Artificial Lighting affecting Focus?

[James Farrell]

MY QUESTION: This post inquires whether LED lighting (as the sole light source) would adversely affect the auto focus system of a typical, current generation Nikon body, e.g., a D750 or a D7200. I recall reading somewhere about incandescent lighting possibly affecting focus. But I don't know about LED lighting. Please review the following background info about the specific issue that has me baffled.

BACKGROUND: Most of you are familiar with some of the different kinds of 'third-party', auto focus adjustment fine tune accessories on the market like the Lens Align-Focus Tune system. With this system, one aligns a high contrast target parallel to the sensor plane of the camera, take a series of shots and examine where upon the slanted ruler (attached to the target) is the most sharply-focused point - in front of the square target (the zero line) on the ruler would be front focusing, behind the zero line would be back focusing. You're supposed to shoot Jpg fine, standard picture control (and default in-camera sharpening), aperture wide open, tripod mounted, shutter delay, remote release, with the target properly illuminated (white balance set for the temperature of the lighting) in AF-S mode, and center focus point only with a high enough shutter speed to avoid blurring of the target but not too high an ISO setting as to create a lot of noise. (Note: Focus Tune software can be used to evaluate the test shots at different in-camera focus adjustment settings, but this post is about just the process of focusing on the target itself under artificial lighting.)

MY ISSUE: The issue I am seeing is that the auto focus performance for the same overall settings, is quite different when shooting these test shots indoors with my focus test rig illuminated by a 3,000 lumen LED light with a 4000K temperature rating versus taking the same shots outdoors in daylight (highlights NOT blown out) with the camera set to auto white balance. The significant difference for both normal PDAF focus AND live view focus between the test results indoors versus outdoors has me wondering if these cameras react (focus) vastly different depending upon the kinds of light sources. Yes, I understand that a JPG image will look very different depending upon the light source and the white balance settings (which normally I don't worry about since I shoot exclusively raw), but he difference I am seeing in a controlled (as much as I can) test environment has me completely baffled. The words "different wave lengths of light" come to mind. But I am not schooled in the science of lighting. Thanks for reading this convoluted mess. >> Jim     

[Frank Fremerey]

Your question can be broken down to:

Does the spectral composition of the lightsource influence AF or is the AF color blind?

[Bjørn Rørslett]

No optical system is entirely "colour blind". Fluorescent light is troublesome for most digital cameras so should not be used for this kind of tests. Continuous-spectrum sources are better. Using a green filter over the lens might improve accuracy more.

[Andrea B.]

If you were using Matrix Metering, then repeat the experiment using Spot Metering. I was thinking that how the camera applies the results of the Matrix Metering algorithms could perhaps affect auto-focus.

[James Farrell]

No optical system is entirely "colour blind". Fluorescent light is troublesome for most digital cameras so should not be used for this kind of tests. Continuous-spectrum sources are better. Using a green filter over the lens might improve accuracy more.

I don't understand the term "continuous-spectrum" ... I am simple 'meat and potatoes' Irishman (even though I hate green beer and don't like Guinness).

If artificial light does affect focus performance, how do studio photographers adjust? What do they do? 

[Akira]

The LED light may be flickering at higher rate than the frequency of the AC current (50Hz) to lengthen the life of the LED and to save the power.  Like the image sensor, the PDAF sensor is also working at a certain "frames per second" to read out the image info.  There can be some interference between them.

[Bjørn Rørslett]

The sun, an incandescent light bulb, or an electronic flash delivers a continuous spectrum out. A fluorescent tube has narrow spikes in various parts of the visible spectrum and almost nothing in between.

I don't think studio photographers are too much worried by fine tuning. If there really is a consistent focusing discrepancy, they let a repair shop recalibrate their gear.

The modern DSLRs usually have a user-controllable setting for flicker reduction (50/60 Hz).

[James Farrell]

The LED light may be flickering at higher rate than the frequency of the AC current (50Hz) to lengthen the life of the LED and to save the power.  Like the image sensor, the PDAF sensor is also working at a certain "frames per second" to read out the image info.  There can be some interference between them.

Thanks you Akira for your thoughts about the LED lighting. And thanks to the others who responded as well.

[James Farrell]

The sun, an incandescent light bulb, or an electronic flash delivers a continuous spectrum out. A fluorescent tube has narrow spikes in various parts of the visible spectrum and almost nothing in between.

I don't think studio photographers are too much worried by fine tuning. If there really is a consistent focusing discrepancy, they let a repair shop recalibrate their gear.

The modern DSLRs usually have a user-controllable setting for flicker reduction (50/60 Hz).

Bjorn: Just to clarify ... I am not using fluorescent lighting - using LED. Having just read about the LED flicker issue posted by a cinema photographer in Switzerland, and now understanding more about it (and what Akira had to say), my question is would one try to have their flicker reduction setting in their DSLR be the same or as different as possible than the flicker rate/frequency of the light source? Sounds like a naive question, I know, but I'm closer to getting a grasp of the issues having read the responses here.

[Frank Fremerey]

James.

"Contiuuous Spectrum" means that all colors of the rainbow contribute to the mixture we call "white"

That is usually the case if light is emitted as a side effect of heated matter (as in the sun or a light bulb or a flash unit)

But this is not the only way to "make light"

a fluorescent tube makes Ultraviolet light first and then converts
it using a fluorescent foil that emits slightly smeared out single
colors. 3 colors in older. 4 colors in typical. 5 to 6 colors in so called
"Full Spectrum" tubes, like the ones I use in my studio.

LED light is similar to that. Single colors smeared out by another
physical effect. If you combine LED of different colors you might
also have a white light of several colors.

but

the AF system might work better in green light than in red light or in blue.

Questions?

[Bjørn Rørslett]

James: set the flicker reduction in-camera to that of the mains: 60 Hz in USA, 50 Hz in Europe etc.

[Akira]

There are two types of "white" LEDs: the one with combined red, blue and green LEDs and another one with a UV emitting chip to induce the UV-fluorescening substance.  The former is the better one and the latter is generally used for cheaper products.

If it is an RGB one, you shoule be able to see the RGB spots when you put a piece of white paper right in front of the LED and observe the reflection (some ND filter should help you see the reflection without damaging your eyes).  If it is a UV one, you would observe some yellow tint in the reflection if you do the same.

[Øivind Tøien]

Is the distance to target the same for outdoor scenes and indoor scenes? If too close to the target, the distance between the central focus point and the laterally positioned scale could come into play if the lens has field curvature. For instance my Nikon 12-24 mm will focus very differently depending if a peripheral focus point is used vs. the central focus point, while other lenses are insensitive in this respect, so it is not a sensor adjustment issue.

I have yet to detect any difference in focusing accuracy of the central point whether my D7100 and D5100 is used with a warm LED bulb/warm fluorescent bulb mix or used in outdoor in cold blue winter light.

[James Farrell]

There are two types of "white" LEDs: the one with combined red, blue and green LEDs and another one with a UV emitting chip to induce the UV-fluorescening substance.  The former is the better one and the latter is generally used for cheaper products.

If it is an RGB one, you shoule be able to see the RGB spots when you put a piece of white paper right in front of the LED and observe the reflection (some ND filter should help you see the reflection without damaging your eyes).  If it is a UV one, you would observe some yellow tint in the reflection if you do the same.

I made an image (attached) of the LED light that I was using and to which I referred in my original post in this thread. Note the small 'yellow' squares (about 3 millimeters square - shown enlarged in the inset on the lower right) in the middle of the light fixture that's about 18cm in overall diameter. All light from this fixture emanates from these yellow squares. It's extraordinarily bright - 3,000 lumens using only 40 watts (US 120 volts AC). So, I assume this to be the cheaper LED light source and one not recommended as an artificial light source for indoor photography. Or, is that an overstatement?

My Nikon D750 was set for 'auto' flicker reduction. I could set it for 60Hz (US grid). But I would assume that the auto flicker reduction setting (which is the default) would have made the appropriate in-camera adjustment. Perhaps that's a flawed assumption.

In any case, I appreciate everyone's input on this. Once again, I thought I was being clever and tried to do something 'on the cheap' (buying inexpensive lighting). Another lesson learned. I'll just stick to Mother Nature's light source from now on. Thanks again, Jim

[Frank Fremerey]

These are HiPower LED as you will finbd them in car lights or flash lights

If you can identify who made the light you might also find technical data. In the technical data you will also find a "spectral emission curve" that tells you which colors are present in the light and in which proportions.

I did never hear that LED "flickr" and I do not know how they should "flickr".

I guess LED are part of an electronic array with condesors and that they fire at random in such a way that the light emission is more or less the same at every point in time.