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Simple DoF tables for focus stacking Simplifies the mechanics of macro photography and explains how to make your own table.

On March 20th 2018 I updated and re-organized this story and realized there's the 'camera movement' aspect to consider. I've included my experiences with macro rails in section 6 so I hope it helps with your stacking explorations.

1. Introduction

A mobility issue means I shoot inside using a tripod on a well lit table in a spare bedroom so I have lots of time to setup my shots. My subjects are insects after they've died, flowers, weeds and inanimate objects of all kinds so I don't have to deal with climatic or movement issues.

At the start, I found many theoretical discussions but they were beyond my understanding. I just wanted a simple and reliable way to get really good images - this is for my own enjoyment. My stacking usage evolved from occasionally to always as I could stack a few or all of the images to get the desired effect - the downside is that the process takes time so there's never a 'quick' shot.

The depth-of-field for any shot Is a function of focal length, aperture, subject distance and sensor size so it does depend on the camera and lens used. The description of how easy it is to make your own table(s) is in section 2 for nearly any camera and lens combination. These techniques aren't limited to just macro shooting but rather how big the subject is and how much of it you want in focus - see section 3.2 for its use with a normal zoom lens.

1.1 Sample shots

This technique may not be for everyone but it's met my needs for over four years. The following images are examples taken using these tables so check them out to see if it can meet yours too. They've been downsized to facilitate easier downloading.

A nearly three inch piece of copper ore - 100mm macro
A two inch sea urchin shell - 100mm macro
A four inch murex shell - 100mm macro
A three-quarter inch cube of pyrite ore - 100mm macro
The inside of a side-lit, four inch loofah sponge -100mm macro.

2. Making your own DoF table

The easiest way to create one is to use a spreadsheet so it can do the math for you. A table should be created for each focal length that you use so yes it's tedious, but only has to be done once. Before starting, find the closest focusing distance for your lens and decide what aperture(s) you'll use and subject distances you'll shoot at. Each aperture needs three rows:

The DoF table for a 100mm macro lens.
  • The first shows the total DoF value in inches taken from the online calculator at http://www.dofmaster.com/dofjs.html.
  • The second multiplies that value by 25.4 to convert it to millimeters.
  • The third multiplies the second row value by two-thirds (0.66) for the step-size in millimeters to allow enough overlap for the stacking process.

After opening the calculator page:

  1. Use the camera dropdown box to find and select your brand and model which sets the sensor size. If it isn't listed, find a model with a similar sensor in your brand.
  2. Choose the subject distance measuring unit you want to use - I prefer inches.
  3. Use the lens focal length dropdown box to find and select it.
  4. Use the f-stop dropdown box to find and select it.
  5. Enter the subject distance.
  6. The Total depth-of-field is displayed on the right of the screen so use it and ignore the Near and Far limit distances. Enter it in the first row of the spreadsheet and the other two rows should fill in.
  7. Repeat steps 5 and 6 for each additional distance.
  8. If you want to include another aperture in the table, setup three more rows, go to step 4 and again repeat steps 5 and 6 for each distance.

Congratulations, you've finished making your table so save and print it. To use it, all you have to do now is:

  • Focus on the closest required element.
  • Measure the approximate distance from it to the camera's focal plane.
  • Find that distance in the table for the required step size.
  • Repeat the 'move and shoot' using the same step size every time until the required distance has been covered.

3. Equipment

I'm a Canon shooter but brand plays no part in making or using a DoF table. For any given focal length, Canon, Nikon, Sigma, Tamron or any other all have the same DoF so other factors canl determine your lens selection.

I use:

  • Canon T3i 18 MP camera with APS-C sensor 22.2 x 14.8 mm. I got it used after being highly recommended for what and how I shoot. Its vari-angle display makes using live-view easy for framing and focusing so I never use the viewfinder.
  • Canon 100mm f2.8 non-L version macro lens.
  • Sigma 18-250mm f3.5-6.3 DC zoom lens for general purpose and some close-up work.
  • This extreme lens made me a Canon shooter. The MP-E 65mm f2.8 super-macro lens (1-5X life size) isn't a normal macro lens but once you get used to its operation and capabilities, it really can open up whole new worlds. Section 5 is dedicated to it - Its table and usage are in sections 5.2 and 5.3. I got it used so it too was much more affordable.
  • Cognisys Stackshot motorized focusing rail. This is the 'icing on the cake' as it does the very small steps required for the MP-E and eliminates ALL of the frustrations involved with a manual rail.
  • Photoshop CS5 to do the stacking and any post processing as I already had it.

3.1 Canon 100mm macro

Its closest focusing distance is 12 inches where it produces a life-size image. Most things aren't that small, so I opted for 2 inch increments up to 24 inches and then 3 inches up to 48 inches - most of my shooting is in the 12-24 inch range. I did many tests when I got this lens and for me, f11 gives the best results after stacking but do your own tests and if need be, make a table for your own shooting requirements.

3.2 Sigma 18-250 zoom

Because of my studio's layout I have to be less than five feet from the subject. If it's too big to shoot with my 100mm macro, then I use this set at 35mm, 50mm or 80mm to get closer.

Its closest focusing distance is 13.8 inches where it produces a one-third life size image. I used a 4 inch increment over a distance range of 12-32 inches. I don't use it often so didn't test it as much as the other lenses and I also use it at f11.

4 A butterfly example using the 100mm and its table

Note: Butterfly's scales don't all have clearly defined edges so there will be areas that aren't 'tack sharp' - blame mother nature, not me.

  • I turned the Blue Clipper butterfly so its wings were at about 45 degrees to the camera to ensure there was a significant depth to shoot. The width of the whole butterfly is about three and a half inches
  • I setup the shot and focused on the closest element, the bottom of the right wing by the body - excuse the clamp that holds it in place.
  • I measured the distance from there to the camera's focal plane and it was about 17" which isn't in the table. At that distance, the lens provides just over a one-third life size image.
  • The table shows the step size at 16 inches is 3.52mm and 4.69mm at 18 inches, a difference of 1.17mm. I find focus isn't linear so I add one-third of the difference or 0.40mm, not one-half, to the smaller value to get 3.92 mm and use it - that's only about one-sixth of an inch.
The closest in the series showing the right wing's scales by the body.
The furthest in the series showing the antenna on the left.
After stacking the 8 shots covering nearly an inch and a half - this is much more impressive in high quality and zoomed into 100%..

Suggestions:

  • Leave some space around the subject to help the stacking program choose which areas of the images to use - some cropping of the final image will likely be required because of the small perspective changes that occur as the lens moves forward.
  • The focus stacking program rarely has a problem choosing which areas of an image to use but if it does, you'll see 'blurry areas' in the final image. This can be fixed by careful masking-in with the appropriate layer(s) in post-processing.

5. Canon MP-E super-macro (1-5X)

This lens is different enough to warrant its own section as it starts where other macros end - at life size. The Owner's Manual is available online for free at Canon's website so you can use it to answer any questions you may have.

Clarification: Canon calls the movable ring on the lens body the 'focusing ring' but it actually adjusts magnification - there's no focusing in the normal sense. At any given magnification, there's only one distance where the subject is in focus and the camera MUST be moved to it.

5.1 Sample shots

Mosquito - 5X
Leopard Lacewing - 2X
Brown Clipper - 2X

5.2 DoF table for MP-E

Its contents show just how different its characteristics are:

  • The depth-of-field values are actually printed in the owner's manual by Canon so no calculations of any kind are needed but the one-third reduction for overlap still applies.
  • The lens has a maximum aperture of f2.8 but it introduces the concept of 'effective aperture' which ranges from f5.6 to f96 depending on the magnification level and the camera's aperture setting. I don't understand the theory but the results are visible in the final image so the table has a row showing my preferred f-stop for each magnification - arrived at by observation rather than any scientific method.
  • It's length varies from 4 inches at 1X out to 9.25 inches at 5X. It uses 'working distance' which is the distance from the front of the lens to the subject. Canon prints the value at each magnification factor on the actual lens.

It is possible to use accessories like extension tubes, bellows, face-to-face lens pairs, etc. to get larger than life size but I didn't know how to make a DoF table for any of them. As a result, I chose the easy way and got this lens - my main requirement was always a simple way to take great photos.

5.3 Observations:

  • At life size, I find the 100mm macro is much more convenient as it has a focusing ring.
  • At higher magnifications, the slightest movement is apparent and even floor flexing shows if you change your standing position while waiting for a series to finish - at least in my house.
  • Between 1X and 3X, finding the start and stop positions for a series is usually easy but beyond that, it can be difficult depending on the subject's size and texture.
  • The largest series I've ever stacked is fourty-one JPG images and from start to finish, the processing took just over twenty minutes on my quite respectable laptop.
  • It's easy to get carried away when using a motorized rail at higher magnifications. To cover a quarter of an inch of depth at 3X requires 40 images, at 4X it's 53 and at 5X it's 106. Be aware or be patient.
  • I usually shoot at the marked magnifications but sometimes have to change it slightly however it's never far enough that I've had to pro-rate the step size.

6 Macro rails

After the tables, I then needed a means to move the camera by a repeatable distance in a continuous plane and discovered that they're not all the same. The unit should be:

  1. Calibrated to show distances.
  2. Precise enough to allow very small step sizes to be set.
  3. Have little or no play while, and after, it's being moved.

6.1 Manfrotto 454 - Micrometric Positioning Sliding Plate

This was my first device and it allowed me to explore stacking's possibilities to see if I'd even enjoy it without having to invest too much money.

Pro's

  • It is calibrated
  • One turn of the adjustment knob moves the camera platform 1 mm so smaller step sizes, like 1/4, 1/3, 1/2, 2/3, 3/4 mm can be set fairly easily.

Con's

  • There's a LOT of play in the rails so no matter how careful I was while moving it, there was usually small misalignments between shots. To be fair, there is a locking mechanism but using it every time becomes very cumbersome, very quickly. The stacking program deals with the misalignments but cropping of the final image will likely be required.
  • It means having to round-down step sizes to the nearest 1/4 turn(mm) which may increase the number of shots.
  • The adjustment knob had no marking on it so for instance trying to move the platform 3 1/4 turns is awkward. I filed a grove in the edge to make an index to solve that problem but even so I'd occasionally loose count and then have to start all over again.
  • Determining the end position of a series requires looking through the viewfinder after each new position is set to see if anything of interest has been included.

Summary - It's a good introduction but has too many usage issues so it spoils the creativity and explorations. After about a month, I was fed up with it and started looking for its replacement as the inconveniences actually impacted my desire to shoot.

6.2 Cognisys StackShot motorized macro rail

This is the ultimate tool and it actually made shooting fun again. I use it in 'Auto-Dist' mode so all I have to do is:

  • enter the step size from the table into the controller
  • position and set the start point
  • move the camera to position and set the stop point
  • get it going

It calculates how many shots are required to cover the distance and the control unit triggers the camera at each one. I use the standard rail which has 4 inches of travel but there's an 8 inch unit.

Pro's

  • Uses a stepper motor and everything about it is rock solid.
  • It can lift up to 10 pounds of equipment vertically.
  • Accuracy is up to 3 decimal places so even the smallest step sizes can be used. It's not calibrated but it isn't a problem as the platform's positions are set visually using the camera's live-view display and the controller knows where the platform is on the rail.
  • There are a number of delays available to allow for various shooting scenarios.
  • There is an optional battery pack available so it can be used outdoors.

Con's

  • It's more expensive than manual rails but it is SO worth the cost.
  • There's no on/off switch but it can be plugged into a power bar with a switch.

Summary - More than anything else, this tool helped my photography as it eliminates all of the mechanics so I can concentrate on what I'm shooting rather than how. It actually encourages exploration as shooting a new series is so easy.

I hope these techniques have helped so Have fun and happy exploring.
Created By
Roy Beauvais
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