your desired aperture, hit the EV button at the top of the camera near the Green button, and voila--perfect exposures. The camera has selected the proper shutter speed.

With all of the documentation here about using manual m42 lenses on Pentax DSLRs and setting the Green button for Tv shift, no one seems to have updated the important fact that on a kx, all you have to do is hit the EV button, and you can assign a different function to the Green button.
How do you like that. I thought I had read a bunch of posts saying that Av mode works great with M42s. I have just uploaded the photos and my conclusion is still the same. Using viewfinder, the metering stinks.

I was aware of the EV button and had tried it out in M mode before. I now realize that that is The One True Way for using the K-x w M42s.

Let's see if I can post some photos. Every thread is better with photos, right???

1.
Av Mode
1/50
Live View mode
Eye pressed up against the viewfinder
Photography For Photography's Sake - SamtheEagle's Photos


2.
Av Mode
1/200
ViewFinder mode
Eye pressed up against the viewfinder
Photography For Photography's Sake - SamtheEagle's Photos



3.
M Mode
1/30
Viewfinder mode
Eye pressed up against the viewfinder
Photography For Photography's Sake - SamtheEagle's Photos



Conclusions:

Photo #3 is the winner. Use M mode and use the EV button (or the green button if you want to use it for that function).

Photo #1 is also fine using Liveview. But liveview is pretty slow and clunky.


Thanks for your speedy help!
- Sam

(PS struggling with that whole attaching a photo thing. I'll have to review the rules on that one. Guess my methods from other forums are not quite working as expected here. :-\ )
04-29-2010, 05:55 AM   #8
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Another thing to keep in mind and to EXPECT:

On some of my m42s (even the Taks), there are one or two F stops that just don't "understand" what the hell is going on. All the other stops meter perfectly, except for this one or two, which will underexpose. (Usually real small apertures which I'm not using anyway.)

Another thing you'll notice is that smaller than 8, 11--depends on the lens--you won't get focus confirmation. So you just focus wide open and then stop down before metering and pulling the trigger.

I think using M mode is the only way to go with these lenses, especially because of the exact exposure control it gives you. You meter on an 18% grey/midtone of the scene (or the tone in the scene that you want exposed perfectly), and you're locked in at that exposure when you reframe/recompose the image.

I'm not going to say I get PERFECT exposure every time, but this method works consistently well for ALL of my M42 lenses, the long list of which you'll see in my signature.

04-30-2010, 08:04 PM   #9
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I use a 50mm, f/1.4 Super Takumar on my K10D and I've never noticed any real exposure problems.

I have verified that, with my K10D, when shooting on a tripod, with my face not at the viewfinder, light entering the VF causes an approximately 2 stop underexposure. The K10D included a VF cover, to prevent this. Does the K-x include such a cover? If it does, use it. IMHO, covering the VF when shooting on a tripod is mandatory.

I suggest that you repeat your experiment with the VF covered. I'll bet that the exposure will be very close to that in live view.

As for focusing an M42 lens, I grew up on a Spotmatic, so stop-down metering is not a problem for me. I focus wide open, half-press the shutter, stop down to the desired aperture and press the shutter the rest of the way. I use Av mode with my 50, almost exclusively.
04-30-2010, 08:47 PM   #10
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Interesting. On my K200D with SMC Tak 55 1.8 lens, in manual mode using the green button, exposure is spot on. With everything the same except AV mode, exposure is consistently 1.5 - 2 stops under in AV mode. Eye up to the viewfinder or away makes little difference.

Here is the reason:

I read this in the Takumar lens club thread. The Tak 55 1.8 lens diameter is just small enough (or maybe its the painted mount) so that it doesn't short out the data pins on the lens mount. When the appropriate pin is shorted, AV mode exposure is the same as M mode. If I cover the data pins with aluminum foil before mounting the lens, AV mode works perfectly.
05-03-2010, 06:32 AM   #11
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QuoteOriginally posted by cfraz Quote
Interesting. On my K200D with SMC Tak 55 1.8 lens, in manual mode using the green button, exposure is spot on. With everything the same except AV mode, exposure is consistently 1.5 - 2 stops under in AV mode. Eye up to the viewfinder or away makes little difference.

Here is the reason:

I read this in the Takumar lens club thread. The Tak 55 1.8 lens diameter is just small enough (or maybe its the painted mount) so that it doesn't short out the data pins on the lens mount. When the appropriate pin is shorted, AV mode exposure is the same as M mode. If I cover the data pins with aluminum foil before mounting the lens, AV mode works perfectly.
Really? I have to try this, with aluminum tape.

On my 50 1.4, Av mode just can't be used.
05-03-2010, 07:27 AM   #12
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I have spent a lot of time trying to figure out metering.

First of all, I find that many M42 lenses will tend to under expose using the metering on the camera, so when you shoot, you need to consider exposure compensation as part of you rM42 set up. I believe this is due to the failure to short out the contacts on the camera body lens mount.

Second, the K10D specifically is one of the worst metering bodies when it comes to manual aperture lenses. I did some tests on the K10D and compared it to both my K7D and *istD

The results are attached


WHat I STRONGLY recommend anyone who is shooting manual glass to do is to check each lens they use with a simple test.

Shooting a uniformly lit surface (I like concrete block walls, sidewalks and paved roads) test the lens by cycling through every aperture setting, and taking a shot with the camera, using the camera's metering (AV Mode for M42,or green button for K mount) to set the exposure.

then, using your photo editor, check the exposure byu measuring the grey scale value in the middle 10% of the frame area. (usually you can select the histogram to be either the full image or a selection box)

Plot the grey value vs aperture (as in my graph)

Each 40-50 grey scale represents 1 stop when between about 25 and 225.
05-03-2010, 07:39 AM   #13
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QuoteOriginally posted by Lowell Goudge Quote
I have spent a lot of time trying to figure out metering.

First of all, I find that many M42 lenses will tend to under expose using the metering on the camera, so when you shoot, you need to consider exposure compensation as part of you rM42 set up. I believe this is due to the failure to short out the contacts on the camera body lens mount.

Second, the K10D specifically is one of the worst metering bodies when it comes to manual aperture lenses. I did some tests on the K10D and compared it to both my K7D and *istD

The results are attached


WHat I STRONGLY recommend anyone who is shooting manual glass to do is to check each lens they use with a simple test.

Shooting a uniformly lit surface (I like concrete block walls, sidewalks and paved roads) test the lens by cycling through every aperture setting, and taking a shot with the camera, using the camera's metering (AV Mode for M42,or green button for K mount) to set the exposure.

then, using your photo editor, check the exposure byu measuring the grey scale value in the middle 10% of the frame area. (usually you can select the histogram to be either the full image or a selection box)

Plot the grey value vs aperture (as in my graph)

Each 40-50 grey scale represents 1 stop when between about 25 and 225.
I'm too stupid to understand this.
05-03-2010, 08:39 AM   #14
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QuoteOriginally posted by Ira Quote
I'm too stupid to understand this.
I will try to explain it as it has been a hotly discussed issue over the past 3 years.

When the K10D came out, there were a lot of issues relating to exposure, both with Manual aperture lenses and with teleconverters.

What it all came down to in the end, and is the origin of the chart shown here, is the focusing screen used in the K10D and K20D. These screens seem to have some odd characteristics when it comes to how light reflects off the screne to the metering sensors.

This characteristic is somewhat all over the map, but is consistant and repeatable as a function of the aperture.

This was ultimately proven out by swapping the focusing screen our of an *istD and putting it into a K10D (they are interchangeable) and being able to duplicate the *istD's metering behavior in the K10D.

Since that time, I have carefully checked most of my lenses for exposure accuracy using a uniform surface as the test subject.

a uniformly lit uniform color surface should have a greyscale histogram value of about 110-120 (the histogram ranges from 0-255)

Through other testing, I established that for all camera settings set to neutral, a change of 1 stop (by either stopping down a lens or by changing shutter speed) would result in a change of about 45 greyscale.

So basically, if you consider a line through 120 as the correct exposure, values above 120 are over exposure and those below 120 are under exposure, AND when you get to a value of 165 you are at +1 stop and when you get to a value of 75 you are at -1 stop.

The attached chart shows the exposure accuracy of my SMC P 50mm F1.4 on each of my 3 bodies with the origonal screens, and 2 additional curves for the K10D, one curve showing the K10D with the focusing screen from the *istD, and the other showing the K10D with my Chineese Diagonal split screen installed.

essentually the curve shows that with a K10D and stock screen, and the SMC-P 50mm F1.4 I can have under exposure by a little over a stop at F2, and over exposure by almost 2 stops at F9.6. The behavior is predictable from day tyo day, etc, but unless you know the behavior, the general observation would be to conclude as others in this thread have posted, that exposure is all over the map.

The K7 takes about 1/2 stop off the under exposure, and almost a full stop off the over exposure and therefore could be considered as metering more accurately, although it is still not perfect.

The *istD stays within +/- 1/2 stop over the entire aperture range, which is why I conisder it to be the best metering camera out there for manual lenses.
09-06-2010, 05:50 PM   #15
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QuoteOriginally posted by Ira Quote
Piece of cake:

First, did you actually upload images to your computer, or are you using the LCD as your guide for exposure? If the latter, bad boy. But even if the former, here's the deal that I found with ALL of my Taks:

Av mode STINKS on the kx--always underexposes.

Instead, set camera to M, lens on M (not A), turn to your desired aperture, hit the EV button at the top of the camera near the Green button, and voila--perfect exposures. The camera has selected the proper shutter speed.

With all of the documentation here about using manual m42 lenses on Pentax DSLRs and setting the Green button for Tv shift, no one seems to have updated the important fact that on a kx, all you have to do is hit the EV button, and you can assign a different function to the Green button.

You are, in effect, shooting in Av mode, although the camera is set to Manual. You've selected your aperture, but you just have to hit the Ev button to lock in the exposure, which is the way to go anyway. Once you hit that button, it won't change on you.

So, with center weighted metering, you can lock in your proper shutter speed on a neutral gray part of the scene, recompose/reframe, and you're cool. Unlike Av mode, where the slightest turn of the camera changes your settings before you pull the trigger.
Thanks for the info, Ira. I was having exposure problems with a Chinon 55mm 1:1.7 lens. I was trying to shoot AV, and consistantly under exposing. Set camera to M, lens to M, and locking exposure with the A/V button did the trick. Thanks again!
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