Nikon D7100 left focus issue

Started Apr 19, 2013 | Discussions thread
PepsiCan
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How to test for focus issues
In reply to SaturnN, Apr 20, 2013

One question not yet answered is "how do you test for left-/right-focus issues"? So, here it goes.

When your pictures are not sharp, there are a number of potential issues.

1) You're settings are wrong and they prevent the camera from acquiring focus on the target you intend. Examples are: shooting with shutter priority, which can cause the camera to fire before focus is acquired. Or having AF-area set to Auto-Area where the camera, not you, selects the focus point.

2) Something is up with your lens. It is decentered, has front or back focus and in the case of really fast lenses (f1.8 or faster) you can have focus shift where the lens will focus slightly differently when stopped down a little. Or you could have a case of field curvature (more prevalent in super wides) where the focal plan is not straight.

3) The camera's focus system is off. Or rather, the camera's Phase Detect Auto Focus is off (PDAF).

Now, how do you know which of the three issues it is (or combination of)? Enter Live View and Contrast Detect Auto Focus (CDAF). Let's first understand how the various focus types work. Every digital SLR has two focus systems build in. Phase detect and Contrast detect.

PDAF is a mechanical focus system where through a system of tiny mirrors two views of the same scene are made to overlap. It is fast, but there are small margins of error. If you would do the focus twice, there is a change that the second time round the PDAF system ends at a slightly, almost indistinguishable different setting.

CDAF is based on the principle that an image is sharp when you reach maximum contrast. So, the system moves the focus system along and compares the current view against the previous view for higher contrast. Once the contrast number starts to go down, go back to the previous position and the sharpest focus has been achieved. It is slower but more accurate than PDAF.

For the purpose of testing focus, it is important to understand that with PDAF and CDAF your camera has two different ways of focusing and therefore one can serve as a reference point for the other. We therefore now have a way forward to find out what type of issue we have with focus.

Let's assume we suspect a problem with the PDAF system. To establish that we'd need to compare the results of shots taken with PDAF against shots taken with CDAF. So, here is how this will work.

1) Get out your tripod.

2) Put a prime on your camera (or some other fast lens - f2.8 or faster) and set it to its widest aperture. The AF-S 50mm f1.8 G is popular lens for this. 35mm f1.8 DX should also work as it is the APS-C equivalent of 50mm (roughly)

3) Put the camera on the tripod and set up single-shot AF, Single point, matrix metering. Ensure the camera is set to Focus Priority. Consult your manual on how to do that. If you have a D5xxx or D3xxx, the fact that you use AF-S will force it into Focus Priority.

4) Ensure good light, normal iso and a rapid shutter speed (1 / (2x focal length)). Do not use VR. Do not use flash. Shoot RAW and ensure you are far enough away from your focus target (think 2x minimal focus length of the lens).

5) Your target must have a lot of contrast. Focus charts are good. Also, your target must be FLAT (so, no three batteries in a row stuff. It can confuse your focus) and your camera must be aimed at the target at 90 degrees! No sloping angles! Dead on frontal!

5) Use things like mirror up and remote releases (or timer) to ensure the mirror slap and/or pushing of the shutter button do not shake the camera

6) In PDAF (so, using the view finder), use the center focus point. Acquire focus and take your shot. Then repeat 2-4 more times.

7) Switch to the most left focus point. Repeat step 6. Then switch to most right point. Repeat step 6.

8) Switch to Live View. Focus. Take the 3-5 test shots when focus is acquired. Repeat for focus on the right side and left side.

9) In View NX do the following:

- Check the RAW files (View NX will by default load the embedded JPEG, but those will be biased because of applied picture controls and possible ADL)

- Check for Focus Point and confirm it is at the spot where you aimed at

- For each of the groups, compare the PDAF pictures between themselves (all the center shots for example) and compare the PDAF picture to their CDAF counterpart (live view center shot against view finder center shot)

There are now couple of possible outcomes.

PDAF = CDAF = in focus --> There is no issue

PDAF Center = CDAF center but either Right PDAF <> Right CDAF or Left PDAF <> Left CDAF. We have a problem with the PDAF system. Your center is sharp, in live view things are also sharp but the PDAF system cannot get sharp focus on the outer edges. This is the symptom that D800 users had and it is know as the left-focus issue. Basically, your PDAF system is miscalibrated and you need to send the camera in for a tune up.

The third possibility happens when both PDAF and CDAF do not give you sharp results. That points towards a problem with the lens rather than the camera. There may be some decentering if one side is weaker than the other. Or if left, right and center all show out-of-focus results, there may be front- or back focus. Which one it is, requires further testing.

I trust that helps somewhat.

 PepsiCan's gear list:PepsiCan's gear list
Canon PowerShot S95 Adobe Photoshop Lightroom 4 Nikon D7100 Nikon AF-S DX Nikkor 16-85mm f/3.5-5.6G ED VR Nikon AF-S DX Nikkor 35mm f/1.8G +5 more
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