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04-03-2009, 03:12 AM   #1
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K20D exposure and anti-shake

I am new to DSLR and I just bought the K20D for a month. Everything is fine so far. I have two questions here:
1. After buying the K20D, I looked through the Web so as to know more about the camera (and that's why I am here). I was informed that the camera is a little bit under-exposed, but I want to know how much. That is, if I set the EV compensation, but how much (1/3? 1/2?) should I set so that I can get normal exposure?
2. I used to use a non-DSLR camera that also has anti-shaking property and timer. Now if I want to take a shot with long exposure time, I would simply activate the 2 sec timer so that my finger would not blur the photo. By this way normally I can have the exposure time a step slower and still get satisfactory result. Now for K20D, by some reason (the forum has discussed this), when I activate the timer, the anti-shake would automatically deactivate itself, which renders the above technique useless. Any thoughts?

Best wishes,
KCC

04-03-2009, 04:32 AM   #2
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Hello, and welcome!
Well, regarding the exposure problem, let's say it's related to the lens used.
For f/3.5-5.6 lens, exposure will be quite good.
But with fast lenses, you'll have a variable amount of underexposure...
- f/1.4 needs about +1 1/2ev
- f/2.8 needs about +1ev

and so on...

This is due to the fact that the focus screen does not respond in a linear way to the aperture of the lens...
You should have a direct relation between shutter speed and aperture (i.e. going from f/1.4 to f/2 should halve the shutter speed, going from say 1/1000 to 1/500).
But with the original k20 focus screen, this relation is not respected (going from f/1.4 to f/2 won't halve the speed).

Note : I'm talking about using actual, stop-down apertures.

For A lenses, the exposure is mesured wide-open, then calculated for the given aperture you want to use, so selecting f/2 will always gives half the speed of f/1.4.
BUT the wide-open reading will be underexposed by the 1 1/2ev mentionned above, so the lens will always underexpose, whatever aperture you're using.

Using an old lens (with aperture blades actually moving when you select an aperture on the lens), you'll see that closing one stop won't halve the speed.

You can lessen this behavior by using a LL60 screen (from the *ist line). They are fully compatible with the k10/k20, and lessen the discrepancies to the point where you won't really need to compensate the exposure anymore.

Regarding SR, I don't have any idea about it...
04-03-2009, 04:47 AM   #3
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QuoteOriginally posted by dlacouture Quote
You can lessen this behavior by using a LL60 screen (from the *ist line). They are fully compatible with the k10/k20, and lessen the discrepancies to the point where you won't really need to compensate the exposure anymore.
Hi,

how does the changing of the screen affect the exposure, and does the LL-80 for the K10/ K20 not do the same?

Bill
04-03-2009, 05:39 AM   #4
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QuoteOriginally posted by dlacouture Quote
Hello, and welcome!
Well, regarding the exposure problem, let's say it's related to the lens used.
For f/3.5-5.6 lens, exposure will be quite good.
But with fast lenses, you'll have a variable amount of underexposure...
- f/1.4 needs about +1 1/2ev
- f/2.8 needs about +1ev

and so on...

This is due to the fact that the focus screen does not respond in a linear way to the aperture of the lens...
You should have a direct relation between shutter speed and aperture (i.e. going from f/1.4 to f/2 should halve the shutter speed, going from say 1/1000 to 1/500).
But with the original k20 focus screen, this relation is not respected (going from f/1.4 to f/2 won't halve the speed).

Note : I'm talking about using actual, stop-down apertures.

For A lenses, the exposure is mesured wide-open, then calculated for the given aperture you want to use, so selecting f/2 will always gives half the speed of f/1.4.
BUT the wide-open reading will be underexposed by the 1 1/2ev mentionned above, so the lens will always underexpose, whatever aperture you're using.

Using an old lens (with aperture blades actually moving when you select an aperture on the lens), you'll see that closing one stop won't halve the speed.

You can lessen this behavior by using a LL60 screen (from the *ist line). They are fully compatible with the k10/k20, and lessen the discrepancies to the point where you won't really need to compensate the exposure anymore.

Regarding SR, I don't have any idea about it...

That is the first time I have ever heard that the focus screen is why Pentax cameras underexpose. Where did you get that info? I've heard of some of the 3rd party ones affecting exposure but not the default Pentax one,

To the OP. The general consensus is Pentax choose to have things slightly underexpose to save blown highlights. Everyone has a slightly different opinion on how much Pentax underexposes, I find in most situations that exposure is pretty accurate and the underexposer to be minimal.

I'd say learn your camera and lens and make the determination if your combo has the need to dial in exposure compensation for the type of shooting you do. You may find that you never really need to.


Oh for your second question. Yes the 2 second timer disables SR. The reason for the 2 second timer is that it actually raises the mirror for those 2 seconds before it releases the shutter. This is used in situations where your shutter speed is SO slow that the mirror slap would actually cause motion blur. With shutter speeds that slow you really have to be on a tripod, hence the camera shutting down SR.

John

04-03-2009, 05:41 AM   #5
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The exposure meter is located on the pentaprism, just above the viewfinder, so is impacted with a focus screen change.
And the LL80 is, AFAIK, based on the same fresnel lens formulae as the original focus screen, and so will suffer from the very same problem... (and, on an aside note, the LL80 is not a "rule of thirds" screen as the LL60, but a less useful "rule of fourths" screen).
04-03-2009, 06:00 AM   #6
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Palmor : it's just my own cross-interpretation of the two following points:

1 - an 18-55 f3.5/5.6 expose "quite right" (without any underexposure).
2 - the original focus screen is not linear, thus inducing heavy underexposure with fast lenses.

So, "highlight savings" is IMO just a byproduct of their focus screen design (made to improve viewfinder brightness with all A-lenses, by the way, which would be a good idea if the meter was calibrated accordingly).
You increase the screen's brightness, the meter will think it has a brighter scene, and finally underexposes the scene...

They calibrated the meter to expose "just right" with the kit lenses (and that makes sense to me).
And when they finally tried with fast lenses, they surely went "hey, okay, this underexposes quite a bit, but this way we won't overexpose things, so we won't get flamed by customers"...
As there is no lenses slower than f/5.6 in the current pentax line (and the occasional 1000/8 lens does not qualify, I think!), the overexposure problem below f/5.6 went unnoticed.

I'm quite sure they went right to the "anyway, 95% of customers will stick to kit lenses, so what's the point of changing this now?".

I still think their decisions (even if not for the reasons I gave above) were good, but they could have documented this a little more clearly, and mentioned that a LL60 screen solves the problem for those of us that want to use stop-down or PK-m lenses...

Anyway, now, my FA50 f/1.4 exposes just like my 18-55...
04-03-2009, 06:12 AM   #7
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QuoteOriginally posted by dlacouture Quote
Palmor : it's just my own cross-interpretation of the two following points:

1 - an 18-55 f3.5/5.6 expose "quite right" (without any underexposure).
2 - the original focus screen is not linear, thus inducing heavy underexposure with fast lenses.

So, "highlight savings" is IMO just a byproduct of their focus screen design (made to improve viewfinder brightness with all A-lenses, by the way, which would be a good idea if the meter was calibrated accordingly).
You increase the screen's brightness, the meter will think it has a brighter scene, and finally underexposes the scene...

They calibrated the meter to expose "just right" with the kit lenses (and that makes sense to me).
And when they finally tried with fast lenses, they surely went "hey, okay, this underexposes quite a bit, but this way we won't overexpose things, so we won't get flamed by customers"...
As there is no lenses slower than f/5.6 in the current pentax line (and the occasional 1000/8 lens does not qualify, I think!), the overexposure problem below f/5.6 went unnoticed.

I'm quite sure they went right to the "anyway, 95% of customers will stick to kit lenses, so what's the point of changing this now?".

I still think their decisions (even if not for the reasons I gave above) were good, but they could have documented this a little more clearly, and mentioned that a LL60 screen solves the problem for those of us that want to use stop-down or PK-m lenses...

Anyway, now, my FA50 f/1.4 exposes just like my 18-55...
Hmm, that is interesting. I do shoot with fast lenses (FA50 f1.4 and FA 35 f2.0, Sigma 17-70 f2.8-4.5, DA*300 f4 among others) and I don't see any more underexposure with those lenses as I do with my slower f/5.6 ones. What I do see is that the stop down metering of old A lenses is off from my istDS.

Now I do tend to use center weighted or spot metering 99.9% of the time so I wonder if using the matrix metering has more impact by the focusing screen like you're seeing?



John
04-03-2009, 09:54 AM   #8
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The exposure problems due to the focus-screen only affect lenses that have to stop down to meter. Basically, that's older manual focus lenses without an "A" position on the aperture ring, and those lenses *only*. And it also is only an issue with the K10D & K20D - the other Pentax DSLR's seem to meter just fine even with those older lenses.

Since KCC didn't mention anything about using older manual focus lenses, I doubt any of that is relevant, though. I'm guessing he's just referring to the general fact that Pentax cameras tend to expose so that highlights are not blown out, on the assumption you might then brighten the shadows in post processing. If you would prefer to have your highlights blown out in order to not have to lighten shadows later, you can switch to center-weighted metering instead of multi-segment, which does a pretty good job of that. Or you can dial in positive exposure compensation - but the amount to dial in depends entirely on the scene. Some will require none, or even negative compensation (eg, a black cat will come out gray by default). Others will require a little, some will require a lot (eg, a scene with a bright light source within the picture). It's something you learn with experience. Any good book on photography that talks about exposure will give advice on this.

Oh, as for the two-second timer - it turns off SR because the assumption is that that if you're using the timer, you're on a tripod. A tripod is of coruse the best way to eliminate shake. When handholding, assume that SR works well enough that you don't need to bother with the timer. Just press the shutter gently.

04-03-2009, 07:49 PM   #9
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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
04-04-2009, 02:36 AM   #10
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Thanks for all your expert advice. My lens is Tamron 18-250mm F/3.5-6.3. I'll experiment myself to see if I need any EV compensation.

kcc
04-04-2009, 04:07 AM   #11
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The focus screen IS linear. The brightness of the screen doesn't change with light, ergo it is linear. The meter in Multi-segment metering is set in such a way as to avoid blown highlights. If you take a picture of a scene with no highlights in it, it will expose properly, but if only one segment register highlights, it will prioritize that segment to avoid blown highlights. Proof:take a flash picture with nothing that reflects the flash (like a mirror or chrome object) and your flash exposure is going to be correct. Now, take the same picture and introduce a shiny object somewhere, and the system will try to avoid blowing that object. If you use center weighted metering, the exposure system won't try to save the highlights.
04-04-2009, 11:48 PM   #12
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Marc: I've said as much in my post :
"For A lenses, the exposure is mesured wide-open, then calculated for the given aperture you want to use, so selecting f/2 will always gives half the speed of f/1.4."

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.

This can be extrapolated to all lenses... The "Highlight saving"/underexposure amount is dependant upon the largest aperture of the lens.
I'm pretty sure those A* 1000/8 lenses do overexpose...

Flyer : I'm not talking about linearity against amount of light coming in (of course, at a given aperture, twice the light results in twice the speed), but linearity against the aperture used.
Focus screens are in fact no longer just ground glass, but are now fresnel lenses... Those fresnel lenses are designed to optimize light transmission for given apertures, and so are not aperture-linear anymore.
LL80 focus screen family are designed to increase perceived brightness at any aperture above f/5.6... And this results in an underexposure. And, negative point of this design, aperture below f/5.6 (rarely seen in PKA lenses) now suffer of a darker viewfinder, and thus an overexposure...
04-05-2009, 01:07 AM   #13
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QuoteOriginally posted by kcc Quote
I am new to DSLR and I just bought the K20D for a month. Everything is fine so far. I have two questions here:
1. After buying the K20D, I looked through the Web so as to know more about the camera (and that's why I am here). I was informed that the camera is a little bit under-exposed, but I want to know how much. That is, if I set the EV compensation, but how much (1/3? 1/2?) should I set so that I can get normal exposure?
No fixed setting. Under or over exposure of the K20D, or simply exposure inaccuracy and inconsistency, varies with different lenses used and for different scenes. There is no straight rule to overcome this issue, except on a trial and error basis for each shot to be made. See some statistics on the DPR sample photos made by them and you'll know more for what I inform:-

RiceHigh's Pentax Blog: K20D Exposure Inaccuracy and Inconsistency

As for how different lenses' exposure tendency, see my homepage for some of the Pentax lenses of different generations:-

RiceHigh's (Pentax) DSLR and Lens Measurbation Page on Exposure Accuracy and More..

QuoteQuote:
2. I used to use a non-DSLR camera that also has anti-shaking property and timer. Now if I want to take a shot with long exposure time, I would simply activate the 2 sec timer so that my finger would not blur the photo. By this way normally I can have the exposure time a step slower and still get satisfactory result. Now for K20D, by some reason (the forum has discussed this), when I activate the timer, the anti-shake would automatically deactivate itself, which renders the above technique useless. Any thoughts?

Best wishes,
KCC
2 sec timer is supposed to be used for the camera mounted on a tripod and the user wants to lock up the mirror 2 seconds before. As the SR cannot work on tripod, it disables it automatically to avoid issues.

Hope all the above do help.
04-05-2009, 04:47 AM   #14
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QuoteOriginally posted by dlacouture Quote
Focus screens are in fact no longer just ground glass, but are now fresnel lenses... Those fresnel lenses are designed to optimize light transmission for given apertures, and so are not aperture-linear anymore.
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.
04-05-2009, 04:53 AM   #15
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[QUOTE=dlacouture;547892 And, negative point of this design, aperture below f/5.6 (rarely seen in PKA lenses) now suffer of a darker viewfinder, and thus an overexposure...[/QUOTE]

On older Pentax lenses, the diaphragm actuating mechanism was not linear, and that is what confuses the metering in newer cameras.
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