Large Format Lens: Flange to Plane distance

[wnnj]

hello, I am new to LF photography and trying to find my way...

need to find out if there is a database / link / source for figuring out flange to plane distance of a lens?

or is there a formula ?

so, if I have a 10" lens or a 8" lens, how do I figure out what is the flange to photo plane distance;

thanks you in anticipation...

wn

 

[BradS]

For a standard design lens (which most large format lenses are) focused at infinity, the distance from the flange to the plane of focus (film plane) is approximately equal to the focal length of the lens. If you focus closer than infinity, the distance from flange to film increases. The relationship is very closely approximated by the following...

1/(focal length) = 1/(object distance) + 1/(image distance)

The object distance is the distance from the flange to the the actual thing that you are focused on and the image distance is the distance from flange to film. Again, this is approximate but for most large format lenses, it is very close.

 

[shutterfinger]

The rear Nodal point of the lens to the film plane is the focal length of the lens for standard lens. The position of the shutter / aperture blades is where/very close to where the rear nodal is on standard lens. Telephoto and retro focus lens have their rear nodal point in different positions.

 

[awty]

Use a ground glass.

 

[David A. Goldfarb]

Fortunately, this isn't something you normally need to think about, unless you're trying to use a very wide lens on a camera that isn't really set up for that for mechanical reasons, like the bellows doesn't compress adequately, or there is some obstruction that prevents the front and rear standards from getting close enough. Alternately, if you want to use a longer lens than you have bellows/rail for, you might have to think about the flange focal distance. If you're using modern lenses, the manufacturers provide spec sheets with this information, and for older lenses, there are often charts with this info on the internet.

 

[Bob S]

Fortunately, this isn't something you normally need to think about, unless you're trying to use a very wide lens on a camera that isn't really set up for that for mechanical reasons, like the bellows doesn't compress adequately, or there is some obstruction that prevents the front and rear standards from getting close enough. Alternately, if you want to use a longer lens than you have bellows/rail for, you might have to think about the flange focal distance. If you're using modern lenses, the manufacturers provide spec sheets with this information, and for older lenses, there are often charts with this info on the internet.

Or, you want to focus closer then infinity and need to know how much extension you need.

 

[wnnj]

Gentlemen, thank you all;

your collective wisdom has cleared all the issues and questions I had on this topic;

I am working with bellows that do not support the lens on the infinity focus and might needs to look into either recessed lens board or longer lens; but, before I venture in either direction; I needed to sort the problem on the flange to film plane distance understanding on the LF lenses.

Thank you all for your prompt guidance.

much appreciated.

 

[Dan Fromm]

Hmm. wnnj, where a barrel lens' flange is placed is quite arbitrary. I've seen lenses in barrel with the flange at the front of the barrel -- these used to be called deep mount lenses -- and at the very rear. There's not a general rule.

Lenses in shutter are another matter. The flange is often, not always, at the rear of the shutter. This is close to the diaphragm. But lenses in shutter for, e.g., bayonet mount cameras, adapted to LF cameras can have the flange at the rear. It all depends on the lens.

Manufacturers' catalogs often show lenses' mechanical designs including where the flange is and flange-to-film distance when the lens is focused at infinity. The first post in this

https://www.largeformatphotography....to-look-for-information-on-LF-(mainly)-lenses discussion has a link to a list of links to information of interest to LF photographers, including lens catalogs. Use it.

 

[Ian C]

The flange distance is the distance from the mounting flange (coincides with the front mounting surface of the lens board on LF cameras) to the image plane when the lens is focused at infinity. This is also measured from the rear mounting surface of the shutter housing where it mates to the front of the lens board.

You can determine the flange distance by first focusing on a distant object. The moon at night makes an ideal infinity target. Then measure the distance from the flange to the front surface of the focusing screen (the image plane). This is the flange distance of the lens.

If you know the flange distance and the focal length of the lens, you can determine the fixed position of the second nodal point of the lens relative to the flange surface.

If the flange distance is greater than the focal length (common on wide-angle lenses), then the second nodal point is behind the flange by the flange distance – the focal length.

If the flange distance is less than the focal length (common on telephoto lenses), then the second nodal point is forward of the flange by the focal length – the flange distance.

Bear in mind that the marked focal length of a lens can be rounded off as much as plus or minus 5% from its true value. Usually, this roundoff error in the marked focal length is much less than 5%.

As stated in post #5, knowing the flange distance is useful in determining whether a particular camera and lens combination is practical and whether an extended or recessed lens mount is needed. In some cases, a rail or bed extension with an extra bellows or extension-length bellows might be needed to use an unusually-long lens on a given camera.

 

[Alan Edward Klein]

Is there a simple guideline that reminds you when you need to worry about losing stops of light when focusing closer due to extending bellows?

 

[Alan Edward Klein]

Does tilting have any effect on calculating additional stops of light needed?

 

[wnnj]

As stated in post #5, knowing the flange distance is useful in determining whether a particular camera and lens combination is practical and whether an extended or recessed lens mount is needed. In some cases, a rail or bed extension with an extra bellows or extension-length bellows might be needed to use an unusually-long lens on a given camera.

Thanks !

 

[Ian C]

Regarding post #10:

In one of my old Kodak Data Guides, I read the suggestion that for negative films, exposure compensation can be ignored for subject distances (subject-to-first nodal point) greater than 7 focal lengths.

For example, a 100 mm lens at a subject distance of 700 mm (7 focal lengths), the exposure compensation is 0.44 stops.

At a 600 mm subject distance (6 focal lengths), the compensation is 0.53 stops.

At 800 mm subject distance (8 focal lengths), the compensation is 0.39 stops.

Exposure compensation in stops = 2.885*ln(s/(s – f))

Where s = subject distance and f = focal length.

 

[wiltw]

Is there a simple guideline that reminds you when you need to worry about losing stops of light when focusing closer due to extending bellows?

One simple guideline is based on the extension/FL

1/10 to 1/5 = +0.5EV
1/4 to 1/3 = +1EV
1/2 = +1.5EV
1/1 = +2EV
2/1 = +3EV​

 

[maltfalc]

Is there a simple guideline that reminds you when you need to worry about losing stops of light when focusing closer due to extending bellows?

treat it exactly like f-stops. a 200mm lens extended to 280mm from the film plane costs you one stop, two stops at 400mm, etc.