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Discussions about cameras, lenses, accessories, and image-processing.
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TOPIC: For lenses that are bit short or long sighted

For lenses that are bit short or long sighted 3 years 8 months ago #150

  • Les Peters
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Another useful tool that isn't needed often, but is handy to get your hands on it, is a focus calibrator. The one I use is called "Lensalign", but again there are others to choose from. I got mine after I bought my first teleconverter.

I generally use a 70 to 200mm F2.8 zoom for pottering about, when birding isn't on my mind -which isn't often. I use a 300mm F2.8 VRI when I feel the urge to photograph birds. For small birds I add a TC17CII, which gives you roughly a 510 mm lens.

As a pure prime, I haven't found the 300mm has any measurable faults at all. However, if any of the three teleconverters available are added to it, the focus point will be moved towards me. It no longer focuses at the right spot.

The lens align tool has a target which you aim at. It allows you to see where the camera is focusing and so study how the depth of field is being used.
An equal amount both in front and behind the target is what you want to see. But as I've said, with a teleconverter in place, I find that most of the focus is in front of the target, which isn't something you want. If you were aiming at a bird's head, a lot of the focus would be on the air in front of its bill.

Nearly all DSLRs have an adjustment you can use to correct this. They have a menu item where you'll find an adjustment scale to move the focus point to the correct place. It's easier than it sounds and the result, after using the tool, is you won't waste a drop of focus.

Once again, it wouldn't be a bad thing to share in your local club. You don't need to use it often.

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Re: For lenses that are bit short or long sighted 3 years 8 months ago #156

  • Lindsay Cooke
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Hi Les, this is a link that may assist in explaining this system, with details about two types of Autofocus microadjustment. Namely LensAlign and Datacolor Spyder. www.the-digital-picture.com/Photography-Tips/af-microadjustment-tips.aspx

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Re: For lenses that are bit short or long sighted 3 years 7 months ago #161

  • Les Peters
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Thanks Lindsay.

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Re: For lenses that are bit short or long sighted 3 years 7 months ago #182

  • Graham Cam
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Hi All,

For those of you reading this thread by les and Lindsay - I can't stress enough the importance of using one of the hardware/software tools for accessing the focal point of your camera/lens combo(S). I have two pro bodies and 8 pro lenses and whilst it can be a task initially to perform an AF assessment it is imperative to do so if you want a sharp image.
The task is more laborious for zoom lenses as you need to make the calculations throughout the zoom range and remember the calculation also varies for distance of the target from the camera sensor. So it is always handy to know the general range your bird targets will be from the camera. In general the prime lenses are far closer to the zero point than zooms - it is all about the physics in designing a prime lens as compared to that of a zoom.

I am a bit of a fanatic on AF fine adjust and so I also had all my lenses and camera bodies re-calibrated by Nikon in Sydney using the standards supplied by Nikon in Japan. I still assess the focus with each lens before a major photo-shoot. You can be lucky and match a camera body and lens and find the focus is spot on - sheer luck. Each camera body and lens varies during the manufacturing process and provided each is within tolerance they meet QC. There is nothing wrong with this. Every piece of equipment that is manufactured has a manufacturer's tolerance specification linked to the product. So if you had a lens that is 3 microns back focusing and a sensor alignment that is 2 microns forward focussing the net result is 1 micron back focussing - no issue. If however your lens is 4 microns forward focussing and your sensor 4 micron forward focussing the net result is 8 micron forward focussing - you should adjust the focus using AF fine tune in many of the newer and high end cameras. It is not uncommon to find a camera lens combination exceeding 12-15 microns.

The AF is dependent on the contrast in the scene and hence low light and minimal contrast will impact upon AF. Similarly, there will be minor differences between AF each time you engage the focus button - whichever one you use (focus/shutter or rear focus button). Hence you need to assess the focus over several shots, usually about 5. Why re-assess every so often. It depends how often you use the lens/camera - it is also about wear and tear on the internal mechanisms. Remember we are talking 'microns' here and so it doesn't take much a micron of wear to be introduced over time or a couple of microns difference each time you activate the focus button.

A tip I received for one manufacturer is switch your VR or IS to OFF when not using the lens or carrying it around, even in the car, as the mechanism is then locked. When VR or IS is ON there is movement in the mechanism and over time this creates more wear and hence AF tolerances increase.

I regularly use FocusTune and the reason I prefer this tool over Focal is that it employs a piece of 3D hardware linked to a software algorithm to assess the focus. Focal uses a 2D target rather than a 3D target and also has software for assessing the focal point - my preference is a 3D taget. Datacolor has a 3D target and no software and is relatively simple if you have had some practice assessing the results from either of the other two tools. The Datacolor tool is also the cheapest.
There is another quick and cheap method (just a few dollars) to assess your camera/lens combo and I would be happy to provide details if anyone is interested. I often use this method if I want a quick answer rather than undertaking a full set-up of LensAlign.

What do you do if you don't have a camera body that does not allow you to make an AF adjustment? Not all is lost because if you know that you camera is forward focussing then you can focus not on the eye but a little further back on the bird. If back focussing is the issue then perhaps focus on the birds shoulder or wing. I agree that this is not ideal but I have seen first hand photographers using this technique and it does improve the sharpness. Of course the other option is to upgrade to a camera body with AF adjust.

Hope this helps.
Cheers
Graham

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