Focus On…. Sharpness

1. Focus On….Sharpness Stan Prevost Huntsville Photographic Society http://photo.sprevost.net/Downloads/HPS/Sharpness.pdf stan@sprevost.net

2. What Do We Mean By “Sharpness”? Let’s not worry too much about a precise definition. Everyone has a reasonably good intuitive understanding of what sharpness means. We use expressions like “razor sharp”, “tack sharp”, etc. The tack or razor analogy points to the fineness of definition of edges as the central concept. There are other related technical terms, including resolution, acutance, modulation transfer function, high frequency detail, and so on. We will invoke these only when necessary. Maybe sharpness is like good music. Maybe you don’t know how to define it, but you know it when you see/hear it!

3. Where In The Photographic Process Is Sharpness Influenced? • Capture Phase • Camera/Lens Settings, Handling, etc. • Lens type and quality (incl. tilt/shift lenses) • Subject • Environment (atmospheric conditions, vibration, wind…..) • Processing Phase (Photoshop, etc.) • Downsampling • Noise Reduction • Sharpening • Focus Stacking (multifocus imaging) • Presentation Phase • Monitor • Projection • Photographic Print • Ink Jet Print • Press Print

4. Factors In The Capture Phase Influencing Sharpness • Lens quality • Camera Motion • Subject Motion • Focus Accuracy • Depth of Field • Diffraction • Camera Settings (JPEG) • Noise Focus and Motion effects usually dominate by far over lens quality issues. Focus your efforts on controlling those before spending money on lenses.

5. Lens Quality Effects • Spherical aberrations • Chromatic aberrations • Coma • Curvature of Field • Distortions Most lens aberrations are reduced as the aperture is stopped down, and no longer dominate as the lens-related issues that limit sharpness when the lens is stopped down two or three stops from maximum aperture. As the lens is stopped down, lens aberrations are reduced in effect and diffraction increases in effect (but diffraction is not a lens quality issue). • What can you do? • Get the best lenses you can afford. • Stop them down when you can. • Use lens correction in Adobe Camera Raw if you shoot raw.

6. Subject Motion Children and animals are rarely still. Flowers and leaves blow in the wind. People and cars on the streets move. Sports participants move vigorously. • Aside from some specialty techniques, capturing a sharp image of a • moving subject requires either • Fast shutter speed • Electronic flash • To use a sufficiently fast shutter speed, you may have to use a large • lens aperture (small f-number) and/or high ISO setting. • One way is to set the camera autoexposure mode to Tv (shutter priority), • manually set the shutter speed to as high a value as necessary, increase the ISO as necessary to achieve an acceptable f-stop setting.

7. What’s Wrong With This Photo?

8. Camera Motion Revealed!

9. Camera Motion • Just as with subject motion, camera motion effects can be reduced by using a fast shutter speed and/or electronic flash. • Stabilization features of the camera or lens are also very beneficial. Image stabilization is a significant advance, providing a 2-3 stop advantage. • When possible, use a sturdy tripod, even in bright conditions. • If a tripod is not available or conditions prevent its use, other techniques can be used. • Brace the camera or your body against a rigid object (even when using a monopod). • Use a monopod. • Make a tripod with your two feet and the monopod. • Use a shoulder against a fixed object plus your two feet arranged as a tripod, aided by the monopod. • Keep your arms against your body. • Use a bean-bag-type support. • When handholding: • Control your breathing. • Use a practiced shutter button press. • Use a shutter speed at least two stops faster than the old rule-of-thumb 1/focal length. With image stabilization, the rule-of-thumb is OK. For the rule, use 35mm equivalent focal length.

10. Using a Tripod • Use as sturdy a tripod as you can. • Sturdy = big & heavy, but do what you can. • Avoid extending the center column when you can, or avoid having a center column. • Wiggle the feet down into a stable position if in soft dirt, sand, gravel, etc. • Hang a weight from the center column to stabilize against strong winds. • If using a DSLR, use remote shutter release, mirror lockup, and 2-second timer. • If using a compact digicam, use the timer. • Consider using a custom settings feature of the camera, if available, to configure the camera for tripod use. • Example: ISO 100, Aperture Priority, Single Shot, Mirror Lock Up, 2-sec Timer • Lens and camera manufacturers usually recommend disabling image stabilization when using a tripod. It may still be advisable to use it in certain conditions.

11. Tripod Demo

12. Focus • There are two parts to the focus issue: • Where to focus • How much should be in acceptable focus and how to achieve that • Those two issues are not completely separable • But first, let’s look at some camera techniques for achieving precise focus on a desired point.

13. Autofocus • If you are using autofocus, you need to really learn your camera. How does it choose the point to focus on? How can you control what it does? • Cameras may focus on the nearest object, or the nearest object near a chosen focus point. It may detect faces and focus on one. • If its logic is too hard to understand and control, set it to focus at the center of the frame and to lock the focus with shutter half-press or other dedicated focus button. You may want to disable face detection, etc. if you can’t predict or control how it behaves.

14. Manual Focus • When you set the lens or camera to manual focus, you can precisely control what you focus on and how accurately you focus. This may be more difficult with some compact digicams, but, again, learn your camera. • For tripod work, manual focus is almost always best, IMO. • If your DSLR has Live View, with a zoom feature, that is absolutely the best way to achieve precision focus. Use manual focus. Frame your shot, enable Live View, zoom to 10X, focus, turn off Live View (or not, depending on the camera), and shoot. This is best done on a tripod, but can also be effective handheld. • Demo • Your compact camera may have a zoom inset during manual focus. Enable it if it is controlled by a menu option.

15. Setup for Photo Experiments Camera about 20 feet away.

16. Handheld, Manual Focus 1 2 3 Three trials,each setting. 1/60 s ISO 100 All at f/5.6in Av mode. 100mm lens 1/250 s ISO 400 No image stabilization Focusing poor,Camera stabilitypoor at 1/60, hardto judge at otherspeeds due to badfocus. 1/1000 s ISO 1600

17. Handheld, Automatic Focus 1 2 3 Three trials,each setting. 1/60 s ISO 100 All at f/5.6in Av mode. 100mm lens 1/250 s ISO 400 No image stabilization Focusing good,Camera stabilitypoor at 1/60, good at otherspeeds. 1/1000 s ISO 1600

18. Smearing of Detail Not Always Obvious In Large View This is the full frame of the top right photo in the previous slide.

19. Image Stabilization - DSLR Trial 1 2 3 Handheld, autofocus. ISO 100 f/5.6 1/30 sec f = 100mm Stabilization Off Stabilization On

20. Image Stabilization – Compact Digicam ISO 100 f/4 1/100 sec f = 17mm (71mm equiv)

21. Depth of Field • Objects at only one distance from the camera can be in focus. • Objects at any other distance will be out of focus and will be blurred (not sharp) to an extent depending on the camera parameters and the distances from the point of perfect focus. • One can define quantitative criteria for how much Out-Of-Focus (OOF) blurring is acceptable. The OOF limits will be reached at certain knowable distances before and behind the focus point. The difference in distance between these limits is known as Depth of Field (DOF) in subject space (or object space). • It is sometimes inaccurately stated that all objects inside the DOF are “in focus” and objects outside the DOF are “out of focus”. Actually, everything removed from the plane of perfect focus is OOF; we just decide how much OOF we are willing to tolerate, and that defines our criteria for sharpness and DOF.

22. Where Do You Draw The Line Between In and Out of Focus? The transition between sharp focus and extreme blur is continuous and gradual.But we need some kind of rational criterion to work with.

23. DOF Criteria • There is little need to strive for sharpness that is beyond the limits of human visual acuity or beyond the capability of our equipment or media to capture. • But, in most cases, we likely will want to strive for sufficient sharpness that the important areas of our images are not perceived as unsharp, given how they will be viewed. • Sharpness criteria definitions have been approached from many directions. We will explore several of these and compare them. • The two basic approaches have been based on human vision or on capabilities of film.

24. Normal (average) human visual acuity is widely considered to be one minute of arc (1/60 degree) Snellen Chart (1862) (20/20 vision) This is the minimum feature size. Acuity can also be expressed as 30 line pairs per degree or 30 cycles per degree (cpd) Depends on lighting, subject contrast, and the individual Thus when a person is viewing an image, details which subtend an angle of less than one arcminute will not be perceived. Also, blurring on a scale less than that will also not be perceived and the image will be considered sharp. Since the size of the minimum discernable detail depends on the viewing distance, viewing distance becomes a primary factor in defining sharpness criteria. Human Visual Acuity

25. Viewing Distance • It is often said that 250 mm (9.8”) is the “normal” viewing distance for an 8X10 inch print. • Sometimes reference is made to a “comfortable viewing distance”. The closest comfortable viewing distance may also be called the “near distance for distinct vision”. • Sometimes defined in terms of distance, usually 250mm. • Sometimes defined in terms of angle, often sixty degrees (viewing distance of 11.1” or 282mm for 8x10). • It is also often said that the “normal” viewing distance for a print is equal to the diagonal dimension of the print, which for an 8X10 inch print is 12.8” or 325 mm. • Another view is that a print is properly viewed from its “center of perspective” which is the distance at which objects in the print subtend the same angle from the viewer’s eye as those objects did from the camera lens, and the field of view is the same. This distance is equal to the focal length of the taking lens multiplied by the enlargement of the print from the negative, or equivalently, from the image captured by a digital sensor. • While this may maintain the “apparent perspective” in prints or projected images, it results in awkward distances for images captured with very wide angle or very long telephoto lenses.

26. What Is A “Normal” Lens? There are a number of definitions floating around. (Reference to 135 format: 24x36mm, diagonal 43mm) • Angle of View equal to that of the human eye. • Too hard to usefully define. Total visual field is about 180 degrees horizontally and 135 degrees vertically, but the regions of sharp vision is the central 13 degrees (macula) and 3 degrees (fovea) • Angle of View = 60 degrees f = 37.5mm • Focal Length = Film/Sensor diagonal f = 43mm • Angle of View = 53 degrees • In practice, 50mm is widely considered to be the “normal” lens for 35mm format. AOV = 46.5 degrees • Center of Perspective for 8x10 (m = 8) using “normal” lens is • 300 mm (11.8”) for f = 37.5 mm • 344 mm (13.5”) for f = 43mm • 400mm (15.7”) for f = 50mm

27. Minimum Discernable Detail versus Viewing Distance (based on visual acuity of 1 arcmin) Note: 250 mm is a commonly referenced viewing distance for 8x10. The corresponding minimum detail is 73 microns (~0.003”). This is like each dot on a 300dpi print. For an enlargement ratio of 8, this corresponds to 9 microns on the film or sensor, equivalent to 56 lp/mm.

28. Film and Digital Sensor Resolution • Fuji Provia 100F slide film resolves about 42 line pairs per mm. • Kodak Professional Tri-X: 50 lp/mm • Kodak T-Max 400: 80 lp/mm • Kodak Professional Ektachrome 30-50 lp/mm (film resolution based on MTF50) (digital resolution based on 2 px/lp, not considering antialiasing filter)) • Canon 5D full-frame digital SLR 4368x2912 pixels, 61 lp/mm • Canon 1DS MkIII DSLR 5616x 3744 px, 78 lp/mm. • Canon Rebel XSi DSLR 96 lp/mm (equiv. to 60 lp/mm on ff35) • Canon A650 IS compact digicam 263 lp/mm (sensor is 4.7X smaller in long dimension). Equivalent to about 56 lp/mm on ff 35.

29. Resolution Criteria • From Carl Zeiss company: “According to international standards the degree of blur tolerable is defined as 1/1000th of the camera format diagonal, as the normally satisfactory value. With 35 mm format and its 43 mm diagonal only 1/1500th is deemed tolerable, resulting in 43 mm/1500 » 0.030 mm = 30 μm of blur.” • A formula called the “Zeiss Formula” is widely published but its origin seems to be lost. It is d/1730, where d is the image diagonal. For 35mm it gives a blur of 25 μ. (It is interesting that if the image is viewed at a distance equal to its diagonal, the Snellen criterion of one minute of arc corresponds to d/1719 for one line pair.) • Some of these criteria date from pre-WWII, when films and lenses did not have the resolving ability of their modern counterparts. Some people say we should be using criteria three to seven times more demanding today. • For 35mm, the image plane blur corresponding to the limit of normal human visual acuity is 18 μm (based on 8X enlargement and 250mm viewing distance). • The most common numbers for 35mm are in the range of 25-35 μm, but you are free to choose what you want. • The resolution in lp/mm is generally taken to be the reciprocal of the blur circle in mm. • For purposes of this presentation, let’s use 50 lp/mm for image plane resolution, corresponding to 20 microns per line pair.

30. Effects of Sensor Size • Images captured by smaller sensors (such as in compact digicams) require greater magnification to final viewing size than for larger sensors. • Full-frame 35mm: 24x36mm. 1/1.7” sensor: 5.7x7.6mm. 4.2X more magnification required • Blur circles get magnified as well, so a smaller CoC should be chosen for smaller sensors. • To maintain an equivalent level of sharpness, the ratio of image height to CoC should be the same for all formats. This is a sharpness quality factor, CoCs per picture height. • (The same concept applies equally to the final product, whether printed or projected image, as long as viewing distance is also scaled by the same factor.) • Note that cropping an image reduces the image height but not the CoC, lowering the quality factor. • To find your sensor size, look in your camera instruction manual under Specifications. If it just says something like 1/1.7”, refer to the table athttp://www.dpreview.com/learn/?/Glossary/Camera_System/sensor_sizes_01.htm

31. Viewing Distance For An Enlargement • 1 arcminute visual acuity • 50 lp/mm image plane resolution This is the distance beyond which the average viewer will not be able to distinguish a “perfect” print from one made from a resolution-limitednegative or digital capture.

32. Moving On……. • Now that we have explored some limits on sharpness due to equipment and vision, and how to view our images to avoid seeing those limitations, we can move on to Depth of Field: what it is, how it is controlled, and how we set the criteria for it. • We will make use of geometrical optics and ray trace diagrams to illustrate the principles involved.

33. Review of Basic Lens Principles f Incoming parallel light rays from infinity f = focal length

34. Focusing Points at Infinite and Finite Distances Image Space Object Space ∞ f do di Thin-lens equation:

35. More Basic Imaging Principles so si do di m = magnification By similar triangles:

36. Far Object Near Object Blur Circle Points Not At The Exact Focus Distance Image As Blur Discs Rather Than Points Point of Focus Image Plane

37. Diameters of Blur Circles of Out-Of-Focus Points The farther a point is from the point of perfect focus, the larger the blur circle it makes on the image plane. Focus Point Image Plane

38. Stop Stopping down the lens reduces the size of the blur circle of an Out-Of-Focus Point OOF Point Point of perfectfocus Image Plane

39. Depth Of Field • One makes a choice of what constitutes acceptable focus (sharpness). This ultimately comes down a choice of maximum allowable size of blur circles (Circles of Confusion) on the final viewed image. We usually then relate that to the size of the CoC on the image plane. • Given that the camera is focused at a certain distance, the blur circles from points closer and farther will grow as they are farther removed from the perfect focus distance. The distance between the nearest point at which the blur circles are equal to the chosen maximum CoC and the corresponding most distant point is call the Depth Of Field. • Within the DOF region, not all objects are in perfect focus, they are just deemed to be in acceptable focus.

40. Effects of Sensor Size on DOF • Consider two cameras, one with a large sensor and one with a small sensor. • Each camera takes a picture of the same subject, at the same distance (same perspective) and with the same framing (Field Of View). • The smaller sensor will require a shorter focal length, scaled by the sensor size ratio, to match the framing (FOV) of the larger sensor. • To maintain the sharpness quality factor in CoCs per image height, the CoC for the smaller sensor must be reduced by the same factor as the sensor size ratio. • Result: For the same f-number on each lens, the smaller sensor will exhibit greater depth of field. • Then scaling the f-number by the sensor size ratio will result in the same DOF as for the large sensor. (Note: the actual shooting aperture diameter will be the same for both lenses.) If the sensor size ratio is 4, then there will be four stops difference in f-number (e.g., f/16 for large sensor, f/4 for small sensor). This can make it difficult for a small-sensor camera to achieve the same shallow DOF of the larger-sensor camera. • Summary: Scale focal length, f-number, and CoC by the sensor size ratio. Using the same perspective and framing, the images will be identical, including DOF and diffraction (relative to each CoC).

41. DOF The Plane of Focus Can Be Tilted To Allow The DOF To Cover The Subject Plane Of Focus DOF

42. DOF Calculations and Charts • DOF formulae are in abundance on the internet (ask Dr. Google). Or make your own equations, the math is straightforward. With these you can make your own DOF calculator and chart printer. • There are many ready-made DOF calculators and chart printers as well. • I like the Bob Atkins calculators http://bobatkins.com/photography/technical/bokeh_background_blur.html or http://www.bobatkins.com/photography/technical/depth_of_field_calc.html • For graphs I like the Nicholas Sushkin programhttp://www.dof.pcraft.com/dof.cgi • http://toothwalker.org/optics/vwdof.html by Paul van Walree • An online calculator that will print tables ishttp://www.dofmaster.com/doftable.html • Jonathan Sachs’ calculator: http://www.dl-c.com/dof.zip . Gives DOF in terms of “lines/mm “, which I think is actually line pairs per mm, and is equal to the reciprocal of the CoC.

43. DOF Table Example From dofmaster.com - some rows deleted to fit table on slide

44. DOF Plot Example

45. DOF Calculator for Field Use http://www.dofmaster.com/custom.html

46. ∞ HFD/2 HFD Image Plane Hyperfocal Distance • For a lens focused at infinity, the Depth of Field extends from infinity inward to a finite distance called the hyperfocal distance. This distance depends on the focal length, f-number and the chosen CoC, • If exact focus is set at the hyperfocal distance, the DOF extends from half the hyperfocal distance to infinity, and everything from half the hyperfocal distance to infinity will be in acceptable focus, based on the choice of CoC. • The formula for hyperfocal distance (HFD) is: HFD = f2/Nc + f or HFD = f(D+c)/c , where f = focal length, D = lens aperture diameter, N = f-number = f/D, and c = CoC. For almost all cases these approximate forms can be used: HFD = f2/Nc or HFD = fD/c.

47. Using Hyperfocal Distance • The hyperfocal technique is normally used when it is desired for the depth of field (based on a chosen CoC) to extend to infinity. • By consulting hyperfocal distance charts, an f-stop and focus distance is selected so that the near edge of the DOF encompasses the desired subject material, considering the effects of diffraction. • The sharpest part of the image will be at the hyperfocal distance, growing progressively less sharp as distance from the hyperfocal distance increases. • Observe, however, that perceived sharpness of a subject will depend on the size of the blur circle relative to the size of the subject, and subject material at great distance has details that are smaller in the image, while the blur circle grows with distance. Thus distant subjects will appear to be much less sharp than near subjects. • Another characteristic of the hyperfocal distance is that it is the near edge of the DOF when the lens is focused at infinity. Some authors define hyperfocal distance this way. Focusing at infinity will improve perceived sharpness of distant subject material.

48. Hyperfocal Distance(cont’d) • Here is still another way to think of hyperfocal distance: With the lens focused at infinity, it is is the distance at which the image magnification is such that the CoC on the image plane corresponds to an object having diameter equal to the lens aperture. • And a way to remember the formula: Divide the lens aperture diameter by the CoC. That is how many focal lengths are in the hyperfocal distance. • A hyperfocal distance plotter can be found at http://dofmaster.com/charts.html

49. Seeing the DOF Criteria To talk about DOF or hyperfocal distance, you have to specify a CoC. So you have to know what you are willing to deem as being in acceptably sharp focus. We have discussed some more-or-less objective criteria, but the proof is in the seeing. This scene was set up based on 100mm focal length, ten foot focus distance (the center target), f/8, and two CoC values: 15µ and 35µ. The two targets on the left side of the group were set up based on 15µ and distances determined from DOF tables. The two on the right are based on 35µ.

50. Seeing the DOF Criteria (cont’d) The shot was resized to 8X enlargement (8x12) at 300dpi and cropped to 4X6 inches. It was also resized to 16X (16x24) enlargement at 300dpi and cropped to 4X6 inches. The images on this slide are inadequate for critical examination but are included for reference. Costco prints are available for inspection at the HPS meeting.