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Archive 2012 · Focus Accuracy with different lighting
  
 
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p.1 #1 · p.1 #1 · Focus Accuracy with different lighting


Issue: 85L is misfocusing (front focusing usually) when I am shooting with a single tungsten light source in the room giving 1/100 to 1/200, f/1.2, ISO1600. Misfocusing on 2x 5D and my 5D2. All misfocusing.

Go into the kitchen with fluorescent lighting, and the focus seems much more accurate. Lighting is about ISO400-800, 1/100 sec, f/1.2.

Brighter focus points near the tungsten light source in room 1 do not focus much/any more accurately than the distant subjects, despite them being closer to ISO800, f/1.2, 1/160.

So it's not just the brightness level. And fluorescent cycles and is claimed to throw off focus systems...but the tungsten is consistently giving me less accurate autofocus.

I suppose the main question I have is....WTF?



Apr 07, 2012 at 06:58 AM
jcolwell
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p.1 #2 · p.1 #2 · Focus Accuracy with different lighting


I suspect that it's because you're at or below the level of light that the 5D and 5DII can accurately focus.

I estimate the amount of light in the two situations as about 3 Exposure Value (EV) and 5 EV:

t 1/125s, f/1.2, ISO 1600, EV = 3
t 1/100s, f/1.2, ISO 600, EV = 5

The AF metering range for both the 5D and 5DII is shown in the manual to be -0.5 to 18 EV, using a 50/1,4 and ISO 100. To me, it figures that increasing the ISO that you're using to take photos doesn't increase AF accuracy in low light, as it simply multiplies the noise at the low levels of sensitivity. If you scale the AF metering floor with ISO (ignoring the f/1.4 vs. f/1.2 difference), you get the following equivalent EV levels for minimum AF metering:

ISO 100, EV -0.5
ISO 200, EV 0.5
ISO 400, EV 1.5
ISO 800, EV 2.5
ISO 1600, EV 3.5

So, in your first condition with EV = 3, you may have been below the AF metering range of teh camera, while in the second at EV = 5, you were within the AF metering range.

I'm not sure if this line of reasoning is correct, but it really doesn't matter to me. In these conditions, I use focus assist from a Speedlite or ST-E2.



Apr 07, 2012 at 10:14 AM
Gochugogi
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p.1 #3 · p.1 #3 · Focus Accuracy with different lighting


Back in the day (1998) I used a ST-E2 on my EOS 3 for AF assist in low light. And while AF systems have improved, they still need light and contrast to lock, so the ST-E2 (or a disabled Speedlite with enabled AF assist) is still very useful under murky conditions.Sadly, AF assist has disappeared from many DSLRs. My 1992 EOS 5 had near IR AF assist and could focus on anything in pitch darkness. Heck, my Oly E-P3 has a wee light on the front for AF assist. So, like jcolwell, I find the ST-E2 still welcome on top of my 5D2.


Apr 07, 2012 at 10:28 AM
Daan B
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p.1 #4 · p.1 #4 · Focus Accuracy with different lighting


Yes, try an AF assist light. Really makes a big difference in low light conditions.


Apr 07, 2012 at 10:43 AM
Eyeball
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p.1 #5 · p.1 #5 · Focus Accuracy with different lighting


You may already be considering this in your informal tests but remember that contrast at the focus point is a factor as well as quantity and color temperature of the light. If one scene has more diffuse lighting or it is hitting the subject direct-on, that could result in poorer AF performance.


Apr 07, 2012 at 01:37 PM
morganb4
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p.1 #6 · p.1 #6 · Focus Accuracy with different lighting


I get more inexplicable focussing errors under tungsten.


Apr 07, 2012 at 02:43 PM
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p.1 #7 · p.1 #7 · Focus Accuracy with different lighting


jcolwell wrote:
I suspect that it's because you're at or below the level of light that the 5D and 5DII can accurately focus.

I estimate the amount of light in the two situations as about 3 Exposure Value (EV) and 5 EV:

t 1/125s, f/1.2, ISO 1600, EV = 3
t 1/100s, f/1.2, ISO 600, EV = 5

The AF metering range for both the 5D and 5DII is shown in the manual to be -0.5 to 18 EV, using a 50/1,4 and ISO 100. To me, it figures that increasing the ISO that you're using to take photos doesn't increase AF accuracy in low
...Show more


Just for your reference, I put a 580EX on the cameras and it never activated the AF assist beam, which should go on whenever needed. The cameras also don't do the dim-light "hesitation" that I have seen them do whenever it approaches being too dark.



Apr 07, 2012 at 02:56 PM
 

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p.1 #8 · p.1 #8 · Focus Accuracy with different lighting


Eyeball wrote:
You may already be considering this in your informal tests but remember that contrast at the focus point is a factor as well as quantity and color temperature of the light. If one scene has more diffuse lighting or it is hitting the subject direct-on, that could result in poorer AF performance.


All objects I attempted to focus on had noticeable contrast. Doorknob against the door, dark blue light switch against a nearly white wall, black letters/music notes on a white background...



Apr 07, 2012 at 02:58 PM
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p.1 #9 · p.1 #9 · Focus Accuracy with different lighting


Found info about this from an old post on FM:

theSuede wrote:

The explanation is very straightforward, and actually quite simple. The AF sensor is monochromatic, and follows the natural color band sensitivity of silicon. This means that it is much more sensitive to red than to blue (but some filters above the actual sensor balance things out a bit at least...).

In this case (very warm WB 2200-2800K vs daylight 5000-6500K WB) you shift the amount of light energy very much towards the "only red" end of the bandwidth. And since longitudinal chromatic aberration makes a bigger difference the further out you get, the point of "best focus" also shifts. Unfortunately then
...Show more

If I understand correctly, this means that white-balance contrast in my first scene is very limited and is causing the problem.


Edited on Apr 07, 2012 at 03:05 PM · View previous versions



Apr 07, 2012 at 03:02 PM
Bruce Sawle
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p.1 #10 · p.1 #10 · Focus Accuracy with different lighting


There are tons of information n the Internet regarding this. Bottom line something with the artificial lighting cause focus shift with some lense and camera combos. Happens with me on my 85 drives me nuts. Outside or with flash it works fine.


Apr 07, 2012 at 03:04 PM
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p.1 #11 · p.1 #11 · Focus Accuracy with different lighting


I wish there was a solution to make it not happen because I shoot a fair amount in tungsten lighting as required in my job.


Apr 07, 2012 at 03:11 PM
dwweiche
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p.1 #12 · p.1 #12 · Focus Accuracy with different lighting


form wrote:
I wish there was a solution to make it not happen because I shoot a fair amount in tungsten lighting as required in my job.

I suppose on your 5DII you could use the cameras focus adjust and come up with a "tungsten offset" to use only in those situations...



Apr 07, 2012 at 06:10 PM
mttran
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p.1 #13 · p.1 #13 · Focus Accuracy with different lighting


Daan B wrote:
Yes, try an AF assist light. Really makes a big difference in low light conditions.


+1, this is how you AF 5D(1/2) with fast lenses



Apr 07, 2012 at 06:27 PM





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