Hyperfocal distance

[Tim Shawcross]

Hi,

I've got a couple of quick questions regarding hyperfocal distances -

1. I've got a Komura 75 F6.3 lens which I believe is a retrofocal (not sure that's 100% correct term, but I hope you know what I mean) design. I believe it focuses at infinity at around 90mm - does this affect how I calculate hyperfocal distance (ie should I calculate for a 90mm lens or a 75mm lens?)

2. When measuring the the focal distance from lens to film plane, where abouts on the lens do you measure from? Is it from the back of the rear element?

thanks

Tim

[Ole Tjugen]

1. No, it's still a 75mm lens.

2. It's from film plane to the front nodal point of the lens, which may or may not be anywhere near the physical lens. Most lenses have this somewhere near the shutter, telephoto lenses may have it way out in thin air in front of the lens.

[Dan Fromm]

Tim,

What are you trying to get at in your second question?

[Tim Shawcross]

Thanks for replys -

Is there a sound method of finding the front nodal point of the lens? My first though was focus my 75mm at infinity and then measure 75mm from the film plane to the lens and call that point the nodal point - I have a feeling this wont work as the 75mm doesn't actually focus at infinity at 75mm but closer to 90mm because of the retrofocal design

Using DOFmaster, a 75mm lens with CoC of .1, at f22 hyperfocal is about 2.5 meters. How do I convert this into a corresponding distance between the film plane and front nodal point of the lens? Is there a formula that can be used, or do I have to figure it out manually (by say focussing on objects in .5 meter increments and then charting the distances so I have a reference?)

Tim

[Dan Fromm]

Tim, reverse the lens, focus on a distant subject. The front node will be 75 mm from the film plane.

[Michael S. Briggs]

When the lens is focused on infinity, the distance from the image (i.e., film) to the REAR nodal point or principal point will be 75 mm. So you can simply focus on infinity and measure 75 mm forward from the ground glass. From the original question, I think this is the location sought, and that the front nodal point is not correct. You don't need to reverse the lens.

I'm not sure why this measurement would be useful in practice. Do you want to establish depth of field by measuring the position of your lens so that you can input the image distance into a calculator?, and thus need to establish the "zero" point? If so, just focus on infinity and measure some convenient point. Then when you focus at another distance, measure to that same point, and do a delta to get the same in image distance from 75 mm.

If you goal is guidance in picking your taking aperture, it is much easier on most LF cameras to measure the focus spread and not bother with any calculations in object space. See http://www.largeformatphotography.info/fstop.html

The front nodal or principal point is where you measure from for object distance.

[Michael S. Briggs]

Thanks for replys -

......

Using DOFmaster, a 75mm lens with CoC of .1, at f22 hyperfocal is about 2.5 meters. How do I convert this into a corresponding distance between the film plane and front nodal point of the lens? Is there a formula that can be used, or do I have to figure it out manually (by say focussing on objects in .5 meter increments and then charting the distances so I have a reference?)

Tim

The equation relating object distance So and image distance Si is given in the Lens Tutorial at http://www.photo.net/learn/optics/lensTutorial. So for f=75 mm and So=2500 mm, Si= 77.32 mm. The distance Si is measured from the image to the rear principal point or nodal point. You could simply focus the lens at infinity, which will make Si = f = 75 mm, then advance the focus by 2.3 mm.