? on Recessed Lens Board

[Lee Christopher]

SIGH ... after weeks of reading, research, uncertainty, deliberation, and non-stop daily scouting and asking around, I finally got a Nikkor 65mm for my Horseman LX from a very nice member here.

Got the bag bellows at the same time, but now I find that even with the standards flipped and the base blocks touching each other, it just refuses to focus past a certain point.

Logic tells me that I would need a recessed board in addition the the bag bellows, but what size should I be looking at?

This additional and unexpectedly needed item is already a hit on my budget, besides more delays.

There are only a few 3rd party recessed boards on ebay right now. I can't afford to make a mistake.

Would this work and get me infinity focus? 400052315307

[Archphoto]

It much depends on how much room you are missing right now: 27mm as quite a bit.

Measure the distance you have now between the focusing screen and the back of the lens board with both standards tight together.
You will need aprox. 7cm for this lens.

With this board: your mesurement minus 2.7 cm should be less than 7cm and you will be OK.

Peter

[Lee Christopher]

Surface of fresnel to back of board (standards tight together) = 73mm

73mm - 27mm = 46mm

This should get me infinity focus?

[Archphoto]

SHOULD !

Then you would have about 20mm to play with for bellows and exact position of the aperture, that theoreticly be at 65mm from the ground glass at infinity.

Peter

[Lee Christopher]

Thanks so much Peter.

I frankly do not yet know how to calculate these things, but I will slowly learn.

CHEERS!

Edit: I found another recessed board that's 47mm deep. Would this make a better choice? I have a very very flexible cabel release.

[rdenney]

Thanks so much Peter.

I frankly do not yet know how to calculate these things, but I will slowly learn.

CHEERS!

Edit: I found another recessed board that's 47mm deep. Would this make a better choice? I have a very very flexible cabel release.

I seem to have answered this already in your WTB post, and this thread answered some of the questions I asked there.

You'll want the least recess that gives you the movements you need. Working with recessed boards is a bit of a pain, because you have to stick your fingers down into the recess to change exposure settings, and you have to see where the aperture pointer is pointing on the side of the shutter case. That will be pretty hard with a two-inch recess--it's annoying enough with a 1" recess.

It's not the flexibility of the cable release, it's the clearance for the stiff part that includes the plunger. There are fixes for that, but if you can avoid the problem, so much the better.

With all these lenses, the optical center is usually a bit in front of the lens board. The focal length is defined as the distance from the optical center to the film when focused at infinity. So, the arithmetic is pretty straightforward. A 65mm lens will require 65mm from the optical center to the film, and the optical center is a few mm in front of the lens board. So, you want a lens board that will put the lens mount at least as close as 60mm. You can look up the exact numbers, but in my opinion you don't want to be so close as to require that much precision, or you won't be able to move the lens much. If the recessed board will get you under 50mm, you'll have plenty of room. If you wanted to use a 47mm lens, you'd have to be more precise--that 27mm recessed board might not be deep enough.

Rick "going to post in the thread about minimum focal length accommodation to keep this information together" Denney

[aduncanson]

Lee,

I got called away by a more urgent matter while drafting an answer for you and so I am not timely. However, my approach is different and so I think it is worth posting anyway.

The key to answering the question "How much recess do I need?" is how far away can you currently focus with a flat board? You do not need to know with any great precision.

Say you can presently focus only out to 900 mm (~3 feet). The very handy lens equation:

1/Focal Length = 1/Object Distance + 1/Image Distance ;

gives an image distance of 70 mm. To get to infinity with a 65 mm lens, you need an image distance of 65mm and so you would need a recess of only 5 mm.

The great advantage of this approach is that it provides better accuracy since you do not try to measure image distance directly. (Which is almost never possible to do with with precision since image distance is defined as the distance from the film to the rear nodal point of the lens which is usually not marked, is rarely documented and is usually buried at some unknown place inside of the lens.)

Furthermore since the object distance appears in the 1/OD form, if it is significantly larger than the focal length, then a significant error in the measured object distance makes for only a small error in the calculated image distance. The biggest source of error in this approach is probably the accuracy to which you know the focal length. Actual focal lengths are often as much as 1% off of the nominal value. However, even that error tends to cancel with this approach if the current far focus distance is significantly more than the focal length.

You will want a recessed board some what deeper than the calculated value since you will want to swing or tilt the front or rear standard sometime. I would not go to the 47mm recessed board since it will make accessing the lens controls difficult.

[Lee Christopher]

Thanks guys!

I'm going to copy what you've shared and read it slowly.

I pulled the trigger on this 27mm recessed board: 310145524997

It appears to offer more allowance around the lens controls than any of the other recessed boards listed at the moment.

You guys are the best!

- Aduncanson, I LOVE whet you have written under your handle! LOL!

[aduncanson]

...
With all these lenses, the optical center is usually a bit in front of the lens board. The focal length is defined as the distance from the optical center to the film when focused at infinity. So, the arithmetic is pretty straightforward. A 65mm lens will require 65mm from the optical center to the film, and the optical center is a few mm in front of the lens board. So, you want a lens board that will put the lens mount at least as close as 60mm. You can look up the exact numbers, but in my opinion you don't want to be so close as to require that much precision, or you won't be able to move the lens much. If the recessed board will get you under 50mm, you'll have plenty of room. If you wanted to use a 47mm lens, you'd have to be more precise--that 27mm recessed board might not be deep enough.

Rick "going to post in the thread about minimum focal length accommodation to keep this information together" Denney

The flange focal distance for most 65mm focal length super-wide lenses, including the Nikon f/4.0 is somewhat greater than 65mm; meaning that the rear nodal point (a more informative term than "optical center") is 5mm to 8mm BEHIND the flange. See http://bigcamera.com/articles/GraflexXLHybridAdapt.htm

[rdenney]

The flange focal distance for most 65mm focal length super-wide lenses, including the Nikon f/4.0 is somewhat greater than 65mm; meaning that the rear nodal point (a more informative term than "optical center") is 5mm to 8mm BEHIND the flange. See http://bigcamera.com/articles/GraflexXLHybridAdapt.htm

Thanks for the information. That will only help the situation.

Rick "facts are good" Denney