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Thread: Image circle too large?

  1. #1

    Image circle too large?

    Hello guys and girls,

    I am just getting started in large format photography, I bought and assembled my own "Bulldog" LFC and recently I bought a Fujion 180mm/5.6f lens of ebay (Image circle was 280phi according to seller). Could someone tell me, how many milimeters that is?

    And most importantly, it seems that I have to move the lens very far away from the camera, before I get a sharp image (at infinity). This point is almost the outer limit of my forward extension. Now, is this a normal phenomenon? Is there a formula that, given a focal size and image circle, tells you which distances from the lens to the focusing screen make an object which is x meters away from the camera look sharp?

    Thank you very much
    Kind regards!

  2. #2
    Jon Shiu's Avatar
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    Re: Image circle too large?

    Focal length of 180mm indicates approximate distance from center of the lens to image plane at infinity focus. For closer objects, the distance would increase. For example at 1:1 close-up, distance between lens and ground glass would be about twice, ie 360mm.

    Jon
    my black and white photos of the Mendocino Coast: www.jonshiu.com

  3. #3

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    Re: Image circle too large?

    The lens focal length is what determines how far from the film your lens will be, not the image circle. As Jon said, the bellows extension when focused at infinity is very close to the lens focal length in MM. I don't know how much extension your camera offers, but you should have around 180mm of extension when focused at infinity.

    If the distance is much longer than that, do you have a front and rear cell, one in front of the shutter and one behind the shutter and lens board? Some lenses will effectively give a longer focal length without the rear lens cell.

    The image circle does not affect how much extension you need. The image circle is the size of the image that the lens projects onto the film plane. A larger image circle will allow you to use more camera or lens movements (rise and fall, etc.).

    Not to confuse the issue, but lens extension does affect image circle. As you focus closer, the lens will be further from the film plane. This means the lens will project a larger circle onto the film plane. That's why image circle specifications are usually quoted for infinity focus.

  4. #4

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    Re: Image circle too large?

    didn't bulldog make 4x5 and a 10x8 diy build cameras? Which one have you got and what are the specs for it. The spec should tell you the minimum lens extension and the maximum lens extension.

    The minimum lens extension will be close to shortest focal length lens that you can use on that camera. Any shorter than that and you wouldn't be able to focus at infinity.
    The maximum lens extension is just that. i.e. as far as you can extend the lens for close up focus.

    Just as an example, If your minimum lens extension was 150mm and your maximum lens extension was 300mm then the shortest lens you could use would be a 150mm lens. At infinity focus it would be all the way back and for a close up doing a 1:1 image it would be extended all the way to 300mm. (you could use a shorter focal length lens but only if its image circle is big enough and you didn't need to focus at infinity).

    The smallest image circle to cover 4x5 at infinity is approx 160mm and for 8x10 its 325mm

    Easiest if you tell us what the minimum and maximum lens extension is on your camera and whether its 4x5 or 8x10 and then suggestions on which lens is good for it will be forthcoming... And also what the capabilities of you 180 lens will be on your camera.

    Also tell us what it is tyou are trying to photograph and close or far away it is. i.e. is it a landscape, a portrait of someone fairly near or a still life of something very close etc.

  5. #5

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    Re: Image circle too large?

    Yes there is lots of information about image circles, flange to film plane distance at infinity and the amount of movement of the lens required to focus at a given film to subject distance.
    A hearty welcome to LF photography
    And
    I appears you have a lot of homework to do to get into the basics
    There are tutorials on this site and references.
    Go to a recent post by Dan Fromm to get a reference to a site that contains a ton of information about Fuji lenses, and many other lens sites.
    There are at least hundreds of books/pamphlets that can be downloaded for free from say google books and Internet Archive.
    A lot of them will be 100 years old, or so, due to copyright laws, but the basic principles of optics haven't changed.
    There are plenty of economical lenses in the 150 mm focal length- normal for 4x5.
    Get one and practice focusing-the shorter focal length should be an easier fit for your camera's extensability.
    Make yourself aware of "depth of Focus, then you can discover how it changes with aperture and focal length
    If part of your question was to ask how to convert inches to millimeters, one inch is 25.4 mm Thus a 'normal' lens for 4x5 is about 6 inches [do the arithmetic and you will see it's a little over].
    If "normal lens" is not a familiar term, please start your homework.
    Half the fun is the learning journey
    regards
    ED

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    Re: Image circle too large?

    Quote Originally Posted by Noah A View Post
    The lens focal length is what determines how far from the film your lens will be, not the image circle. As Jon said, the bellows extension when focused at infinity is very close to the lens focal length in MM. I don't know how much extension your camera offers, but you should have around 180mm of extension when focused at infinity.

    If the distance is much longer than that, do you have a front and rear cell, one in front of the shutter and one behind the shutter and lens board? Some lenses will effectively give a longer focal length without the rear lens cell.

    The image circle does not affect how much extension you need. The image circle is the size of the image that the lens projects onto the film plane. A larger image circle will allow you to use more camera or lens movements (rise and fall, etc.).

    Not to confuse the issue, but lens extension does affect image circle. As you focus closer, the lens will be further from the film plane. This means the lens will project a larger circle onto the film plane. That's why image circle specifications are usually quoted for infinity focus.
    Unless it is a Tele design!

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    Re: Image circle too large?

    Quote Originally Posted by mingliaozi View Post
    Hello guys and girls
    That very polite, but misguided, sadly there are no females active on this forum.

    Now - back to technobabble.

  8. #8

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    Re: Image circle too large?

    Quote Originally Posted by Noah A View Post
    Not to confuse the issue, but lens extension does affect image circle. As you focus closer, the lens will be further from the film plane. This means the lens will project a larger circle onto the film plane. That's why image circle specifications are usually quoted for infinity focus.
    One way to visualize what Noah is mentioning is think of the light coming from the rear of the lens to the ground glass / film plane as a cone.

    The usable cone is shortest when focused at infinity, and the resulting circular base of the cone (image circle) is he smallest. I say usable cone, as you cannot form an image if the lens is placed closer to the ground glass than the infinity focus point. Focusing closer moves the lens further away from the ground glass and thus makes the image circle larger. Hence the reason typically the image circle is quoted at infinity.

    The exception to that situation is where for a particular use (i.e. process lenses) that may be quoted at 1:1.

    Welcome to the community, but suggest you look at the LF Home Page (see tab at top of page) and read some of the articles there.

  9. #9

    Re: Image circle too large?

    Thanks to everyone. Yes, I do realize that I have lots of reading to do. However, as a mathematician I sometimes find some of the articles a bit handwavy and lengthy for the information they provide. So I was asking for a quick formula, and thanks a lot. So, I have a bulldog 4x5 DIY camera , my minimum lens extension is around 100mm and maximimum about 180mm ish. This doesn't seem to be very good, basically only allowing 90mm lenses, am I right?
    And also, which formula relates f stops to depth of focus? For pinhole cameras, everything is in focus. So, by stopping down heavily, I could account for the limitied lens extension ?!

    Thanks to everyone!

  10. #10
    I live in Connecticut now.
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    Re: Image circle too large?

    Quote Originally Posted by koh303 View Post
    That very polite, but misguided, sadly there are no females active on this forum.

    Now - back to technobabble.
    Speak for yourself! :-p

    Quote Originally Posted by mingliaozi View Post
    Thanks to everyone. Yes, I do realize that I have lots of reading to do. However, as a mathematician I sometimes find some of the articles a bit handwavy and lengthy for the information they provide. So I was asking for a quick formula, and thanks a lot. So, I have a bulldog 4x5 DIY camera , my minimum lens extension is around 100mm and maximimum about 180mm ish. This doesn't seem to be very good, basically only allowing 90mm lenses, am I right?
    And also, which formula relates f stops to depth of focus? For pinhole cameras, everything is in focus. So, by stopping down heavily, I could account for the limitied lens extension ?!

    Thanks to everyone!
    As someone else mentioned, things change with "telephoto" lenses, but for normal LF lenses, the rule is.

    Lens FL=bellows extension.

    90mm lens = ABOUT 90mm of bellows extension.

    150mm lens = ABOUT 150mm of bellows extension.

    Etc.

    Although I don't often care for actual calculations, as a more "artist" type, just view the ground glass and estimate based on experience how much depth I want... But here is the "Hyperfocal Distance" calculation, that is, the distance to which you can focus and stop down to have mostly everything in focus.

    Hyperfocal Distance
    Setting focus at the Hyperfocal Distance gives maximum depth of field from H/2 to infinity.

    H = (L x L) / (f x d)

    Where:
    H = Hyperfocal Distance (in millimeters)
    L = lens focal length (ie, 35mm, 105mm)
    f = lens aperture f-stop
    d = diameter of circle of least confusion (in millimeters)
    for 35mm format d = 0.03
    for 6x6cm format d = 0.06
    for 4x5in format d = 0.15

    I hope this helps you.

    That camera does sound very limiting. Look into the "Intrepid Camera" company, they make very cheap 4x5's that are not fancy but have a lot more options than yours seems to.

    If you want to get a fancy one, I recommend a Chamonix 45N-2.

    Hope this helps.

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