Is it me ?

[Steve M Hostetter]

all I can remember about hyperfocal distance is that a 50mm lens on a Mamyia RZ (6x7cm) is roughly 12 feet ,,,, so if I am using a wide angle on any format I tend to use the 12' distance as a starting point..
in practice you focus on an image and then tilt and refocus and maybe tilt some more and refocus till you see a totally sharp image on the GG and just have to say well I must of used the hyperfocal distance somewhere in there ...making it more a ridged camera concern
and as you know a 210mm isn't a wide angle untill you mount it to an 8x10 or bigger format

[Alan Davenport]

Why is this comparison of more than hypothetical interest?

Only because that's the direction the thread went...

[Wally]

I prefer the old Monty Python line...

"A wink is as good as a nod to a blind man."

Vaughn

It must be me. I thought it was a blind horse.

[Kirk Gittings]

In this case there is not change in DOF; the magnification is not changed by cropping the ground glass image using the 6x9 back.

But, in order to have the same framing (and perspective) on 6x9 that you have with 4x5 you must use a shorter focal length on the 6x9; shorter focal length, less magnification, greater DOF.

Bang on. This was part of the reason I switched to 6x9 roll film from 4x5 for commercial architectural photography a few years back. More DoF at a given aperture meant less strobe fill for interiors etc. The DoF I was getting on 4x5 at F16 with a 90mm, I could get at F11 (more or less) on a 65mm on 6x9. That one stop difference meant allot in the real world of shooting.

[bglick]

Here is a relatively simple cheat sheet for DOF analysis between formats which takes all the variables into consideration....


1) Change format size, change lens fl.....

same DOF exists, when you have the same:


a) recording media (same film type or same density digital sensor)
b) composure (half the format diag, = half the fl)
c) camera to subject distance (same angular view of lenses)
d) resolution on same final print size
(double the format diag. = half cc diam.)


Result ..... 2x format diag, double f stop = 2 stops slower shutter speed for the same DOF. So the penalty for equal DOF in larger format is 2 stops of shutter speed.

or, as Kirk corectly mentioned above, very painful when using strobes, as strobes are dependent on f stop only. Hence why LF is certainly not a flash friendly format.



2) Change lens fl, change camera to subject distance,

same DOF exists, when same:


a) format size (film size or digital sensor size, same density)
b) composure (half fl, half subject to camera distance)
c) focus point (but not focus distance)
d) cc (due to same size format)

Result is..... half fl = 1/2 f stop (2 stops faster).


It's interesting to note, and rarely mentioned, when using larger formats, there is a loss of resolution at the point of exact focus vs. the smaller format. DOF assumes only the max. size cc, not the min. sizes.


However, these are general rules. The reality is, nothing is carved in stone with modern optics. DOF formula was created in the late 1800's IIRC.... there was very few optical variables to contend with back in those days. With modern optics, many things have changed, including the use of aspheres, which can radically change the expected DOF output and sharpness at point of focus. Also, different lens designs react differently to DOF. A math formula that lumps all lenses into a single formula is over generalization today. Aberations vary greatly with each lens type... and at each f stop and even at different focus distances. This is evidenced by viewing MTF data of lenses at different focus distances.

Also, the DOF formula is deceiving.... the reason is, you simply plug in variables, and the formula pumps out answers. The Optical chain is not that simple in the real world. For example, you enter a given cc..... what knowledge do you have that cc can actually be recorded on film / sensor? The formula does not prevent you from using rediculous values that are not achievable, as in real world recording, the MTF values of the lens and the film play a critical roll in what's possible. Example, use DOF formula and shoot with color neg film, then the same with B&W high rez film, compare the results.... the color neg film will probably fall short of your expectations, the B&W film will far exceed your anticipated outcome.

cc is also effected by shutter vibrations, wind, film flatness, etc. etc. too many variables to ever expect an over simplified formula to provide accurate result.

So buyer beware on the DOF formula subject....

I have tested a lot of lenses. I have noticed with newer aspheric designs, the resolution at the point of exact focus on 35mm lenses, will often not vary at f2.8 through f11. This was unheard of 10+ years ago. The aspheres change all the rules as we know them, specially the rules (DOF formula) created in the 1800's.

So IMO, its OK to use these general rules of thumb to get close to what you want to achieve, but often today, it requires testing to be sure you meet your desired objectives...

[GPS]

...

However, these are general rules. The reality is, nothing is carved in stone with modern optics. DOF formula was created in the late 1800's IIRC.... there was very few optical variables to contend with back in those days. With modern optics, many things have changed, including the use of aspheres, which can radically change the expected DOF output and sharpness at point of focus. Also, different lens designs react differently to DOF. A math formula that lumps all lenses into a single formula is over generalization today. Aberations vary greatly with each lens type... and at each f stop and even at different focus distances. This is evidenced by viewing MTF data of lenses at different focus distances.

Also, the DOF formula is deceiving.... the reason is, you simply plug in variables, and the formula pumps out answers. The Optical chain is not that simple in the real world. For example, you enter a given cc..... what knowledge do you have that cc can actually be recorded on film / sensor? The formula does not prevent you from using rediculous values that are not achievable, as in real world recording, the MTF values of the lens and the film play a critical roll in what's possible. Example, use DOF formula and shoot with color neg film, then the same with B&W high rez film, compare the results.... the color neg film will probably fall short of your expectations, the B&W film will far exceed your anticipated outcome.

cc is also effected by shutter vibrations, wind, film flatness, etc. etc. too many variables to ever expect an over simplified formula to provide accurate result.

So buyer beware on the DOF formula subject....

I have tested a lot of lenses. I have noticed with newer aspheric designs, the resolution at the point of exact focus on 35mm lenses, will often not vary at f2.8 through f11. This was unheard of 10+ years ago. The aspheres change all the rules as we know them, specially the rules (DOF formula) created in the 1800's.

So IMO, its OK to use these general rules of thumb to get close to what you want to achieve, but often today, it requires testing to be sure you meet your desired objectives...

This is the same type of reasoning as to say that the equations for area calculations used to cover a swimming pool with a piece of material are old and out of touch with reality- the materials changed a lot, a modern plastic cover is not as soft and stretching as a piece of cotton cloth etc...

[bglick]

> This is the same type of reasoning as to say that the equations for area calculations used to cover a swimming pool with a piece of material are old and out of touch with reality-



Sorry, I don't see the parallels....unless fabrics have changed substantially over the past 100 years, like optics have.


I cited specific examples of why DOF can run astray.... such as entering cc values not attainable on the recorded media, the use of aspherical elements which changes the typical spherical pattern of lens performance we are accustom to seeing and different types of lens designs. I would think addressing these specific issues would be more productive than making a swimming pool analogy.... but its a public forum....


When the DOF equation was created, lenses were mostly doublets and film was at its infancy at best. DOF was based on the infamous "thin lens equation" which represents the most simplest of optical design forms, mainly singlets and doublets... a far cry from todays lens designs.


The purpose of the DOF formula is determine the amount of resolution which can be recorded at the near/far distances. However, the amount of resolution recorded is a function of the optical performance of a lens and the performance characteristics of the recording media. Neither of these are taken into consideration in the DOF formula. And over the past 100 years, the improvements in these two variables have been remarkable.


However, since the DOF formulas premise is based on a given focus distance and a given fl lens.... the general premise still applies today, i.e. the point of exact focus will always be the sharpest recorded area of the image, with the near/far being less sharp.... (till you get into very small apt., where the two level out) DOF formula tries to define "how much less sharp" the near/ far points will record. Yet, there is no information on the recording media and lens performance. Therefore, as per the DOF formula, an 1880's vintage lens, and a modern lens should perform identical, when using the same fl, format and focus distance. Draw your own conclusions....

[GPS]

[QUOTE=bglick;472757...

Sorry, I don't see the parallels....unless fabrics have changed substantially over the past 100 years, like optics have.


....[/QUOTE]

Haven't they?? Check modern sports cloths, to say just one example!

What you try to describe as some old fashioned optical equations is rather a simple difference between the theoretical values and the practical values. The fact that a car doesn't have the same practical consumption as the theoretically calculated consumption doesn't prove that the theoretical equations are "too old" - just that there is a difference in the correct theoretical values and the practical values.
A square area calculation equation is not incorrect because the material you cover the area with is less stretching today than it was 100 years ago...