flange focal distance

the super angulon 65/5.6 has a flange distance of aprox 72mm.. am

i correct in assuming that this is the lensboard to the ground glass

at infinity?.. what would the distance from lens board to ground

glass be for focus at three feet?. is it similar to the 135mm i have

setup on the scale of the bed of my crown?.. im trying to figure out

what would happen if i put a 65/5.6 or a 72mm xl on my crown

graphic?.. the 72 xl would have a 82mm distance from gg to lens board

if im correct?.. this is far enough out to rise, (as the board would

not hit the box) using the generous movement of the 72 xl.. but i

cant drop the bed as this makes the lens board jump to about

114mm.. has anyone had 72xl problems on the crown..??? that is the

bed showing on the bottom of the exposure?? if i can shoot at ininity

and closeups with the lens board up id be vary happy whith that..

thanks dave...

<p>Yes, the Flange (focal) Distance is the distance from the front of the lensboard (or back of the shutter) to the ground glass when the lens is focused on infinity. The image distance is measured from the image to the rear principal plane. By definition, when a lens is focused on infinity the rear principal plane is one focal length from the image. For a 65 mm lens with a 72 mm FFD, this is telling you that the rear principal plane is 7 mm behind the flange.</p>

 

<p> Re your question about focusing on 3 feet -- the first step is to convert everything to the same units, so about 900 mm. The basic equation of optics (1/f = 1/Si + 1/So), for f=65 mm and So=900 mm gives Si = 70 mm. Si is the image distance -- the distance you need to position the rear principle plane from the ground glass. From the 7 mm difference between f and FFD, the flange is 7 mm farther from the ground glass and you need to position the lensboard at 77 mm. Only a 5 mm motion of the lens has moved the focus from infinity to 900 mm.</p>

 

<p>Some previous related discussions: <i>Bellow length for 240mm lens</i> at <a href="http://www.photo.net/bboard/q-and-a-fetch-msg?msg_id=006P7r">http://www.photo.net/bboard/q-and-a-fetch-msg?msg_id=006P7r</a>,

and <i>Bellows Extension for a Given Lens to Subject Distance</i> at <a href="http://www.photo.net/bboard/q-and-a-fetch-msg?msg_id=004O3b">http://www.photo.net/bboard/q-and-a-fetch-msg?msg_id=004O3b</a>.

im sure your math is right.. but the infinity distance focus is not 65mm with a 65mm lens.. doesnt this throw the proverbial wrench into the calculations?.. the reason im saying this is 5mm bellows extention from infinity to 3 feet just doesnt seem to make sence.. my 135 lens is about 120mm at infinity, roughly.. there is about a one inch belows extention to 6 feet on the scale, from memory, im not being tecknical here.. one would think that there would have to be around one half inch (12mm )to 35 mm extenion to focus at three feet with a 65mm lens.. math can be real squirrely, and im not doubting your calculations, it looks like something else is comming into play here.. can any 65mm lovers confirm or deny the calculations?? by measreing the actual infinity focus distance to around three feet focus difference?? dave..

Dave, by definition the distance from the film plane to the lens' rear principal plane when the lens is focused at infinity equals the lens' focal length.

 

What's bothering you may be that where the rear principal plane is isn't always obvious. Many of us approximate its location as the diaphragm's position, but this is rarely quite correct. Sometimes, as with Super Angulons, it is very incorrect. In proof of which, consider the little 65/6.8 Raptar I no longer have and the 65/8 Ilex I now use. The Raptar wouldn't make infinity on my 2x3 Speed. The Ilex, in the same design family as the Super Angulon, makes infinity with mm to spare on that camera.

 

Why do you think Michael is mistaken?

 

Cheers,

<p>Lenses of different focal lengths needed to be positioned differently for every intended object difference. The two easy cases are infinity and 1:1 (life size image): for the former, the image distance is one focal length, for the later, the image distance is two focal lengths.

So to move a 65 mm lens from infinity focus to 1:1 requires a position change of 65 mm, while for a 135 mm focal length it requires 135 mm.</p>

 

<p>Comparing lenses of different focal lengths, the inbetween positions are not linearly proportional. You have to use the equation 1/f = 1/Si + 1/So. For So = 900 mm, Si = 70.1 mm for a 65 mm lens and 158.8 mm for a 135 mm lens -- changes compared to infinity of 5.1 and 23.8 mm.</p>

 

<p>You might want to read the first section of the Lens Tutorial, <a href="http://www.photo.net/learn/optics/lensTutorial">http://www.photo.net/learn/optics/lensTutorial</a>.</p>

that sounds much more correct Mike.. the first post said a 5mm difference for three feet,if im reading it right, which i may not be.. your second post said a 65mm difference for 1 to 1. that would mean to me that 3 foot would be closer to 20-30mm movement give or take.. i may not be reading it right of course.. ... im going to restart this post looking for those that have shot the 65mm on the crown and see how they came out.. thanks dave.

im comming up with the bed down i can set the 65mm on the rear rails, and slide the lens mounting board back and fourth to give me a total of about 1 1/4 inch of adjustment...it will take some trial and errer., but now i see also that there is very little movement to get down to 35 feet focus.. and the camera slide on the lower bed can adjust that much easily if im in a proper position on the rear rails, and the bed down.. so Mike thanks for pointing out that the movement is actually small, and John for makeing me realize that i had to put it on the rear rails, i felt this was the case but i was afraid i couldnt extend the lens enough for closer shots.. .. the biggest problem is that the rear rails are not very solid on my camera, but that can be fixed with a wedge gently put between the board and the side of the camera applying only enough pressure to hold the rails in a more solid position.. the next problem is getting the lensboard square with the fresnell, as i am used to the stops taking care of this.. the 65 appears to get more movement from infinity towards a closer focus than the 72mm xl with the bed down, as the rear bed extension is limited, and the board can only be moved so far out on it into the air.... im going to send this in triplicate to reach the different posts and emails that i have sent.. thanks to you all, and ill try a 65 and post results.. as John has said he uses a 65mm on a speed graphic, so ill bet ill be fine with it.. i just needed a little encuragement to put the 600 dollars plus out for a super wide angle lens... thanks dave...

Dave,

 

Your last post was a bit confusing. But in the previous one you still seemed to be saying that you felt the calculation for 3 feet couldn't make sense. But there is nothing contradictory about the calcuation for 1:1 and for three feet. As Michael says it all comes out of the formula

 

1/Si + 1/So = 1/f

 

and the fact that the rear principal plane of the lens is 7 mm closer to the film than the rear of the shutter. Try to visualize the position of that plane. When it is 65 mm from the film, you are focused on infinity. Move it 5 mm further away, and you will be focused at 910 mm or just about 3 feet. Move it so it is 130 mm away, and you will be in the position for 1:1 imaging.

 

In each case the shutter will be 5 mm futher in front of the rear principal plane.

 

Sometimes we start off with faulty intutition. The solution is to study the formulas, work out a lot of examples and try to visualize phjysically what is happening. Eventually your intution will square with the mathematics.

 

There is one additional subtlety. The distance to the subject is not measured from the rear principal plane. It is measured from the so-called front principal plane would would be somewhere further away from the film. Just where it is for your lens can't be determined from the data we have, but it is unlikely to be very far from the lens board. For a subject at 3 feet, it can't make too much difference, but it might make a slight difference for 1:1 magnification. If you want to find out just where it is, you could carefully focus for 1:1 magnification, measuring carefully on the ground glass. The subject would then be 65 mm from the front principal plane, so you can measure back from the subject to find out roughly where it is.