Calculating close focus

[abruzzi]

I recently bought a Nikkor-T with 360 and 500 mm rear elements. I wasn’t too worried that it didn’t have the 720 element, given that I can’t really use it. But I was wondering, the 720 shows a flange distance of 469mm. My Technikardan officially has 485mm of extension. Obviously 19mm, if I even have those 19mm, isn’t going to focus much closer than infinity. But say I put the lens on a 50mm top hat board, so I have just shy of 70mm on a 720mm lens. So the question is can you calculate how close that 70mm would get me? I’m sure there is a relatively simple formula to determine this, but I don’t know what it is.

[Bernice Loui]

Camera before the lens..

Either apply another camera, crop the image (similar effect to increasing focal length) or forgo the 720mm rear element.

Calculating the required focal length for infinity focus or closer (requires more camera-bellows extension) is folly. Real world image making conditions tend to be unpredictable making calculated camera-bellows length vastly... useless.


Bernice

I recently bought a Nikkor-T with 360 and 500 mm rear elements. I wasn’t too worried that it didn’t have the 720 element, given that I can’t really use it. But I was wondering, the 720 shows a flange distance of 469mm. My Technikardan officially has 485mm of extension. Obviously 19mm, if I even have those 19mm, isn’t going to focus much closer than infinity. But say I put the lens on a 50mm top hat board, so I have just shy of 70mm on a 720mm lens. So the question is can you calculate how close that 70mm would get me? I’m sure there is a relatively simple formula to determine this, but I don’t know what it is.

[Kiwi7475]

Here’s the way to find out with some examples included:

https://www.kennethleegallery.com/html/tech/bellows.php

[sharktooth]

Similar to what Kiwi7475 posted.

The raw formula is 1/(focal length) = 1/(Subject to lens nodal point) + 1/(lens nodal point to film plane)

To make it easier to view, I'll call the focal length F, the Subject to lens nodal point S, and the lens nodal point to film plane P.

The formula then becomes 1/F = 1/S + 1/P

For your 720mm lens the focal length F is 720mm. This is the distance from the nodal point of the lens to the film plane when focused at infinity.

In your case, your camera has enough bellows draw to move the lens nodal point another 19mm, so the maximum distance from the lens nodal point to the film plane P is 720 + 19 = 739mm

This means that you know both F and P, so you can solve for S.

1/S = 1/F - 1/P

S = 1/(1/F - 1/P)

Now insert the values for F and P

S = 1/(1/720 - 1/739)

S = 1/(.0013888 - .00135318)

S = 28004mm or 28.004m (92 feet)

The distance from the subject to the film plane is S + P, so that would be close to 29m. That means the theoretical minimum focus distance with your camera is about 29m from the film plane when using the 720mm lens.


If you were to use a 50mm extended lens board, then P would be 789mm. This would result in an S of 8.23m (27 feet).

[abruzzi]

Here’s the way to find out with some examples included:

https://www.kennethleegallery.com/html/tech/bellows.php

Thanks, that’s kind of what I needed, what’s not clear though is how telephoto lenses should be treated. That says 1/f+1/d=1/b. Since it is a telephoto, for f do I use 720 (the effective focal length) or so I use the flange distance? Or is there is a more complex version of formula to account for differing f and ffd? Using the ffd only suggests that the closest focus with a 50mm top hat would be around 3.8 meters.

[Dan Fromm]

Use focal length. Flange-focal distance is strongly affected by mechanical design, has little to do with focal length. Measure extension from the front standard's position when the lens is focused at infinity.

By the way, by a simple rule of thumb, 70 mm of extension from the infinity position with a 720 mm lens will get you almost 1:10 magnification.

[peter brooks]

By the way, by a simple rule of thumb, 70 mm of extension from the infinity position with a 720 mm lens will get you almost 1:10 magnification.

Does that roughly 10% extension to give 1:10 apply across the board? Would 30mm extension (from infinity) on a 300mm lens give an approx 1:10 magnification?

[Dan Fromm]

Does that roughly 10% extension to give 1:10 apply across the board? Would 30mm extension (from infinity) on a 300mm lens give an approx 1:10 magnification?

yes

[peter brooks]

yes

Thanks - that's a really useful rule of thumb.

[Dan Fromm]

Thanks - that's a really useful rule of thumb.

Peter, in fact it isn't a rule of thumb. Its stronger than that, its the law. E = (m + 1)*f, whence (e/f) - 1 = m. e means extension from the infinity position, m means magnification, and f is focal length.