Diffraction?

[LJH]

No it is well documented on books on optical engineering and lens design. See Kingleslake for examples. Also Modern Optical Engineering by Warren Smith. Optics Eugene Hecht. Try doing your homework in advance instead of shooting from the hip sonny.

Is there a chapter on "Making guesses on the intended end format of the lens" in these books?

Try not making ridiculous, assumptive statements, Sonny.

 

[blockend]

People who say "diffraction kicks in at f/whatever" are annoying. Diffraction is always there. It doesn't "kick in" at an aperture, at some aperture it becomes the dominant influence on the blur circle. And this has nothing to do with what's capturing the image.

The only point of any relevance is whether diffraction can be seen in the print at normal viewing sizes - and let's not get into a discussion about what's "normal". One person's kick in, is another's "dominant influence on the blur circle".

 

[Dan Fromm]

I didn't write that cropping changed things. I wrote that it is impossible to put a predetermined criteria on a lens if you don't know its intended application. Now do you understand why your unsubstantiated statement is wrong?

In post 27 you wrote:

In terms of LF lenses, how do they know what format the lens is to be used on? For example, a 14" lens will have different (visual) diffraction character on 4x5 compared to 8x10.

What's the difference between shooting a 14" lens on 8x10 and cropping to 4x5 and shooting it on 4x5?

You still haven't substantiated your absurd statement, you weaseled.

 

[L Gebhardt]

I calculate (or view using the manual stop down lever) the f stop based on the DOF I need. Then I stop down one additional stop to be sure. Is this a bad practice?

If that's what you've found works from actual shooting I'd say you're doing exactly the right thing.

 

[L Gebhardt]

What's the difference between shooting a 14" lens on 8x10 and cropping to 4x5 and shooting it on 4x5?

The difference is you will enlarge the 4x5 crop twice as much as the 8x10 to make the same sized print. So diffraction effects will be twice as visible in the print from the 4x5 crop.

Also the 8x10 will show a larger angle of view, so you would normally choose a lens that was half as long if you were to switch to 4x5 to capture the same image (it won't ever be the same).

 

[Dan Fromm]

Larry, let LJH speak for itself.

More seriously, diffraction's effect on the final print depends on the aperture at which the exposure was taken, the magnification at which the exposure was taken (remember that in closeup work and photomacrography the effective aperture is smaller than the aperture set), and the magnification from negative to final print.

For shots framed identically on 8x10 and 4x5 and taken with the same lens at the same aperture, diffraction's effect on the final print will be the same.

You and I know this and we're in agreement. Where LJH and I part ways is over his assertion "For example, a 14" lens will have different (visual) diffraction character on 4x5 compared to 8x10." On film the effect is the same. If not, my practice of assessing central sharpness of lenses for formats larger than 24x36 by shooting them on 24x36 is wrong.

 

[Ian C]

I calculate (or view using the manual stop down lever) the f stop based on the DOF I need. Then I stop down one additional stop to be sure. Is this a bad practice?

As others have said, if this gives you satisfactory results, that’s all that matters.

Could you do better? Using the largest aperture practical for the situation gives the best resolution due to restricting the percentage of diffracted light forming the image. For critical purposes it is possible to select the optimum focusing distance and aperture for a given format and situation. It involves calculation (which many of us would rather avoid).

The hyperfocal calculations can be used to quickly generate a table of hyperfocal focusing distance and aperture combinations for any lens and format when we require the depth of field to extend from one-half the hyperfocal distance to infinity. We can also calculate the required point of focus and aperture for a finite depth of field.

For a finite depth of field, if you know the following distances from the lens:

A = distance from the lens to the nearest point at which the depth of field must begin

B = distance from the lens to the farthest point to which the depth of field must extend

Then we can calculate the ideal lens-to-subject distance s.

For example, if we’ve composed a scene for which the near limit of DOF is A = 6 meters and B = 47 meters, then s = 10.6 meters. This is true for any format and any focal length lens.

If we also know the circle of confusion diameter c for the format and the focal length of the lens f, we can compute the maximum aperture (minimum f number) that gives us depth of field from A to B. In some cases the required aperture number is impractical or not even available. The calculation gives us the required information.

In the above example, for the 35mm format and using a circle of confusion diameter of c = 0.029mm and 50mm lens, we get aperture value N = 6.3 (aperture closed about 0.31 stop from f/5.6, or opened about 0.69 stop from f/8).

For the 6 x 7cm format using c = 0.059mm and 110mm lens, N = 15.1 (aperture closed 0.83 stop from f/11, or opened 0.17 stop from f/16).

For the 4” x 5” format with c = 0.10mm and 150mm lens, N = 16.6 (aperture closed 0.11 stop from f/16, or opened 0.89 stop from f/22).

This procedure maximizes resolution for a given scene with finite depth in two ways:

1. It uses the ideal point of focus relative to the near and far limits of the field.

2. It uses the largest aperture (smallest f number) that produces the required depth of field and thereby minimizes the percentage of diffracted light forming the image.

For casual shooting the above procedure is overkill. But in some situations, the extra steps might be justified, such as the case of a costly long-planned trip to a distant, hard-to-get-to location of unusual interest and beauty, or a scene that will soon be obliterated due to development giving us one—and only one—chance to capture it on film before it is gone forever.

 

[RalphLambrecht]

As others have said, if this gives you satisfactory results, that’s all that matters.

Could you do better? Using the largest aperture practical for the situation gives the best resolution due to restricting the percentage of diffracted light forming the image. For critical purposes it is possible to select the optimum focusing distance and aperture for a given format and situation. It involves calculation (which many of us would rather avoid).

The hyperfocal calculations can be used to quickly generate a table of hyperfocal focusing distance and aperture combinations for any lens and format when we require the depth of field to extend from one-half the hyperfocal distance to infinity. We can also calculate the required point of focus and aperture for a finite depth of field.

For a finite depth of field, if you know the following distances from the lens:

A = distance from the lens to the nearest point at which the depth of field must begin

B = distance from the lens to the farthest point to which the depth of field must extend

Then we can calculate the ideal lens-to-subject distance s.

For example, if we’ve composed a scene for which the near limit of DOF is A = 6 meters and B = 47 meters, then s = 10.6 meters. This is true for any format and any focal length lens.

If we also know the circle of confusion diameter c for the format and the focal length of the lens f, we can compute the maximum aperture (minimum f number) that gives us depth of field from A to B. In some cases the required aperture number is impractical or not even available. The calculation gives us the required information.

In the above example, for the 35mm format and using a circle of confusion diameter of c = 0.029mm and 50mm lens, we get aperture value N = 6.3 (aperture closed about 0.31 stop from f/5.6, or opened about 0.69 stop from f/8).

For the 6 x 7cm format using c = 0.059mm and 110mm lens, N = 15.1 (aperture closed 0.83 stop from f/11, or opened 0.17 stop from f/16).

For the 4” x 5” format with c = 0.10mm and 150mm lens, N = 16.6 (aperture closed 0.11 stop from f/16, or opened 0.89 stop from f/22).

This procedure maximizes resolution for a given scene with finite depth in two ways:

1. It uses the ideal point of focus relative to the near and far limits of the field.

2. It uses the largest aperture (smallest f number) that produces the required depth of field and thereby minimizes the percentage of diffracted light forming the image.

For casual shooting the above procedure is overkill. But in some situations, the extra steps might be justified, such as the case of a costly long-planned trip to a distant, hard-to-get-to location of unusual interest and beauty, or a scene that will soon be obliterated due to development giving us one—and only one—chance to capture it on film before it is gone forever.

I'm certainly not afraid of numbers and calculations but this procedure would seriously discourage me from taking any photographs at all. I just aim for f/8-11 and liberalyrely on the DOFscale on the lens.

 

[Dan Fromm]

I'm certainly not afraid of numbers and calculations but this procedure would seriously discourage me from taking any photographs at all. I just aim for f/8-11 and liberalyrely on the DOFscale on the lens.

Ralphie, LF lenses don't have DoF scales. And the DoF scales on lenses for smaller formats are usually calculated for larger circles of confusion than many of us think reasonable.

 

[Sirius Glass]

Please do not insult the weasels! The last thing we need here is PETA* posting on APUG.













* People Eating Tasty Animals

 

[Dan Fromm]

Steve, how about stoats?

 

[Sirius Glass]

I have known some nice stoats at work too.

 

[wiltw]

I calculate (or view using the manual stop down lever) the f stop based on the DOF I need. Then I stop down one additional stop to be sure. Is this a bad practice?

A GOOD practice, and a necessary one. The problem with DOF calculators or even the engraved DOF scale on you lens is that 'manufacturer standard' uses a CofC size which is larger than a person with 20/20 vision can interpret as a BLUR circle rather than perceived as a POINT...poorer assumption of human visual acuity than is the target by the optometrist. While the angular acuity measurement of human visual acuity calculates to CofC size of 0.025mm (but generally CofC runs from 0.029 --assumed by Zeiss -- to 0.033) So, by using one additional f/stop smaller than actual, effectively you are compensating the poor assumption of DOF calculators and scales.

 

[E. von Hoegh]

I have known some nice stoats at work too.

I know a stoat, but he'll be an ermine for the next few months...

 

[Sirius Glass]

or one stroat over the line.

 

[L Gebhardt]

Larry, let LJH speak for itself.

Sorry. Just trying to add a little clarity to the discussion, as it seemed a little blurry.

 

[Dan Fromm]

Sorry. Just trying to add a little clarity to the discussion, as it seemed a little blurry.

Yeah, but a focus error that has nothing to do with DoF.

 

[Sirius Glass]

The difference is you will enlarge the 4x5 crop twice as much as the 8x10 to make the same sized print. So diffraction effects will be twice as visible in the print from the 4x5 crop.

Also the 8x10 will show a larger angle of view, so you would normally choose a lens that was half as long if you were to switch to 4x5 to capture the same image (it won't ever be the same).

Actually greater than a factor of four. It is not linear it is area and therefore as the square of the area.

 

[Sirius Glass]

Sorry. Just trying to add a little clarity to the discussion, as it seemed a little blurry.

Some in the running discussion were and are not what we would call terribly focused to start with.

 

[RalphLambrecht]

Some in the running discussion were and are not what we would call terribly focused to start with.

good focus is a great start,that's why I like AF.Icsnnot beat it manually, as much as I've tried

 

[Sirius Glass]

AF will always win on repeatability as long as there is no confusion about what is the subject of focus.

 

[RalphLambrecht]

that's excactly matches my experience too.