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10-23-2008, 02:39 AM   #1
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K10, split-screen, LF80 & LL60 : Focus and exposure accuracy

Hello all!
For those not willing to go on the other pentax forum, I'll repeat here a post I've made there...

I recently acquired a chinese 45° split-screen for my manual lenses (bought at JinFinance on ebay)...

First thing : focus is accurate... They changed their design, and the screen no longer needs shims to be at the proper place. A slight raise can be felt around the screen, so I guess it's been altered to include this ridge. Anyway, installation is really easy.

Now, about the use:
- Split prisms seem simply huge in the viewfinder! It's a real treat to focus, easier than on FF split-screens...
- Brightness has not really changed...
- Only thing that bother me : the AF red square is really distracting... I tried to disable it, but then it won't even show which focus point you select on a FA lens... Bad thinking, Pentax!

Now, having met the standard exposure problems with the original screen, I then tried to chart exposure with this split screen.

Experiment : display a Gretag chart on my laptop, in a dark room, frame to include (and crop if needed) only the screen, use the viewfinder cover, and shoot away at all apertures...
Hardware :
- k10 with split-screen
- Lenses:
- FA 50 f/1.4 in non-A position
- Porst 55 f/1.2 (PK, M + GreenButton)
- Apollo 135mm f/1.8 (m42, Av mode & M+GB)
- SMC Tak 200mm f/4 (m42, Av mode & M+GB)



This is a reference of what the K10 considered as a "correct" exposure with the FA lens, meaning that it's what you'll always have by controling the aperture on the body... (for information, it's underexposed to my taste, and can cope with +1ev without clipping, as with the original screen).

The chart below show the exposure of each lens compared to the reference exposure (0EV).


As you can see, all the lenses follow the same behavior:
- approximatively "correct" exposures between f/1.4 & f/2.5 and at f/22
- general (and massive!) overexposure between f/2.5 and f/22

Once compensated for proper exposure (+1ev), you end up with:
- approximatively "correct" exposures at f/1, f/4 and f/16
- general underexposure between f/1 and f/4
- general (including a massive +1 1/2Ev!) overexposure between f/4 and f/16
- general underexposure above f/22

These measures follow quite exactly what I found at the time with the original Lx-80 family screen, but with a slight +1/3ev exposure compared to it.

And this is what I experienced on the field, with both the original screen AND this split screen.

My brother (having a K10, the original screen, a new LL-60 screen, the FA 50 f/1.4 and a nice collection of manual lenses) repeated these tests with the original screen and the LL-60.

Our conclusions:
- original screen (and by extension all Lx-80 screens) and the split screen behave exactly the same way, with a slight, constant overexposure (around +1/3ev) for the split screen.
- Split screen had no impact (apart for the +1/3Ev overexposure) on slower lenses...
- LL60 really handles exposure in a better way (I'll post results later today) :
+ quite constant exposure between f/3.5 and f/11 (around +1/3ev from 3.5 to 11)
+ slight underexposure above f/3.5 and below f/11

I guess Pentax will soon run out of LL60s!!!
If only they made a Lx-60 split screen!!!

10-23-2008, 03:34 AM   #2
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Interesting experiment.
I've certainly noticed the drop in exposure with my LL-60 at F/22 and smaller (mostly evident on my FA 100 macro), but not at f/3.5 and larger.
10-23-2008, 04:17 AM   #3
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Ok, suite of the tests...

There are the results with the LL60 installed...


For comparaison means, the blue line is the FA 50 results with the split-screen (or a Lx-80 family screen).

The 50 now is rather well exposed (I can live with a -+1/2Ev error), with reasonably similar readings at f/1.4 and f/4 (apart toward the f/16 end, but I never use those apertures anyway, and this could be an error, I'll have to repeat the test...).
The 135 is a steady -1Ev, which is not bad...

I think we can conclude that the LL60 really improves exposure for manual lenses, without really changing exposure for zoom lenses around f/4...

And that if you're OK with the original screen's behavior, then a split-screen won't change things much, exposure-wise...

Voilą!
10-23-2008, 08:00 AM   #4
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QuoteOriginally posted by dlacouture Quote
First thing : focus is accurate... They changed their design, and the screen no longer needs shims to be at the proper place. A slight raise can be felt around the screen, so I guess it's been altered to include this ridge. Anyway, installation is really easy.
hmmm.... jinfinance was always writing on ebay that his screens are cut from K1000 OEMs that were thinner than regular Pentax screens... so they did not change the design but probably changed the source of screens... can you please give a link to ebay auction w/ the screen you purchased so that I can look at it if you do not mind ? thank you in advance.

10-23-2008, 09:18 AM   #5
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interesting tests ... but as i've confirmed, don't really want to mess with the metering of my K100D with Focus Screens. and I use the Spot Meter a lot.
10-23-2008, 09:27 AM   #6
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QuoteOriginally posted by dlacouture Quote
As you can see, all the lenses follow the same behavior:
- approximatively "correct" exposures between f/1.4 & f/2.5 and at f/22
- general (and massive!) overexposure between f/2.5 and f/22

Once compensated for proper exposure (+1ev), you end up with:
- approximatively "correct" exposures at f/1, f/4 and f/16
- general underexposure between f/1 and f/4
- general (including a massive +1 1/2Ev!) overexposure between f/4 and f/16
- general underexposure above f/22
Excuse me for the inexperienced questions I am going to ask, but as an amateur photographer (hobbyist), I like your experiment, but I need some help understanding what was done and what the carts say and what you stated above.

So, the charts are essentially comparing what one would have to adjust the EV by to get equal exposures between the tested focus screen to the one that comes with the camera with both cameras set for EV+0? i.e. a positive EV means that you'd have to reduce the exposure by that many stops to get to the EV+0 value for the default focus screen?

The problem I have (or perhaps my misunderstanding) is the second part where you are talking about compensating for exposure... Are you saying that if the ideal exposure for the default screen is EV+1, then having the split screen at EV+0 gives you the conclusions you state (e.g. approximately correct at f/4, f/16, etc.)?

I guess this would make sense, but I want to be sure that you comparison isn't with respect to having EV+1 for both screens as it seems to suggest that the results are closer with EV at +1 than with EV at 0.

I hope my questions aren't too confusing (I may have confused myself even). I actually have a jinfinance split screen, but I am such an amateur that I generally use M-mode to shoot at EV +1/3 to +1 with Chimping (the histogram) determining my need for adjustments, so I honestly never really noticed the variations in exposure that come with changing aperture.
10-26-2008, 01:26 PM   #7
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One observation about dlacouture's data: when an exposure drops by 1 EV when the aperture increases by an f-stop, the shutter speed isn't changing. At that point, I'd say that there is a different cause for metering errors, as it is one thing to adjust the shutter speed inappropriately and an entirely different thing to not adjust shutter speed at all.

For example, in dark conditions, my f/4 max lens would read 1/10@f4 and 1/5@f5.6. Great. But for f/8, 11, 16, and 22, it still read 1/5s. This appears to be a different problem than when, in bright conditions, it will read 1/1600@f4 but 1/640@f5.6 and 1/200@f8. For this reason, I'm not going to trust any metering pattern that isn't tested at multiple levels of brightness--in particular, I'm not going to trust tests in low light conditions, like a laptop display in a dark room. Being able to evaluate low light is very important, but it doesn't extrapolate to bright conditions.

Are the axes reversed on the LL-60 graph?

Here's what I've observed: constancy with ISO, constant exposure calculation at the A setting for each f-stop, and variability with everything else. If anything, my camera is getting better the more it meters. I used to see a plateau where it wouldn't set the shutter speed below 1/5s, but now it will set it down to about 3s; honestly, when I started shooting with stop-down metering, exposure was a crap-shoot, but now it is sometimes better than the A metering. I'm probably just hallucinating, though.

Of course, none of this has to do with focusing screens. :-) In my opinion, the LL-60 graph exhibits the same behavior as the split-focus graph, but there are too few data points to draw any conclusions. I'd suggest taking more data points (different lighting conditions, non-sequential tests at the same f-stop) with a single lens and metering mode; reducing the variables makes any conclusion more significant.

Lastly, a question. Does anyone know how to make the rear e-dial useful when using the lens ring to set aperture? I'd love to be able to set aperture with the lens, shutter speed with the front dial, and ISO or exposure compensation with the rear dial.
10-26-2008, 01:36 PM   #8
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QuoteOriginally posted by emalvick Quote
So, the charts are essentially comparing what one would have to adjust the EV by to get equal exposures between the tested focus screen to the one that comes with the camera with both cameras set for EV+0? i.e. a positive EV means that you'd have to reduce the exposure by that many stops to get to the EV+0 value for the default focus screen?
If I'm understanding things correctly, then, yes...but only assuming that EV compensation works as expected. A point on the chart showing an EX of +1 means that the image is overexposed by one stop; that's all it says. The image is overexposed because the metering mechanism isn't working as we would expect it to. Because it isn't working as expected, I'm not sure you ought to trust EV compensation to work correctly, either. But--here's the holy grail--if we know that the camera will overexpose by 1 EV at a given f-stop and that EV compensation works as expected (and I've seen no indication that it doesn't), then, yes, proper exposure would be attained by setting the EV compensation to -1 when using that f-stop.

QuoteOriginally posted by emalvick Quote
I actually have a jinfinance split screen, but I am such an amateur that I generally use M-mode to shoot at EV +1/3 to +1 with Chimping (the histogram) determining my need for adjustments, so I honestly never really noticed the variations in exposure that come with changing aperture.
The histogram is the best way to evaluate an exposure. Essentially, the exposure meter is predicting what the histogram will look like; if you can look at the histogram of an image, then adjust your exposure settings accordingly, you'll be more likely to get the exposure level that you want. So you're not working at a beginner's level in that respect.

However, aperture is just as important as shutter speed for determining exposure. You would benefit enormously from reading about how aperture and shutter speed both affect the resulting image--or, even better, taking images at different apertures and seeing what happens, and then reading about it to understand what's going on.

Hope this helps.

10-26-2008, 11:50 PM   #9
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Hello Emalvick!
QuoteOriginally posted by emalvick Quote
So, the charts are essentially comparing what one would have to adjust the EV by to get equal exposures between the tested focus screen to the one that comes with the camera with both cameras set for EV+0? i.e. a positive EV means that you'd have to reduce the exposure by that many stops to get to the EV+0 value for the default focus screen?
Well, no, that's not what these charts say.
These charts show Ev differences between a reference exposure with a FA 50 f/1.4 in A mode (thus consistent thorough the whole aperture range, as stop-down exposures are mathematically calculated by the body) and exposures made at actual apertures on the FA 50, m42 lenses and a PK lens.

Ev difference you then see is the fluctuation of exposure computed by the k10 against an ideal, mathematical exposure based on what speed the body computed at f/1.4...

Now these results only chart the behavior of the split screen, with no reference to the original screen, as it was not installed in my k10 anymore.
But my brother did make this experiment with his factory screen, and came to the same charts and conclusion, that's why I said that the split-screen and the factory screen do behave identically.

QuoteOriginally posted by emalvick Quote
The problem I have (or perhaps my misunderstanding) is the second part where you are talking about compensating for exposure... Are you saying that if the ideal exposure for the default screen is EV+1, then having the split screen at EV+0 gives you the conclusions you state (e.g. approximately correct at f/4, f/16, etc.)?
What I implied here is that both the factory screen and the split screen provoke underexposure at usual apertures of f/1.4 (FA 50) or f/3.5-5.6 (18-55), and to have a full use of the histogram, I had to compensate with about +1Ev on both screens.
10-27-2008, 12:10 AM   #10
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Hello Jon PB

QuoteOriginally posted by JonPB Quote
One observation about dlacouture's data: when an exposure drops by 1 EV when the aperture increases by an f-stop, the shutter speed isn't changing. At that point, I'd say that there is a different cause for metering errors, as it is one thing to adjust the shutter speed inappropriately and an entirely different thing to not adjust shutter speed at all.
Well, I have to say that, on this point, you sound true : in fact, the k10 (with LL80 or split) seems to stop metering when it reaches 1/5s at f/16, 400iso... This is about 6Ev, I think, so it's still quite high...

I'll have to repeat this test with a sunny subject instead of a crappy laptop screen to validate this part, but that's all we had at the time (around 11pm with everyone sleeping in the house)...

But the fact is that with the LL60, the tendency is reversed, and the k10 starts to overexpose at f/16, using the same setup...
So, I'm still pretty sure it's screen-related, not body- or experiment-related...
10-27-2008, 12:21 AM   #11
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Well, at last, I had time to do some real-world tests this week-end (about 200 pics in different settings), and here are my feelings:
- exposure is now constant! This is a real pleasure to stop-down, and only see a DOF difference instead of wildly different exposures between two pics.
- there is a constant 2/3Ev underexposure with all my lenses (which makes me think that this LL60 is indeed brighter than the factory screen), and this in wildly different situations (direct sunlight, cloudy day, indoors, shade, backlight...)
- Altough the LL60 is a little bit coarser than the factory screen, Manual focus is not so bad.

Voilą, I hope this will help you using the K10...
Thanks to all those guys that pointed the whole LL60 way to me!
10-27-2008, 05:28 AM   #12
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I am curious as to how yo measurd he exosr error in EV?
While I don't disagree in the overall shape of the curve, it appears that it is shifted about 1 stop
can you tell me what program, and also what the average grey scale reading is from your esposures, or are you making this based on the in camera histogram
10-27-2008, 06:18 AM   #13
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For those of you here in the states, the US, and purchased from jinfinance on ebay how long did it take to get the screen? How long was the shippng time? I almost purchased one a while back and asked the question about exposure which they were very honest about exposure details. I am really condsidering one of these but just can't quite seem to bite the bullet for the Katz Eye.

Thanks

BTW dlacouture, thanks for the testing. I really don't understand it completely but have a general idea. So it does help.
10-27-2008, 06:43 AM   #14
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Hello Lowell!
First, big thanks for all your posts on the subject! It really helped me understanding both the problem and the solution!

Well, as I said, the 0Ev basis of these charts is taken on what the camera found using the 50 FA in A mode... That's all! It was choosen purely on a practical basis, to ensure no external dependency at all (apart from the light source!)...
After all, what I wanted to chart was exposure accuracy through all the aperture range, not accuracy of the metering system of the k10 regarding external conditions (where grey card and controlled lighting is important)...

That's why I said that, for my personnal taste, it would benefit from a +1Ev expo comp (and then we fall back on your results, if I recall right)...
10-27-2008, 06:56 AM   #15
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OrenMc, to France, it was shipped early the next day, and took 2-1/2 weeks to arrive... I guess it won't be really different for you...

I guess the wrap-up on all this would be:
- K10's factory screen cannot produce constant exposure at different apertures (at least on the three bodies I had in hands).
- JimFinance split screen behaves exactly like the factory screen (with maybe a slight, constant +1/3Ev overexposure).
- LL60 does solve the problem (on the two bodies it was installed on), ensuring quite constant exposure regardless of what aperture is used, but with a constant -2/3Ev underexposure.

Now I'm thinking about sending my k10 to pentax for focus calibration (it's 4 weeks old, so well under warranty), secretly hoping that they will also recalibrate the metering system with the LL60... This would be simply great!
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