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Thread: relationship between f-stop & image circle?

  1. #1
    David J. Heinrich
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    relationship between f-stop & image circle?

    I know that in general, image circle sizes increase with increasing f-stop. Is there a formula for this? i.e., given image-circle of size X at f-stop Y, can you guess how large the circle will be at larger f-stops?

    I'm curious how large the G-Claron 305/9's image circle is at f/32, f/45, and f/64 (I know it's 381mm at f/22).

  2. #2
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    Re: relationship between f-stop & image circle?

    It varies with the optical design and mechanical mount, so there's no universal formula.

    With the G-Clarons, it's often said that you can get about 1/3 more usable coverage than the specified image circle at f/22 by stopping down to f/45. But you need to take anecdotal statements like that with a grain of salt - it depends on your intended application and on what you consider "usable". Detailed enough for a contact print? For an enlargement? How large? How critically will you view it? How much do you care about acuity in the corners? FWIW, I've used my 270 G-Claron on 11x14 with acceptable (to me) results, but I've made contact prints only. YMMV.

    Also bear in mind that for some optical designs and lens mountings the circle of illumination is larger than the circle of good definition. So when someone says a lens is giving enormous coverage stopped down, you need to ask which criterion they're using.

  3. #3
    Mark Sawyer's Avatar
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    Re: relationship between f-stop & image circle?

    I'd rather say it reduces the gradual fall-off caused by the mechanical vignetting of the outside edges of the lens caused by the barrel. As Oren said, it varies with design and mount. I'll add to his observations that sometimes the smaller aperture will give sharper definition at the outer edges that would have been too soft at wider apertures...
    "I love my Verito lens, but I always have to sharpen everything in Photoshop..."

  4. #4
    David J. Heinrich
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    Re: relationship between f-stop & image circle?

    Thanks for the response. The lens has plenty of coverage for 4x5, I was only thinking for a possible normal 8x10, if it has more room for movement than it specifies. It has ok movement (9 degrees tilt) on 8x10. On 4x5, it has outstanding movements, 25 degrees tilt (within the 381mm spec).

  5. #5

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    Re: relationship between f-stop & image circle?

    Quote Originally Posted by dh003i View Post
    I know that in general, image circle sizes increase with increasing f-stop. Is there a formula for this? i.e., given image-circle of size X at f-stop Y, can you guess how large the circle will be at larger f-stops?

    I'm curious how large the G-Claron 305/9's image circle is at f/32, f/45, and f/64 (I know it's 381mm at f/22).
    For some lenses (Nikon) two values of the IC were usually published - one for the maximum aperture and one for the f/22 aperture. For practical purposes you could calculate from these values the IC dimension even for the rest of aperture sizes by simple interpolation.
    If you know the IC dimension for one given aperture size only you can measure the IC for the maximum aperture and make the interpolation again.
    Alternatively, you can measure the difference between the known IC and the illumination circle for the known aperture size. For a different aperture you measure the illumination circle and subtract the known difference from it to get the approximative IC from it.

  6. #6
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    Re: relationship between f-stop & image circle?

    Quote Originally Posted by dh003i View Post
    Thanks for the response. The lens has plenty of coverage for 4x5, I was only thinking for a possible normal 8x10, if it has more room for movement than it specifies. It has ok movement (9 degrees tilt) on 8x10. On 4x5, it has outstanding movements, 25 degrees tilt (within the 381mm spec).
    Unless you're planning to do tabletop work that requires pretzel contortions of a monorail camera, you may be overestimating how much of an image circle you need. Even at its rated coverage at f/22, the 305 G-Claron provides ample room for typical field work on 8x10.

    If you're going to routinely use lenses with substantial excess coverage for your format, you may want to learn about using a compendium hood, to minimize internal bellows flare from all that extra light sloshing around inside your camera.

  7. #7

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    Re: relationship between f-stop & image circle?

    Does anyone have experience using the Rodenstock 150mm f/5.6 Apo-Sironar-S on 5x7? It has an image circle of 231mm at f22, which should be OK without movements. I'd like to know how it performs on 5x7 with movements, and/or what the image circle is, at f32 and f45.

    Thanks.
    Cheers!

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