Saturday 24 August 2013

Assembling a focus stacking rig

Okay, you've accumulated all the equipment mentioned in the previous post. How do you put it all together and shoot a low magnification* stack?

(* low magnification here means up to 4:1)
  1. A Digital SLR (Here an Olympus E330)
  2. Ideally a wired or wireless remote shutter release.
  3. A Manual-focus, manual-aperture lens (Here an Olympus OM 50/1.8)
  4. Extension tubes or bellows (Here OM Auto bellows)
  5. Focusing rail (built into OM auto bellows)
  6. Solid mount for camera/bellows/lens assembly (A tripod or copy-stand)
  7. An adapter from the camera mount to the bellows (Here an Olympus MF1)
  8. If necessary an adapter from the bellows to the lens (not needed here)
  9. Some light: a bright window, a desk lamp or a flashgun with off camera cable
  10. To process the stacks you will need a computer and some stacking software.
First, mount the camera/bellows adapter to the camera and then mount to the bellows. Mount the lens on the front of the bellows. We're going to reverse the lens by reversing the whole front stand/lens assembly. (As at higher magnification, this standard lens will work better in this orientation).

Carefully unscrew the bellows retaining screw (circled in red in the next photo). You do not need to remove this screw - just undo until the bellows comes away from the front stand.

Unscrew and remove the end-stop screw from the lens end of the focus rail.

Now wind the lens assembly off the end of the focus rail, reverse it and push it back on a little before winding it back into place.

Tighten the bellows retaining screw to hold the bellows to the front of the (now reversed) lens, and replace the end-stop screw in the end of the focus rail. (This just stops you extending the lens off of the rail, I say 'just', this is quite important!).

You should now have something like this:

Note: From the left: Reversed front stand, reversed OM 50mm/f1.8, Bellows, Rear stand, 4/3 to OM adapter, E330 DSLR.

Now mount this whole assembly onto the tripod/copy stand. Make sure all the mounting screws and the screws that hold each end of the bellows in place are done up tightly. The whole camera/bellows/lens assembly is quite heavy, so I use pliers to tighten the screw that holds all that onto the copy stand as I really don't want that coming undone.

Note the lens aperture control switch on the bellows. When set as in the following picture, pointing down the length of the bellows, the lens will remain wide open.

When set as in the following picture, almost perpendicular to the bellows, the aperture of the (OM) lens will stop down to the setting on the aperture ring.

Notes: The aperture control switch still operates the lens aperture when reverse mounted in the way described above. If you are using a non OM lens via an adapter then this switch will have no effect, in that case you will have to rig some way to stop the lens down manually.

Set your camera up as follows:
  • Manual exposure mode. (or if your camera does not have fully manual mode then use shutter priority mode)
  • Check exposure bracketing is turned off. (in fact make sure all bracketing modes are disabled)
  • Set White-balance to flash (5600k)
  • Set ISO to lowest available setting.
  • If using flash, set manual flash power, and set fastest available sync shutter speed (If using E330 and compatible flash then 1/160th second is the fastest setting available).
  • Set Mirror-lock-up to 5 seconds.
  • Ensure you have fresh camera battery in camera (and fresh batteries in flash if using flash).

My version of this particular lens works best at around f5.6, so set that on the lens aperture control ring. For now, leave the bellows aperture switch to hold the aperture wide open, if you stop down the lens at this point then you will need to put a lot more light on the subject to be able to see what's going on...

Now you're all setup to shoot and focus stack at up to around 4:1. The next article will describe the basic process of shooting the stack and combining the slices in Zerene Stacker.


What equipment do you need to do focus stacking?

The answer to that really depends on what you already have, how much you want to spend, how much time you want to spend on eBay, and how serious you are about micro-photography.

To shoot high magnification studio stacks (say 4x and beyond) reliably, you will need the following equipment at absolute minimum.

  1. A Digital SLR
  2. Ideally a wired or wireless remote shutter release for the camera
  3. A Manual-focus, manual-aperture lens
  4. Extension tubes or bellows
  5. *Focusing rail (or bellows with built in focusing rail)
  6. Solid mount for camera/bellows/lens assembly (A good tripod, copy stand etc)
  7. If necessary an adapter from the camera mount to the bellows
  8. If necessary an adapter from the bellows to the lens
  9. Some light: a bright window, a desk lamp or a flashgun with off camera cable
  10. To process the stacks you will need a computer and some stacking software.
* A regular focusing rail such as the one built into the OM bellows will get you stacking up to around 4:1, beyond this you will not be able to reliably move the unit in small enough step sizes and you will get out of focus bands in your output. To go beyond this magnification you will need a very fine movement such as a microscope focus block or a linear stage. A future article will go into this in more detail.

To go into some more details on these elements, (1) the DSLR does not need to be anything fancy, I'm still shooting with a pair of Olympus E330s from 2007. (2) Suitable lenses will be the subject of an entire future article (or two) but a great place to start is an old manual-focus, manual-aperture 'standard' 50mm lens which will mount on your bellows/extension tubes (if necessary with the aid of an adapter). Ideally the aperture of the lens should open and close as you move the aperture ring while the lens is not mounted to a camera.

(2,3,4&7) To be honest if you have a DSLR and a decent tripod, and you can find a suitable camera adapter, a great start would be an Olympus OM Auto bellows and an Olympus OM 50/1.8 MF lens. Using this particular lens on the OM Bellows gives you aperture control (aperture open/set value) with a switch on the bellows, which is very convenient. You can also reverse the whole front of the bellows with the lens mounted to reverse the lens. The Olympus OM bellows also has a built in focusing rail that you can use for lower magnification work.

Please read my next blog post which will explain how to put this equipment together to shoot a low magnification focus stack.

Although you can happily start out with a tripod (as I did), if you get serious about stacking you will want to build a more permanent and stable rig. At this point a couple of questions arise.
  1. Vertical or Horizontal movement?
  2. Subject or Camera movement?
Personally I almost exclusively move the subject rather than move the camera. This is just because the subject is a tiny insect weighing a fraction of a gram. The camera is a camera, bellows, lens, flash power unit and some cables and weighs probably a kilogram. It just seems obvious to me that moving the subject is easier.

Horizontal vs Vertical is not such an easy decision. I mostly shoot vertical but I do have a horizontal rig available as well, although I do have to move my lighting equipment from one rig to the other.

So a couple of future articles will go into building a horizontal and a vertical rig, both featuring subject movement. (Although the horizontal rig would be quite easy to convert to camera movement).

Why focus stack?

Why Focus Stack, why not simply stop down the aperture? 

You soon realise this is not an option when you start to shoot at extreme magnification - at 10:1 and more, even stopping down to F22 won't give you enough DOF for anything beyond a perfectly flat subject - three dimensional subjects like insect faces are way too deep for one shot to get everything in focus. Also microscope objectives used to shoot at this level of magnification typically do not even have aperture control, they are fixed 'wide open' (and at apertures of around f1.5 or less you can imagine the depth of field of each shot is wafer thin)

The main reason though is that at real macro magnification (Anything much beyond 1:1) stopping down a lens will soon start to cause diffraction. The more magnification, the sooner this starts to degrade the resolution of your photographs.

Here's an example, even resized down for web, the diffraction blur is obvious in this shot of a tortoise beetle, shot at F11 and around 3-4:1 magnification. I'm not talking about the lower part of the subject where the rear of the beetle is going out of focus, that is simply demonstrating that F11 does not give enough DOF to get the whole subject in focus, I'm talking about the general resolution of the "in focus" section of the image.

If the loss of resolution isn't obvious in that shot alone, compare the detail to the following stack of the same bug (admittedly the other side of it), a composite of 47 images shot at f4.0

The reason diffraction starts to become a problem is that the aperture you set on the lens actually appears to become smaller from the camera's point of view as you add extension between the camera and the lens. This is known as the effective aperture.
The formula to calculate the effective aperture is:

Effective Aperture = Marked Aperture x (Focal Length + Total Extension) / Focal Length

As an example, with a 20mm bellows lens and 200mm of bellows extension for 10:1 magnification, the lens is set to f8.0 then:

Effective Aperture = 8 x (20 + 200) / 20 = f88

An effective aperture of f88 is very small, and is enough to cause diffraction on any modern camera (any camera with smaller than medium format sensor).

If the aperture on this lens can open up to f2.0 then the numbers are quite different:-

Effective Aperture = 2 x (20 + 200) / 20 = f22.

An aperture of f22 is still just into diffraction territory on smaller sensor DSLRs like my Olympus, and APS sized sensors, but only to a small degree. This lens would be quite usable for stacking on a DSLR at a set aperture f2.0, but set to f8.0 will produce a seriously degraded image. (At this extension/magnification).

(* This is a slight re-write of an old article, I am currently reorganizing my articles into a simple series of blog posts)

Tuesday 9 July 2013

Impossible Photography

Welcome to my new blog. I am Laurie Knight and I am a keen focus-stacking micro photographer.

What is "Focus Stacking" and why would you be interested in it? And what's with the name, "Impossible photography"?

Focus Stacking is a technique used to gain extended depth of field (most commonly used in the field of macro/micro photography).

The technique involves shooting multiple exposures of the subject changing the focus distance slightly between each shot. This gives you a number of photographs each containing a 'slice' of the subject in focus, which when all combined together using dedicated software, produces a single photograph with more depth of field than would otherwise be possible. E.g.

This shot is of part of the eye of a weevil (Polydrosus species) shot at 40x magnification onto the sensor. This is not a giant insect, the entire weevil is approximately 5-6mm long. The field of view of this shot is roughly 0.45mm on the long side. This is a stack of 44 input images at 4 micron focus steps.

Until about a decade ago, the computing power to do this image combination (in a sensible timescale) was not available to the amateur photographer (or even really professional photographers, who don't normally own supercomputers!). Nor was the image combining software available.

Now things are different, even a basic modern computer is easily capable of focus stacking. And a number of software packages are available, most with a free trial, one is completely free. (I will go into more detail about the various software packages in a future blog post).

So to take the photograph above would have been completely impossible a decade ago and is now possible. Hence the blog name.

Well I hope I've piqued your interest! Please check back for more articles on focus stacking soon.