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04-05-2009, 06:45 AM   #16
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Fresnel Vs Ground Glass Focusing Screens - Case Study

QuoteOriginally posted by flyer Quote
I may be wrong here, but I don't think the Pentax focus screen is a fresnel lens, just a ground glass. On a fresnel lens, you can see concentric rings on the glass itself with a magnifier (or regular pattern like diamond shape), and I don't see any on the screens I have on my Pentax. And I used to have fresnel screen on my Olympus and Konica in the 70's, so nothing is new here. A lot of times, focusing screen were manufactured with a fresnel pattern in the center to make focusing "pop up" when it was dead on.
I think dlacouture is right. Modern focusing screens are no longer ground glass. They react differently with different aperture and will not give the user the correct DoF as viewed, although they always look brighter.

As for the fresnel patterns you've mentioned, you can view them clearly on these pictures taken for the Pentax screens:-

ɫӰ޼

And, here is a case study of mine on different focusing screens for Canon higher grade DSLRs, see this:-

RiceHigh's Pentax Blog: When the Focusing Screen Lies - Part 2

Although its not about Pentax's screens, I found exactly for the different between "ground glass screen" and fresnel's. Canon offers 3 types of screens typically, standard one is Fresnel, "super precision" is ground glass and the "grid type" is just somewhere in between the two. Clever options.

04-05-2009, 08:32 AM   #17
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QuoteOriginally posted by dlacouture Quote
But now, think about the following experiment:
- take a 50/4 and a 50/1.4 PKA lenses...
- check exposure on both at f/4 (in full A mode, not stop down)
- you'll see a difference, as the f/1.4 lens will heavily underexpose wide-open, and this will be propagated on all apertures you select on the body.
That's the part I don't get. I've never heard anyone claim that any PKA lenses "heavily underexpose" wide open (or anywhere else, for that matter). I mean, there is the common beginner complaint of underexposure in general from people who don't understand about protecting highlights, aiming for 13% or so reflectance,etc. But it sounds like you are making a different claim, one I've never heard before, and one not supported by anything in my own experience with several different Pentax cameras. But I don't have a K20D, so I can't directly say you're wrong about this. I can only wonder why no one else has ever reported this.
04-05-2009, 07:43 PM   #18
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QuoteOriginally posted by Marc Sabatella Quote
That's the part I don't get. I've never heard anyone claim that any PKA lenses "heavily underexpose" wide open (or anywhere else, for that matter). I mean, there is the common beginner complaint of underexposure in general from people who don't understand about protecting highlights, aiming for 13% or so reflectance,etc. But it sounds like you are making a different claim, one I've never heard before, and one not supported by anything in my own experience with several different Pentax cameras. But I don't have a K20D, so I can't directly say you're wrong about this. I can only wonder why no one else has ever reported this.
Underexposure is underexposure, even though there are times and times no highlights are needed to be protected, see these examples:-

RiceHigh's Pentax Blog: Underexposure Tendency of K10D and K100D

As anyone can see, the highlights, no matter the R, G or B channels, are far from the risk of being burnt out. The camera just loves to expose to the left!

And, it is a fact with no doubt that the latest Pentax digital lenses underexpose more than the old F and FA lenses. With different generation of Pentax lenses on the same Pentax DSLR bodies, it is almost certain that the exposure levels will be different - something unarguable and widely recognised again.
04-05-2009, 07:59 PM   #19
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QuoteOriginally posted by RiceHigh Quote
Underexposure is underexposure
Exposure is a largely subjective determination, but to the extent it can be made objective, it mostly revolves around shooting for an average reflectance of somewhat less than 18% (typically 12-13%). That's precisely what the picture you posted achieves. So it actually shows how *well* Pentax cameras expose.

If you're curious to actually learn about exposure rather than criticize it based on misinformation, feel free to look up ISO standard 2720:1974. Then maybe you'd understand what proper exposure is, rather than assuming that just because you personally don't like an exposure, that it must be incorrect.

In any case, no one denies that a lot of folks are under the impression that Pentax cameras underexpose in general - we all know that this is a common perception. What I found surprising was the claim that this is specifically due to the focus screen - that's something I hadn't heard before.

04-05-2009, 08:11 PM   #20
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QuoteOriginally posted by Marc Sabatella Quote
That's the part I don't get. I've never heard anyone claim that any PKA lenses "heavily underexpose" wide open (or anywhere else, for that matter). I mean, there is the common beginner complaint of underexposure in general from people who don't understand about protecting highlights, aiming for 13% or so reflectance,etc. But it sounds like you are making a different claim, one I've never heard before, and one not supported by anything in my own experience with several different Pentax cameras. But I don't have a K20D, so I can't directly say you're wrong about this. I can only wonder why no one else has ever reported this.
I like to shoot wide open for most of my shots and that typically requires + 2/3 to +1 EV exposure compensation just to get the histogram balanced. If I stop down to f/8 I have to dial back to zero exp. compensation for most shots. It's something I have learned to live with but it is pretty clear to me that Pentax DSLRs require more attention from their users than some other cameras do. And that's fine with me
04-05-2009, 08:20 PM   #21
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QuoteOriginally posted by Robert S Donovan Quote
I like to shoot wide open for most of my shots and that typically requires + 2/3 to +1 EV exposure compensation just to get the histogram balanced.
OK, but if by "balanced", you mean, centered, that's actually half a stop or so overexposed according to the classic standards of exposure (should be around 12-13% reflectance, or somewhat left of center on the histogram), suggesting that the actual exposure was probably pretty accurate.
04-05-2009, 08:25 PM   #22
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I really like this article.... ;)

QuoteOriginally posted by Robert S Donovan Quote
I like to shoot wide open for most of my shots and that typically requires + 2/3 to +1 EV exposure compensation just to get the histogram balanced. If I stop down to f/8 I have to dial back to zero exp. compensation for most shots. It's something I have learned to live with but it is pretty clear to me that Pentax DSLRs require more attention from their users than some other cameras do. And that's fine with me
So, in the perfect world of a spot-meter calibrated to ISO standard and pure gamma = 2.2 transform of perfectly linear data coming from the sensor, the neutral surface should render 101 RGB if exposed according to the spot-meter; next stop is 138 RGB, then comes 189 RGB, and finally we are getting clipping increasing exposure full 3 stops instead of 2.97 stops which are the theoretical limit as it was shown above.

Headroom in Highlights : Where is Zone V in The Digital World? | LibRaw
And a really nice article on the Canon...
http://www.libraw.org/articles/Canon-5Dmk2-headroom.html
Hidden correction in the middle gray level in RAW converters
Many RAW-converters apply correction to the middle gray level so that if the shot is exposed according to the exposure meter (which is 7.5% of the saturation level for the discussed camera), it will be brought to the standard middle gray of 15-18% necessary for display and printing. This will happen even when all the RAW converter controls are in default neutral position.
This means that behind the scenes the RAW converter adjusts the placement of the middle gray by approximately 1 EV. To get closer to the “truth” it is necessary to manually apply -1 EV to the standard value. In other words, a correctly exposed shot that places the middle gray not to the arbitrary level decided by the camera manufacturer but to the standard zone V 18% gray will most of the cases look overexposed and blown out when in fact it is not..........................
The headroom in the highlights and per-channel sensitivities are vastly different for different manufacturers and camera models, and even one manufacturer can set those differently for two cameras released nearly at the same time (for example, Canon 450D exhibits the headroom in highlights half a stop lower than 5D Mark II, while both of these cameras sport 14-bit ADC and were launched during less than one year interval). As a consequence, measurement of the placement of the middle gray should be accomplished for each individual camera. The good part is that the measurement is easy to do and does not take much time.

It is important to conduct the measurements similar to those described above not only for the sunlight but also for other different sources of light. Furthermore, if we are going to use filter to balance per channel sensitivities, it makes very good sense to repeat the measurements with such filters as well.

P.S. Yes, and by the way, if you see distinct shoulders in highlights present on the graphs – it is some RAW converter that took care of moving the middle gray and compressing highlights. Have another look at DPReview tests.

Last edited by jeffkrol; 04-05-2009 at 08:32 PM.
04-06-2009, 12:42 AM   #23
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QuoteOriginally posted by Marc Sabatella Quote
That's the part I don't get. I've never heard anyone claim that any PKA lenses "heavily underexpose" wide open (or anywhere else, for that matter). I mean, there is the common beginner complaint of underexposure in general from people who don't understand about protecting highlights, aiming for 13% or so reflectance,etc. But it sounds like you are making a different claim, one I've never heard before, and one not supported by anything in my own experience with several different Pentax cameras. But I don't have a K20D, so I can't directly say you're wrong about this. I can only wonder why no one else has ever reported this.
I observed this with both a k10 and a K20, and on both, changing the focus screen greatly improves the exposure.
My FA50/1.4 easily underexposes 1 stop compared to the kit 18-55 set at 50, for the same scene...
When you say : "you are making a different claim, one I've never heard before, and one not supported by anything in my own experience with several different Pentax cameras", you surprise me, as you know about the stop-down problem of the k10, and this is just another aspect of this problem...

To clarify my example:
- do you agree that a stop-down m42 50/1.4 set to f/4 will expose just like a regular 50/4 lens (apart from slight discrepancies in light transmission)?
- do you agree that the same m42 50/1.4 will expose differently when set at f/1.4 and f/4?
- And last, do you agree that a FA50/1.4 will then underexpose, compared to the 50/4 above, even when the aperture is set to f/4 on the body (as the exposure is metered wide-open)?

That's what I noted on my Pentax K10 & k20 bodies.
Mind you, the exposure problems appeared with the k10/K20 generation. It was perfectly fine with the *ist generation, and is, luckily, linked to the focus screen, as changing it for the *ist LL60 screen solves the k10/k20 exposure problem.

I don't know about the k1x0-k200-k2000 family, though...

04-06-2009, 12:51 AM   #24
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My FA lenses are all "brighter" than my DA lenses.

I agree that different focusing screens do have a different effect on exposure, though.

QuoteOriginally posted by dlacouture Quote
I observed this with both a k10 and a K20, and on both, changing the focus screen greatly improves the exposure.
My FA50/1.4 easily underexposes 1 stop compared to the kit 18-55 set at 50, for the same scene...
When you say : "you are making a different claim, one I've never heard before, and one not supported by anything in my own experience with several different Pentax cameras", you surprise me, as you know about the stop-down problem of the k10, and this is just another aspect of this problem...

To clarify my example:
- do you agree that a stop-down m42 50/1.4 set to f/4 will expose just like a regular 50/4 lens (apart from slight discrepancies in light transmission)?
- do you agree that the same m42 50/1.4 will expose differently when set at f/1.4 and f/4?
- And last, do you agree that a FA50/1.4 will then underexpose, compared to the 50/4 above, even when the aperture is set to f/4 on the body (as the exposure is metered wide-open)?

That's what I noted on my Pentax K10 & k20 bodies.
Mind you, the exposure problems appeared with the k10/K20 generation. It was perfectly fine with the *ist generation, and is, luckily, linked to the focus screen, as changing it for the *ist LL60 screen solves the k10/k20 exposure problem.

I don't know about the k1x0-k200-k2000 family, though...
04-06-2009, 03:24 PM   #25
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QuoteOriginally posted by dlacouture Quote
When you say : "you are making a different claim, one I've never heard before, and one not supported by anything in my own experience with several different Pentax cameras", you surprise me, as you know about the stop-down problem of the k10, and this is just another aspect of this problem...
I'm not saying it isn't; it's just that it's new information to me - information that doesn't seem to be consistent with what others have reported (or more to the point, what they *haven't* reported). I admit there is a certain logic to what you say, but I'm not prepared to accept it at face value until someone can demonstrate how this could be true but not lead everyone who has ever owned one of these cameras to complain about underexposure with the FA50/1.4.

QuoteQuote:
- do you agree that a stop-down m42 50/1.4 set to f/4 will expose just like a regular 50/4 lens (apart from slight discrepancies in light transmission)?
I don't have a K20D, so I can neither agree nor disagree based on experience, but sure, that sounds completely logical.

QuoteQuote:
- do you agree that the same m42 50/1.4 will expose differently when set at f/1.4 and f/4?
No idea. The common complaint I hear involves stop down metering at smaller aperture.

QuoteQuote:
- And last, do you agree that a FA50/1.4 will then underexpose, compared to the 50/4 above, even when the aperture is set to f/4 on the body (as the exposure is metered wide-open)?
Again, I can neither agree nor disagree based on personal experience, as I don't own a K20D. The complaints I hear about erratic exposure involve only manual lenses, only at apertures smaller than f/4, and then the usual complain is *overexposure*, not underexposure.

I'd also observe that while virtually everyone who owns the K20D does note the issue with manual exposure lenses doing a poor job of stop-down metering, but I've never heard any of these folks have had any similar complaints about the FA50.

So again, I agree what you say has a certain logic to it, but it doesn't seem to fit the "facts" as I see them. These "facts" are all hearsay to me, though, since I have never used a K10D or K20D for any length of time. So I admit to not really having devoted a ton of though to this whole issue, since the cameras I have used are immune to these issues (DS, K100D, K200D).
04-06-2009, 05:42 PM   #26
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QuoteOriginally posted by dlacouture Quote
I observed this with both a k10 and a K20, and on both, changing the focus screen greatly improves the exposure.
My FA50/1.4 easily underexposes 1 stop compared to the kit 18-55 set at 50, for the same scene...
When you say : "you are making a different claim, one I've never heard before, and one not supported by anything in my own experience with several different Pentax cameras", you surprise me, as you know about the stop-down problem of the k10, and this is just another aspect of this problem...

To clarify my example:
- do you agree that a stop-down m42 50/1.4 set to f/4 will expose just like a regular 50/4 lens (apart from slight discrepancies in light transmission)?
- do you agree that the same m42 50/1.4 will expose differently when set at f/1.4 and f/4?
- And last, do you agree that a FA50/1.4 will then underexpose, compared to the 50/4 above, even when the aperture is set to f/4 on the body (as the exposure is metered wide-open)?

That's what I noted on my Pentax K10 & k20 bodies.
Mind you, the exposure problems appeared with the k10/K20 generation. It was perfectly fine with the *ist generation, and is, luckily, linked to the focus screen, as changing it for the *ist LL60 screen solves the k10/k20 exposure problem.

I don't know about the k1x0-k200-k2000 family, though...
I've noted "anomalies" at both ends (wide open or fully shut) on both the *ist-D and k200D in "A" positions (A50 f 1.7 and others). Either slight under or over exposure depending on camera/lens combination. nothing spectacular. The metering systems seem to favor f4 or a bit slower lenses (except the kit for reasons unknown).... My guess is at wide apertures you get light scatter that is not handled well by either screen types (orig had a tendency to overexpose a tad as f1.7). There probably are multiple factors (from hardware to software tweaks) that cause this and are a non issue for most. Especially the D series and it's larger latitude for exposure adjustments.. (my own thoughts btw.)
FUN chart:
http://www.photozone.de/active/survey/dslroutput2.jsp
04-06-2009, 07:26 PM   #27
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QuoteOriginally posted by Robert S Donovan Quote
I have to dial in +2/3 EV as a baseline using my kit lens on my K10D. Use the histogram and you'll get the hang of it pretty quick. Now I pretty much know how much exp comp each shot will require just looking through the viewfinder. Maybe I should just start shooting in full manual
Can I just add that what I meant to say is that I have to dial in positive exposure compensation to get images that look properly exposed to me?

Last edited by Robert S Donovan; 04-07-2009 at 09:36 AM. Reason: added photo for clarity
04-07-2009, 12:40 AM   #28
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Marc : oups, sorry if I sounded patronizing, but I was sure you had a K10...

About the FA50/1.4, it will behave exactly like any manual lens once it taken from the A-setting, and will underexpose between 1.4-4.5 and overexpose above 5.6...

It would be interesting to know the focus screen used in the K100/K200 line... (EDIT : from the Japanese site, it seems like it's the same).

Jeffkrol : What you're refering to is, I think, a difference between light transmission and physical aperture... A given PK-A lens will have a slight difference between the aperture selected and the actual amount of light let through the lens at this aperture. This causes slight variations of exposure with A lens, as the theorical exposure (calculated wide open) will be slightly off. Others know way more details about this, so I'll let them explain...
04-07-2009, 11:35 AM   #29
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QuoteOriginally posted by dlacouture Quote
Marc : oups, sorry if I sounded patronizing, but I was sure you had a K10...
No problem - and similarly, I don't mean to imply that you must be wrong. I'm just expressing surprise at what comes to me as new information that does seem to explain some things I've heard, but leaves me with new questions, too. If I did have a camera with these kinds of problems, I'd definitely be off investigating this further based on your ideas!
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