Ophrys Photography

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Tutorial - Lens Micro adjustment

Background

The later generation cameras from Canon and Nikon (and some others) have the facility to fine-tune the autofocus of their lenses to the camera, to remember the adjustments necessary and apply them when that lens is attached again. Cameras are smart as they recognise and report which lens has been put onto them and automatically apply any adjustment that you dialled in previously. They can even detect the presence of a teleconverter and remember these settings too.

Before lens micro-adjustment, if your lens wasn't quite calibrated accurately to your camera, you had no choice but to send it back to the manufacturer for calibration. Now you can do it yourself provided the errors are not way off spec.

So what are we talking about here exactly ?
Well, if the lens and camera's autofocus are precisely matched to each other then the camera will focus accurately on the subject, but if the point of focus wrongly falls just in front of the subject, then camera is said to be "front focusing" and if it falls just behind the subject it is said to be "back focusing". These focusing errors may not be immediately obvious but will make your images look soft and you will not be getting the best out of your equipment.

So how come the manufacturers are selling equipment that front or back focuses ?

Well, if you consider that pretty well all manufactured goods are produced to a tolerance range, it is possible to be unlucky and pair a camera at the positive limit of its acceptable manufacturing tolerance range with a lens that is also at the positive limit of its acceptable tolerance range and have a combination that is way out of spec. To clarify this point let's imagine that both cameras and lenses have a manufacturing tolerance of zero plus or minus 5 arbitrary units. Now let's say that the camera we buy turns out to be +5. It is still in spec (just) and will be released for sale. If we pair this with a lens which has a value of -5 then the errors will effectively cancel each other out as the overall value will be zero and the pairing will focus beautifully despite both being at the tolerance limit extremes. However, if you paired a +5 camera with a +5 lens then then resultant +10 would cause focusing errors that are sufficient to produce soft images. Lens micro adjustment enables us to dail-out these errors in camera.

Which cameras can be micro adjusted ?

In the Canon range that I am most familiar with, the following cameras with Liveview have this ability. Certainly those cameras from the EOS 50D, 7D, 5DII, 1DmkIII and 1DSmkIII and presumably all models onwards will have the facility.

In the Nikon ranges I believe that the D300 onwards, D700 and the D3/D3x onwards have this facility. The Sony A900 and the Pentax K20 also have this facility.

If your camera predates the ones listed above then you will not be able to adjust your lenses, but you should still check for front or back focusing. If you have evidence that you have a bad combination then the only solution is to send the lens and camera back to the manufacturer for calibration.

How do you set the lens micro adjustments into the camera ?

Whichever method you use, you will need to set the adjustments into the camera for each lens using the appropriate custom function. Below is how you do it with a Canon 1DmkIV, just check the handbook to find it on your camera. For example, on a 5DmkII it is CFn III-8, on a 1DmkIV it is III- 7 and on 7D it is III-5.

Select Autofocus /Drive in the appropriate custom function
Select Adjust by lens
Press Info to change the value (if you have trouble here you probably pressed Set instead of Info)
Dial in the forward adjustment (-) i.e. toward the camera or backward adjustment (+) i.e. away from the camera that you determine from one of the methods described below)
Press the Set button to accept the value. Every one of my Canon lenses on every camera I have used has required either zero or (+) adjustment, none of them have required (-) adjustment.

Set CFn to adjust
by lens and press Info to change
Micro adjust screen. This lens front focused and required a value of +9 to correct it

What methods are available for lens micro-adjustments ?

These range from expensive ready made focusing kits that you can purchase to DIY versions using sloping focusing charts or focusing on interference patterns on a computer. These are all covered below but my favourite method using a tethered laptop described at the end of this tutorial.

I have tried all of these methods but by far the best one in terms of speed and accuracy is the "tethered" method described at the very end of this tutorial.

However, this method only relevant to one of the latest generation cameras that are capable of autofocusing whilst in liveview. If you are lucky enough to have one of these and have access to a laptop computer, skip to the tethered technique instructions below. I know it sounds a bit scary, but trust me on this - it is really easy and uses the Canon software provided with your camera to run it.

I should add that none of the methods described here are recommended by Canon other than their own one - but don't let this deter you!

Quick check using the latest cameras

This is a quick check or temporary method if you are short on time. It capitalises on the fact that the latest cameras have such high resolution rear screens that you can zoom in and check critical focus without resorting to looking at your images on a computer. Once you have determined whether you have 1) front or back focusing and 2) the approximate value you can fine tune around more thoroughly later.

With the camera mounted on a tripod you can focus on a fixed target (a banknote taped to a window is fine) and then switch the lens carefully to manual focus. Zoom in to 10x using the + magnifying button on the back of the camera and see if you can improve on the camera autofocus by focusing yourself manually. If you can better what the camera can do yourself you need to dial in a bit of lens micro adjustment as described in the other methods here and then recheck it. When I first got my Canon 24-105 lens I thought that the images from it looked a little soft, so after doing a 5 minute check and adjust I ended up with quite a large +10 adjustment that made a big difference to sharpness. When I had time to set the lens up accurately using the tethered method I found that I needed +9 so I was not far out.

Canon's method

* Mount the camera on a good tripod.
* Set up a target for the camera to focus on. The reference target should have sufficient contrast for the AF system to detect. It should be flat and parallel to the camera's focal plane, and centred.
* Lighting should be bright / even.
* Camera-to-subject distance should be no less than 50 times the focal length of the lens. For a 50mm lens, that would be at least 2.5 meters.
* Set the lens for AF and the camera for One-Shot AF, and manually select the centre focusing point.
* Shoot at the maximum aperture of the lens via manual mode or aperture-priority. Adjust exposure level to get an accurate exposure. Use low ISO setting.
* If the lens has an image stabilizer, turn it off.
* Use a remote switch or the camera's self-timer to fire the shutter. Use mirror lock up as well.
* Take three sets of images at microadjustment settings of -5, 0 and +5, i.e, three consecutive images at -5, three consecutive images at 0, and three consecutive images at +5.
* Look at the images on your screen at 100% magnification.
* Take additional sets of test images at different microadjustment settings if necessary until the sharpest image is achieved.
* Register the corresponding microadjustment settings in the camera.

Notes from Canon

For best results, manually set the focus on the lens to infinity for every exposure before allowing the camera to autofocus the reference target.

Expect some minor variations in focusing accuracy within each set of three test images, even though they were all taken at the same microadjustment setting. This is completely normal, and is due to the tolerances of the camera's AF system.

Expect smaller microadjustment settings to have a greater effect with telephoto lenses, and vice versa for wide-angle lenses.

If you are attempting to set microadjustments for a zoom lens, it is important to realize that the camera's setting may only be accurate for the focal length setting you test. The instruction book suggests testing at the longest focal length of the lens, but you may find it more efficient to choose the focal length you use most often.

Sloping chart method

Both this and the Canon method can be used to check the focus on a camera that lacks lens micro adjustment but you will just not be able to make any adjustments of course. If you find significant errors you might need to send the camera and/or lens back to the manufacturer for calibration.

The method follows Canon's recommendations above other than it uses a sloping target. You focus on a thick black focusing line and look to see if the text is equally in focus in front and behind the focusing line. I use a target that you can download from Tim Jackson's website here: Tim Jackson lens adjustment chart . I glued it using spray mount to a flat board that I propped up at a 45 degree angle with a triangular piece of wood (also glued to the board using wood glue).

lens adjustment chart

 

Front focus requiring (+) adjustment
Back focus requiring (-) adjustment


The benefit over the flat target is that you are able to see straight away which way (front or back) to adjust the camera whereas the flat target just tells you it is out of focus. The minus is that it is less accurate as it depends on the camera and chart being aligned very well in every plane. Some of the commercial products use this method but have a means of accurately aligning the camera. However, I have used the home made chart for some time, just aligning it by eye - with considerable success.

Adjustment is best carried out when the light is not constantly changing and is good but not too bright. I often do this indoors on a dull day indoors using a desklamp as the light source. You may have to override your camera's meter as the chart is white and will fool the metering into under-exposing.

The lens under test is mounted on the camera/tripod and using a cable release, set the camera to One shot in Av mode. Focus at the lens's widest aperture to minimise depth of field on the black line at the centre of the test chart. I usually place the tripod at a distance from the target that seems relevant to the lens focal length, that is, where I can read the chart numbers and determine if they are in or out of focus. Canon recommends using a vertical target and testing at 50x the lens focal length. I find that this is often a bit excessive. If you were testing a 600mm lens at 50x 600mm that means 30 metres - at which you would need a very big target. I do all my testing at the kind of distance I usually shoot at which is much closer and seems to work well for me. For a 100mm macro lens, the recommendation is 50x100 i.e. 5 metres. I would test a macro lens at about 1 metre.For zoom lenses Canon recommends setting the zoom to the maximum focal length or alternatively to the focal length you use most and that is what I do.

Next you need to roughly know whether the camera is front or back focusing so take a quick series of photos at various plus and minus micro adjustment settings, then view them on the back of the camera. If you have one of the later high-res screens you can determine whether the camera appears to be front or back focusing. If the screen is not of sufficiently high resolution to tell this then you will need to view them on a computer screen before you can go any further.

Next, if you think that the camera is front focusing for example, take a series of images in triplicate from zero backwards and then view them on the computer screen at 100%. From this you can determine which settings are required to make the focusing error zero. The appropriate camera lens micro adjustment value is then dialled into the camera and rechecked so that the numbers in front and behind the focus line on the chart are equally in focus. It is all done by trial and error and is admittedly a bit time consuming !

Interference pattern (Moire fringe technique)

This method is much the same as those above but uses a novel target viewed on a laptop screen- so is best viewed indoors. It utilises a clever test pattern that was developed by by Bart Van der Wolf. It is not a printable target - you must use a computer screen to view it.You are striving to achieve "maximum interference" at which point lots of circular patterns appear on the laptop screen and you will see colour fringing which looks a bit like oil on water. At this point the lens is extremely accurately focused.

 

Tethered technique for later cameras that can autofocus in Liveview

Finally I get to my favourite method, and the one that I currently use. It was first described by Arash Hazeghi on the Birdphotographers.net website and is quite ingenious.

It is only relevant to cameras that can autofocus in Liveview. This includes:
Canon cameras: 50D, 1DmkIV, 5DmkII, 50D and 7D and may be possible on some others too. Presumably later cameras will also have this function. In the Nikon range, D300, D300S, D700, D3, D3s and D3x are all apparently OK.

You will need a computer loaded with the Canon EOS Utilities software that came with your camera. If you shoot Nikon you will need to have purchased and loaded Nikon camera control Pro2 software. A laptop is most practical as you are not restricted to working off your desk with it.

I use a banknote as the target taped to an angle bracket from a hardware store, alternatively you could stick it to any vertical surface with masking tape. A banknote has lots of very fine engraved detail on it which is excellent for checking focus.

My usual target - a banknote taped to an
angle bracket with masking tape

There follows Arash Hazeghi's method. He describes using a mirror to check alignment but I must admit I skip this step for speed and have not found this to be necessary if I set everything up reasonably by eye.

Arash Hazeghi's method (my notes in red)

"Pin the target to the wall at a distance ~25-50 times the focal length, for a zoom lens choose the focal length that you use most often.

First use the spirit level on the ball head to level the camera in XY plane. It is essential that sensor plane is parallel to target. To do this you can use a small mirror, tape it to the target look through the camera’s finder until you can see the centre of your lens in the mirror and lock down your tripod. Optical axis is now perpendicular to target and you are done with the mirror.
I skip this step, just setting up quickly by eye.

You must have Canon EOS utility application installed on your machine, this is included with every Canon camera, if you have an older version make sure to go to Canon’s website and download the latest update.

You are ready for micro adjust now. Follow these steps:
First set the camera to Av mode and select the centre AF point. Dial in any necessary exposure compensation to get the exposure correct on your target.
1) Connect the camera to your computer via the USB cable that came with the camera and cancel any image download pop-up/application
2) Run EOS utility.
3) Click on Camera setting/remote shooting icon
The camera should now magically switch into liveview
4) Click on Remote Live View Shooting. This will open a new window with live sensor video feed 5) Make sure AF is in phase detect mode (Quick mode AF) that uses the camera’s main AF sensor. This is found within the Liveview menu
6) Choose the centre AF point and make sure the white rectangle is centred on the AF point
7) Click on the magnifying icon for a full size view .
8) Click AFON button the camera will now perform AF.
If you have assigned any function to the camera AF ON button within the custom functions then you will need to reset it temporarily back to the default or the camera might not focus!
9) Click on 200% magnification, you are now viewing the sensor output at 2:1). Note it is essential that the tripod be placed on a solid surface plus nobody should walk in the proximity of the setup or you will see vibrations on the screen!
10) Now click on the ( > ) or( < ) buttons to shift focus back or front one click at a time until image appears sharpest on the screen, notice the contrast edges, you want them as crisp as possible. Write down how many clicks you have moved relative to the centre, infinity symbol indicates far direction
11) Repeat this a few times until results are consistent.
It is best to defocus the lens and get the camera to refocus within EOS Utility a few times to check for consistency.
12) Each click on the ( > ) or ( < ) corresponds to one unit in the AF micro adjust scale in the camera.
e.g. if you clicked the > icon 3 times in the + direction then dial in +3 into the camera Lens microadjustments
13) Disengage LV by clicking "close" in the Zoom View and Remote Live View Windows.
14) Go to MA menu option in your camera and dial in the exact value noting the back or front direction.
15) Go back to step 3 and perform AF again, if the image is already as sharp as possible when you click 200% you are done, if not iterate until you can repeatedly get the sharpest image.
Your camera and lens static AF should be adjusted with great accuracy now. "



To read more and also see the EOS utility screenshots click here: Arash Hazeghi's lens micro adjustment technique

The beauty of this method is that once you have everything set up, you can wizz through all your lens and camera combinations very quickly and accurately. You will know if the camera is front or backfocussing as soon as you start clicking the > icons and can instantly see the effect of each adjustment without actually making them in camera. Once you know the value all you have to do is set it into the camera once and double check it - brilliant ! Unlike the other methods, I also find this to be highly reproducible.