OptimumCS-Pro User Guide

George Douvos

OptimumCS-Pro calculates the settings that minimise the combined effects of defocus blur and diffraction, for the sharpest images the laws of physics will allow. This is functionality, based on solid optical science, available nowhere else. The difference between using the optimum aperture and using the aperture derived from a standard depth of field scale or calculator is not a trivial one. Using the latter, you can, in some situations, lose half or more of your image resolution for near and far objects.


1.   Instructions in Brief

2.   Notes

       Setting Focal Length

       Measuring Distances

       The Aperture Scale in Cinematography

3.   Settings

       Customising the Distance Scale


       Object Space or Image Space?

4.   Other Factors Affecting Sharpness

5.   Optimum Settings for View Cameras

6.   Optimum Settings for Tilt-Shift Lenses

7.   Techniques and Alternatives

8.   Optical Science

9.   Contact

1. Instructions in Brief

Inputs couldn’t be simpler:

•   Set the focal length of your lens.

•   Set the distances to the nearest and furthest objects that you want to appear sharp in your image.

The black markers indicate the distance on which to focus and the aperture to use in order to obtain the sharpest possible photograph. The blur spot diameter display (on the right on the main screen) shows how sharp that image will be. (For information on exactly what it is that this display shows, tap the “info” button immediately above the scale.) If you are satisfied that the image will be sharp enough to meet your needs, set your camera at the indicated focus distance and aperture, and shoot.

The indicated aperture is the optimum aperture for the shot you’ve composed. Using a smaller aperture (larger f-number) will degrade the image due to the effects of diffraction, while using a larger aperture (smaller f-number) will degrade it due to the effects of defocus blur.

Optimum camera settings are independent of camera format. This means that, in order to calculate the optimum focus distance and optimum aperture, OptimumCS-Pro does not need to know anything about your film/sensor.

2. Notes

Setting Focal Length

Set the actual focal length, as printed on the lens. Photographers should not carry out any “35mm / full-frame equivalent” conversions.

The rapid selection buttons, located above and below the focal length scroll wheel, are intended for very rapid setting of particular focal lengths. Choose what you want these particular focal lengths to be by first pressing the “f Presets” button on the Settings screen.

Measuring Distances

Some cameras provide a digital readout of focus distance in the viewfinder. If your camera does not provide this functionality, you may use your lens’s distance scale to measure distances: focus on the object and read it’s distance on the scale. Use this scale to set the focus of your lens as indicated by OptimumCS-Pro’s blue marker.

If you prefer to use an external distance measuring device, such as a laser rangefinder, that’s perfectly OK, but not if you’re utilising movements on a tilt-shift lens. For procedures in working with such lenses, see Section 6—Optimum Settings for Tilt-Shift Lenses.

The Aperture Scale in Cinematography

If you’re a cinematographer, you’ll know that f-stops determine depth of field, while T-stops are intended for exposure control. For obvious reasons, the aperture scale on this app is calibrated in f-stops. Cine lenses, however, are calibrated in T-stops, and most lenses these days do not include a secondary f-stop scale. Is this a serious problem? No. For modern prime lenses and relatively simple zoom lenses, light transmission is so high that T-stops are close enough to f-stops as to render the differences unimportant in practice, certainly for our purposes (being a wee bit off the ideal aperture setting has little effect on image sharpness).

If you’re using some insanely complex zoom lens (with its attendant transmission losses), and you definitely want to get image sharpness spot on, you’ll have to determine the relationship between f-stops and the marked T-stops for that particular lens. To do that, you’ll turn to the manufacturer’s documentation or conduct appropriate tests.  

3. Settings

Customising the Distance Scale

You may choose between 3 different distance scales via the buttons at the top left of the main screen. Each scale is available in units of meters or feet (you may toggle between units by tapping the “m” or “ft” button on the distance scale) . All scales read from 0.3 m (1 ft) to infinity. The first scale emphasises the near distances and is more useful for work with very wide angle lenses. The second scale is a general purpose scale. The third scale emphasises the far distances and is therefore more useful for work with quite long lenses.

Note that the first scale is not intended for precise macro work (nor is any other scale, for that matter). Depth of field in macro photography is hideously complex and cannot be calculated accurately for today’s asymmetrical, floating element (internal focusing) macro lenses, unless one is a lens manufacturer and thus privy to the details of the particular lens design. Do not try to use this app for macro work.


The facility to set the wavelength used in the calculation of optimum aperture is intended for the benefit of those photographers and cinematographers who shoot in the infrared or ultraviolet parts of the spectrum. For general work, leave the wavelength at the default value. If you are shooting in the infrared (or ultraviolet), select a wavelength roughly in the middle of the spectral range in which you are working.

Your spectral range will be governed by two things: The transmission range of the filter you are using and the spectral sensitivity of your film/sensor. Please consult the manufacturers’ literature for this information.

For example, you may be using an infrared filter that transmits radiation only beyond 700 nm, while your sensor (with it’s infrared-rejection filter removed) may be sensitive all the way to 1200 nm. That sensitivity, however, steadily decreases as it approaches that limit. So, peak sensitivity of your filter-sensor combination may be at, say, 900 nm. That, therefore, is the wavelength you would set.

Clearly, making changes to the wavelength setting is for users who are happy to do some research. Be aware, though, that there is no need to seek great accuracy in determining where on the spectrum the peak sensitivity of you filter-sensor combination lies. In finding optimum aperture, the effects of changing wavelength are not huge. If you set, say, 800 nm instead of 900 nm, the difference in calculated optimum aperture will be too small to be of practical concern.

Object Space or Image Space?

You have a choice of thinking and working in object space or in image space. If you choose “Object Space” you will have access to the usual selection of distance scales for setting distances to objects and reading the optimum focus distance. If you choose “Image Space” you will be presented with a “focus shift” scale. This is a very different type of scale specifically for use with view cameras. For info on how to use this scale, please see Section 5—Optimum Settings for View Cameras.

4. Other Factors Affecting Sharpness

Creating a sharp image also entails attention to:

•   the quality of the optical system

•   movement of the subject

•   vibrations in the optical system

At the sorts of apertures you’ll be using to achieve maximum sharpness over a large depth of field, the better lenses by reputable manufacturers will be quite up to the task of delivering the image resolution indicated on this app’s resolution display.

Motion blur can be overcome to some degree (if desired, of course) by shooting at a faster shutter speed. This may entail shooting with a wider aperture than the optimum aperture indicated by this app. For information on shooting at other than the optimum aperture, tap the info button above the Blur Spot Diameter display on the main screen.

With regard to vibrations, we all know what to do: Use a quality tripod or, at least, a monopod. Be careful about applying various “hand-holdability” rules, for they tend to be far too generous. Image-stabilising lenses can help.

5. Optimum Settings for View Cameras

If you’re using a view camera, you don’t need to bother with distances to objects — it’s much easier to deal with depth of field matters by working in image space rather than object space (set this option on the settings screen).

You’ll need a ruler, calibrated in mm, attached to your camera so that it can be used to measure the distance between the front and rear standards (some cameras come equipped with suitable scales).

Make sure that, on the Settings screen, you have selected the Image Space option.

Your working method:

1. Compose your shot.

2. Carry out all necessary movements (this must be done before the following steps).

3. Focus on the nearest object that needs to be sharp. Note the distance between the front and rear standards.

4. Focus on the furthest object that needs to be sharp. Again, note the distance between the front and rear standards.

5. The optimum focus distance is half way between the two distances.

6. The difference between the two distances is the focus shift. Input this into OptimumCS-Pro to instantly find the optimum aperture.

Optimum focus distance and aperture, when found this way, are independent of focal length. The focal length input function is therefore disabled.

This astonishingly simple, elegant and uniquely effective method was developed by Paul Hansma and serves as the basis for the optical science behind OptimumCS-Pro.

6. Optimum Settings for Tilt-Shift Lenses

With these lenses, you’ll be working in Object Space. Make sure that, on the Settings screen, you have set Object Space as the distance scale option.

The distance scale on a tilt-shift lens measures actual distances to objects only if no lens movements have been made. For our purposes, that’s of no concern. Here’s the procedure:

1. Compose your shot.

2. Carry out all lens movements (this must be done before the following steps). In this step, you’ll adjust the tilt so that focusing from one extreme of your desired depth of field to the other results in the smallest possible rotation of the lens’ focus collar.

3. Use the lens’ distance scale to measure the “distances” to the nearest and furthest extreme of your desired depth of field (it is important that you do not measure distances using some external device, such as a laser rangefinder).

4. Enter those values into OptimumCS-Pro, even if they seem very different from the actual distances to the objects.

5. Set the optimum focus distance, as indicated by OptimumCS-Pro, using the lens’ distance scale.

6. Set the aperture to the value indicated by OptimumCS-Pro.

Why does this procedure work? It works because it’s a simple translation into object space of the procedure described above for use with view cameras.

7. Techniques and Alternatives

OptimumCS-Pro is not a depth of field calculator—its functions are quite different. There is, of course, still a place for depth of field calculators. Read about technique and about when it is best to use OptimumCS-Pro or the TrueDoF-Pro depth of field calculator in the article Best Practice in Working With Depth of Field, available at www.georgedouvos.com.

Also, please be aware that OptimumCS-Pro is designed for single-shot photography. If you are prepared to use the focus stacking technique, whereby you take multiple shots of the same scene and stack those shots in software to obtain astonishingly sharp images over a huge distance range, you’ll need the FocusStacker app. Again, more info is available at www.georgedouvos.com.

8. Optical Science

There’s a great deal of lovely optical science behind all this. Feel free to ignore it—your creativity won’t suffer. If you’re interested in how the relevant quantities are calculated, though, you may check out the info available at www.georgedouvos.com. You will find that OptimumCS-Pro is based on mathematically sound and empirically tested methods, and may thus have confidence that it performs as advertised.

© 2016-2018 George Douvos

Home          Pro Tool Comparison

Quick Start


Use the scroll wheel to set the focal length of your lens.


Slide the pointers to set the distances to the nearest and furthest objects in the scene you are photographing.


Set your camera to the indicated focus distance and aperture... and shoot. You've made the sharpest possible image.


This marker shows the blur size for the objects at your distance extremes (so you know how sharp your image will be).

User Guide in Full