Sorry, don't want to go too deep into this as it's not really what the OP was asking, but this is wrong. The resources monopix linked are a great start to understanding focus and dof. The fact that an SLR has a fixed lens mount and film plane is of no consequence, with a standard (not internally focusing) prime lens the whole barrel will move in and out during focusing. The fixed flange merely allows the use of an infinity stop and footage markings on an interchangeable lens system. The depth of focus zone is positioned around the image of the object you consider to be the subject of interest and this can be anything in front of the lens. Focus is then achieved by moving the lens to bring that zone into position on the film plane.
Flange to focus?
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Sorry, don't want to go too deep into this as it's not really what the OP was asking, but this is wrong. The resources monopix linked are a great start to understanding focus and dof. The fact that an SLR has a fixed lens mount and film plane is of no consequence, with a standard (not internally focusing) prime lens the whole barrel will move in and out during focusing. The fixed flange merely allows the use of an infinity stop and footage markings on an interchangeable lens system. The depth of focus zone is positioned around the image of the object you consider to be the subject of interest and this can be anything in front of the lens. Focus is then achieved by moving the lens to bring that zone into position on the film plane.
The OP very specifically asked does it "make a difference between the lens and the film?"
- In front of the lens is where Depth of Field applies
- Behind the lens is where Depth of FOCUS applies
Depth of focus - Wikipedia
"The phrase depth of focus is sometimes erroneously used to refer to the depth of field (DOF), which is the area in front of the lens in acceptable focus, whereas the true meaning of depth of focus refers to the zone behind the lens wherein the film plane or sensor is placed to produce an in-focus image.
Depth of focus can have two slightly different meanings. The first is the distance over which the image plane can be displaced while a single object plane remains in acceptably sharp focus;[1][2][clarify] the second is the image-side conjugate of depth of field.[2][clarify] With the first meaning, the depth of focus is symmetrical about the image plane; with the second, the depth of focus is greater on the far side of the image plane, though in most cases the distances are approximately equal."
To state it again, from another source https://www.masterclass.com/articles/depth-of-focus-vs-depth-of-field"What Is Depth of Focus?
In photography, depth of focus describes the relationship between the camera lens and the image plane (the film plane or camera sensor). It has to do with how the focus changes when the image plane is moved closer to or further from the lens. "Note the similarity between the first sentence of this specific post (the OP question) and the last sentence preceding this paragraph (what Masterclass defines)
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And my point was that you said the ENTIRE IMAGE will be out of focus. Believe as you wish I merely didn't want others going away with the wrong idea regarding DOF!
And no I'm not mixing anything up that's why I carefully worded my statement as the Image of the object of interest.
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Steve, believe what you will, in spite of quoted sources
Depth of Focus - Introduction and Meaning
Depth of focus in optics is a measure of the tolerance of placement of the image plane with respect to the lens. In photography, depth of focus refers to the tolerance of the films displacement within the camera.
www.azooptics.com
Lastly, the stated subject of this thread is 'Flange to focus'... the space which is within the camera
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Steve, belied what you will, in spite of quoted sources
"In photography, depth of focus refers to the tolerance of the film's displacement within the camera. "Depth of Focus - Introduction and Meaning
Depth of focus in optics is a measure of the tolerance of placement of the image plane with respect to the lens. In photography, depth of focus refers to the tolerance of the films displacement within the camera.
I provide this information so others do not get confused by mistaken beliefs, in reading this threadl
That's how 'bokeh' also has become so incredibly commonly misused, in spite of what even Zeiss has to stay on the subject.
Your quoted sources ARE correct !! and agree with my statements.
OK, this explanation
In depth: depth of focus and depth of field
Update: The original website with its digitally-altered ‘miniworld’ pics seems to have been taken down. I’ve constructed a few of my own ‘miniworld’ pics to give the …
skullsinthestars.com
"What happens if our film (or retina) is not placed at the ideal image distance?
The image will be increasingly blurry the further we move from the ideal image distance. There will be, however, a range of distances around the ideal distance
where the blur will not be noticeable. This range of distances is known as the depth of focus."
Outside of the range of distances (around the ideal distance), the entire image is out of focus, the blur is noticeable.
Depth of Focus vs. Depth of Field — Differences Explained
Depth of focus refers to the space between the lens and the image sensor where the image being filmed will appear in focus to the human eye.
www.studiobinder.com
"Depth of focus, meanwhile, is rarely altered in service of a creative choice. If your image sensor is outside the acceptable range of focus, your whole image will be blurry, not just certain objects within it."
The issue of film flatness, and its bowing away from the ideal plane of perfect focus is precisely why some manufacturers have sought different film flatness solutions engineered into cameras, particularly in medium format sizes, not merely for large format sheet film.Film Flatness in MF Cameras
I have read much of the archived stuff,but can find no clear answer.If a medium format camera(Pentax 6x7 or Mam 645)is left loaded for a week(or longer) mid roll will this create focus ruining curl?Does the direction of wind matter?The M645 reverses off the roll,whereas the P67 feeds straight lik...
www.photo.net
"According to our lab results, film flatness errors in roll films can reach a magnitude of 300 to over 450 microns (millionth of a meter), this is almost 0.5 millimeter. If you regularly stop your lenses down to f/16 you may continue to ignore and neglect the phenomenon. If you, however, also use apertures of f/5.6 and wider and expect to find the higher image quality of medium format in your photos, you would be foolish to do so."
A solution was brought to market in the Contax 645 medium format SLR, "Film flatness problems are addressed for the first time in any medium format camera through the development of a Real Time Vacuum system" in its 220 vacuum film insert MFB-1B. Other solutions were launched by independents to fit other brand camera bodies https://www.horolezec.cz/blog-photography/hasselblad-vacuum-backs-magazines-for-roll-films-220
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OK, this explanation
In depth: depth of focus and depth of field
Update: The original website with its digitally-altered ‘miniworld’ pics seems to have been taken down. I’ve constructed a few of my own ‘miniworld’ pics to give the …"What happens if our film (or retina) is not placed at the ideal image distance?The image will be increasingly blurry the further we move from the ideal image distance. There will be, however, a range of distances around the ideal distanceOutside of the range of distances (around the ideal distance), the entire image is out of focus, the blur is noticeable.where the blur will not be noticeable. This range of distances is known as the depth of focus."
Depth of Focus vs. Depth of Field — Differences Explained
Depth of focus refers to the space between the lens and the image sensor where the image being filmed will appear in focus to the human eye.www.studiobinder.com"Depth of focus, meanwhile, is rarely altered in service of a creative choice. If your image sensor is outside the acceptable range of focus, your whole image will be blurry, not just certain objects within it."The issue of film flatness, and its bowing away from the ideal plane of perfect focus is precisely why some manufacturers have sought different film flatness solutions engineered into cameras, particularly in medium format sizes, not merely for large format sheet film.
Film Flatness in MF Cameras
I have read much of the archived stuff,but can find no clear answer.If a medium format camera(Pentax 6x7 or Mam 645)is left loaded for a week(or longer) mid roll will this create focus ruining curl?Does the direction of wind matter?The M645 reverses off the roll,whereas the P67 feeds straight lik..."According to our lab results, film flatness errors in roll films can reach a magnitude of 300 to over 450 microns (millionth of a meter), this is almost 0.5 millimeter. If you regularly stop your lenses down to f/16 you may continue to ignore and neglect the phenomenon. If you, however, also use apertures of f/5.6 and wider and expect to find the higher image quality of medium format in your photos, you would be foolish to do so."A solution was brought to market in the Contax 645 medium format SLR, "Film flatness problems are addressed for the first time in any medium format camera through the development of a Real Time Vacuum system" in its 220 vacuum film insert MFB-1B. Other solutions were launched by independents to fit other brand camera bodies https://www.horolezec.cz/blog-photography/hasselblad-vacuum-backs-magazines-for-roll-films-220
We seem to be going around in circles here, these articles when read in full do agree with my statements. I think the studiobinder one, when it says 'your whole image will be blurry' could be better stated as the whole of the plane you wanted to be in focus will be blurry.
As one thing at one distance in front of the lens goes out of acceptable focus another at another distance will come into focus.
Everything in front of the lens is in focus at some point behind the lens ALL of the time, moving the lens (or film) just selects which is recorded onto the medium in use.
Maybe we should move to a conversation to discuss this further if you wish.
MattKing
Moderator
+1
The only practical exception to this might be a subject where there is focusable detail at one particular distance, and no focusable detail either closer than that detail or, in some cases, farther away than that detail.
Something like aerial photos, or microscope slides.
If there is foreground detail and background detail, something will be in focus.
Yes, and of course I'm not taking into account extremes of field of view and other distortions at the limits of the lenses inherent errors.
But the plane of the Depth of Focus is so very thin that when a large format camera focusing screen is not in exact agreement with the film magazine placement of the film (perhaps due to inappropriate placement of the fresnel lens), the resulting photo taken i results in an inacceptably focused image.
This same phenomenon occurs when medium format film bows because the pressure plate is not sufficiently cancelling out deviation of the film from the perfect plane of focus.
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Well, going back to the original question: if the depth of focus has a thickness I assume it will be thicker at smaller apertures (as is the depth of field)?
Very impressed with all the responses!
From AZO optics quoted by wiltw:
"In photography, depth of focus refers to the tolerance of the film's displacement within the camera. "
Key word is 'tolerance' - how much and what affects it (not wanting figures just general principles).
"In photography, depth of focus refers to the tolerance of the film's displacement within the camera. "
Key word is 'tolerance' - how much and what affects it (not wanting figures just general principles).
- There have been threads in the past, talking about observed lack of film flatness in rollfilm cameras, and in rollfilm backs for sheetfilm cameras.
- There have been optional vacuum backs for rollfilm to hold film flatter
- ...but the rollfilm industry in general had not seen sufficient need for such offerings
Contax 645 was the first to even offer a vacuum back for 220 film (which could use vacuum hold down because it lacked paper backing) - ...and the Contax RTS III 135 format SLR incorporated, for the first time a Real Time Vacuum Back and ceramic pressure plate for the ultimate in film flatness, in 1990
- And in general vacuum back or adhesive back had not been offered to hold sheetfilm, primarily because of thick base...
- yet some 4x5 photographers did find that shooting at f/16 could cause sufficient visibility of 'not-exactly-at-plane-of-focus' compared to shooters why routinely stopped down to f/22 of f/32 to shoot sheet film.
- Discussions also were held regarding the flatness of Readyload and Quickload films in dedicated holder from the makers vs. in Polaroid backs.
One example https://www.photo.net/forums/topic/80361-film-flatness-of-contax-645/
More than one report quantifying the amount of film curl seen in normal rollfilm backs.
Contax 645 Review: The Most Popular Film Camera for Wedding and Portrait Photographers » Shoot It With Film
In this in-depth review of the Contax 645, we take a look at one of the most popular medium format film cameras for wedding and portrait photographers.
shootitwithfilm.com
"Film Flatness
Another big complaint of Contax 645 users is issues with film flatness.
Remember those film inserts we spoke about earlier? They wear out and must be serviced.
If you are getting scans back from the lab where one half of the frame is sharp but the other half of the frame, even if it is on the same focus plane, is not sharp, you may have a film flatness problem.
Zeiss conducted a study on the issue of film flatness, reported here https://www.horolezec.cz/blog-photography/hasselblad-vacuum-backs-magazines-for-roll-films-220"The "altitudes" of that kink is typically in the magnitude of 350 microns!!! after the 120 film sat around the feed roller for one hour. With 220 film, we found the kinks to be around 200 microns high. Kinks start to become detectable after just one minute of time sitting around the feed roller. Thus our advice: Go through your roll quickly if you want to reduce film un-flatness. Or, discard the first frame after having paused for more than 15 minutes.
The worst our lab at Zeiss found was the magazines of the new Mamiya 645 AF, where the kink was right in the center of the image, the worst conceivable position for both landscape and portrait shots."
This happens when the spring in the insert wears out and needs to be replaced"
One person reported that his Mamiya had film flatness issues, and Mamiya changed from 16-exposures to 15-exposure backs to address a film flatness issue in its backs.
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Maybe I don't understand this remark, but yes, depth of focus is like depth of field, increasing in depth/thickness at smaller apertures. I see some nuances being explored in what exactly the term 'depth of focus' is referring to, but I will take it to mean the 'thickness' of images rendered that fall within the acceptable circle of confusion (and yes, the difference between focusing system plane of absolute focus and film emulsion plane(s) of absolute focus is a part of this acceptable thickness issue). The following is from a discussion of rifle scopes but shows this nicely (google exposes you to all sorts of new things if your search terms hit the algorithm sideways enough
I provide the original link for copyright type respect, not to recommend people study ar15 sights ) (and yes, the image uses 'depth of field' where 'depth of focus' is the proper term as discussed here)For your purposes, Beejay, remember that these tolerances can be cumulative. This is what leads a company to work to .05mm tolerance in one part of their system. Other parts of the system can introduce more slop, etc. Like film flatness!
The Ultimate Guide to the AR15 Iron Sights Ver 1.1 - The New Rifleman
Irons these days are just misunderstood. You want to master iron sights before moving on to a optics? You need knowledge. Where is the knowledge? Right here.
thenewrifleman.com
For the general principles I do recommend you read that book I linked to. As to what affects it - pretty much everything. Depth of focus/depth of field is constantly varying. Yes you can quantify it but, to do so, you have to make a number of assumptions about the resolving power of the lenses, the resolving power of the film, the degree of enlargement, the display medium and who is going to view the result and from what distance. All these things get rolled into a definition for the maximum acceptable size of the circle of confusion. But what is acceptable for you is not going to be the same for someone else and what works for one picture may not work for another. Yes, using smaller apertures will increase your depth of focus and may, sometimes, get you out of trouble but you should not be relying on it.
If you suspect there is a difference between your ground glass and film backs, test for it. Shoot an oblique brick wall and focus on a specific marked point. Take the shot and check the negative to see if the point of focus on the negative coincides with what you focussed on. If it's the same, you don't have a problem, if it's different, you need to fix it maybe by shimming either the glass or the film back. You say you can't work to the required tolerances - yes you can. You can buy shim material of 0.05mm thickness. By testing and adjusting and re testing you should be able to get it where it needs to be.
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Key statement highlighted in blue bold text. The Circle of Confusion is aptly named, because folks are so confused by the concept. Just as some folks have vision poorer than 20/40, some folks can achieve 20/15 or even 20/10, while their (US) optometrists set 20/20 vision as their goal for correction of vision.
The usual CofC size assumes somewhat poor vision of 20/30 and the inability to see a CIRCLE of blur (rather than perceiving it as a perfectly focused pinpoint of light). Poorer vision than what someone with 20/20 will perceive as a blur circle!
This illustration shows the reality of the plane of focus concept, which applies both in front of the lens and behind the lens...the one behind the lens is microns thin, not feet (although the illustration (made aany years ago) mentions 'feet' to illustrate Depth of Field!
Any image NOT in absolutely perfect focus is always a blur circle...that is, a piece of film which is not precisely located at the CENTER of the Focus plane is ALWAYS capturing an image which is not perfectly sharp!
Unless the flim is pressed perfectly flat and in the center of the plane of focus its quality is compromised...you simply might not be perceiving that imperfection. Flatness is the reason why glass negative carriers are preferred by so many for their enlarger, in lieu of glassless negative carriers (which allow the film to bow)...if perfection is wanted in the enlarger, why not also in the camera?!
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<snip>
I'm sure in your own mind, you made a point in there somewhere, but damned if I can make it out.
Not sure what the issue is with what Steve wrote. I don’t see a problem with it. He clearly indicated image space when discussing Depth of Focus. It is, btw, directly proportional to the Depth of Field times the system magnification.
Somewhere here in photrio I posted a stickie discussing DoF, boiling it down to the necessities for photography.
Btw, to OP’s original question: yes, in addition to directly increasing depth of focus, stopping down the lens reduces the magnitude of optical aberrations such as spherical aberration, field curvature, spherochromatism, and coma. Doing so further decreases the minimum blur size towards (and eventually to) the diffraction limit.
Both effects combine to increase allowable focusing errors. This is therefore a very important consideration in designing a producible, manufacturable imaging system.
Somewhere here in photrio I posted a stickie discussing DoF, boiling it down to the necessities for photography.
Btw, to OP’s original question: yes, in addition to directly increasing depth of focus, stopping down the lens reduces the magnitude of optical aberrations such as spherical aberration, field curvature, spherochromatism, and coma. Doing so further decreases the minimum blur size towards (and eventually to) the diffraction limit.
Both effects combine to increase allowable focusing errors. This is therefore a very important consideration in designing a producible, manufacturable imaging system.
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By the way, you can flex your mathematical prowess by backing out the lens manufacturers’ assumed circle of confusion from the depth of field scale on your lenses. All the information necessary is there on the lens.
1. Depth of Focus/Depth of Field ability for any observer to perceive "blur circle, not a 'pont' varies especially based upon an individual observors vision. The Circke if Confusion size itself is variable, based upon the individual program writer, and is typicall based on substandard human visual acuity -- not even 20/20 vision.
2. If many folks think they need to have glass negative carriers to hold film flat and in-focus across the entire negative, in the enlarger how is it that most photographers ignore equivalent need for film flatness in cameras?!
Yes, I am working my way through them.
The oblique wall is a brilliant idea - exactly the sort of thing I come here for.
Many of the posted responses have not only answered my original question but clarified the whole situation (both in terminology and by way of solution).
Back to the kitchen table / garage.
1. That correlates with what I said so I guess you were just trying to add detail. But thanks for clarifying that the standard coc is based on sub-standard vision. That makes my point even more relevant as the dof is likely to be actually less than thought so, again, dof should not be used as a way of overcoming system focus errors.
2. There is a distinct difference between the effect of film flatness in the camera and film flatness in the enlarger. If the film in the camera isn't flat and assuming the subject is a typical one (not a flat wall) then something will be in focus, maybe not exactly what the photographer intended but, in many cases, it has no consequence. So the effect of lack of flatness of the film in the camera often has minimal consequences and goes unnoticed. But lack of flatness of the negative in the enlarger is a whole different matter. If the negative isn't flat, the image on the paper will be out of focus in places and sharpness of the image, in places, will be less than it could be. In the enlarger you are dealing with two flat planes so any error of flatness becomes easily apparent. In the camera, you're usually dealing with only one flat plane and a subject that has depth so any error is, usually, less apparent but specific circumstances can change that.
I do not disagree...but the difference between the enlarger and the camera is that film flatness concerns in the enlarger HAVE a solution, whereas in-camera there is usually no solution available. FIlm flaness has been acknowledged by photographes in BOTH film cameras and enlargers.
Having done this, I'll make one suggestion- use a black and white target, similar to the charts used to check front and back focus on digital cameras. like linked below. Not necessarily this exact design, but do use something like this to make a reusable, easily read standard target. I used brick walls with a quarter, or electric meters, but from experience I can tell you that such rough targets are hard to interpret. Especially since you are involved in an ongoing process of improvement, etc. Maybe devote a wall section in your home, develop a standard distance, etc.? I do think that 'real world' distances will be best for your cameras, not a desktop target like used for digital cameras.
Heck, at $5, this chart might be worth using itself, just mount flat to a wall...
[looks like they don't allow links to Amazon. Fair enough. Go to amazon and search this term:
DSLRKIT Lens Focus Calibration Tool Alignment Ruler Folding Card ]
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