ExpoDev’s Depth of Field calculator was designed with the large format photographer in mind. There are many takes on DOF calculators but a lot of them are either too simplistic or make it needlessly complex for everyday use. ExpoDev’s DOF calculator strives to remain simple to use yet flexible enough for large format photographers who really want to maintain the clarity and visual integrity in their prints. Depth of Field is a complex topic though and I’m not going to go too deeply into it here. Instead, I’ll explain how ExpoDev’s DOF calculator works and how to use it to the greatest advantage. First though, I can’t escape a little background on DOF:

One key aspect of an effective DOF calculator is letting the photographer choose one very important parameter, the Circle on Confusion size (CoC). CoC values are typically expressed in millimeters. ExpoDev offers a selection of CoC values along with the suggested film format size they should be used with*.

There are a lot of opinions though on what CoC values should be used for particular film formats but it’s really a matter of personal choice and greatly depends on the enlargement factor to be used when making prints from your negatives. For instance with contact printing, a larger CoC can be used since there is no enlargement factor. With smaller film formats that will be enlarged, a smaller CoC should be used. The greater the enlargement factor, the smaller the CoC value needs to be.

Typically though, we don’t always think about enlargement factors while working in the field. To make things simpler (but not necessarily less flexible) ExpoDev makes one general assumption: that the larger the print size, the farther away the typical viewing distance will be. ExpoDev therefore assumes that the typical viewing distance will be roughly 1.5 times the diagonal measurement of the print size. This generally holds true for most viewers, whether it is a large print on a wall or a smaller print in their hands. This assumption allows ExpoDev to use one CoC value for each film size without making you think too much about enlargement factors. If however you want to increase the perceived sharpness of a print it is as simple as choosing a smaller CoC value in ExpoDev. This method retains the flexibility without making the user interface needlessly complex.

The important take-away here is that choosing the correct CoC value is what determines final perceived print sharpness and since ExpoDev gives you that control, you can make the DOF calculator work to suit your needs.

Now that we got that out of the way, we can get back to the features of ExpoDev’s Depth of Field calculator.

ExpoDev has three DOF calculators. The first one, Check mode, is the simplest and can be thought of more as a DOF checker. You simply tell ExpoDev what aperture you are using and the subject distance and it will calculate the Total DOF, the Near and Far planes, and the Hyperfocal distance for you.

The next DOF mode, Distance mode, allows you to tell ExpoDev the Near and Far focal planes and it will then tell you the Minimum Aperture to that can be used along with the Focal plane and total DOF. It also tells your one more important thing, the Optimal Aperture.

The Optimal aperture is the concept that print sharpness is really a balance between perceived sharpness and diffraction. There is both a minimal aperture to get the perceived print sharpness you want but there is also a point at which a smaller aperture starts to degrade the sharpness due to diffraction. The Optimal aperture is the aperture which balances these two effects. It is the midpoint where you get the greatest DOF with the least amount of diffraction. You can use either aperture when making your exposure to get the DOF you need, but for optimal print sharpness you sometimes might want to go with the optimal aperture vs. the minimum aperture.

The final DOF mode is called Focus Spread mode and is probably the most practical one to use as it requires no estimations or measurements of subject distances. Focus Spread mode is a way of easily focusing a view camera by measuring the distance that the focus rail has traveled between focusing on both the nearest and farthest points. If you measure and enter this distance (in mm), ExpoDev can then calculate both the minimum and optimal apertures that will make everything from the nearest point to the farthest point fall within the DOF. This makes it really easy to focus a view camera as it does not require you to stop down the lens at all until you are ready to make an exposure. You do all focusing while the lens is wide-open. The really great thing about this method is that it automatically takes into consideration any view camera movements that you are using. This makes figuring out which movements are improving the DOF as simple as measuring the changes in focus rail travel (i.e. if the distance is getting smaller, the movements are helping). You can find more information about this method here, http://www.largeformatphotography.info/how-to-focus.html in the “Procedure II” section.

Once ExpoDev has calculated an aperture to use for the desired DOF, it makes using that aperture easy by automatically settings the Exposure aperture to that value (if you are using Aperture exposure mode). There is also an app setting to tell ExpoDev which aperture to use when it sets one for you, either the minimum aperture or the optimal aperture.

Of course you can change the automatically selected aperture to use on the exposure tab at any time but if you do and that change negatively affects the calculated DOF (i.e. it decreases the DOF), ExpoDev will warn you when you are doing so.

That about covers the DOF calculator in ExpoDev for iOS. It is a significant improvement over the older Palm version of ExpoDev in many ways. It is quite powerful and flexible yet remains as easy to use as ever.

* Right now ExpoDev offers a few commonly used CoC values for different film formats. In the next version, ExpoDev will let you create your own set of CoC values to choose from much like it does for filters and lenses.

ExpoDev is very unique in that is uses highly accurate and test-produced reciprocity data for over 90 film and developer combinations. Phil Davis did the work of testing each and every film developer combination using selected exposure times up to 1,000 seconds or roughly 16 minutes. He also field-tested these results to ensure their accuracy. This is far above and much more specific than the data that film manufacturers publish for their films and developers, not to mention that ExpoDev's reciprocity data covers film and developer combinations across different manufacturers. Correction is not a one-size-fits-all curve; it is very unique to each film and developer combination.

What is meant by "selected exposure times" though. What this means is, when you tell ExpoDev that the shutter time you want to use is anywhere within the range of 1 to 1,000 seconds,  ExpoDev would then apply reciprocity failure correct and give you a corrected exposure time to use. This corrected exposure time may be a small adjustment for a selected exposure time of a few seconds or it may be several hours for a selected exposure time of 16 minutes (from 1 to 8 hours, depending on which film and developer combination you use).

There is a second part to reciprocity failure correction though and that is that the overall resulting negative density also changes with extended exposure times. Unlike exposure times negative density can change either direction, either increasing or decreasing depending on the film and developer combination used. ExpoDev's reciprocity data for these film and developer combinations includes how development time should be adjusted as well. This reciprocity data is used to apply an adjustment to the Average Gradient that is calculated as part of the exposure. It is this Average G value that is used to look up the development time that should be used to produce a negative that matches the target density desired (so that printing or scanning remains consistent and easy across all of your negatives).

Each Film Profile that you import into ExpoDev contains a two-part code expressed as a letter/number combination (e.g. D4, A6, etc…), The first part is for the exposure time adjustment code and the second part is the development adjustment code. For the exposure time adjustments there are 5 codes, A-E; for development adjustments, there are 7 codes, 1-7. When you export film test data from Plotter for Windows you select which code to use for that film/developer combination (Plotter has a table for all the film and developer combinations that were tested). Since you get to choose the code to use when you export your film test profile this also means that you can override the codes if your working processes need adjustments. You can even choose to export without any codes at all which will cause ExpoDev to skip reciprocity adjustment altogether.

In the next article I'll cover ExpoDev's adjustable Depth of Field calculator.

Until then I'm happy to say that ExpoDev version 1.0 has reached the final testing stages and if all goes well, it should be available in the Apple iTunes App Store very soon.

ExpoDev is unique among analog exposure calculators in that it uses actual personal film test data to drive the exposure calculation process. This probably makes ExpoDev the most accurate B&W exposure calculator in existence as it uses your personal data that is derived from your equipment and your working processes.

Here are some details on what that means exactly and how it works.

Getting film test data into ExpoDev is now easier than ever

With previous versions of ExpoDev (for Palm and PocketPC devices), getting data onto a device was cumbersome at best. ExpoDev had to rely on the software tools supplied with each of these devices to sync the data. For Palm devices, that software won’t even run on modern operating systems like Windows 7 or OS X.

I’m happy to say that those issues are a thing of the past now. ExpoDev for iOS can open film test data files directly with no computer intervention. That makes this new version of ExpoDev truly platform independent. Since all Apple iOS devices have both Internet and email connectivity, you can simply email the film export files as attachments and open them directly on any iOS device. ExpoDev would then import them directly into your library of films.

That’s not the only way though. If you already sync your iOS device with Apple’s iTunes (available for both PCs and Macs), you can also import and export data files for ExpoDev directly using iTunes. Down the road, there will be other options like an online film test database to allow you to retrieve your stored film tests. This last option will be especially useful to people who have used the View Camera Store’s film testing service in the past as it will allow them to retrieve their test results at any time.

How do you get personalized film test data?

ExpoDev film test data is produced by a companion software program, Plotter, that analyzes your actual film tests to produce the charts that drive ExpoDev. These film tests are unique in that they very effectively test the film and developer combinations you actually use. This process will yield very personalized film test data that is tied to your personal working methods and equipment. Once Plotter analyzes that test data, it can then export it to an Export Film data or .XDF file that can then be used by ExpoDev. You can get your own personalized film test data files in one of two ways:

  1. By owning Plotter for Windows and performing your own film test and analyses
  2. By using the film testing service from ViewCameraStore.com, who will help you by sending you pre-exposed test film to develop that you then send back for analysis. In return, they send you your personalized film test files.

Either method is effective, it just depends on how much you want to personally do. If you’re serious about testing materials, shoot lots of different film/developer combinations or perhaps want to target several different printing processes, you might think about owning and using Plotter yourself (available through ViewCameraStore.com). However if you just want to get better negatives than you do now, you might consider using the View Camera Store’s film testing service to quickly get up and running.

What does an Export Film data file contain and how is it used?

Film analysis produces two main charts that are used by ExpoDev when calculating exposures, the Average G (Average Gradient) to EFS (Effective Film Speed) lookup chart and the Average G to Development Time lookup chart. For each scene that you meter, ExpoDev will calculate the SBR of the scene (Subject Brightness Range) and the corresponding Average G. It then uses the film test charts for your selected film and developer combination to lookup the EFS and the Development Time to use when calculating the exposure. This process directly ties the film test, your equipment, the scene, and the resulting exposure together in a closed-loop which makes for very accurate exposure calculations. In actuality, there are many other things that are also taken into account such as the Paper ES (that printing paper’s Exposure Scale, which controls the target density of your negatives), Lens Flare, Filter Factors, and film test reciprocity data (which will be covered in another article as it is its own topic).

In the meantime if you want to do a “deep dive” on understanding and reading a film test as produced by Plotter, there is an article on BTZS.org written by Phil Davis that gets into many of the details. That article is located here: How to read a film test – by Phil Davis

If this is the first article on ExpoDev you’ve seen, be sure to check out the others on this site (and check back again for future articles).

Next time I’ll cover how ExpoDev uses the reciprocity test data as produced by Phil Davis to adjust both exposure and film development for over 90 different film and developer combinations.

Today I’m going to start a series of short articles on ExpoDev for iOS. My goal is to talk about some of the unique features that set ExpoDev apart for other analog photography apps on the market. I’ll try to get something new out each week.

ExpoDev has many unique features that you won’t find anywhere else, such as calculating exposures based on personal film test data, the only exposure calculator which applies reciprocity adjustments that are based on actual lab-produced film tests for a multitude of film and developer combinations, the ability to calculate exposure and development to produce negatives which will closely mesh with different printing processes whether that be silver, platinum, digital hybrid/scanning, or any other alternative process. This is just the tip of the iceberg and I hope to talk about them all in upcoming articles.

But first an update: Beta 2 of ExpoDev for iOS is nearing release in the next few days. This release is about 95% functionally complete with the only thing left to complete being ExpoDev’s Depth of Field calculator. At it stands now, it is completely usable in the real-world to calculate and catalog your film exposures and development information. The exposure calculation system within ExpoDev have been extensively field test on other platforms for almost 10 years now (I’m referring to ExpoDev for PocketPC which has a smaller, private release). It is also a significant upgrade to the exposure calculation system used in ExpoDev for Palm, used by many of photographers today. I’ve personally produced hundreds of 4x5 B&W negatives using ExpoDev’s exposure calculator and I have never had a bad exposure (at least not due to ExpoDev – photographer error is another story).

After I get Beta 2 out the door I will write a few articles on ExpoDev’s unique system of exposure calculations and reciprocity adjustments, which is probably the most accurate reciprocity adjustment calculator in existence. Until then, here are some previous articles to catch up on things.

Previous ExpoDev for iOS articles:

I’m happy to announce that ExpoDev for iOS (for the iPhone and iPod Touch devices) was recently released into Beta testing. Things have really progressed since the initial previews were posted here. ExpoDev is now functional enough to use in fields tests with everything working with the exception the Depth of Field calculations (which will be working by the next Beta version).

What makes ExpoDev so special (and different) from other exposure calculators is that ExpoDev uses your personal film testing data to determine the proper exposure and development time for each and every scene, matching the scene contrast to a target film contrast. Since this personal film test data is keyed to your personal working methods, material, and processes, what this means in a practically sense is that B&W negatives produced using ExpoDev will be far more consistent and far easier to print (or digitally scan).

ExpoDev for iOS also makes it even easier to get your personal film test data imported than previous versions. There are two ways to import film test data into ExpoDev, via email or via iTunes. For those of you that use the View Camera Store’s BTZS film testing service this is as simple as receiving the email with your test results and then opening the film test files on an iPhone or iPod Touch that has ExpoDev installed. ExpoDev will then offer to import the film test data into your library of films to use.

For those of you that produce your own film tests using Plotter, you can connect you iPhone or iPod to iTunes and simply drag over the exported film test files to import them directly into ExpoDev. Details on that process are here on Apple’s website.

In addition to making it even easier to manage your film library, ExpoDev for iOS also manages your list of lenses and filters (along with their factors) for easy selection in the field. It’s new Depth of Field calculator offers several different methods of working and gives you complete control over image sharpness by letting you choose the Circle of Confusion size for your film format and printing preferences.

I’ll cover all of this and more in future blog posts. In the meantime keep checking back here often as well as over at the new ExpoDev for iOS forum to join in the discussion.

Flux and Mutability

The mutable notebook of David Jade