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Thread: Measuring the focus distance.

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    Arrow Measuring the focus distance.

    Greetings,

    This topic came up in a different discussion and it seemed like it deserved its own thread so others can benefit. Here goes...

    When you read the specifications for a lens, one of the specs is its minimum focus distance. It tells you how closely you can get to your subject and still focus. Unfortunately, the specs don't provide much more information than that and this can create confusion. Is the focus distance measured from the front of the lens or the optical center of the lens? Answer: Neither.

    Most camera and lens makers measure the focus distance from the focal plane inside the camera as illustrated below (I prepared these illustrations for a software manual that I've been writing). In other words, they measure it from the surface of the image sensor if its a digital camera or the surface of the film if its a film camera.


    Some cameras like the E-1 mark the location of the focal plane on the outside of the camera body for the convenience of photographers. It is shown below (the circle with a line through it: ). I haven't seen this label on many consumer cameras and I'm not sure if any of the other 4/3rds cameras have this label. Even on the E-1, Olympus doesn't explain it anywhere in their manual so most owners who don't know what it means have no idea why the symbol is there.


    You can estimate the location of the optical center of a lens by measuring forward from the focal plane of the camera by the focal length as shown below. In other words, the optical center of a 50mm lens will be about 50mm in front of the camera focal plane.


    This is only a close approximation of the optical center because of the complexities of modern lenses. But there's more: The optical center of many lenses changes. Obviously, it changes with the focal length setting of a zoom lens. But what is not obvious is that it can also change with different focus settings---even with a prime lens---because many modern lenses shift the focal length when you focus at distances closer than infinity. In fact, if you read the fine print from many lens specs you'll see that the focal length rating is almost always referenced to a focus distance of infinity.

    For most of us, knowing exactly how the focus distance is measured and where the lens optical center (also called its "null point") is unimportant. But two photo specialties that need this information are: macro photographers and panoramic photographers.
    Best regards, FL

    Pursuing excellence...

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    Default Re: Measuring the focus distance.

    Fantastic explanation First Light.Explains everything in a very understandable way........many thanks for the post!
    [SIGPIC][/SIGPIC]
    E1,E300,14-45,40-150

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    Default Re: Measuring the focus distance.

    The E-500 also has a focal plane indicator to the right of the left strap loop. Great explanation.

    Ron

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    Default Re: Measuring the focus distance.

    Well done, FL. What a superb illustration! Top notch stuff!

    Roger

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    Default Re: Measuring the focus distance.

    Great explanation firstlight. The E300 also has the mark. Hurrah for optical companies making cameras! More companies should do it. It sure can't cost much.
    thanks
    barondla

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    Default Re: Measuring the focus distance.

    Quote Originally Posted by First Light View Post

    ... But what is not obvious is that it can also change with different focus settings---even with a prime lens---because many modern lenses shift the focal length when you focus at distances closer than infinity. In fact, if you read the fine print from many lens specs you'll see that the focal length rating is almost always referenced to a focus distance of infinity.
    The definition of 'Focal length' is the distance of the image from the lens optical center, when the lens is focused at infinity. For a prime lens it's a fixed number and for zooms it changes with the zoom setting.

    As you focus at closer subjects, the distance of the image to the optical center increases. In classical optics the relation is given by:

    1/f = 1/a + 1/b

    Where: f is the focal length, a is the subject distance and b the image distance (in the same units of length). If a tends to infinity, f = b (as defined). The number 1/f, when f is in meters, is called the lens 'Diopter'.

    Since in a practical camera the location of the image (sensor, film) is fixed, the optical center (or entire lens) moves forward. It's not the 'focal length' which is shifted. The focal length is a fixed number (for a given zoom).

    The closest focusing distance is determined by how much the lens can be moved forward. Adding extension tubes or bellows moves the lens further forward, thus enabling closer focusing.

    Moshe

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    Default Re: Measuring the focus distance.

    Hi Moshe,

    Thanks for contributing to this thread. While your explanation is clear, I still think we can use the label "focal length" at focus distance less than infinity---tell me if you think this line of reasoning is mistaken:

    (Let's talk about prime lenses to keep it simple.) The focal length rating of a lens is calculated at infinity. But what do we call the distance between the lens optical center and the camera focal plane when the focus is less than infinity? I believe it is still correct to call this distance the "focal length". But it is no longer the "reference" focal length specification of the lens. Therefore I think it is true to state that the focal length changes as the lens if focused.

    Further support for this view is the fact that the AOV changes as we focus less than infinity. The support comes by solving backward through the AOV equation for the focal length.

    Maybe we should make a difference between a "reference" focal length at infinity as used in a lens spec and an "actual" focal length with the lens focused to any specified focus distance.
    Best regards, FL

    Pursuing excellence...

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    Default Re: Measuring the focus distance.

    Quote Originally Posted by First Light View Post

    ... But what do we call the distance between the lens optical center and the camera focal plane when the focus is less than infinity?
    Hello First Light,

    That distance is referred to as 'image distance'.

    'Focal length' is a physical attribute of the lens, a fixed value (for primes) determined at manufacturing time, independent of the camera the lens will be used on or application.

    Image distance depends on application (macro, normal, astro-photography, etc.). It is a value which normally is of lesser interest to the photographer since normally he doesn't know where the lens optical center is, anyway, and it is not something he has to deal with directly. It may help him calculate bellow length in macro photography.

    I found it more convenient to use technical terms accurately. Using them loosely may lead to misunderstandings.

    Moshe

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    Default Re: Measuring the focus distance.

    Hello again Moshe,

    Yes, I agree completely about using technical terms accurately and "focal length" is used as a technical term. But "image distance" seems imprecise because it does not imply focus. The length of focus is the length of focus whether the optical system is focused to infinity or not.

    One thing does seem clear: We can avoid this muddle by describing this variable in terms of the AOV rather then the focal length. In other words, "the AOV of a prime lens changes with focus distance". This puts the emphasis on what the photographer observes.
    Best regards, FL

    Pursuing excellence...

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