The Nikon D800 has serious AF accuracy problems if the AF assist light of the SB-900 is enabled (part 2)

As mentioned first in a previous article, the D800 has a serious auto focus accuracy problem if the camera is using an external flash and the flash AF assist light is used.

I contacted Nikon after I wrote the first article and just a few minutes later my case was closed with a short message saying the following:

"I am sorry to learn about your issues with the D800 focusing when the AF assist light is used.
I would like to confirm that this phenomenon is known to us. It is caused by the following:

1. The wavelength of red light is longer than approximate wavelength of white light.
2. Lenses (including the one in the AF sensor) are calibrated to provide best sharpness for the whole spectrum, however are not ideally corrected for all colours.

This leads to a wider degree of tolerance when focusing using red light. This will be common to any flash that uses red light as a focusing aid, but assuming the camera is not already back focusing this is within specification. Shooting wide open (especially with fast lenses) on high resolution cameras will highlight this issue, which is why we would recommend stopping down between 2 to 4 stops from maximum aperture, especially on a D800.

We apologize for the inconvenience this may cause you."

Of course, I was not happy about the answer or the fact that Nikon decided to close the case without doing anything more, so I explained that their answer is not satisfying and is totally incorrect since the AF problem is there, even when the AF assist light is covered, which rules out the wavelength theory, and that stopping down the lens is not helping since the camera has a problem even with the 24-120/4 lens, so stopping down the 50/1.4 to f/2 would make no difference at all. Besides, without the AF assist light, I have no problems getting accurate focus with the 50/1.4G at f/1.4. After this message my case was reopened and Nikon Support asked me to take a few test shots according to their own recommendation on how to test auto focus.

https://nikoneurope-en.custhelp.com/app/answers/detail/a_id/51633/

Three unedited raw images was requested, each taken on tripod with the 50/1.4G at f/1.4, one without flash, just using available light, one with the flash and AF assist light and a third with the AF assist light covered. I decided to provide them with two "bonus" images as well, one with the flash but with AF assist light turned off in the flash, and one with the camera in P mode. I choose P mode because in this mode the camera should set whatever it chooses regarding ISO, shutter speed and aperture, knowing that it will stop down the aperture to f/5.6. I decided also to take six images with each settings, and would force to start focus in three from infinity to target and the other three from nearest focus to target. This would rule out the possibility that focus direction would be the cause of the problem.  I considered that if all six images in each set demonstrates the same behaviour then the case is clear.

I was no longer surprised by the results, I knew that the focus problem was not caused by my own hand shake or camera movement, and using tripod did not improve the results. To eliminate any possibility of focus error caused by movement, I even used a wire trigger so that I did not need to touch the camera at all after I offset the focus between each shot.

However, there was one thing which surprised me a lot. That was the shots in P mode, with the aperture set to f/5.6. I expected that this would hide any focus problems, so seeing that severe back focus was hard to believe, but it was there, showing that stopping down is not helping at all, the focus is just moved even more back, so the end result is even worse than at f/1.4 since the target is even more out of focus. This is actually indicating a pure firmware error, so there is hope...

After I submitted my test images, Nikon changed the status of my case to "Researching" and it is currently in that state.

Hopefully this will soon result in a firmware update and a fix of at least this problem.

The submitted test images and the results

This is the requested test target scenario. The focus was on "D800" of the D800 / D800E user manual. The ruler is there to clearly indicate the direction of focus, not to measure the back focus.

Image 1: D800 without flash, on a tripod, 50mm f/1.4 lens with wide open aperture, focused in viewfinder mode (not Live View) and with the central AF field on the "D800" of the D800 manual. The white balance is wrong because the ambient light was too low and the camera did not manage it well but once that is corrected you will see that the image is OK if you disregard the high ISO noise.

Image 2: Taken as above, with help of the SB-900 AF assist lamp and with flash.

Image 3: Taken as above, with the SB-900 AF assist lamp covered and with flash. It seems that by covering the light the focus is moved very far back. I don't know how that can be, but this is the case always when the AF assist light is covered.







Image 4: Taken as above, with the SB-900 AF assist lamp off and with flash. No question about that the auto focus is accurate if the AF assist light is not on.

Image 5: Taken as above, with the SB-900 AF assist lamp on and with flash. This image is clearly the worse, even though the aperture was f/5.6. This is actually indicating a pure firmware error since it is obvious that when the lens is stopped down the focus is moved even more to the back, so much that the target is seriously blurred. As you can see, best focus is very far back, on the ruler it is somewhere between the digits 19 and 20 on the scale nearest the books. It is simply horrible.


What is interesting also that reading the EXIF data using Kuso EXIF viewer shows a focus distance of 0.84m for all five images, which is obviously not the case since there is several cm back focus between the accurate and the back focused images.

This problem is now confirmed by several other people, and not only when the SB-900 is involved but also with other flashes, even third party. Hopefully Nikon will take this problem seriously and fix it very soon. There is no doubt in my mind that this has nothing to do with the light spectra, just on the fact that the external flash AF assist light is turned on by the camera.

The Nikon D800 has serious AF accuracy problems if the AF assist light of the SB-900 is enabled

In a discussion on a large forum about camera gear somebody mentioned that there might be a problem with the D800 AF accuracy if the camera uses the SB-900 flash and the AF assist light of the flash is enabled and turns on for the image. The person who brought this up mentioned also that  it can have something to do with the colour temperature of the light, since the in-camera, white LED does not produce the same results and the AF works accurately with it. Initially I dismissed this as farfetched because I had hard time to believe that the accuracy of AF can have anything to do with the red AF assist LEDs, other than the fact that the LEDs allow the camera to focus at lower light level than it would be possible without. Never the less, I wanted to test before completely dismissing this as a myth.

Sadly, I have to admit that I was wrong, it is not a myth and it is definitely true that the D800 has serious AF accuracy problems when the SB-900 AF assist light is enabled, the problem is definitely confirmed and very easy to repeat. Though, it is not the colour of the light which is the problem, since even if the LEDs are totally covered the accuracy of the AF is still very bad, the camera consistently back focuses, i.e. the actual focus is after the focus area.

I have not noticed this problem before since I am not using the flash too often, but not so long ago I was out shooting a local event and came home with a bunch of badly focused images, which I thought was my own fault. I had enough usable images from that event, so I gave not much thought about the high number of out of focus images then, but now I know that the problem was the flash AF assist light, not a user error. I realize now that every image which needed the red AF assist light was out of focus and useless.

This is an extremely serious problem for every professional photographer, since the famous event photographer camera Number 1, the Nikon D800, has a very serious flaw regarding the auto focus because especially during events, the AF assist light is very often used. My otherwise perfectly well functioning camera definitely behaves strange with the AF assist light on, and the problem is apparently not just with my camera and my flash, but so far every other D800 user I have been in contact with, and not just the SB-900 flash but also the SB-700 has this problem.

Test setup

Camera set to single centre AF area in AF-S mode.
Focus priority is set to ON.
Camera in M mode, 1/200 second shutter speed and lens wide open, ISO800.
For the sample images here, I used the 24-120/4GVR and the 50/1.4G lenses but tested also with the 105/2.8GVR macro.

The flash was on top of the camera, bounced from the roof.

The ambient light in the room was set to fairly low level, so that the camera could focus even without the AF assist if the AF assist light is turned off in the flash, but uses the AF assist light if it is set to on. This way the tests are consistent, ambient light does not have to be changed to allow focus without AF assist light, so it should result in identical images both with and without the AF assist light.

A3 size test chart was used with a target area which gives focus confirmation in 100% of the cases under the low ambient light. The actual AF target was the center of the test chart, the circular pattern, which is a "good" AF target for the center AF spot. Distance was about two meters to the target with the 120mm, less with the 50mm lens. I was standing to the right from the target to be able to verify consistency in AF accuracy error.

About 50 images with, and 50 without AF assist light were taken with each lens, manually offsetting the focus between each image to make sure that it is necessary for the camera to focus each time the shutter is released.

 
The camera was previously tested by me with all the lenses and was considered accurate. The camera and lenses are all one year or older, used frequently and working perfectly well. No problems whatsoever with lenses, flash or the camera. The flash was used also on a D300s before, without any problems and even the AF assist light gave consistently good AF accuracy with that camera. The images under Results are cropped images of the AF area and the size is 100%. 

Results

Every time the SB-900 AF assist light is ON the camera back focuses. The image to the left is taken with the 24-120/G lens at f/4. This is consistent, easily repeatable, and the amount of back focus is always the same.

Also tested covering the AF assist LEDs so that no red light reached the test target. Even so, the camera still back focuses, just like when the red light is on the target. This excludes the possibility that the problem has anything to do with the colour of the light. Something else must be wrong, not the colour.

I have even removed the red Plexiglas filter which is in front of the LEDs but all that did was that the red light became stronger, no improvement of the focus accuracy. It is not the red filter that gives colour to the light, which I was hoping for. The three LEDs under the huge lenses are red in their own. It is also not the filter that produces the AF assist light pattern, even that is produced under those lenses.

Fast lenses become completely useless, creating totally out of focus images in 100% of the cases. This is totally unacceptable. With the 50/1.4G set to f/1.4, as in the image to the left, the focus is so out of target that one might easily suspect user error and camera movement, but no, that is not the problem at all. All the 50 images were equally out of focus when the AF assist light was on, and in focus when I turned it off in the flash menu. Imagine if the image on the left would represent a portrait and the eye lashes would be where the AF target circle is... The ears of the model would be in perfect focus but not the eyes. Who would pay for such image? 

Disabling the AF assist LED light in the flash and enabling the in-camera white AF assist LED resulted in perfect accuracy, as the image to the right shows, which is taken with the 24-120/G lens at f/4, consistently the same for all the images. Disabling even the in-camera white LED gave the same consistent accurate results, so the problem is gone as soon as the AF assist light of the flash is set to OFF.

Everything is of course repeatable and the focus error is consistently wrong, always the same direction (back focus) and always the same amount.

With the 50/1.4G the error was obviously the worst and most disturbing, since at f/1.4 the depth of field is considerably narrower than the other lenses, so back focus becomes a really serious problem with this lens, rendering the lens useless if the AF assist light of the SB-900 is activated. Never the less, if the AF assist light is disabled, as in the image to the left, even the 50/1.4 is very good, even at f/1.4 producing accurately focused images.

Conclusions

The AF assist light of the SB-900 is totally useless with the D800 and should be switched off until this error is fixed by Nikon. I suspect this is the case even with other flashes, not just the SB-900.

This is a very serious problem and it needs to be fixed ASAP. Regardless if the D800 AF module is very sensitive and in many situation the AF assist light is not necessary at all, but there are situations when it becomes necessary to use it and in any case, it should not make the camera worse.

This problem makes the D800 considerably less usable for professional work, since many type of work demands the use of AF assist light since the ambient light is often too low. 

Nikon must fix this as soon as possible

I don't know what causes the problem, but it is definitely not the colour of the light, since covering the LEDs does not change anything, the camera still back focus when the AF assist light is turned on.

Can it be the pattern? Possible, but not likely. The SB-900 AF assist light projects a pattern and unfortunately that pattern is projected even if the red filter is removed. On the other hand, the in-camera LED is not only white, but also not projecting any pattern, just a light beam. However, if the AF assist LEDs are completely covered and all the light from it is prevented from reaching the AF target then there is no pattern on the target yet there is still the same back focus problem when the AF assist light is turned on.

Can there be a problem with the new AF module? That is possible too, but as far as I know, the D4 uses the same AF module and in that case there should be a problem with that camera also.

The AF accuracy problem does not seem to be connected to the light at all, it seems as the problem is connected to the fact that the external flash AF assist LED is turned on, not necessarily providing light on the AF target since the problem is there even if the light is blocked by a thick cardboard sheet. Also, when the ambient light in the room is strong enough so that the AF assist LED never comes on due to the decision the camera makes, the camera again focuses with high accuracy without any problems. So it seems that the only criteria for the back focus is that a flash with external AF assist light is on the camera and that the camera forces the light to be used.

This problem should be brought to Nikon's attention in hope that they will fix this somehow. Some people say that this issue has a connection to the last firmware update, in which case it should be easy to fix. I don't know how to reverse the firmware to the previous release, my camera has the latest firmware and I cannot find any documentation on how to reverse an update.

Hopefully Nikon solves this soon. Contact Nikon and make them aware of this problem. Use the following link, or contact them in your own country.

https://nikoneurope-en.custhelp.com/app/ask/locale/en_GB 

Status update regarding my flash adapter for the Nikon V1/V2

Well, after I made the first seven I considered the appearance not to be good enough for selling these adapters and I realized that the tools I have are not enough to make it better looking. So that gave me three options.

1. to let some company make the part which I cannot do on my own
2. to buy or build a CNC (computer controlled milling machine) which could be used to make better looking adapters, all repeatable and very similar to each other as opposed to my first seven, which are all individuals and have a "hand made" look
3. to just ignore the project all together and give up

I opted for the first two, starting with the first one. I asked a local company make me 10 pieces. These are just partially ready and still needs some finishing plus the final installation on a hot shoe. To let a company make part of the adapter is very expensive and strictly speaking, I know I am going to make a financial loss on this, but I will test the market and sell them once I am ready with all the 10. It took some time to find a company which could make my 10 pieces at a reasonable price since I was not ready to pay any amount. Low volumes have very high prices normally, and this is no exception, but I am not ready to order higher volume at this moment, partially because at this moment I am not sure I can sell them, and partially because I am also working on option 2, building a CNC which can be used for this and for some other projects I have in mind for the N1 and other cameras. All that plus the fact that it is summer in this part of the world with a lot to do around the house, being outdoors and so on, resulted in a halt (or at least a break) in the progress. I will not start selling before I have all the ten ready, so that might take some more time. If I sell all 10 within a short period of time I may decide to continue and make them in higher volumes, but I may also just give up if I get tired of it because of all the trouble and all the time it takes plus the costs involved.

I hope this is for answering the questions I receive about why it is taking so long. Anyway, there is no product for sale yet. Sorry to have to keep you waiting a little longer, be patient, it is on the way.

Updated information:

The adapter is now available. Read about the details here:

http://adapting-camera.blogspot.se/2015/08/the-v1-f1a-is-dead-long-live-v2-f1a.html


If you are interested, here is a short video about it as well:

 

Beware of the flash trigger voltage

If you are going to use a flash other than the original Nikon flashes with the Nikon V1/V2 cameras with the help of my adapter then you must make sure that the flash you are intended to use is suitable for the purpose and won't fry your camera. Please note that Nikon does not publish the maximum allowable trigger voltage, so it is an intelligent guess that you should not exceed 12V. This is based on my own tests since my old flashes have a diversity of trigger voltage and the highest I used on the V1 has 12V. It is an old Olympus T32 flash, and while it is from the late 1970's, it does not mean that every flash made after that date is safe. Olympus was early in using low trigger voltage, most flashes from that age and even long after that, used high voltage which may cause harm to many modern digital camera, so it is entirely up to you to check that the selected flash has safe trigger voltage level.

 

How to measure the flash trigger voltage

To check the trigger voltage is very easy. All you need is a digital volt meter. One terminal must be connected to the ground contact of the flash and the other to the center pin, the actual trigger signal. After that the flash must be turned on and the increasing voltage should be monitored. Once the flash is fully charged the voltage should stabilize around the final trigger voltage. The displayed value should not be higher than 12V. I don't know what is happening if it is higher and I am not willing to try at this moment, all I know that my flashes have a large variety, from 5V to 350V, so if you just pick any flash you might end up with the same variation.

 

Four of my flashes as examples

 
The lowest trigger voltage has the Osram VS280 Studio which has 5V and is safe to use. The age of this flash is unknown, but I am guessing it isn't very old.

Next is the Olympus T32 which has 12V trigger voltage. It is from the late 1970's and in spite of the duct tape, I still use it fairly often.

The third is the Metz Mecablitz 30BCT4. The trigger voltage is over 150V, so I regard this unsafe, even though it is a very nice flash.

The fourth flash has even higher trigger voltage, almost 190V. It is the Sunpak Auto zoom 3000. It is very versatile but in direct trigger mode, using a cable, it is a NO NO.

I also have some other flashes which have even higher trigger voltages. Of course, these flashes can be used if you are using a radio trigger with a receiver which can trigger high voltage flashes. 

Please note that all the modern flashes I know of have low trigger voltage and also of course, all modern Nikon SB flashes, like the SB-600, SB-900 also are safe to use.

 

It is very important that the trigger voltage is checked prior to using a flash with my adapter on the Nikon V1/V2 cameras. The adapters will contain a protection diode, but the best level of protection is your own carefulness. The diode I am using is a 33V zener diode. Why did I select 33V zener? Because that's what Canon is using in their hot shoe cables, and I figured if it works for Canon it works for Nikon also. In other words it is just a guess from my side, so measuring the trigger voltage of unknown flashes is an important safety measure to avoid accidents, and the only thing I know for sure is that 12V (or less) trigger voltage is safe.