Flange to focus?

[BJC0000]

Not sure if this is in the right forum so if not feel free to move it!

Probably a stupid question – optics isn’t my strong point.

I know that stopping down increases the depth of field in front of the lens but does it make any difference between the lens and the film?

In other words would a smaller aperture improve sharpness if the flange to focus distance wasn’t spot on?

I realise any difference must be minimal (fractions of a millimetre) but is there any at all?

 

[Ian Grant]

Are you focussing on a ground glass screen, unless a lens has a fixed Infinity position it makes no difference. If it does have a fixed Infinity stop you adjust with shims or with set screws (on TLRs).

Ian

 

[Dan Daniel]

I know that stopping down increases the depth of field in front of the lens but does it make any difference between the lens and the film?

In other words would a smaller aperture improve sharpness if the flange to focus distance wasn’t spot on?

Must admit that I am not clear what you are asking. But since this is the internet, I won't let that stop me!!

When you say that depth of field increases in front of the lens, I get confused. Are you thinking that from the point of focus, depth of field happens between object focused on and the space behind the object focused on? And only in this direction? Because that isn't how it works. Below is a lens DOF table and you can see that it is from in front of the object to the back of the object, slightly lesser distance to the front and larger to the back.

Yes, depth of field can cover for a lens not being properly focused. Remember that most lenses are, for all practical purposes, unit focusing, meaning that the whole lens group moves forward and backward. Depending on the camera, flange distance may or may not be critical as it relates to the focusing method of the camera itself.

Well, I could be going down a rabbit hole when you are hunting foxes, so I'll stop here. Maybe say more? Also look up 'circle of confusion' to get an idea of what depth of field is if you don't know. E.g. in the chart below there is a note that Circle of Confusion if 1/200 inch. Depth of field is a slop factor.

 

[MattKing]

Yes.
If you use a smaller aperture, the range of positions at the film plane that will result in some part of the subject being in apparent focus expands..
All of us who focus enlarger lenses have observed that.

 

[gone]

In other words would a smaller aperture improve sharpness if the flange to focus distance wasn’t spot on?

Yes, but you should always make sure the lens is capable of sharpness at infinity if possible. I had a Leica R 90 Summicron lens that wouldn't quite get to infinity. Wide open it wasn't that sharp due to the focus error, but stopped down to f8 or more it seemed as sharp as it could ever get even w/ that problem.

So if your focus is a little off, stopping down cures a lot of ills.

 

[Lew_B]

Short answer, no! You need to differentiate “object space” from “image space”. The “depth of field” change that occurs with aperture happens in “object space” (the thing you are imaging). The “flange to focal plane distance” is in “image space” and is fixed for any given lens. “Stopping down” the aperture, only increases the depth that is in-focus in the “object space”.

Hope that helps!

 

[wiltw]

1. The depth of the zone of acceptable focus blur is the 'Depth of Field' on the subject side of the lens
2. The depth of the zone of acceptable focus blur is the 'Depth of Focus' on the film/sensor side of the lens.
3. Yes, Depth of Focus is a larger/deeper zone with smaller f/stop than it is with larger f/stop (that is, the same relationship as applies to Depth of Field)

If your film/sensor is positioned outside the acceptable range of Depth of Focus, your whole image will be blurry, not just certain objects within it.

 

[BJC0000]

Thanks for all the replies.

Perhaps I should have been clearer

Yes, Ian, focussing on groundglass.

Comprehensive reply Dan, I do understand DoF relates to both sides of the object focussed on and I understand Circle of Confusion (especially as relates to hyperfocal settings).

To clarify with an example:

My Symmar-S is listed by SK as having a flange – focus of 229mm in Copal / Compu #3 shutter.

If I focus (at infinity) presumably the flange to ground glass is 229mm?

Whatever it is inserting a film holder should bring the film into the same plane as the ground glass?

But what if it doesn’t – if the holder isn’t perfect and places the film at another (not 229mm) distance? Will stopping down improve the focus on the film at all? If so, how much error can be compensated for and to what degree?

Barry

 

[Dan Fromm]

Yes, Ian, focussing on groundglass.

My Symmar-S is listed by SK as having a flange – focus of 229mm in Copal / Compu #3 shutter.

If I focus (at infinity) presumably the flange to ground glass is 229mm?

Whatever it is inserting a film holder should bring the film into the same plane as the ground glass?

Barry, you are a worrywart. As long as you focus on the GG the only thing that will cause problems is operator error. Been there, done that, don't recommend it.

Lenses' focal lengths and ffd's are all nominal. Actual lenses' FLs and FFDs will deviate from the nominal figure. I once bought 20 38/4.5 Biogons in F.135 cameras. Each lens was marked with actual focal length. Nominal FL for that lens is 38.5 mm. Actuals varied from 38.3 to 38.8 mm. The F.135 is a fixed focus camera. Each lens came with a shim marked with the shim's thickness and the lens' serial number. The shims went between the rear of the shutter and the camera main casting.

Film holders are made to fairly tight specifications.

 

[btaylor]

Longer focal length lenses have greater depth of focus. If you’re using sheet film, you’re likely using longer focal length lenses, so that helps. BUT! Ground glass and film depth should be congruent. You can use a depth gauge to check.

 

[Ian Grant]

Longer focal length lenses have greater depth of focus. If you’re using sheet film, you’re likely using longer focal length lenses, so that helps. BUT! Ground glass and film depth should be congruent. You can use a depth gauge to check.

As you increase Focal Length you reduce the DOF, you get significantly shallower focus at similar apertures.

Ian

 

[Dan Daniel]

Flange distance is useful to see if a lens will work on your camera. For example, I had a Horseman VH which has issues with focal lengths shorter than 65mm (as I remember). I found a NIkkor 75mm lens with a flange distance of 81mm as I remember. It allowed me to have the bellows extended and get decent movement for architectural work- win! A long focal length like 210mm lens's flange distance let me know what if my extension tube board would let me focus.

Other than this kind of info, flange distance on a view camera is not a factor. You simply focus on the ground glass. Now the relation between your focusing system and the actual lens focus point is a factor and seems to be more what you are concerned with. I can tell you that Rolleiflex repair manuals have a tolerance of 0.05mm for the film plane to lens plane flatness (0.002 inch). This kind of measurement might be more what you are looking for.

All discussions like this deal with what is considered acceptable. Theoretically a lens has one sharp focus plane, period. A theoretical Euclidean focus plane- width and length but absolutely no depth. Everything away from that, the wavelength of the light, nanometer or millimeter or whatever is 'out of focus.' But then you add in film grain. Magnification for final image (Rollei manuals give a DOF table for 8x10 prints and say to reduce by one stop for larger prints). Subject matter- will anyone notice out of focus by .01mm for fog? Film flatness will matter... maybe, depending on final use.

And I guess that would be my attitude- final use? Is it out of focus for you? Then start honing in all the factors that could be causing that. If the images are working, if you never go 'Is this tilted/oof/motion blurred?' then your camera is working well for you.

 

[BJC0000]

Film holders are made to fairly tight specifications.

Yes, but my home-built roll film backs for LF cameras may not be.
I've built several panoramic cameras on the stretch limo principle and all seem to deliver acceptable images (at least to me).
Also built 6x24 film back for a 4x10 camera (also home built - do you see a theme here?). Pic attached.
I'm just about to start on a 6x17 back for my Grundlach 13x18 camera but thought to be a bit more scientific about it than previously. Dan Daniel's mention of tolerance of 0.05mm makes me need to go have a lie down.
I'm not an engineer / modeller; these are kitchen table / garage projects but I try to do it to the best of my ability.

Thanks again for all the replies. Don't stop.
Barry

 

[btaylor]

I came up with a simple visual way to test ground glass and film plane agreement. I took a film holder removed the dark slides and drilled 5 approximately 1/2 inch holes though the septum separator, one on each corner and one in the center. Then I placed a processed negative of a gray card in the holder, removed the ground glass and put the holder in the camera. Using a loupe I focused the camera on a target about 10' (3 meters) away using the exposed film like a ground glass. I then removed the holder and replaced the ground glass to see if they agreed. In my case they did, but if they had not, I would have shimmed the ground glass until they did.

 

[grain elevator]

Yes. The answer is simply yes.
As you know, focusing (as long as we're leave out internally focusing lenses) is done by changing the distance between film and lens. If it's slightly off, that just means focus is slightly off. You already know how slightly off focus is impacted by stopping down.

 

[AnselMortensen]

I came up with a simple visual way to test ground glass and film plane agreement. I took a film holder removed the dark slides and drilled 5 approximately 1/2 inch holes though the septum separator, one on each corner and one in the center. Then I placed a processed negative of a gray card in the holder, removed the ground glass and put the holder in the camera. Using a loupe I focused the camera on a target about 10' (3 meters) away using the exposed film like a ground glass. I then removed the holder and replaced the ground glass to see if they agreed. In my case they did, but if they had not, I would have shimmed the ground glass until they did.

Boom!
That is an excellent idea.
You win the interwebs for today.

 

[Steve906]

I have found it useful to remember that with a lens that is adjusted as a whole to bring an image of an object in front of the lens to a focus at the film plane, which I consider to be many if not most, then all of the objects in front of the lens (within reason) are in focus at some point behind the lens ALL of the time. One merely adjusts the relative position of the film to the lens to select the object that is required. All other objects are still in perfect focus somewhere in front of or behind that position. The objects not focused on that selected plane will still be recorded with varying degrees of blur relative to their distance from it. With care it is possible to obtain all acceptable levels of blur in front of or behind the object of interest, so acceptable depth of field can be entirely in front of, or behind the subject as required.

 

[Dan Fromm]

Film holders are made to fairly tight specifications.

Yes, but my home-built roll film backs for LF cameras may not be.

I don't understand why you worry about fine points and build slop in.

 

[BJC0000]

I don't understand why you worry about fine points and build slop in.

Well, I don't deliberately build slop in but as I've said I'm not an engineer and tolerances of 0.05mm are beyond me and my equipment.
That shouldn't stop me trying to do the best job I'm able (and enjoy doing it and the results).
Accepting my limitations and looking for ways to still get decent results was the whole point of the original question.
Obviously there's no point in trying to build an LF camera and accessories if they are going to be so crude as to be useless.
I wouldn't dream of trying to build an SLR for example - I don't have the skill or equipment, but I would like to get the best out of what I can do.

 

[Steve906]

Short answer, no! You need to differentiate “object space” from “image space”. The “depth of field” change that occurs with aperture happens in “object space” (the thing you are imaging). The “flange to focal plane distance” is in “image space” and is fixed for any given lens. “Stopping down” the aperture, only increases the depth that is in-focus in the “object space”.

Hope that helps!

The depth of field in, as you say 'object space' is directly linked to the effect recorded on the film and so cannot really be differentiated from 'image space'. Flange to focal plane distance is only relevant if the lens has a fixed infinity stop or position.

 

[Steve906]

1. The depth of the zone of acceptable focus blur is the 'Depth of Field' on the subject side of the lens
2. The depth of the zone of acceptable focus blur is the 'Depth of Focus' on the film/sensor side of the lens.
3. Yes, Depth of Focus is a larger/deeper zone with smaller f/stop than it is with larger f/stop (that is, the same relationship as applies to Depth of Field)

If your film/sensor is positioned outside the acceptable range of Depth of Focus, your whole image will be blurry, not just certain objects within it.

There is no reason the whole image would be blurry unless one was photographing a flat wall. Something will be in focus if in view at the correct distance for the current film to lens position. Reducing aperture reduces the effect of spherical aberrations increasing the acceptable range of lens to film distances and hence increasing the acceptable range of object distances as they are inherently linked.

 

[Lew_B]

The depth of field in, as you say 'object space' is directly linked to the effect recorded on the film and so cannot really be differentiated from 'image space'. Flange to focal plane distance is only relevant if the lens has a fixed infinity stop or position.

Lots of good responses here…and I must admit that my earlier answer was biased towards “fixed” systems like 35mm, where you can’t (easily) charge flange to focal plane distance. Of course, in a system like LF ( with a bellows), it is simple to change that distance, and my argument doesn’t hold water anymore! Thanks for making me think more broadly!
Best, Lew

 

[monopix]

You might find this useful.

'The Ins and Outs of Focus' by Harold M. Merklinger

or this

'Focusing the View Camera' by Harold M. Merklinger

 

[Dan Daniel]

Dan Daniel's mention of tolerance of 0.05mm makes me need to go have a lie down.
I'm not an engineer / modeller; these are kitchen table / garage projects but I try to do it to the best of my ability.

Rollei was making thousands of each camera model. It was worth their while to hone each part precisely. And in case you don't know, the final tolerance was achieved with little brass washer serving as shims. Even with some of the best machining of its time their cameras needed to be tweaked like someone setting valve clearance on a Chevy engine in their garage.

And Rollei had the advantage of an integrated body and lens board. One issue with folders like view cameras is the lens board itself. Add swings and tilts and the slightest slop of resetting to 'zero' and a 17cm total horizontal distance; I bet most any view camera will be relying on depth of focus to get 'sharpness' across the film plane. And we haven't even dealt with differences in film base thickness and film holder slot slop, etc. What if you tilt the camera forward and the film tilts in its slot on the top edge .05mm more than the back edge? Will you find yourself throwing out the negative? I doubt it.

Another factor is that Rollei had the ability to measure .05mm. There are lots of ways to measure things with varying degrees of precision. It was worth it to Rollei to aim for certain tolerances considering their desired goals. Holga obviously has different standards of precision in manufacturing, and their cameras are happily used by many.

Some time ago I did a summer project using a 4x5 view camera put together from a kit of wooden parts that were perfectly well made but not precisely machined. I had a great time and got some sharp images. When I have done some camera hacking, I used manufactured parts from old cameras as much as possible because the manufacturer had capabilities that I don't. Need a holder for a 120 roll film back? Find an old 6x9 press camera with a Graflex back and cut out the part I need. Do what you can, understand the trade-offs, and if a camera is not giving you what you want make adjustments to it. Beyond that, don't go chasing numbers and be honest that most times if you are not getting what you want it is not the camera's fault

 

[wiltw]

There is no reason the whole image would be blurry unless one was photographing a flat wall. Something will be in focus if in view at the correct distance for the current film to lens position. Reducing aperture reduces the effect of spherical aberrations increasing the acceptable range of lens to film distances and hence increasing the acceptable range of object distances as they are inherently linked.

If the Depth of Focus zone is not where the placement of the film plane is located, the ENTIRE IMAGE will not be in focus!
Of course, this is something that could occur with Large Format, but not with a 135 format SLR which has a machined position of film plane vs. lens mount.