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Archive 2011 · fast prime focus shift under tungsten light?
  
 
Jim Levitt
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p.1 #1 · p.1 #1 · fast prime focus shift under tungsten light?


I'm a longtime Canon user who picked up some Nikon in the past year, looking for better full-frame focusing than the 5D2 provides. For much of my low-light music performance work, I've come to prefer the D700 - although I still remain most comfortable with the Canon controls, just from so much more time using them.

Recently I bought the 85/1.4G for the added light-gathering ability. Shallow DOF portraiture is not my main concern: I just wanted a couple of stops more light than the f/2.8 zooms can give. The lens and cameras seem to work fine in natural light. It was a different story when I tried them at a local jazz club, where the 14-24, 24-70 and 70-200/2.8 lenses have done great. So last night I spent some time trying out photos around my kitchen, under tungsten light, at the distances (mfd to about 15 feet) where I'm most likely to use the 85mm. The light was actually a couple of stops brighter than what I often encounter in performance venues, on the order of 1/800th, f/1.4, iso 800. Much to my dismay, I had to set the AF fine-tune on both my D700's to +18 or +20 to get accurate focus. I pulled out the Canon 85/1.8 and 5D2. They focused just fine, without any micro-adjust necessary.

This morning I tried some of the same shots, with natural light in the room. The D700/85G combo focused well without the need for moving the AF fine-tune off zero (at least fairly close anyway - I won't pretend this is a controlled test on a tripod.) I used only the centerpoint, in single-shot mode, no focus-recompose.

There are other threads on the web discussing this issue; I guess I'm not alone. This is a disappointment, as the f/2.8 zooms do not exhibit this sort of strange behavior under artificial light.

How do you deal with such a radical shift in focusing accuracy from daylight to tungsten? I can't imagine having to remember to engage the AF fine-tune when I walk indoors, and then to switch it off again when I go outside again. Is there a firmware fix? Can Nikon service either the lens or camera to eliminate the problem? I'm confident working with the 85G in daylight - but that's not what I need it for!

Is this a known issue with the 24/1.4G and 35/1.4G as well?



Dec 02, 2011 at 11:09 AM
Jammy Straub
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p.1 #2 · p.1 #2 · fast prime focus shift under tungsten light?


I've seen this issue under certain types of lighting even with f/2.8 zooms.

The best solution I've found is to make sure in daylight your focus tuning is dead on, use a chart and tripod and get that as accurate as you can. I get better results indoors in very dim light using AF-C and making sure the camera has a chance to AF to the best of it's abilities before I trip the shutter.

I don't have experience with the D700 85G combo, maybe a user of them can chime it.

You might see if the Sigma 85 f/1.4 gives you the same issue. Different lenses AF programming can have radically different effects on the same body.



Dec 02, 2011 at 11:48 AM
Bernie
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p.1 #3 · p.1 #3 · fast prime focus shift under tungsten light?


When you're shooting at f1.4, the focal plane is so razor thin that even your own body sway is going to affect whether a shot is in focus or not.

I'm with Jammy, shoot AF-C when shoting wide open so that the camera can track YOU.

f1.4 is a double edge sword. It also tends to be softer on a lot of lenses even when in focus.



Dec 02, 2011 at 01:07 PM
wickerprints
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p.1 #4 · p.1 #4 · fast prime focus shift under tungsten light?


It's not focus shift but rather AF error you are observing. It may be specific to this lens but it may be that this phenomenon is present for all your lenses, but you are only able to see it clearly in the 85/1.4 because of its fast aperture and the way you are using it.

There is not much you can do about it, I'm afraid to say. Because you report that AF is accurate in natural lighting but inaccurate under tungsten light--even when there is sufficient scene EV--I am inclined to conclude that the issue is with the camera body's AF system, not the lens. As such the only thing I can suggest is to send the body in for service and maybe they might find something to calibrate for you.



Dec 02, 2011 at 01:20 PM
colincarter46
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p.1 #5 · p.1 #5 · fast prime focus shift under tungsten light?


no problems here with the 24 f1.4 or 85 f1.4 on my D3's

looks like it could be the camera



Dec 02, 2011 at 07:14 PM
MalbikEndar
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p.1 #6 · p.1 #6 · fast prime focus shift under tungsten light?


I have read other claims of poor focus in tungsten light and good focus in sunlight. However I have trouble imagining a explanation for this sort of problem.


Dec 02, 2011 at 07:19 PM
Engelsen
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p.1 #7 · p.1 #7 · fast prime focus shift under tungsten light?


My 85G is spot on, knock on wood, but that doesn´t mean I and/or the focus will hit the target every time, especially not at f1.4. Shooting hand held at f1.4 will not give you reliable results for AF testing/fine tuning.


Dec 02, 2011 at 08:37 PM
 

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Jammy Straub
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p.1 #8 · p.1 #8 · fast prime focus shift under tungsten light?


MalbikEndar wrote:
I have read other claims of poor focus in tungsten light and good focus in sunlight. However I have trouble imagining a explanation for this sort of problem.


The explanation I remember from dealing with the problem years ago when I shot Canon was that it had something to do with the red shift of the light and it's interaction with the AF sensor under certain incandescent light. Something about the wavelengths doing weird things.



Dec 03, 2011 at 02:12 AM
eSchwab
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p.1 #9 · p.1 #9 · fast prime focus shift under tungsten light?


It is all cameras that do this. They focus differently in tungsten and fluorescent lights.


Dec 03, 2011 at 04:38 AM
MalbikEndar
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p.1 #10 · p.1 #10 · fast prime focus shift under tungsten light?


> the red shift of the light and it's interaction with the AF sensor

> It is all cameras that do this. They focus differently in tungsten and fluorescent lights.

Thanks for trying guys but I was hoping for something a bit more, well, specific, and physics-based.




Dec 03, 2011 at 04:50 AM
MalbikEndar
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p.1 #11 · p.1 #11 · fast prime focus shift under tungsten light?


Being somewhat curious about the physics of this problem I did what I should have before, that is, consult Dr. Google.

Found one site

http://circ-of-conf.blogspot.com/2011/01/a580-focus-shift-under-fluorescent.html

Not sure I buy this either, I'm still thinking on it.



Dec 03, 2011 at 04:59 AM
Jan Brittenson
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p.1 #12 · p.1 #12 · fast prime focus shift under tungsten light?


MalbikEndar wrote:
I was hoping for something a bit more, well, specific, and physics-based.

The AF sensor(s) or assembly doesn't have an IR cut filter, and incandescent lights throw out a ton of IR?

The lens has jittery bokeh near the plane of focus and the AF system is focusing on this sometimes in AF-C mode?

The lens isn't an apochromat and so is in focus in different planes for different wavelengths?

The first step might be to try an IR cut filter like a B+W 486 on the lens...



Dec 04, 2011 at 01:13 AM
theSuede
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p.1 #13 · p.1 #13 · fast prime focus shift under tungsten light?


MalbikEndar wrote:
I have read other claims of poor focus in tungsten light and good focus in sunlight. However I have trouble imagining a explanation for this sort of problem.


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 you counterweigh the shift by whitebalancing your picture (making the green 4x stronger and the blue 10x stronger). But you don't do this for the AF-sensor - it's one color, one balance only.

If you check the separate channels of a picture taken at F1.4 (you can do this in photoshop or any other editing program), you'll see that the Red, Green and Blue channels don't focus at the same distance. In normal WB conditions the green channel is ~about what the AF sensor sees. As the light temperature changes toward warmer, the AF sensor will see more and more what only the red channel sees.

At 2400K the WB ratio is about 4:1 between R and G. At 6500K it is about 1:2. That changes the AF dependence on the red channel with a ratio of about 8:1

So it is indeed a type of "focus shift". You can in fact easily replicate the problem systematically by setting up two lights, one incandescent and one D65 type light. With the camera on tripod, pointed towards an "easy" target, the focus point will a different distance depending on which light you have switched on for the moment.

As the effect is a lot smaller for lenses that existed when the D700 was introduced, I guess (but cannot say for certain) that the effect is somewhat compensated for by software. I do know that the 7D and the 1D4 has got temperature compensation in the AF correction system, but I don't know how well it works in the 5D2. I know it was introduced with the 50D and the 5D2 though. I also know it's built in to the D3, which makes it natural to assume that it's present in the D700 too.



Dec 04, 2011 at 02:09 AM
CGrindahl
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p.1 #14 · p.1 #14 · fast prime focus shift under tungsten light?


My education continues... Thanks Joakim.


Dec 04, 2011 at 02:51 AM





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