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View Poll Results: Do you get focusing problems in tungsten lighting?
Yes 4152.56%
No 3747.44%
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12-25-2008, 12:34 PM   #1
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POLL: Who has front-focusing in tungsten lighting?

It honestly bugs me that I get focusing differences in different lighting color temperatures on my K20D. I did some tests with the following conditions:

1. On a tripod for exact scene
2. Largest aperture on lens (DA 70mm f/2.4)
3. One room in tungsten and one room in flourescent lighting
4. 3 shots with no AF adjustment, 3 shots with -9 AF adjustment
5. Before each shot I completely de-focus using the focusing ring
6. Using center AF point
7. Used a target that's parallel to the sensor plane with text for testing legibility

Here are the results:

Tungsten room scene:


No AF adjustment:


With AF adjustment:


Flourescent room scene:


No AF Adjustment:


With AF Adjustment:


----------

I'm using a K20D. Do you get this too?

12-25-2008, 02:35 PM   #2
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I believe this is physics. Different color lights have different wavelenths and are diffracted at different angles. Particularly noticeable if you focused in tungsten or flourescent and shot with flash. Flash being a different wavelength, comes to focus at different plane. Try these tets in daylight and see if there is any focusing issues. Cheers.
12-25-2008, 02:49 PM   #3
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Original Poster
I didn't use the flash in the shots. I did test in daylight and the focusing is perfect without AF-assist. I couldn't do it with the tests I did above since it's 5am here. So far focusing is fine in daylight, flash (flash for AF-assist), and flourescent.

Tungsten seems to be the one causing the funky-ness. I tried using a CTO gel on the flash in a dark room (AF-assist) to simulate tungsten lighting and it did front focus again.

Anyone else get this? I'm thinking of sending it to the service center here early next year
12-25-2008, 03:21 PM   #4
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It might also be interesting to test the same setup, manually focused, with different lighting, without changing focus.

BTW I have noticed front focusing with tungsten also.

12-26-2008, 11:22 PM   #5
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QuoteOriginally posted by soccerjoe5 Quote
It honestly bugs me that I get ... (snip)

Try the same large, relatively flat, two-dimensional subject (the calendar) in each room. With the Blu-Tack box (a small three-dimensional object surrounded by other objects at slightly different distances within the area covered by the camera's selected focus point), I suspect the camera is either intentionally focusing at some point other than the front of the box or attempting to find a compromise between the various distances within the selected focusing area.

stewart
12-27-2008, 08:03 AM   #6
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No Poll is actually Needed!

It has been verified by so many different Pentax DSLR users and for so many times and this problem is well known. And, it has never been changed for any Pentax DSLRs of which users up to the K20D still reporting the same.

Note that some users haven't noticed that doesn't mean the problem is non-existent, say, if they use slow lenses with small apertures and/or at wider angles.

More summarised info: RiceHigh's Pentax Blog: AF Accuracy's Dependency on Lenses and Yellow Light
12-27-2008, 08:07 AM   #7
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Physics?

QuoteOriginally posted by polur101 Quote
I believe this is physics. Different color lights have different wavelenths and are diffracted at different angles. Particularly noticeable if you focused in tungsten or flourescent and shot with flash. Flash being a different wavelength, comes to focus at different plane. Try these tets in daylight and see if there is any focusing issues. Cheers.
If it is Physics (in general sense), I just cannot understand why my MZ-S and MZ-30 both have no problem with the same lens (my FA*85/1.4) whilst with my *ist D, *ist D and K100D are all having front focus problem.

Yes, its Physics but its the Physics inside the DSLR bodies but not in general.
12-27-2008, 08:11 AM   #8
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Normal!

QuoteOriginally posted by soccerjoe5 Quote
It honestly bugs me that I get focusing differences in different lighting color temperatures on my K20D. I did some tests with the following conditions:

1. On a tripod for exact scene
2. Largest aperture on lens (DA 70mm f/2.4)
3. One room in tungsten and one room in flourescent lighting
4. 3 shots with no AF adjustment, 3 shots with -9 AF adjustment
5. Before each shot I completely de-focus using the focusing ring
6. Using center AF point
7. Used a target that's parallel to the sensor plane with text for testing legibility

Here are the results:

Tungsten room scene:
http://i52.photobucket.com/albums/g9/soccerjoe5/scenes.jpg

No AF adjustment:
http://i52.photobucket.com/albums/g9/soccerjoe5/tungstennoadjust.jpg

With AF adjustment:
http://i52.photobucket.com/albums/g9/soccerjoe5/tungstenwadjust.jpg

Flourescent room scene:
http://i52.photobucket.com/albums/g9/soccerjoe5/scenes-2.jpg

No AF Adjustment:
http://i52.photobucket.com/albums/g9/soccerjoe5/flourescentnoadjust.jpg

With AF Adjustment:
http://i52.photobucket.com/albums/g9/soccerjoe5/flourescentwadjust.jpg

----------

I'm using a K20D. Do you get this too?
AF Adjustment can NEVER help you to solve the problem. You can EITHER choose to have AF to focus correctly (in general) under yellow light OR white light. You CAN'T have BOTH!

The achieved focus point by the AF of Pentax DSLRs will always be FRONTER under YELLOW LIGHT! No matter when and where and no wonder!

12-27-2008, 09:44 AM   #9
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QuoteOriginally posted by RiceHigh Quote
It has been verified by so many different Pentax DSLR users and for so many times and this problem is well known. And, it has never been changed for any Pentax DSLRs of which users up to the K20D still reporting the same.

Note that some users haven't noticed that doesn't mean the problem is non-existent, say, if they use slow lenses with small apertures and/or at wider angles.

More summarised info: RiceHigh's Pentax Blog: AF Accuracy's Dependency on Lenses and Yellow Light
And Canon's (some) backfocus under flourescent light (for some). What's your point?
I have just discovered this exact same anomaly with my 30D. I just got a used 85L and a 30 f/1.4 (the latter of which I was sure backfocused, so I sent it to Sigma service for calibration...came back much better today...but now I think it may have been this all along.) The 85L performed beautifully during the day when I was taking shots outside. The 30 f/1.4 was spot on when taking shots of my daughter under incandescent light. I went up to my office, which is lit with fluorescents, and took shots in my kitchen, which also has them, and in both cases, both the 30 f/1.4 and 85L exhibited noticeable backfocus. Tried again this morning...same story. Shots in my living room were spot on...in the kitchen...backfocus.

Remembering previous 'wow, a lot of my lenses appear to backfocus all of a sudden' thoughts (100 f/2, 28 f/1.8, even though both performed well in real world shooting)' ..I was taking test shots in my office at home, and at work, which has fluorescent light too. Also, like the OP, it doesn't show up on lenses f/2.8 or slower.
Did you ever find a solution to this interesting anomaly?
30D backfocusing under fluorescent light - FM Forums

No AF sensor is perfect.
Care to try this w/ your 5D and report results? Make sure you use a good fast lens.
don't forget film has "thickness" and AF errors can be covered up. CCD/CMOS are essentially 2 dimensional and are more prone to AF "close but not perfect" focus.
A starting point to understanding AF......
AF can compensate for most of the chromatic aberration - the AF sensors see the same colour as the main sensor and control the focus the same way as with white light. However, one of the biggest weaknesses of the Phase Detection AF system is that it cannot compensate for spherical aberration - the shift in focus between the edge of the lens and the centre. This should be fairly obvious with reference to the previous diagram of the Canon AF system - the AF only uses the parts of the lens corresponding to f/5.6 (or f/2.8 in the central cross sensor), while the image is produced by the full aperture. Since this focus difference gets worse at the red and blue ends of the spectrum, so does the AF error.

As I hope I have explained, this chromatic focus shift isn't a fixed amount for all lenses, it is different for each lens design - and obviously different depending on which aperture you are actually shooting at.

So, to make use of the colour of the incoming light, the AF sensor would also have to know how much chromatic and spherical aberation each lens that could be fitted to the camera actually had, and compensate for it accordingly based on some a-priori knowledge. There already is some pre-programmed data in the AF system about the white light spherical aberation of each Canon lens but doing that across the spectrum requires a lot more data and a much more complex (and hence expensive) test and calibration procedure. I doubt we will ever see that.
--
Its RKM

http://forums.dpreview.com/forums/read.asp?forum=1032&message=29849436&q=foc...ors+light&qf=m
If you are going to use MF regularly - and you can do a better job with MF than AF achieves on the outer points at f/2.8 or faster - then you would be well advised to get hold of the Ee-S focus screen. It makes the viewfinder image slightly darker than the standard screen, but focus is much crisper.

Also, if you want to know why this trade-off between screen brightness and focus precision occurs, Doug Kerr's explanatory document of MF and AF is a great start. http://doug.kerr.home.att.net/pumpkin/Split_Prism.pdf

Last edited by jeffkrol; 12-27-2008 at 06:11 PM.
12-29-2008, 07:44 AM   #10
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Thanks for the input everyone!

I conclude for me, almost exclusively manual focussing with the original matte screen on K100D:

- slight daylight backfocus could be the rule (that's so with my cam)

- basic 'lens laws' would dictate this problem to be especially delicate with high speed apertures and/or longer focal lengths (that's so with my cam)

- stated in the linked articles, and most important for me: It depends on the lens (even between same FL/speed lenses). I don't like this fact, cause I happily change between say 30 lenses or more, so I should really concentrate on some of them and really know their behaviour - Know thy lens!

- last, I've read somewhere that you can already estimate a lenses' color-sensitivity (performance-wise as well as focus-wise) if you look at the distance from the red infrared-mark to the 'daylight' mark (only same FL/speed is comparable)

If Pentax and Nikon should be in the same camp on this one, and Canon would be on the other side, somebody doing some basic comparative testing would be nice :-)


(if picture shows: f1.2/55 at near distance. Tough focus anyway)

Thanks again for the valuable info,
Georg (the other)
12-29-2008, 11:21 AM   #11
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The results of my testing showed front focussing in tungsten light, but only at wide apertures (1.4, 2.8). In daylight all were fine at large apertures, and at F4 all my lenses focussed perfectly, even under tungsten light.
12-29-2008, 07:39 PM   #12
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QuoteOriginally posted by audiobomber Quote
The results of my testing showed front focussing in tungsten light, but only at wide apertures (1.4, 2.8). In daylight all were fine at large apertures, and at F4 all my lenses focussed perfectly, even under tungsten light.
Canon whitepapers have some interesting info re: this. Pentax is probably a little behind in this area (fast lenses.). Technically they are still working on it as can be seen from the newer models.
As on the EOS 50D, the EOS 5D Mark II’s AF system is sensitive to light levels as low as
EV -0.5, and capable of locking on to a moving subject and tracking it across the diamondshaped
AF area. Improved precision over previous AF systems stems from each camera’s
ability to adjust for different light sources while calculating focusing distance. Here’s how it
works: during Phase Detection AF, the AF engine rapidly measures the density of horizontal
and vertical details in a scene and the camera processor uses this data to determine where
the sharpest edges and details are located.When found, the AF is locked on target. But
under fluorescent lighting and some other artificial lighting sources, a rapid, imperceptible
flickering of the light occurs along with sudden color temperature shifts. These tend to
throw off the density measurements and therefore the accuracy of the AF calculations. In both
the EOS 50D and EOS 5D Mark II, the type of lighting is taken into effect, and readings
are averaged and processed with the help of the super-fast DIGIC 4 Image Processor.
The result is improved AF accuracy and speed when shooting under lighting conditions that
could fool the AF systems found on earlier Canon and competitive models alike

http://www.usa.canon.com/uploadedimages/FCK/Image/White%20Papers/EOS%2050D%2...%20II%20WP.pdf

Sorry to keep adding to this but I wanted to collect a few choice tidbits.............
> A lot of dslr systems don't have backfocus or
> calibration issues.


A lot? Nikon, Canon, Sony, Pentax, and Panasonic all have focus calibration issues.

Do you know what the main cause of focus calibration issues is? An optical phenomenon called "spherical aberration". That means that the rays of light from the outer portions of the lens's exit pupil (the widest parts of the aperture) converge on a different plane than rays from the inner portion. The AF sensors are aimed at the inner portion, typically f5.6, so they can focus lenses f5.6 and faster. The focusing screen (remember the pictures Jay Turberville posted) also only scatters light enough to see the inner (f4 or 5.6) portion of the screen.

What do those systems all have in common? Fast lenses. Nikon, Canon, and Sony also have very high MP count cameras, which makes the focusing errors even more noticeable. Canon being the most insane, with two 21+mp cameras, two f1.2, 3 f1.4, and 7 f2.0 lenses.

And until Oly brought out the two f2.0 zooms, what did they have? An f2.0 macro (like all macros, weighing more and costing more than lenses a stop or two faster, in order to correct SA, CA, distortion, and field curvature) and a well corrected, but insanely priced 150mm f2.0.
And still no f1.4 under their own name.


http://forums.dpreview.com/forums/read.asp?forum=1018&message=30505523

Last edited by jeffkrol; 12-30-2008 at 01:01 PM.
12-29-2008, 08:47 PM   #13
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Here is my proposed solution to this... if shooting indoors under tungsten w/o flash, I switch to my tungsten calibrated focus settings. Otherwise, I leave it unadjusted.

https://www.pentaxforums.com/forums/pentax-dslr-discussion/44548-proposed-sol...ont-focus.html
12-30-2008, 06:33 AM   #14
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QuoteOriginally posted by laissezfaire Quote
Here is my proposed solution to this... if shooting indoors under tungsten w/o flash, I switch to my tungsten calibrated focus settings. Otherwise, I leave it unadjusted.

https://www.pentaxforums.com/forums/pentax-dslr-discussion/44548-proposed-sol...ont-focus.html
So, you mean you adjust / change the settings each time you should under different colours of lights? And if you go indoor and outdoor and in between rooms with yellow and white lights??
12-30-2008, 10:18 AM   #15
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QuoteOriginally posted by RiceHigh Quote
So, you mean you adjust / change the settings each time you should under different colours of lights? And if you go indoor and outdoor and in between rooms with yellow and white lights??

This is exactly what I do with my K10D. When in daylight, I use a setting of -140. When in incandescent light, I use a setting of -90. I've not worked out the number to use under compact flourescent yet.

When in mixed light, I stop down to at most f2.8 and the focus point is usually good enough.

PS. Yes, it's an annoyance to remember each time.
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