Hyperfocal 'acceptable sharpness'

[Withnail73]

I was under the impression that if a lens was set to its HF distance that the sharpness of objects at infinity would the same as if I'd focused at infinity. But several tests whilst walking in the lake district recently seem to prove otherwise. It's clear but nowhere near as sharp - is that what is termed 'acceptably sharp'? If not then I guess it's down to my technique, but I just want to know if it's achievable.

Thanks.

Canon 550d - sigma 17-50 2.8

 

[AnandaSim]

I was under the impression that if a lens was set to its HF distance that the sharpness of objects at infinity would the same as if I'd focused at infinity.

It's clear but nowhere near as sharp - is that what is termed 'acceptably sharp'? If not then I guess it's down to my technique, but I just want to know if it's achievable.

DOF calculations are calculations - they're not visual assessment. Did you try a more conservative distance or one, two more clicks on the f/no scale?

--



Ananda
http://www.dpreview.com/articles/6861540877/a-compilation-of-tips-for-beginners
http://anandasim.blogspot.com/
http://gplus.to/anandasim

'Enjoy Diversity - Live a Little or a Lot'

 

[sherwoodpete]

I was under the impression that if a lens was set to its HF distance that the sharpness of objects at infinity would the same as if I'd focused at infinity. But several tests whilst walking in the lake district recently seem to prove otherwise. It's clear but nowhere near as sharp - is that what is termed 'acceptably sharp'? If not then I guess it's down to my technique, but I just want to know if it's achievable.

Thanks.

Canon 550d - sigma 17-50 2.8

Any lens produces an image which is only truly sharp at just one distance. The use of 'depth of field' relies entirely upon the concept of acceptable sharpness. This in turn is based upon specific viewing conditions, that is a specified print size, viewing distance and visual acuity of the user.

It sounds from your question that you may be 'pixel peeping' or scrutinising the image very closely in order to examine sharpness. As such, you will be breaking the conditions on which the DOF is built, that is, the viewing conditions will be changed.

When composing an image, it it necessary to decide what are the priorities in terms of sharpness. If the part of the subject at infinity is the most important, then set the focus there. On the other hand, if the scene as a whole needs to be sharp, then setting focus at the hyperfocal distance may be a better choice.

Some reading on the concepts involved:
http://toothwalker.org/optics/dof.html#hyperfocal

http://www.cambridgeincolour.com/tutorials/depth-of-field.htm

Regards,
Peter

 

[Deleted1929]

The usefulness of hyperfocal distance is when you want more or less everything some point relatively close to infinity reasonably sharp. It's pretty much an assumption that you're more interested in the foreground than the background.

Here are some notes on how DOF values are derived if you've a mathematical inclination.

http://toothwalker.org/optics/dofderivation.html

[ There are quite a few articles of note on that site - see the links at the bottom of the page. ]

There is an approach you can try to get more sharpness from a final image, if you like, which is to use something like focus stacking or image blending ( layers and masks ) to merge images taken with the focus on different distances within the scene. This is more commonly used in macro shots.

http://www.cambridgeincolour.com/tutorials/focus-stacking.htm

--
StephenG

 

[sherwoodpete]

The usefulness of hyperfocal distance is when you want more or less everything some point relatively close to infinity reasonably sharp. It's pretty much an assumption that you're more interested in the foreground than the background.

I'd say it assumes that you are equally interested in foreground and background, since neither will be at the actual point of focus, but instead rely on DOF for sharpness.

Here are some notes on how DOF values are derived if you've a mathematical inclination.

http://toothwalker.org/optics/dofderivation.html

[ There are quite a few articles of note on that site - see the links at the bottom of the page. ]

There is an approach you can try to get more sharpness from a final image, if you like, which is to use something like focus stacking or image blending ( layers and masks ) to merge images taken with the focus on different distances within the scene. This is more commonly used in macro shots.

http://www.cambridgeincolour.com/tutorials/focus-stacking.htm

--
StephenG

 

[Withnail73]

I've not stopped down yet, no. So in your opinion it should be achievable?

 

[AnandaSim]

As the others have said, there is only one plane that the focus will be exact. The Depth of Field is for acceptable sharpness and it is based on a chosen Circle of Confusion which is chosen for an enlargement and the viewing distance.

If you push your nose so close that it touches the screen and pixel peep at 100%, the Depth of Field could be so shallow that it could be thin plane.

I've not stopped down yet, no. So in your opinion it should be achievable?

--



Ananda
http://www.dpreview.com/articles/6861540877/a-compilation-of-tips-for-beginners
http://anandasim.blogspot.com/
http://gplus.to/anandasim

'Enjoy Diversity - Live a Little or a Lot'

 

[Withnail73]

You're right, I am pixel peeping, but only out of curiosity. The image when viewed at normal size is fine. I'm just wondering if optically it is possible to have same ludicrous sharpness front to back throughout the entire dof.

 

[Jirka-]

When I used a Pentacon six MF camera with with Zeiss Jena lenses which had a fairly precise distance scale I used one stop smaller aperture than the DOF scale indicated. E.g I would determine that both the near point and infinity were covered by f16 at certain distance and than set the aperture ring to f22. Otherwise the picture would not be quite sharp enough for 30x40 cm prints.

 

[sherwoodpete]

You're right, I am pixel peeping, but only out of curiosity. The image when viewed at normal size is fine. I'm just wondering if optically it is possible to have same ludicrous sharpness front to back throughout the entire dof.

The short answer is: no - it's not possible.

A longer answer is that you would need to stop down the lens to a smaller aperture, to give the extra sharpness when the image is viewed closely. But - there is a limit. As the aperture size gets smaller, the image gradually gets softer due to diffraction. Because you are scrutinising the image very closely, the effect of diffraction will very quickly become apparent.

For more on diffraction, see this page:
http://www.cambridgeincolour.com/tutorials/diffraction-photography.htm

There's a useful calculator here which shows the depth of field as well as the hyperfocal distance for a given camera/focal length/aperture combination. http://www.dofmaster.com/dofjs.html
Two points to note here:

• the DOF calculator does not take diffraction into account
• the calculations are based upon 'standard' viewing conditions. If the image is examined more closely, then the DOF gets less and the hyperfocal distance changes too.

Regards,
Peter

 

[dsjtecserv]

For more on diffraction, see this page:
http://www.cambridgeincolour.com/tutorials/diffraction-photography.htm

There's a useful calculator here which shows the depth of field as well as the hyperfocal distance for a given camera/focal length/aperture combination. http://www.dofmaster.com/dofjs.html

Good links. To actually see and experiment with the relative effects of depth of field and diffraction, see the calculator on this page, which shows a graphical representation the two factors: http://www.tawbaware.com/maxlyons/calc.htm

One interesting thing to remember, which is illustrated by the chart on that page, is that diffraction limits the maximum sharpness; that is the "best" sharpness at the plane of focus. It has little or no effect on the sharpness of things that are fore and aft of the plane of focus.

Dave

--
http://www.pbase.com/dsjtecserv

 

[Withnail73]

Thanks!

 

[Deleted1929]

I'd say it assumes that you are equally interested in foreground and background, since neither will be at the actual point of focus, but instead rely on DOF for sharpness.

Hyperfocal distance includes a region in the foreground that is in what might be called "ordinary" focus region and which abuts onto the region to infinity that is in acceptable focus.

So the foreground region is everything from (near DOF limit) to (far DOF limit) and as a freebie you get everything in (far DOF limit) to (infinity) in "acceptable" sharpness. You are, however, really focusing on at a distance of twice the hyperfocal distance into the scene.

For simplicity we think of this as one region starting at the hyperfocal distance, which is actually the near limit of "ordinary" depth of field in this special case.

It is implicit in using hyperfocal distance that we tend to use narrow apertures and this does mean we loose some sharpness to diffraction anyway.

Also bare in mind that distant objects are more likely to be obscured by haze and that's another reason why absolute sharpness is not as critical for them.

It's really the arrival of "pixel peeping" that has made this relative lack of sharpness an issue, IMO.

--
StephenG

 

[Mike Davis]

There is no single f-Number at which diffraction will begin to inhibit a desired print resolution - it varies with your anticipated enlargement factor, because the Airy disks recorded at the sensor (or film) plane with a given f-Number will suffer varying degrees of magnification for different size prints examined at an anticipated viewing distance.

If you intend to make 4x6-inch prints or even smaller wallet photos, to be viewed at a distance of 20-inches, you'll have a lot less concern for the possibility of either diffraction or defocus inhibiting whatever you personally consider to be an acceptable level of subject detail, than you would when making larger prints. You can feel free to shoot at just about any aperture available on your lens when the enlargement factor is small, the viewing distance is great, or your personal requirement for resolution in the final print is low.

If instead you intend to make 8x12-inch prints, that must survive scrutiny at a viewing distance of only 10 inches, you'll likely have a preference for greater resolution in the final print, and will therefore suffer a greater possibility of diffraction or defocus inhibiting the print resolution you desire.

Thus, it's impossible for anyone to recommend an aperture at which "diffraction begins to destroy sharpness" for any given camera, without knowing the enlargement factor at which you plan to print your images -and- your desired print resolution (or, at the very least, your anticipated viewing distance, so that we can recommend a desired print resolution.)

Again: There is no single f-Number at which diffraction "becomes a problem" for any camera at all combinations of enlargement factor and desired print resolution.

Note that an image for an enormous roadside billboard does not have to be shot with a 10,000 Megapixel camera because the associated viewing distance is typically hundreds of feet. A 24x36-inch print viewed at a distance of 20 inches can similarly appear to have every bit as much subject detail as when the same file is printed to a 12x18-inch print for viewing at half that distance.

Yet somehow, anticipated enlargement factor and specification of the resolution one personally hopes to record in the final print are, more often than not, completely ignored in discussions of aperture selection for controlling either diffraction or DoF.

A good number of DoF calculators don't even allow user-specification of a custom CoC diameter, instead allowing the user to specify only the focal length plus the near and far distances of the subject space. Countless people have been disappointed by the results had when they make use of the DoF scales engraved on their lens barrels. Why are they disappointed? Because enlargement factor and the amount of resolution they personally hoped to secure in the final image (which itself should take viewing distance into consideration) are ignored by such tools!

Have a look at this formula from Wikipedia's Circle of Confusion page, http://en.wikipedia.org/wiki/Circle_of_confusion

CoC (mm) = viewing distance (cm) / desired final-image resolution (lp/mm) for a 25 cm viewing distance / enlargement / 25

For example, to support a final-image resolution equivalent to 5 lp/mm for a 25 cm viewing distance when the anticipated viewing distance is 50 cm and the anticipated enlargement is 8:

CoC = 50 / 5 / 8 / 25 = 0.05 mm

Notice that the only variables in this equation for calculating the maximum CoC diameter to be used in DoF calculations are those for viewing distance, desired final image resolution (in lp/mm), and enlargement factor, but again, these variables are seldom proclaimed when discussing aperture selection.

A viewing distance of 25 cm is about 9.84 inches - about the closest that a person with healthy vision can focus with the naked eyes. If you are willing to confine your specification of a desired final image resolution to always satisfy a viewing distance of 25cm, you can reduce the Max. CoC calculation to this equation:

CoC (mm) = 1 / desired final-image resolution (lp/mm) for a 25 cm viewing distance / enlargement factor

Here's a similar formula for determining the aperture, f/N, at which diffraction's Airy disks will begin to inhibit your desired final image resolution (in lp/mm) at the anticipated enlargement factor for a 25cm viewing distance:

N = 1 / desired print resolution (lp/mm) / anticipated enlargement factor / 0.00135383

Notice that it's just the calculated CoC value divided by the constant 0.00135383.

See David M. Jacobson's Lens Tutorial for the origin of this constant: http://graflex.org/lenses/photographic-lenses-tutorial.html .

This same math is employed by Sean McHugh's diffraction tutorial at http://www.cambridgeincolour.com/tutorials/diffraction-photography.htm .

(Continued below...)

 

[Mike Davis]

(...Continued from above)

For calculating either the CoC diameter that you can specify in your choice of DoF calculator (of those that permit user-specification of max. CoC) or for calculating the f-Number at which diffraction will begin to inhibit a desired final image resolution at an anticipated enlargement factor, here are some definitions of the two variables used in each equation:

Enlargement factor is simply the ratio of the final image dimensions to your sensor or film dimensions. (Divide print diagonal by sensor diagonal, or print width by sensor width, for example.)

For your desired final image resolution, I recommend a value of 4- to 8-lp/mm when you want the print to survive scrutiny at a viewing distance of 10 inches. 4 lp/mm will satisfy most people at that viewing distance and anything greater than 8 lp/mm would be overkill (despite Ctein saying that some people can appreciate resolutions as high as 25 lp/mm in a print viewed at 25cm). If you only want to satisfy a viewing distance of 20 inches, you can cut my recommendation for desired resolution (at 10 inches) in half.

A desired final image resolution of 2 lp/mm requires an unresampled image file resolution of 100 dpi (for a sensor that lacks an AA filter) or 144 dpi for a typical CMOS sensor, due to a 30% loss of resolution typically imposed by the RGBG Bayer algorithm and AA filter.

A desired final image resolution of 3 lp/mm requires an unresampled image file resolution of 150 dpi (no AA filter) or 216 dpi (with 30% Bayer and AA losses).

A desired final image resolution of 4 lp/mm requires an unresampled image file resolution of 200 dpi (no AA filter) or 288 dpi (with 30% Bayer and AA losses).

A desired final image resolution of 5 lp/mm requires an unresampled image file resolution of 250 dpi (no AA filter) or 360 dpi (with 30% Bayer and AA losses).

A desired final image resolution of 6 lp/mm requires an unresampled image file resolution of 300 dpi (no AA filter) or 432 dpi (with 30% Bayer and AA losses).

A desired final image resolution of 7 lp/mm requires an unresampled image file resolution of 350 dpi (no AA filter) or 504 dpi (with 30% Bayer and AA losses).

A desired final image resolution of 8 lp/mm requires an unresampled image file resolution of 400 dpi (no AA filter) or 576 dpi (with 30% Bayer and AA losses).

Before you can select a "desired" final image resolution, you have to be realistic when selecting an enlargement factor.

For example, the Nikon D300's 15.7 x 23.7mm sensor captures 4288 x 2848 pixels (12.21 MP) at a moderate pixel density of 181 pixels/mm.

Taking into account the loss of resolution caused by the RGBG Bayer algorithm and AA filter, typically a 30% loss relative to actual pixel count, if you "desire" to record subject detail in the final image at a resolution of 5 lp/mm, you'll be limited by pixel count to the following print dimensions:

Max. 5 lp/mm Width: 4288 pixels / 360 dpi = 11.91 inches
Max. 5 lp/mm Height: 2848 pixels / 360 dpi = 7.91 inches

Assuming you plan to make 7.91 x 11.91-inch prints (where the Pixel Count will support a desired print resolution of 5 lp/mm), your enlargement factor (without cropping) would be 11.91 inches / 23.7mm = 302.54 mm / 23.7mm = 12.8x

Now let's run the two equations for an anticipated viewing distance of 25cm (9.84-inches):

FOR CONTROLLING DEFOCUS:

CoC (mm) = 1 / desired final-image resolution (lp/mm) for a 25 cm viewing distance / enlargement factor

CoC (mm) = 1 / 5 / 12.8 = 0.0156 mm

Your largest CoCs, which will occur at the near and far limits of DoF, must not exceed 0.0156 mm at the sensor, before enlargement. Plug this CoC diameter into your choice of DoF calculators (www.dofmaster.com, for example), then follow the calculator's recommendations for setting both your working aperture and your focus distance.

FOR CONTROLLING DIFFRACTION:

N = 1 / desired print resolution (lp/mm) / anticipated enlargement factor / 0.00135383

N = 1 / 5 / 12.8 / 0.00135383 = 11.54 (or f/11.54)

Thus, even when your DoF requirements indicate that you should use an f-Number larger than f/11.54, doing so would cause diffraction to inhibit your desired goal of 5 lp/mm in prints that require an enlargement factor of 12.8x.

(Continued below...)

 

[Mike Davis]

(...Continued from above)

Unfortunately, the loss of subject detail caused by diffraction impacts the entire image uniformly, where loss of subject detail caused by defocus occurs only at the near and far sharps of DoF. Thus, it's more difficult to discern a loss of resolution that impacts the entire print vs. a loss of resolution that occurs only at the nearest or farthest distances in the subject space, unless you make a side-by-side comparison of two prints.

Any print can be made "sharp" by enhancing its acutance (edge sharpness), through the use of tools like unsharp mask in PS, but rendering actual subject detail at resolutions that cause people to gasp in appreciation, requires that you control both defocus and diffraction, to ensure that both the circles of confusion caused by defocus -and- diffraction's Airy disks are kept small enough, after enlargement, to satisfy your desired print resolution (which should have been chosen with consideration for an anticipated viewing distance).

For a better understanding of the difference between acutance and resolution, see Sean McHugh's tutorial on "sharpness" at http://www.cambridgeincolour.com/tutorials/sharpness.htm

Remember that if the DoF calculator calls for an f-Number that you can't use (because the corresponding shutter speed would be too slow, or because that f-Number is larger than the f-Number at which diffraction will become visible, or because your lens simply doesn't offer that f-Number), all you have to do is....

Back away from the nearest subject until you're far enough away to use a viable f-Number (you'll have to recalculate DoF for this new range of subject distances).

-OR-

Go to a shorter focal length without moving the camera (you'll have to recalculate DoF for this new focal length).

-OR-

Resign yourself to making a smaller print: Reducing the enlargement factor by 1.414x (making a 10-inch print instead of a 14-inch print) allows you to open up one stop and get the same apparent resolution. Reducing the enlargement factor by 2x (making a 7-inch print instead of a 14-inch print) allows you to open up two stops and get the same apparent resolution).

-OR-

Resign yourself to hanging the print in a location (over a piano?) where people can't examine it at your originally anticipated viewing distance: Increasing the viewing distance by a factor of 1.414x allows you to open up one stop. Increasing the viewing distance by a factor of 2x allows you to open up two stops. (This last solution is difficult to enforce, so it's not practical.)

Sometimes, in the interest of maintaining your intended composition (as defined by your original choice of camera position and focal length), it's best to just go for a smaller print (an enlargement factor lower than that specified when you calculated the Circle of Confusion diameter used to produce your DoF calculators) rather than backing away from the nearest subject or selecting a shorter focal length - both of which can change the composition drastically. But you've got to remember that you made this choice, in the field, to go with a smaller print. Don't cop out later and produce the full sized print only to suffer visible degradation caused by defocus and/or diffraction. Stay the course.

Lastly, please realize that there are many factors other than available pixel count, defocus and diffraction that can prevent you from achieving a "desired" final image resolution. What I've written here only describes an approach to controlling defocus and diffraction, with no attention given to lens resolving power at various apertures, the smearing caused by subject or camera motion at inadequate shutter speeds, film resolution and in-camera flatness (for those who are still shooting film), etc. Defocus and diffraction are, however, among the most controllable of factors affecting final image resolution, if you're willing to exercise that control instead of just rolling the dice every time you make an exposure.

For more on how I approach landscape photography (only one of many ways to boil the proverbial egg), read these five posts (or the entire thread) from the Canon Digital Photography Forum:

http://photography-on-the.net/forum/showpost.php?p=2756604&postcount=25

http://photography-on-the.net/forum/showpost.php?p=2780643&postcount=39

http://photography-on-the.net/forum/showpost.php?p=2784205&postcount=45

http://photography-on-the.net/forum/showpost.php?p=2786373&postcount=54

http://photography-on-the.net/forum/showpost.php?p=2788383&postcount=57

The complete thread starts here:

http://photography-on-the.net/forum/showthread.php?t=279951

--
Mike Davis
http://www.AccessZ.com

 

[Mark S Abeln]

…then focus at infinity, and stop down the lens to get the near stuff in acceptable focus.

I’ve often noticed blurry infinities, and found that quite annoying, even when I was using hyper focal distance. Using infinity focus has helped quite a bit when I found out about that technique.

Regarding diffraction, you can reduce its effect by using Unsharp mask in post processing. Newer software can reduce diffraction greatly, but as of yet this is computer-science grade software as far as I know. I’ve never found diffraction to be much of a problem, at least with my current equipment.

--
http://therefractedlight.blogspot.com

 

[TapioH]

I was under the impression that if a lens was set to its HF distance that the sharpness of objects at infinity would the same as if I'd focused at infinity.

As others have said, there's only the focus plane that is sharp and you must decide what is the critical place to put it.

Of course, focus stacking and tilt-lenses are alternatives for better results:

http://en.wikipedia.org/wiki/Tilt-shift_photography#Tilt

--
Tapio

 

[sacundim]

I was under the impression that if a lens was set to its HF distance that the sharpness of objects at infinity would the same as if I'd focused at infinity.

And therein lies the your false assumption. Anything not on the plane of focus will not be optimally sharp. Period.

In the context of depth of field, "acceptable sharpness" doesn't mean "just as sharp as the plane of focus"; it means circles of confusion less than a somewhat arbitrary standard, usually derived from human visual acuity by assuming a display size, a viewing distance, and doing the math roughly. Change those parameters and the DoF calculations need to change.

 

[jrtrent]

If you need infinity sharp... then focus at infinity, and stop down the lens to get the near stuff in acceptable focus.

I've often noticed blurry infinities, and found that quite annoying, even when I was using hyper focal distance. Using infinity focus has helped quite a bit when I found out about that technique.

I've found Harold Merklinger's articles and books helpful. Here are links to a couple in case you or the OP hasn't seen them yet:

http://www.trenholm.org/hmmerk/DOFR.html
http://www.trenholm.org/hmmerk/TIAOOFe.pdf

There was a discussion of this last year at the link below, with tko making some interesting counterpoints.
http://forums.dpreview.com/forums/read.asp?forum=1018&message=35185459