AF calibration

Auto-focus calibration

(and how Khromagery can help you)

When cameras use “live” focus (contrast detection based on the image being captured by the main image sensor) then obviously when the auto-focus system thinks the image is in focus, it actually is. However when using separate AF sensors (as DSLR cameras not in Live View do) there exists the risk that when the AF sensor thinks the image is in focus, that it’s not actually in focus on the image sensor. The result is the image being focussed slightly behind or in front of the target. When this error is large the result can be obvious, but even small errors can result in your “focussed” area losing a lot of sharpness. The effects can be seen with all lenses, but are most obvious with telephoto lenses and especially when used at wide apertures.

(sample photos)

Historically it’s been possible to send your lenses and camera body together to the manufacturer and get them to resolve any front-focus or rear-focus issues. But many cameras now offer in-camera adjustments where the user can set an adjustment value to be used with each lens. Nikon call this “AF Fine Tune”, Canon calls it “AF Microadjustment”. Other manufacturers have their own names. See the list below for some of the camera models which support this calibration. These adjustments are necessarily limited in range, and it is still possible that a lens/camera pair might not be calibrated successfully because they need to be serviced (but this is unusual).

Page contents

In-camera AF calibration
We can calibrate your camera’s AF to each of your lenses.

Sigma in-lens AF calibration
We can calibrate recent Sigma lenses for optimal AF, with more control than offered by the camera bodies.

Costs
And how to order.


In-camera AF calibration

While photographers can make their own calibration adjustments, and tools such as LensAlign and FoCal are available to help, many photographers find it easier to get an expert to do the calibration for them. Thus Khromagery offers a calibration service where you provide us with your cameras and lenses and we calibrate them all (or just the combinations you require). How long it takes depends on the number of camera/lens/teleconverter combinations, but a camera and a few lenses is usually turned around within a day or two. Obviously you’ll need to coordinate with us to ensure we’re not going to be away on a LuminOdyssey or tied up with other jobs, but we can often fit calibrations in on fairly short notice.

We provide a detailed report at the end of the process, so you can re-program the cameras if for some reason you lose all the settings.

Our workshop is located in Box Hill, Victoria (Australia). You will have to bear the cost of transport to/from us, so obviously it’s most convenient for Melbourne-based photographers.

Calibration distance

The required adjustment for each lens can vary based on the focus distance, though as you move away from the minimum focus distance (MFD) towards infinity the adjustment quickly settles down. While Canon recommends adjusting focus at 50x the focal length (thus a 400mm lens would be calibrated for accurate focus at 20 metres) the most accurate recommendation is actually to calibrate at the distance you’re most likely to use the lens. Unfortunately that’s hard to predict, so by default we will calibrate your lenses at around 25x the focal length (thus a 400mm lens will be calibrated at 10 metres) but if you have a special request we can calibrate at a specific distance.

Aperture

Obviously as you stop your lens down the depth of field (DOF) increases and more of the subject will be in focus. As a rough approximation the DOF will extend a third towards you and two thirds towards infinity. However some lenses are very soft overall at their widest apertures (some faster-than-f/2 lenses spring to mind as examples) and we usually find it best to calibrate them stopped down slightly. As an added complication some lens models exhibit “focus shift” with the point of maximum focus changing as you stop down. Not only does the DOF increase, but it also moves!

As a result we usually calibrate focus 2/3rds of a stop down from wide open (e.g. the Canon 100-400mm at f/7.1, and the Sigma 120-300 at f/3.2). We can of course accommodate specific requests.

Focal length (zooms)

The focus behaviour of a zoom lens changes as you zoom. The required AF adjustment can (and does) vary. Older camera bodies (e.g. D700 and EOS 5D Mark II) only have one adjustment value which is applied at all focal lengths, while newer bodies have a value for each end of the zoom and scale between the two values as you zoom. While this is a significant improvement, the linear scaling of the value is only an approximation. Recent Sigma lenses offer calibrations at four points through the zoom, which is even better (see below for details on these lenses).

For cameras with a single adjustment value for zooms many people calibrate the lens at the telephoto end: DOF is smaller there and for telephoto zooms most people end up using the long end of the lens the most. We can calibrate the lens at any focal length you prefer (and for new lens models will analyse how the required adjustment varies at several points across the zoom range with their corresponding calibration distances).

Nothing’s perfect! (disclaimer)

This is fairly complex technology. As we’ve already mentioned there are variables of focal length, subject distance, and aperture, and the cameras force us to make compromises and pick adjustment values which will have the best chance of maximising focus accuracy.

But also remember that “accuracy” is a different thing to “precision”. Each time the camera focusses on the same object it will get a slightly different result. The results will vary, but hopefully grouped around an accurate result. We make many tests in order to settle on an appropriate “average” calibration, but occasionally the camera won’t hit that average point. In general the higher-end bodies and lenses will have higher precision than the cheaper equipment (and some bodies have higher-precision AF sensors that activate with faster f/4 and f/2.8 lenses), but it’s not always that simple. For example Canon seems to have made improvements in the last few years, where we see that recent camera bodies (such as the 5D Mark III) when used with recent lenses (for example the ‘II’ series of super-telephotos, the lenses with STM focus motors, and even the EF 70-300mm/4-5.6 L IS USM zoom) achieve much higher precision than the same lenses with older bodies (or the same bodies with older lenses).

Also, while we have found that most lenses and cameras benefit from some amount of AF calibration, don’t assume that all your shots will then magically be in focus. We often see photographers thinking their lens is front/back-focussing when in fact the problem can be due to misunderstandings about the size of the AF sensors (they often extend past the little boxes shown in the viewfinder) and about the way the cameras select from multiple AF points in various focus modes. It’s definitely complex technology!

Camera models with in-body adjustments

This list will obviously evolve as new models are introduced, but provides a starting point. We can calibrate all of these cameras with their appropriate lenses.

Canon EOS 50D, 70D, 80D, 6D, 7D, 7D Mark II, 5D Mark II, 5D Mark III, 5Ds, 5Ds R, 1Ds Mark III, 1D Mark III, 1D Mark IV, 1D X, 1D X Mk II
Nikon D7000, D7100, D7200, D300, D300s, D500, D600, D610, D700, D800, D800E, D810, D3, D3s, D3X, D4, D5
Olympus E-30, E-620, E-5
Pentax K20D, K200D, K-5, K-5 II, K-30, K-50, K-7, 645D, 645Z
Phase One DF+, XF
Sony SLT-A77, SLT-A99, A850, A900


Sigma “Global Vision” lens AF calibration

As an extension of calibrating the AF in camera bodies to accommodate a variety of lenses, we also provide a service to calibrate the internals of the current generation of Sigma lenses. These are the “Art”, “Contemporary”, and “Sports” models. The older Sigma lenses do not support this function.

(photo of Sigma lens?)

 

Whereas some camera bodies provide the ability to set a single adjustment value which is used throughout the lens settings, and other (more-recent) bodies have two values for zoom lenses and interpolate between these as you zoom, the Sigma lenses offer much finer adjustment. Prime lenses have four adjustment values for four different distances, and interpolate between them as you focus. Zoom lenses have sixteen adjustment values: four distances at each of four zoom settings. This turns into quite a bit of work to calibrate, but it’s well worth the effort. As David says about his own calibrated 120-300mm/2.8 lens: if an image is not in focus it’s probably his own fault in technique!

(crop of screencap from SOP)

We have found with multiple copies of these lenses that each needs its own set of adjustments. And after one lens had its AF motor replaced by Sigma under warranty we recalibrated the lens and unsurprisingly found that the adjustments were quite different.

When we calibrate a lens at the same time as calibrating the camera body it will be used with, the in-camera adjustment is usually set to zero. However if we are calibrating multiple bodies with the lens the other bodies may need non-zero adjustments. Also note that even after the lens is adjusted, if used with a teleconverter the camera will need an in-camera calibration for that combination.

If you wish you can send us just your Sigma lens and we can calibrate it with our own camera body (although we only have Canon cameras) and then take care of an in-body adjustment yourself: some photographers are comfortable doing that themselves in the field. However the best results are usually found if we can do the camera/lens/teleconverter together.

This calibration is done using Sigma’s Optimization Pro software and their USB Dock (and our own procedures and analysis software). We have the Canon-mount and Nikon-mount versions of the Sigma USB Dock here: if you have one of these lenses with a different mount you will probably have to supply your own USB Dock. Obviously if you have your own USB Dock you can do all this yourself (as well as update lens firmware and on the Sports lenses you can adjust AF speed, OS behaviour and focus limiter configuration), just as you can do in-camera AF calibration yourself. However doing it accurately and efficiently is what we can do for you. We’ve had a lot of practice!

(photo of USB Dock)

Of course the report you receive with your calibrated lens includes all the adjusted values, so if you reset your lens to default values you can restore it to the calibrated settings.

During the calibration we can also update the lens firmware to the latest version at no added cost, although we will check with you prior to doing this just in case you have special requirements.


How much does it cost?

The work we have to do can scale a lot from one camera with a prime lens all the way up to several cameras with a collection of primes and telephotos and teleconverters. But we’ve put together a basic formula:

Camera body: AU$40
Each prime lens combination with that body: AU$20
Each zoom lens combination with that body: AU$30

Thus a 5D Mark III body with a 70-200/2.8 zoom and 400mm/2.8 telephoto and 1.4x teleconverter would cost $140. If you only use a teleconverter with specific lenses you should identify that so we don’t do unnecessary work: obviously we clarify all these choices before we start.

Sigma prime lens calibration: AU$50
Sigma zoom lens calibration: AU$75

This is only for the Sigma “GlobalVision” lenses, and is instead of the above per-lens fee, although adding a teleconverter would involve another “regular lens” calibration (as would using the lens on a second camera body). One camera plus Sigma 120-300/2.8 with 1.4x teleconverter would cost $140 (body plus Sigma-zoom plus lens+TC).

Please don’t hesitate to contact us to discuss your specific needs and we can clarify a quote derived from the above numbers. These prices include GST. As noted you will also be responsible for transport costs to/from Box Hill, Victoria (postcode 3128).

Payment can be made via PayPal or direct deposit (or cash in person) although we can also process credit cards directly.

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