Bipod

The answer it gave was for color film!

If you do not specify the number of megapixles for "35 mm (full frame)" the calculator assumes film
(and apparently, color film!). f/22 gives "Diffraction limited: NO".

But if you plug in 12 megapixels, it says: "Diffraction Limited: YES"

The calculator is useful, but the user interface is a bit confusing. It should probably
have separate "Camera types" for "35 mm film" and "Full-frame sensor".

Also, it treats all films and film speeds are the same resolution--which is definitely
not true. And it seems to be based on color film--I suppose to "compare apples with
apples" (it alos only lists color image sensors). But the formats go up to 8" x 10"!
I'm trying to think of somebody who shot color film in an 8" x 10" view camera,
but no luck. If they ever made color film that big, they certainly don't anymore.

It is a nice tool--it just needs a little more work on the film side. But the page
does a very good job of spelling out the assumptions the calculator makes about digital
sensor resolution.

Also, the term "diffraction limited" is somewhat misleading, since in optical
engineering this means a lens that is so good that it is only limited by diffraction.
But get the aperture small enough, and any cheap plastic lens is "diffraction limited"--
for that tiny aperture.

I hope the authors keep improving this calculator--it's very nice, but could be even
better.

Bipod

Addendum: there is no documentation on how the "circle of confusion"was chosen for film.
It seems to depend on format size--which makes very little sense.

There is no way to caclulate the circle of confusion of lens -- the lens has to be tested. In the
best case, a good lens will exceed the resolving power of any sensor or medium (except
possibly a daguerreotype plate). If you have a poor lens -- lucky you, you'll notice less
diffraction. But a calcultor shouldn't assume it's a poor lens. Some lenses have a very,
very small circle of confusion in the center of the image.

Basically, what you care about is the resolving power of the medium, As long as the Airy
disk produced by diffraction is smaller than that, you won't see it.

The circle of confusion of a pinhole can be calculated, based on geometry. But in a camera
lens, the aperture is located at a focal node -- the light is focused on the aperture. The aperture
does not create a circle of confusion: only aberrations in the lens do. The only thing the
aperture contributes is diffraction.

The page explains how the circle of confusion is calculated for digital cameras (ie., if you
enter a number for megapixels). But if you don't enter megapixels, it assumes film and there
is no clue how the circle of confusion is calculated.

Different films vary greatly in resolution (on "standard" development). The difference between
color and B&W should not be ignored.

selmslie Loc: Fernandina Beach, FL, USA

There is plenty of documentation. It's based on the resolution of the average human eye. See Circle of confusion and Google for more.

The basic assumption is that the eye can resolve about 2 minutes of arc.

For the sake of further simplification and standardization, it is assumed that the result will be viewed from a "normal" perspective and that eliminates focal length from the equation. It then becomes a function of only the format. A lens does not have its own circle of confusion.

When you work through the math that is simplified to about 1/1500 of the diagonal measurement of the format of the film or sensor.

The airy disk is not the same as the CoC.

burkphoto Loc: High Point, NC

Bipod

Just a note to users of Reikan FoCal and similar products:
These are primarily intended for focus calibration--and very good for that.
But lens testing requires carefully controlled conditions and a battery of tests
using different test targets and illumination.

In some cases special equipment may be needed: collimator, star test jig,
Twyman-Green interferometer, etc. However, if one doesn't have the
required equiopment, often there is an alternative test that can be performed
(but is less convenient and/or less accurate).

Neither FoCal, Imatest or published MTF data tells you what is wrong with
a lens: the specific aberration(s) from which it suffers. But this is crucial
information for the lens user to have.

For example, a lens whose performace drops off at the periphery becaues of field
curvature is quite useable. But one whose performance drops off because of
chormatic aberration is not--except as a paperweight.

Another example: zoom lenses all have geometric distortion, but some have
more than others. It's important to know how much. A lens with a lot of
distortion can be fine for certain shots, but horrible for others (e.g, architecture).
A grid test target (or sheet of pegboard) makes distortion obvious.

To find out what's wrong with a lens, you have to look for every potential
problem: starting with a careful physical inspection. It is not sufficient just
to measure resolution. Low resolving power is a symptom, not a diagnosis.

Simply shooting a star from a tripod (a start test) can tell you more about the
aberrations than these automated tests do. If you magnify and examine the
image of a point source. the shape is quite informative (though it can be hard to
interpret if several different aberrations are present).

Even published MTF data is incomplete and has to be used with caution.
And the conditions under which these tests are conducted do not match field
conditions in important ways. The data -- while valid--is limited by its methodology
and incomplete and can be misleading.

MTF testing uses low spacial frequency as a measure of contrast. That's fine,
except it ignores flare---a major problem with some lenses. To know how the
lens will really perform in the field, one has to test contrast both with and
without a bright light outside the angle-of-view. Light from the sky is a
significant cause of flare.

Moreover, it does not measure the dimness of the lens--how much light
is absorbed. Manufactures used to measure the "t-stop" of lenses -- the
effective aperture. Back in the 1930s - 1950s, an "anastigmat" lens might
only have 3 or 5 elements, so there was very little glass. But the surfaces were
uncoated--so there was quite of specular reflection.

When coated lenses became common the 1960s, this was no longer necessary,
since the the amount of light being reflected became negligible. But these
lens still had only 5 to 7 elements so absorbtion of light by the glass also was
negligible.

But now we are seeing lenses with 20 - 30 elements with 10 - 15 groups.
It once again is necessary to measure t-stop and check for flare. Manfuactures
don't do it: so photographers need to.

The largest I've heard of in a consumer lens is 33 elements in 15 groups:
a true "glass log" with 30 surfaces. Caveat emptor.

Obviously, it is only common sense to test such a lens for dimness and loss
of contrast due to flare. Then you'll know how fast the lens really is and
whether it can be used outside without a lens hood. But most people simply
wonder why their photos lack contrast, and why they aren't getting the
speed they paid for.

No lens is perfect. Knowing the specific strengths and weaknesses of a lens
enables one to make better use of it. It's not just a matter of a lens being
"good" or "bad". A very well-corrected lens can be a bad lens in actual use.

TriX Loc: Raleigh, NC

Certainly not contending that Focal is a laboratory grade testing device, that it tests CA, center vs corner distortion or any of the myriad of characteristics that define a lense’s performance, and while an MTF chart is marginally useful, it doesn’t chabracterize all of those parameters either. But, it does relatively accurately test accuitance, and more importantly, it tests that for YOUR specific lens. That combined with a good test target, can show center to edge sharpness and makes poor CA pretty obvious. Are those 2 evaluations a complete and accurate picture of your lens? No, but they can provide a great deal of helpful information on a topic that is constantly discussed here - sharpness, and it’s certainly more informative, accurate and objective than just taking a shot or two and making subjective judgements. Not that isn’t useful as well, but not as objective as a graph. In short, while not perfect, it’s certainly more information than you’d have without it.

MauiMoto Loc: Hawaii

That looks like half stop increments. My cameras came preset for 1/3 stop increments. There should be a menu setting for exposure adjustments to select 1/2 or 1/3 stop increments for both aperture/shutter and iso. It can be confusing since doubling or halving the iso value or shutter speed is one stop but doubling the aperture value is two stops. f1.4 to f2 is one stop, f1.4 to f2.8 is two stops down.

Bipod

That only works if you enter the size of the print and the viewing distance. Then you can use the
angle-of-arc to come up with Lines/mm. But I didn't enter that data.

The question was: if I just select "35 mm (full-frame)" and "f/22", what circle of confusion is
it using and why?

Print size and viewing distance only apply to a specific project or photo. Often, you don't know the
viewing distance or people will walk right up to the print.

Sure, a lens has a circle-of-confusion, but as I explained, you don't know what it is unless you test
the lens at its's sharpest aperture. An then it will probably vary between the center of view and the
periphery. But for most modern lenses, the resolving power of the lens in center of view at its sharpest
aperature will exceed that of the film/sensor. So lens CoC is not the limiting factor: the film or
sensor is.

The only safe assumption--which will always work--is to try to equal the resolution of the film/sensor.
So the narrowest aperature that doesn't degrade the image is the one that produces an Airy disk smaller
in diameter than the resolution of the film/sensor.

If the Airy disk is larger than the resolution of the film/sensor, then the diffraction is reducing the
resolving power of the lens. In the calculator's terms: "Diffraction limited: YES".

This is how the calculator works when you enter a magapixel number (and don't enter a print size or
viewing distance). It tries to calculate the resolution of the sensor.

Each type of film also has a maximum resolution (expressable in Lines/mm). If they can go to all the
bother of calculating digital sensor resoltuion (with a bunch of simplifying assumptions), then why not
look up the resolution of different film types? That would be good enough.

In other words: unless you screw up the lens by selecting a very narrow aperture, it will meet or exceed
the resolution of the film or sensor. So we'd like to know the narrowest aperture we can select without
reducing the resolution of the optical system. Very simple question--but not what the calcualtor is giving us.

If you know how the calculator is choicing the CoC when the user doesn't enter a megapixel number
and doesn't enter a print size and doesn't enter a viewing distance, I'd love to hear it.

Bipod

That's a really good point: production runs of lense vary -- even within tolerances --
and stuff happens to lenses (e.g, they get dropped). It's a super good idea to test
one's lenses.

I agree: Focal, IMAtest, etc. are very useful tools. The problem is when patial or incomplete data
(e.g. MTF graphs) are used to market lenses.

You buy a lens to do certain jobs: portraits, landscapes--whatever. The MTF chart
is lacking key information that would enable you to decide the best use for the lens.
Using the high spacial frquency resolution data to classify lenses as "good", "bad"
and "indifferent" is not very helpful. As Tolstoy said, "All happy families are alike,
but each unhappy family is unhappy in its own way."

Some of my favorite lenses are "unhappy" lenses: poor MTF performers. But great
for portraits or other speciifc uses. That's because they have aberrations that help
(or at least don't hurt) and don't have the aberrations that would make them unusable
for that purpose.

It's only fair to tell the buyer what aberrations the lens has.

And as I mentioned, low-spacial-frequency data should not be interepreted as a measure
of contrast unless one always uses exactly the best lens hood (which is not feasible for
any zoom lens). Flare is a really common problem--particularly with zoom lenses.

Zoom lenses are now the best selling lenses. The whole marketing system seems to be
designed not to rock that boat. And prime lenses that are much cheaper to build are
priced comparable to zoom lenses, in order to avoid undercutting the sales of super-
expensive zooms.

If manufacturers were trying to make photrography bad, they could hardly come up
with a better strategy:

* Don't build any camera with monochroom (higher res, higher contrast, lower noise) sensors
(only Leica does)

* Price people into using only miniature format and subminiature format cameras
(remember, Kodak Brownies used to be medium format: 120, 620, 616 etc.).

* Price people into using zoom lenses

selmslie Loc: Fernandina Beach, FL, USA

Circle of Confusion - A bunch of photographers talking about depth of field.

Lens sharpness and Circle of Confusion (CoC) are not the same thing. You can read more about CoC at TUTORIALS: DEPTH OF FIELD. (DoF)

CoC is an assumption based on viewing a standard 8x10 print from about 10 inches with normal eyesight made from the uncropped image. All of the derivations of CoC are based on similar assumptions. It is commonly expressed as the diameter of the circle in mm. Normal viewing distance can also be 10 feet for an 8x10 foot print or 10 meters for an 8x10 meter print, etc. CoC assumes a perfect lens and it applies to the entire image, corner to corner. It is tied to the size of the uncropped image regardless of the lens focal length. CoC is the basis for calculating hyperfocal distance (HD) and DoF. If you change any of the basic assumptions that determine CoC, both HD and DoF change.

Sharpness is an entirely different concept. It is commonly expressed as line pairs per millimeter. It is a property of the lens, not the format. It varies across the surface of the image - usually best in the center and worse in the corners. It can be degraded by diffraction (aperture too small) and several different factors (various aberrations and vignette) when the aperture is too large.

When we think of a "sweet spot" we are usually looking for the aperture where the softness from wider apertures is minimized and before diffraction becomes noticeable. If we concentrate on the center of the image, this aperture will be wider than if we consider the entire image. For landscape, the best aperture is usually when all other forms of degradation are minimized and just before diffraction becomes a problem.

Bipod

I wish you would present your own argument instead of making a distracting remark accompanied by a link.

"Sharpness" is not a technical term--it can mean acutance, subjective perception of acutance or resolution,
depending on the context. But CoC is a technical term from optics.

No, this is incorrect. CoC is defined in optical engineering as an optical spot caused by a cone of light rays from a lens not coming to a perfect
focus when imaging a point source. It is also known as disk of confusion, circle of indistinctness, blur circle, or blur spot.
https://en.wikipedia.org/wiki/Circle_of_confusion

CoC is an actual phenomon, observable in all optical systems that project an image of a point source.
It respresents the combined effect of lens aberrations. For example, the "caustic"--or failure to focus at a sharp
point--of spherical aberration, will create a CoC on the image plane.

The diameter of the CoC is inversely proportional to the resolution of the lens: resolution in L/mm = 1 / diameter in mm

What you are describing is the method of working backwards from print size and viewing differece to a "maximum" CoC
that is permissable. The next step is to allow any aperture for which the airy disk is smaller than this calculated CoC.
Several websites follow this procedure.

But as I explained before, this makes assumptions:
1. The print will never been seen closer than a certain distance
2. The lens has no aberrations -- it's a perfect lens
3. Printing doesn't degrade resolution.

1 is usually false: people will walk right up to prints if they can.
2 is always false: there are no perfect lens (just very good ones).
3 is also false: printers of types are far from perfect.

For a given size print and a given lens, you may have barely enough resolution at sharpest aperture
to survive close viewing. I can tell you that in a gallery, if your prints looks fuzzy or pixellated
to people who walk up to them, you won't sell any. If you don't believe me, ask Bruce Barnbaum.

"Landscape photography is the supreme test of the photographer - and often the supreme disappointment."
--Ansel Adams

That kind of "good enough" reasoning helps explain why landscape photography so often
disappoint: because photographers cut corners. If one tries for perfection, one just might
get a print that's good enough.

As I said before, the only safe criterion is to match the resolution of your sensor or film.
When resolution counts, do not use a lens or an aperture that degrades resolution. After all,
there is barely enough resolution in a "full-frame" image to get a decent 8" x 10" print
of a detailed, well-lit subject.
"The test charts shall consist of groups of parallel straight
lines and spaces of equal width; the resolving power is the
reciprocal of the center-to-center distance of the lines that
are just distinguishable in the recorded image.
Sharpness is an entirely different concept. It is commonly expressed as line pairs per millimeter.
[/quote]
No that is resolution. In fact, the test targets for resolution (e.g, USAF 1951) are parallel lines.
Here is what Schneider Optics has to say about MIL-STD-150A, "Military Standard Photographic
Lenses" (US Department of Defens 12 May 1959 (Rev A)):

"The resolving power of a lens is a measure of its ability
to image closely spaced objecds so that they are recognizable
as individual objects. The resolving power shall be expressed
in lines per millimeter, usually in the short conjugate plane.
Resolving power is measured by photographing or observing
suitable test charts at specified angular distances from the
center of the field.

"The test charts shall consist of groups of parallel straight
lines and spaces of equal width; the resolving power is the
reciprocal of the center-to-center distance of the lines that
are just distinguishable in the recorded image. . . . "

Lenses have a maximum resolution, which certainly varies across the image.
But so do sensors and films.

Resolution of a given size projected image does not depend upon format--it depends
upon "pixel density" (photocell spacing) and other factors. But resolution of lines
in the subject of the photo ("object" in optics) very much depends on the format
(provided we don't crop).

When we take photographs, we try to use as much of the frame as possible.
That's because for a given film or sensor technology, the larger the area on the sensor
the image covers, the greater the maximum resolution can be.

The first limit is the sensor or firm: size and "pixel density"
The second limit is the lens
The third limit is the aperture
Then comes everything else (focus, motion blur, haze, etc)
that reduces resolution.

You may think one has a enough, but by the end of the process
(including resolution lost in printing), you'll wonder where it went.
But if you don't print large, you'll never know the difference.
All thumbnails look sharp.

Exactly so. (Although one has to be careful with "noticable" -- some people notice
more than others--gallery owners and dealers in fine art prints, for example.)

In other words, if the lens is fuzzy, we can get away with more diffraction. True enough.
But I hope to God the lens is not fuzzy, and the system is limited by the resolution of the sensor.
In that case, we do not want to reduce the resolution firuther with diffraction.
If an aperture creates an Airy pattern large enough to reduce the resolution of the system
(which -- we hope -- is limited by the sensor), then that aperture is too narrow.

With full-frame and highest quality sensor or B&W film, f/22 is too narrow.
You can test it with a good lens (I use Nikon primes) and a resolution test chart.

Yep. It's a trade-off between reducing certain aberrations and increasing diffraction.
Diffraction is always present, but as long as the Airy disk is smaller than the resolution
of the system, it makes no difference.

So once you've found the sharpest aperture, if desired you can narrow it to create greater
depth-of-field up until the point when it starts to degrade the resolving power of the system.

When resolution matters--as in landscape photography--one aims to achive the full resolving
power of the film or sensor. A good lens, lens hood, tripod, mirror lock, cable release -- all
work towards this goal. The last thing one wants to do is ruin it with too much diffraction.

selmslie Loc: Fernandina Beach, FL, USA

It's ironic that you would complain about me presenting a link to the article describing Circle of confusion and then present the exact same link a few lines later, "It is also known as disk of confusion, circle of indistinctness, blur circle, or blur spot. https://en.wikipedia.org/wiki/Circle_of_confusion"

Rather than go to great lengths to argue with me, I suggest you read the article itself. Specifically, look at the section titled Circle of confusion diameter limit in photography where the derivation of the CoC is explained and culminates with the table showing Circle of confusion diameter limit based on d/1500 for different format sizes.

You have spent a lot of time talking without listening. You are going to be embarrassed when the penny finally drops for you.

selmslie Loc: Fernandina Beach, FL, USA

No, CoC is a theoretical value based on the acutance of the human eye. The article on CoC that you and I both cited says, "For full-frame 35 mm format (24 mm × 36 mm, 43 mm diagonal), a widely used CoC limit is d/1500, or 0.029 mm for full-frame 35 mm format, which corresponds to resolving 5 lines per millimeter on a print of 30 cm diagonal. Values of 0.030 mm and 0.033 mm are also common for full-frame 35 mm format."

CoC is a constant used in the calculation of DOF for a given format. It assumes a perfect lens - no lens aberrations or diffraction. CoC is the same for all lenses used on a specific format, regardless of focal length, acutance or other lens characteristics. Otherwise we would never be able to calculate DOF.

Diffraction is measured against the theoretical CoC to figure out whether it will be noticeable in film, which is difficult because of the randomness of film grains. But for digital see LENS DIFFRACTION & PHOTOGRAPHY, "When the diameter of the airy disk's central peak becomes large relative to the pixel size in the camera (or maximum tolerable circle of confusion), it begins to have a visual impact on the image." The "maximum tolerable circle of confusion" can span several pixels in high resolution crop sensors or it can be smaller than the pixel size.

Absolutely not! CoC is never calculated from the resolution of the lens.

If the resolution of a lens is 40 lp/mm, it will be 40 lp/mm whether you use it on a crop sensor, full frame, medium format or large format camera. What changes with the format is the total number of line pairs that can be recorded with each format. But now we are talking about system resolution, You can read more on that in Photographic System Resolution.

Lens resolution is measured by focusing precisely on a target that is parallel to the film/sensor plane. DOF is not a factor in this case.