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09-17-2014, 06:40 AM   #1
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Focus confirm with fast primes in low light

Hi All

I have with me the following pentax camera bodies - k100ds, k200d, k10d, k-x

I like to do low light photography using a pentax-m 50mm f1.4 and f1.2. Now this combination works in bright daylight conditions and I am able to get accurate focus most of the time. But when I use this under low light conditions it doesn work fine all the time. Say I am in a dimly lit street I look for some object within the focal distance which has some light falling on it. after a few seconds the camera confirms its in focus and i click it but when I look at the pic I can clearly see its not in focus. I also tweaked focus using debug/secret menu and tried various settings but most of the times it doesnt focus right. I was wondering whether I need to replace the existing focusing screen (which is the standard matte finish screen which comes in all pentax bodies) and replace it with a ground glass or split prism screen in order for more light to get through and focus confirm to work right in low light condition. else do i need to do something else?

replies appreciated and thank you for your time

09-17-2014, 07:33 AM   #2
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I'm gonna guess you have ruled out the motion blur possibility. 50mm at 1.2 is an insanely shallow depth of field for relatively close subject. Any movement on your part after focus confirmation and before the picture will make the picture blurry. Have you tried with a tripod, just to make so test?
Also, your K-x has live view, so you could try using the magnified view to focus and take the picture, just to see what sort of result your get.
It could well be that the Tungsten lighting messes up your camera's focus calculation, but at 1.2 - 1.4, the margin of error is so small that "user error" cannot be ruled out entirely.
Good luck!
09-17-2014, 07:34 AM   #3
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QuoteOriginally posted by aruk5 Quote
I was wondering whether I need to replace the existing focusing screen (which is the standard matte finish screen which comes in all pentax bodies) and replace it with a ground glass or split prism screen in order for more light to get through and focus confirm to work right in low light condition.
The k-x will have three separate focusing systems:
1) The viewfinder AF which also controls the AF confirm light, which uses PDAF
2) The LCD focusing which is independent of the viewfinder system completely and uses CDAF
3) The manual focusing system that shows an image on the focusing screen and allows you to adjust the focus until it looks correct

Changing the focusing screen may improve manual focus as it is rendered on the focusing screen, it has nothing to do with the AF confirm light or either AF system. In some cases, depending on the screen, it may actually make the view darker though most screens do not change things much. I used an after market focusing screen in my k-x for years.

Part of the problem is that you are perhaps relying on the AF confirm light which is not accurate below perhaps f/4. The AF light will come on if the focus is good enough for f/4 but if you are at f/1.4 the depth of focus is much smaller so what is in focus may not be what you intended. I have had some success using the 'edge' of the AF confirm light, meaning that near to far edge or far to near edge may result in consistent focus, whereas the center of the confirm range may no. This will vary depending on lens of course.
09-17-2014, 08:25 AM   #4
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I can say about K200D only. If you take pictures in twilight or so, focus confirmation may work properly, but you may not see where exactly it focuses. And even bright object seems bright enough, still, you can not see clearly. I've been tricked that way, even with magnifying viewfinder eyepiece.
Or with wide open it's shallow DOF. Or even you think your speed is fast enough, it's not, even with shake reduction.
Anyway, for K200 in twilight with manual focus lens tripod must have, imo, since ISO higher than 400 number usually is not good idea for that camera.

09-17-2014, 08:35 AM   #5
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The title of this thread holds the key. Focus confirm uses the AF system to indicate point of focus and unfortunately there are limits to the focus sensitivity (ability to detect an out-of-focus condition) for the PDAF sensors. The PDAF sensors on your cameras have the same sensitivity at f/5.6 as with your fastest lenses. What that translates into is a high probability of missed focus when shooting at wider apertures.

As noted above, you have a couple of options for fine focus with faster lenses:
  • Magnified live view using focus peaking (newest bodies only)
  • Viewfinder magnifier using the stock focus screen. Some users on this site have reported good success with traditional manual focus by simply magnifying the viewfinder image.
  • Switching to an aftermarket focus screen having greater focus sensitivity and/or focus aids (split-image or microprism)
In regards to the stock focus screen...It is nice and bright, but unfortunately has focus sensitivity that is not much better than the PDAF system. Estimates indicate a limit of about f/4. A high sensitivity matte field (e.g. Canon-derived S-type screen) or a split-image viewfinder aide will provide sensitivity adequate for your M 50/1.2.*

An aftermarket screen was my choice several years ago when I could not get acceptable results from any of my faster lenses. I have not regretted that choice, though there were trade-offs. Potential issues include loss of spot metering (split-image types), other metering issues, and calibration (shim) problems. Often, the stock screen calibration requires no changes. If adjustment to the shims is needed, it can be tedious.

Good luck.


Steve

* The most popular screens on this site are the Type-S from focusingscreen.com, the KatzEye screen, and products from various eBay vendors.
09-17-2014, 05:13 PM   #6
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The Pentax O-ME53 1.2X magnifying eyepiece (or similar magnifying eyepieces like the nice Tenpa 1.36X, and others) are indeed viable options. I'd even consider them option 1, due to their ease of installation and use.
09-17-2014, 07:16 PM   #7
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The universal 1.3x magnifier system also is very good - quality multi-coatings - 1 3X Magnifier View Finder Eyecup for Canon Nikon Pentax Olympus Sony DSLR | eBay.

Some of the newer cameras (K5ii/s and K3) have advanced AF precision that is pretty good for f/2-2.8. The older cameras such as the ones you have listed require your talents to focus correctly manually.

Changing focus screens might help in low light, but has a trade-off in normal light - and might degrade exposure consistency. Its a bit of a crap shoot and can be rather expensive.
09-17-2014, 07:26 PM   #8
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It is pretty easy to exceed the Ev limits of the AF sensors. The K100DS AF only works down to Ev 0. At its base ISO of 200, that's 0.5 sec at f1.4, a bit of a risk for handheld shots but I might try it. The other cameras go to Ev -1. That might be the problem.

09-18-2014, 04:28 AM   #9
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Original Poster
QuoteOriginally posted by kp0c Quote
I'm gonna guess you have ruled out the motion blur possibility. 50mm at 1.2 is an insanely shallow depth of field for relatively close subject. Any movement on your part after focus confirmation and before the picture will make the picture blurry. Have you tried with a tripod, just to make so test?
Also, your K-x has live view, so you could try using the magnified view to focus and take the picture, just to see what sort of result your get.
It could well be that the Tungsten lighting messes up your camera's focus calculation, but at 1.2 - 1.4, the margin of error is so small that "user error" cannot be ruled out entirely.
Good luck!
Thank you for posting a reply

The recommended shutter speed I got was 1/125 at iso 800 so I guess i didnt require a tripod. Plus the SR in pentax bodies is good upto 1/15 shutter speed (atleast for me when I hold the camera as still as possible)

K-x live view with magnification is difficult to use handheld plus the live view image is a downscaled version of the sensor's actual resolution and creates false edges which look sharp but when clicked the image appears a bit soft

Its possible the white balancing under tungsten lighting might screw up the camera's focus. will look into it and change white balance and see if it affects focus confirmation

---------- Post added 09-18-14 at 12:35 PM ----------

QuoteOriginally posted by jatrax Quote
The k-x will have three separate focusing systems:
1) The viewfinder AF which also controls the AF confirm light, which uses PDAF
2) The LCD focusing which is independent of the viewfinder system completely and uses CDAF
3) The manual focusing system that shows an image on the focusing screen and allows you to adjust the focus until it looks correct

Changing the focusing screen may improve manual focus as it is rendered on the focusing screen, it has nothing to do with the AF confirm light or either AF system. In some cases, depending on the screen, it may actually make the view darker though most screens do not change things much. I used an after market focusing screen in my k-x for years.

Part of the problem is that you are perhaps relying on the AF confirm light which is not accurate below perhaps f/4. The AF light will come on if the focus is good enough for f/4 but if you are at f/1.4 the depth of focus is much smaller so what is in focus may not be what you intended. I have had some success using the 'edge' of the AF confirm light, meaning that near to far edge or far to near edge may result in consistent focus, whereas the center of the confirm range may no. This will vary depending on lens of course.
Thank you for posting a reply

Its interesting to know the af system is designed for f4 but I must mention when I use the da 50mm f1.8 the af works fine and I have managed to get sharp results in relatively low light conditions

so changing a focusing screen as far as focus confirmation is concerned wont have much of a benefit, thanks for pointing that out.

Can you please explain a bit more about the edge method?

---------- Post added 09-18-14 at 12:42 PM ----------

QuoteOriginally posted by micromacro Quote
I can say about K200D only. If you take pictures in twilight or so, focus confirmation may work properly, but you may not see where exactly it focuses. And even bright object seems bright enough, still, you can not see clearly. I've been tricked that way, even with magnifying viewfinder eyepiece.
Or with wide open it's shallow DOF. Or even you think your speed is fast enough, it's not, even with shake reduction.
Anyway, for K200 in twilight with manual focus lens tripod must have, imo, since ISO higher than 400 number usually is not good idea for that camera.
Thank you for posting a reply

I have a k200d body and apart from higher megapixel and better af it more or less shares same features as the k100d

anyway same problem while trying to shoot in low light

I have tried shooting with a tripod and its same under f1.2 (incase u assumed the focusing was affected due to shallow depth of field and handheld shake)

---------- Post added 09-18-14 at 12:50 PM ----------

QuoteOriginally posted by stevebrot Quote
The title of this thread holds the key. Focus confirm uses the AF system to indicate point of focus and unfortunately there are limits to the focus sensitivity (ability to detect an out-of-focus condition) for the PDAF sensors. The PDAF sensors on your cameras have the same sensitivity at f/5.6 as with your fastest lenses. What that translates into is a high probability of missed focus when shooting at wider apertures.

As noted above, you have a couple of options for fine focus with faster lenses:
  • Magnified live view using focus peaking (newest bodies only)
  • Viewfinder magnifier using the stock focus screen. Some users on this site have reported good success with traditional manual focus by simply magnifying the viewfinder image.
  • Switching to an aftermarket focus screen having greater focus sensitivity and/or focus aids (split-image or microprism)
In regards to the stock focus screen...It is nice and bright, but unfortunately has focus sensitivity that is not much better than the PDAF system. Estimates indicate a limit of about f/4. A high sensitivity matte field (e.g. Canon-derived S-type screen) or a split-image viewfinder aide will provide sensitivity adequate for your M 50/1.2.*

An aftermarket screen was my choice several years ago when I could not get acceptable results from any of my faster lenses. I have not regretted that choice, though there were trade-offs. Potential issues include loss of spot metering (split-image types), other metering issues, and calibration (shim) problems. Often, the stock screen calibration requires no changes. If adjustment to the shims is needed, it can be tedious.

Good luck.


Steve

* The most popular screens on this site are the Type-S from focusingscreen.com, the KatzEye screen, and products from various eBay vendors.
Thank you for posting a reply

Its quite possible due to the focus sensitivity on the pdaf sensor towards f5.6 might be reason why fast primes might not be focusing accurately

The stock screen which comes along with the camera bodies have microprisms which ensure to a certain extent even narrow aperture lenses appear bright in thye viewfinder and when you mount a fast prime the viewfinder displays dof of lens at an aperture of f3.5 or something close to that. I could see this when the bokeh of the f1.2 appears very different when seen thru the viewfinder and when actually clicked

I have tried a cheap chinese viewfinder magnifier attachment but hasnt solved my problem

was planning on going for a split prism screen or the more expensive katzeye but there is still the issue of the viewfinder view still been small for aps-c bodies as compared to film camera bodies which had larger pentaprisms and viewfinder and made manual focusing a breeze

---------- Post added 09-18-14 at 12:51 PM ----------

QuoteOriginally posted by rawr Quote
The Pentax O-ME53 1.2X magnifying eyepiece (or similar magnifying eyepieces like the nice Tenpa 1.36X, and others) are indeed viable options. I'd even consider them option 1, due to their ease of installation and use.
Thank you for posting a reply

will look into this item certainly

---------- Post added 09-18-14 at 12:55 PM ----------

QuoteOriginally posted by ScooterMaxi Jim Quote
The universal 1.3x magnifier system also is very good - quality multi-coatings - 1 3X Magnifier View Finder Eyecup for Canon Nikon Pentax Olympus Sony DSLR | eBay.

Some of the newer cameras (K5ii/s and K3) have advanced AF precision that is pretty good for f/2-2.8. The older cameras such as the ones you have listed require your talents to focus correctly manually.

Changing focus screens might help in low light, but has a trade-off in normal light - and might degrade exposure consistency. Its a bit of a crap shoot and can be rather expensive.
thank you for posting a reply

i am considering trying out that one and hope and hope its not the same as the chinese one i bought sometime back

not planning on going for the new models as money is limited

u r right it takes talent and skill to get precise focus using manual lenses on older pentax aps-c bodies

i have considered changing focusing screen with a split prism screen but the viewfinder is still small and I think I will have trouble viewing if the prism display in the viewfinder has fully turned into sharp focus as compared to older film bodies with split prisms which were easy to view at

---------- Post added 09-18-14 at 12:59 PM ----------

QuoteOriginally posted by Just1MoreDave Quote
It is pretty easy to exceed the Ev limits of the AF sensors. The K100DS AF only works down to Ev 0. At its base ISO of 200, that's 0.5 sec at f1.4, a bit of a risk for handheld shots but I might try it. The other cameras go to Ev -1. That might be the problem.
I prefer the k100d's sensor (ccd) compared to the new cmos sensors on pentax bodies. One thing is the good highlight recovery and 'film like' output you get on the k100d's sensor while the new cmos sensors give out a more digital like look to the pic. Ofcourse under high isos the cmos sensors have a major edge but I rarely go above iso 800 so not an issue for me

i think its very hard to get a sharp non blurry pic at 0.5 second with the k100d. The least I have managed to go is 1/15 (after many tries).
09-18-2014, 05:26 PM   #10
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QuoteOriginally posted by aruk5 Quote
Thank you for posting a reply Its interesting to know the af system is designed for f4 but I must mention when I use the da 50mm f1.8 the af works fine and I have managed to get sharp results in relatively low light conditions so changing a focusing screen as far as focus confirmation is concerned wont have much of a benefit, thanks for pointing that out. Can you please explain a bit more about the edge method?
I was speaking of the AF confirm light. It will come on within a range that IMHO approximates the depth of field of f/4. However, your f/1.8 will indeed focus correctly in good light. To see what I mean focus manually from near to far and take a shot as soon as the AF confirm light comes on. Now do the same by manually focusing from far to near. You will see that the focus is not in the same place as there is a bit of slop in the AF confirm light. This is what I mean by the 'edge' of the AF confirm light. Some lenses are good using the far edge, some the near edge. You have to test to see.
09-18-2014, 05:39 PM   #11
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QuoteOriginally posted by aruk5 Quote
Hi All

I have with me the following pentax camera bodies - k100ds, k200d, k10d, k-x

I like to do low light photography using a pentax-m 50mm f1.4 and f1.2. Now this combination works in bright daylight conditions and I am able to get accurate focus most of the time. But when I use this under low light conditions it doesn work fine all the time.
I think you're up against it, Aruk.

Trusting that green hexagon with low light/wide aperture is like tossing a coin. Check out one of those online DoF calculators to see the leeway you have with f8 compared to f1.4.

A newer body can help ... in Live View on the K-30 you press the OK button and you suddenly get a x6 magnification. Focus peaking is good, but if the overlay colour is white it can still be difficult, and at f1.2 will be very, very difficult with any colour.

I don't know what exactly you're shooting at f1.2 in the shade/dark, but if they're stationary subjects and you do it often enough, you could get a split prism screen and illuminate the target with a very bright torch to set perfect focus (there will need to be some kind of edge or line, won't work for a blank wall), kill the light and take the shot.
09-23-2014, 05:34 AM   #12
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I have never heard of such focusing issues as I do here in these forums. Only in very low light does my ancient D200 ever give a slight hiccup. And in this rare instance, it will not lock on with a phony focus, but simply not be able to confirm focus (again, in very low light). Otherwise, it focuses without fail, which is what I expect from a modern camera. There should be no great mysteries or puzzles in regard to focusing.
09-23-2014, 01:39 PM   #13
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I feel your pain

I actually stopped using my K-x over this past year, partly because of the AF problems (constantly having the background in focus despite using single point focusing and recomposition). When I bought the camera way back when it came out I didn't realize how annoying the AF would be. How I wished for the good ole split circle focus of my SRT, or even those little squares that turn red on a Nikon.

But I have decided it is easier to get a KatzEye screen put in than buy a new camera, and just use my 35/f1.8 in manual and get back into shooting. My original idea was to sell the camera and pick up the SRT101 again, but film is just so expensive to process, and I still want to have something manual and fun.

Long pre-amble to two questions:
1) Are there any good tricks to setting up the K-x to achieve better AF?
2) I am looking into the "Plus" Prism, but for shooting with my fast fifty (equivalent) is there any reason to get the OptiBrite? I am considering this an investment that will let me enjoy the camera for many years, and not buy a new one right now, so price isn't a huge concern in the <$200 range.
09-23-2014, 08:26 PM   #14
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QuoteOriginally posted by neely615 Quote
but for shooting with my fast fifty (equivalent) is there any reason to get the OptiBrite?
Yes. I have owned both versions and OptiBrite provides a view that is a little brighter than the stock screen, but with excellent focus sensitivity across the field. That is important when considering the viewfinders on AF cameras. There is a lot of light lost due to the half-silvered main mirror. The standard KatzEye (no OptiBrite) is less bright than the screen on your SRT 101. (I own a SRT-101 as well.)


Steve
09-24-2014, 07:25 AM   #15
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OptiBrite it is then

Thanks steve, and also the OP for bringing this up.
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