View Full Version : Lens to Film Distance for 180mm?

Garry Madlung

2-Apr-2009, 11:37

I have a question for anyone with a 180mm lens. I'm considering a 180mm for my Ebony SW45. It's a wide angle camera but is supposed to take up to a 180mm. My question is this: With a 180mm how much draw do I need to focus at a relatively close distance - perhaps around 10 feet?

I'd rather not have to attach a bellows extension. If the camera doesn't have enough draw to focus at 10', I'll look at the 150.

Drew Wiley

2-Apr-2009, 12:09

You need to look at the mfg spec for the actual focal length of the specific lens. But

the SW45 only has 190mm bellows draw. You won't need too much to focus at 10ft.

But if it was me I'd mount the lens on a slightly extended lensboard (avail from Ebony,

Linhof, or Bromwell). Too much board extension risks vignetting with strong tilt or rise.

But generally a 180 with give better coverage and more even illumination than a 150.

Bob Salomon

2-Apr-2009, 12:57

It is the flange focal length that you need to know not the actual focal length. As an example a 180mm Apo Sironar S has a flange focal length of 177mm at infinity with a Copal 1 shutter. The 180mm Apo Sironar N has a flange focal length of 173mm with a Copal 1 at infinity.

Bob Salomon

2-Apr-2009, 12:59

"slightly extended lensboard (avail from Ebony,

Linhof, or Bromwell)"

Wista makes extension boards, Linhof does not.

Drew Wiley

2-Apr-2009, 13:05

Sorry Bob, I should have stated "Linhof-style" board. You can also get them custom

made from SK Grimes.

Bob Salomon

2-Apr-2009, 13:15

Sorry Bob, I should have stated "Linhof-style" board. You can also get them custom

made from SK Grimes.

Since Wista makes screw together extension boards that accept additional extensions and come with screw-in 0, 1 and 3 rings it would be hard to see how one would need a "custom" version. The Wista can be as short as 1" and as long as you need.

They also make fixed length extended boards with a 0, 1 or 3 thread.

I have a Nikon 180mm lens. Shall I set up the camera and measure tonight?

drew.saunders

2-Apr-2009, 15:26

Ebony lists this information: http://www.ebonycamera.com/articles/lenses.html

From that list, the 180 with the shortest flange to focal distance is the Fuji CMW at 170.7mm. Most of the rest are 177mm or so.

With 190mm of bellows, rounding the CMW to 171mm, you get (1/171) - (1/190) = (1/distance to subject) and that comes out to 1615mm minimum focusing distance. With 178mm, the minimum focusing distance works out to 2819mm.

Drew

Henry Ambrose

2-Apr-2009, 18:22

You'll be very close. I have an SW45 and had a 180 that I seldom used, so sold it. I find the 150 to be more useful for me. You can add an extension board to get closer focusing but the front movements will get a little goofy since you will be moving the lens's nodal point away from the camera's movement pivots. This is not a huge problem.

Garry Madlung

2-Apr-2009, 19:28

Thank you everyone. (I've been occupied with some prep work this evening.) Wayne, if you have a chance, please test it out.

The extended lens board is an excellent option. I won't have to attach a back extension.

Out of curiosity, a number of months ago I tried to focus a 210. Nowhere close to focusing. The 180 might be more useful to me than the 150, and I might have a use for it on some more detailed work than my usual wide angle pursuits.

Garry

Helen Bach

3-Apr-2009, 09:37

Ebony lists this information: http://www.ebonycamera.com/articles/lenses.html

From that list, the 180 with the shortest flange to focal distance is the Fuji CMW at 170.7mm. Most of the rest are 177mm or so.

With 190mm of bellows, rounding the CMW to 171mm, you get (1/171) - (1/190) = (1/distance to subject) and that comes out to 1615mm minimum focusing distance. With 178mm, the minimum focusing distance works out to 2819mm.

Drew

Is that the right way to do it?

Surely the correct way is to use the location of the rear nodal point (and assume that the nodal points are effectively coincident, if necessary):

If the FFD for a 180 mm lens is 171 mm, the rear nodal point is 9 mm in front of the flange. Therefore with 190 mm of bellows draw, the rear nodal point is about 199 mm from the image plane.

The formula then becomes

1/180 - 1/199 = 1/object distance

(ie 1/f - 1/image distance = 1/object distance)

which gives a result of 1885 mm.

It's not a big difference, but isn't it worth getting the principle right?

Best,

Helen

aduncanson

3-Apr-2009, 23:10

I guess that what I did need to say was that I believe that Helen is correct, and that Yes, it is worth getting the principle right.

drew.saunders

4-Apr-2009, 19:34

Yep. Since it's 1/fl - 1/extension = 1/object distance, where fl and ffd aren't the same, "extension" changes (kinda like a top hat, or recessed board, depending on how it works out), so your math is correct. Hadn't thought of it that way before, so I learned something today, and it's the weekend even!

drew

Gordon Moat

5-Apr-2009, 12:25

Is there a way to use this to come up with distance scales for lenses? The reason I ask is that it can sometimes be convenient to have these on the focus rail of the camera.

Ciao!

Gordon Moat Photography (http://www.gordonmoat.com)

drew.saunders

5-Apr-2009, 14:34

Is there a way to use this to come up with distance scales for lenses? The reason I ask is that it can sometimes be convenient to have these on the focus rail of the camera.

Ciao!

Gordon Moat Photography (http://www.gordonmoat.com)

Sure. Let's keep using a 180mm lens but with a 180mm ffd just to make it a bit easier. Say you wanted a scale of 20m, 10m, 5m, 4m, 3m, 2m, 1m

1/fl - 1/(distance in mm) = 1/extension in mm

1/180 - 1/20,000 = 1/181.6mm

1/180 - 1/10,000 = 1/183.2mm

1/180 - 1/5,000 = 1/186.7mm

1/180 - 1/4,000 = 1/188.5mm

...

1/180 - 1/1000 = 1/219.5mm

etc.

As you can see, for this lens, you might want to only do the 10m and shorter distances.

If your ffd isn't the same as the focal length, adjust the extension accordingly (i.e: for the 171mm ffd 180mm fl lens we discussed, bring the extension back 9mm for every setting, giving you 210.5mm for the 1m focusing distance).

You could even make a ruler that you could hold up to the standards and have it with two lenses per side, two sides. I.e.: for my 165mm lens on my Ebony 45SU, at infinity, the distance from the front of the front metal uprights to the back of the rear ones is 13cm (easier than figuring out where the actual film plane is) so I could work out the distances for a 165mm lens but starting at 13cm of "extension."

Drew (probably got it right this time, but it is still the weekend, so no guarantees!)

Lynn Jones

6-Apr-2009, 11:34

Hi Gary,

The 180 at infinity is from aperture to film is about

7", at 1:1 actual size close up, it is 360mm or a bit over 14" aperture to film. This assumes that the focal length is accurage and very often, long focus large format lenses tend to a little bit shorter than indicated.

A simple formula to remember is Magnification plus 1 times focal length equals aperture to film plane distance (lens to film). Or, Bellows draw divided by focal length minus one equals magnification.

Lynn

Sorry it took forever. Nikkor-W 180/5.6. Zone VI field camera. 12'-15' from my living room window. 184mm face of lens board to film holder minus film.

aduncanson

7-Apr-2009, 08:45

Hi Gary,

The 180 at infinity is from aperture to film is about

7", at 1:1 actual size close up, it is 360mm or a bit over 14" aperture to film. This assumes that the focal length is accurage and very often, long focus large format lenses tend to a little bit shorter than indicated.

A simple formula to remember is Magnification plus 1 times focal length equals aperture to film plane distance (lens to film). Or, Bellows draw divided by focal length minus one equals magnification.

Lynn

Lynn,

Your math is right, but your interpretation requires a bit of refinement. Image distance in these formulas is the distance from the film plane to the "rear nodal point" of the lens. For some lens designs and for some purposes the aperture may be a suitable proxy for the rear nodal point, but this is not true in all cases.

When the manufacturer publishes both the actual (effective) focal length and the flange focal distance you can deduce that the rear nodal point is located a distance of (FL-FFD) ahead (behind if negative) the flange. For telephoto and wide angle retrofocus designs the difference can be large. The FFD is less than 72% of the FL in the latest 400mm Schneider Apo-Tele-Xenar. On the other hand, I have a wide angle lens where the FFD measures approximately 35% greater than the FL.

When calculating close focusing distance with a given bellows, particularly for a short bellows, a design which pushes the rear nodal point forward by a few millimeters can have a measurable, (significant but probably not major) advantage.

Sorry it took forever. Nikkor-W 180/5.6. Zone VI field camera. 12'-15' from my living room window. 184mm face of lens board to film holder minus film.

Correction. My apartment is larger than I thought. 19'/184mm.

Lynn Jones

8-Apr-2009, 13:08

Of course to be precise aducanson, there is the node of admission and the node of emision, commonly called the first and second nodes. The average "photo Cat ain't likely" to find the second nodal point unless he has the necessary equipment to spin the lens in a special way so that the image doesn't offset. From our standpoint as photographers, the aperture in long focus lenses is at that approximate point, otherwise the aperture and/or shutter would distort or vignette that bundle of light energy.

Lynn

This guy (http://cgi.ebay.com/lens-board-extension-set-for-Linhof-technika-96-x-99mm_W0QQitemZ200272531306QQcmdZViewItemQQptZLH_DefaultDomain_0?hash=item200272531306&_trksid=p3286.c0.m14&_trkparms=72:1462|66:2|65:12|39:1|240:1318|301:1|293:1|294:50) makes beautiful extension boards. I've purchased some flat boards of different sizes for Linhof from him recently. I am very happy with the quality of his products.

Lynn,

Your math is right, but your interpretation requires a bit of refinement. Image distance in these formulas is the distance from the film plane to the "rear nodal point" of the lens. For some lens designs and for some purposes the aperture may be a suitable proxy for the rear nodal point, but this is not true in all cases.

When the manufacturer publishes both the actual (effective) focal length and the flange focal distance you can deduce that the rear nodal point is located a distance of (FL-FFD) ahead (behind if negative) the flange. For telephoto and wide angle retrofocus designs the difference can be large. The FFD is less than 72% of the FL in the latest 400mm Schneider Apo-Tele-Xenar. On the other hand, I have a wide angle lens where the FFD measures approximately 35% greater than the FL.

When calculating close focusing distance with a given bellows, particularly for a short bellows, a design which pushes the rear nodal point forward by a few millimeters can have a measurable, (significant but probably not major) advantage.

Aduncanson, you're mistaken in your explication, I'm afraid. While the FFD distance is usually expressed in a precise number (tenths of a mm) the FL is not. As a result, taken with you calculation, the precise position of the rear nodal point suffers from the (sometimes very) rounded value of the FL. Lynn correctly states that to know the exact position of the lens emission point you would need "the necessary equipment", not just one precise value married with another approximative one.

__________________

GPS

Dan Fromm

8-Apr-2009, 15:22

GPS, you're right that for some lenses focal length as engraved is nominal and can vary from actual by enough to matter. FFDs as published in, e.g., Boyer's fiches techniques and all those files in Schneider's archives, are also nominal and can also very from actual by enough to matter.

Other lenses, and I have some, were delivered with quality control slips that give actual focal length to 0.1 mm. And others, e.g., ex-F135 38/4.5 Biogons, of which I've had 20, have focal length to 0.1 mm and FFD to 0.01 mm as measured and marked on the lens by the camera manufacturer.

But the real question is whether approximate is close enough. I can't read the OP's mind, suspect that when he actually gets a lens and uses it he'll find that a SWAG, by definition less precise than a good approximation, is better than close enough.

Cheers,

Dan

Yes Dan, I know that the values are sometimes rounded even when published in tenths of a mm. It is then a real pleasure to have the lengths certified precisely for some lenses. Maybe not because of questions of bellows draw but when constructing your own cameras, it certainly is handy. Cheers!

aduncanson

9-Apr-2009, 10:10

Lynn - I agree and explicitly acknowledged that using the aperture as the location for the rear nodal point is appropriate for many, but not all, purposes.

GPS - I thought that I recognized that the nominal focal length was not useful for this calculation when I suggested that it be done only "When the manufacturer publishes both the actual (effective) focal length and the flange focal distance" Both are typically published to 0.1mm in my experience (where they are published at all.) Perhaps you are suggesting that even when FFD & FL are published, expressed to 0.1mm, they can not be trusted.

For the purposes of the of the OP's question, which was determining the close focus distance of two nominally 180mm lenses, I think that knowing the Rear Nodal Point location to 0.1mm is probably sufficient to let you know if one lens offers a significant advantage over the other. Actually I am dubious about predicting the close focus distance without considering the effective thickness of the front and rear standards. Without that information you may determine that one lens has an advantage over another, but not have precise knowledge of the how close either will allow you to get.

If, alternately, you have the lens in hand, it seems to me that you could find the Rear Nodal Point location by simply focusing on an object at infinity and measuring forward one (actual) focal length from the film plane. Am I wrong?

If you cannot make that measurement with sufficient accuracy for your purposes then I have to ask: "What are you going to do with the location of the rear nodal point once you have it?"

With little regard to whether you use FFD or focal lens on the lens, you can think of the following

Normal lenses (non-telephoto designs, non-retrofocus designs):

1) infinity focus requires <focal length of lens> distance between nodal point of lens and focus on film plane

2) 1:1 image to subject size requires 2*<focal length of lens> distance between the nodal point of the lens and film plane. ((and the distance from the film to the lens nodal point will be the same as the distance from the lens nodal point to the subject.

Telephoto design (not "longer focal length" but telephoto design) are easy to identify by apparent pupil size differing front and rear: (Note the FFL isn't the true focal length of this lens). For example a 1100mm Fine Art XXL by Schneider is a normal design, whereas a Nikor 1200mmT is a telephoto design.

3) infinity focus will take <y, where y is less than printed focal length of lens> distance at infinity

and

4) 1:1 focus will take <y + actual focal length of lens> not 2*<y>

for both:

You will need to make corrections to exposure if the lens nodal point to subject is less than 10 times the actual FFL

These are quick calculations rather than the formula driven versions.

So for your original question of how much bellows you might need, roughly between 180mm at infinity to 360mm at 1:1. If you wish to go closer than 1:1, consider a Macro lens that is optimized for less than 1:1, and continue to add bellows. Your 10 foot distance will be closer to the maximum of 360mm.

Garry Madlung

10-Apr-2009, 17:25

Thanks everyone for your input. This has been very educational. Thanks venchka for checking that! I have what appears to be 185mm. Close! With an extended lens board it'll work for sure.

Sorry it took forever. Nikkor-W 180/5.6. Zone VI field camera. 12'-15' from my living room window. 184mm face of lens board to film holder minus film.

venchka

11-Apr-2009, 00:39

You are welcome. Glad to help.

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