Using Focus Stacking

While I’ve been aware of Focus Stacking for sometime, I’m late to the party in trying it out. I have been using  the Enfuse application for HDR for a short time. As I said in a recent post, I like the results of Enfuse better than Photoshop 6. I have not tested any of the other apps available for HDR work. While paging through the Enfuse manual ( now there’s a radical idea – reading the manual ) I noticed that Enfuse also does focus stacking. I decided to try it out.

Basic Scene
Basic Scene

I had been laboring under the impression that focus stacking was mostly for macro work. It finally dawned on me that focus stacking gives you the capability to stop making a compromise between depth of field and diffraction.

Here’s a quick review: Depth of Field is determined by aperture. Small aperture e.g., f/22, gives the largest depth of field while large aperture e.g., f/2.8, gives a narrow depth of field. So if you want sharp focus from front to back of an image, you would use f/22. ( See why Ansel and gang called their group f/64?) The problem is something called diffraction. With a small aperture, the light tends to get fuzzy ( a technical term – read about diffraction here ) before it hits the sensor. That results in less sharpness, not more. But diffraction goes away at larger apertures. Sadly, so does depth of field. I have tended to compromise by using f/16 – good depth of field – and my camera is just barely diffraction limited at that point.

One more thing – here’s the definition of hyperfocus distance: “The hyperfocal distance is the closest distance at which a lens can be focused while keeping objects at infinity acceptably sharp; that is, the focus distance with the maximum depth of field. When the lens is focused at this distance, all objects at distances from half of the hyperfocal distance out to infinity will be acceptably sharp.” Notice the use of the word “acceptable” here. ( More about hyperfocus distance.)

So, the idea with focus stacking, as I’m describing it here, is that I could use an aperture that is not diffraction limited and still get an image that is sharp from front to back. I tested this idea and my results are very promising.

For the test, I used my 55-200mm lens.  I tested using focal lengths of 55mm and 95mm with apertures of f/16 for the base shot and f/8 and f/5.6 for the focus stacking shots.  I used just three shots at each setting.  The red Fs are the points I used to focus.  The second from the bottom was for the 95mm setting since the building corner wasn’t in the frame.   The church steeple is 1000 ft from where I’m standing while the corner of the building is around 60ft away. The shot I used for comparison was shot at f/16 using the hyperfocus point(more or less) to focus.  Here’s how the results look:

55mm f/16 vs f/8 focus stack
55mm f/16 vs f/8 focus stack

First up, the steeples plus the background.  These are 100% crops.  I think, even with the images converted to jpegs, you can see the focus stacking wins.






But what about the foreground? Here’s the same shot but with the crop set for the foreground.  The focus stacking is better in this case also.  Let’s use an even wider aperture and see how that turns out.55mm-foregnd-f16-vs-f8fs-






Here’s the steeples at f/5.6 with the 55mm focal length.  Not much doubt about it, right?  I haven’t looked a detailed curves for the 55 to 200mm lens but I’d have to wonder if it’s not a little sharper at 5.6 than 8.







And here’s the building corner at f/5.6 compared to the f/16 shot.







So how does 95mm stack up?  (BTW, why 95mm?  Because the marking on the barrel of the lens is not exact, that’s why.)   Here’s the steeples: f/16 vs f/8 focus stack







And here’s the foreground shot:







And, again, same thing using f5/6 for the focus stack













So what’s my conclusion?  Focus Stacking is a excellent way to get sharp focus from front to back. ( I may be late to the party but I can still party down.)  I can use an aperture like f/8 and get sharper images than using f/16.  And, while I didn’t test it, way sharper than f/22.

However, you might not want to use focus stacking for every image.  Sharp front to back focus may not be what you want.  In addition, you must align the images before you use the software.  Notice that all this implies you are using a tripod but, even so, align the images.  There was a fairly high wind when I was shooting these shots on a tripod and they were definitely not aligned.  And, speaking of wind, if there’s significant movement in the frame, this technique doesn’t work.    I used Photoshop to align the images but check out Hugin if you don’t have Photoshop. I’ll test it and edit this post later.  And, if you’re not carrying your tripod, then go with the hyperfocus distance technique.  It’s not a bad compromise and it seems unlikely you can get three, or more, shots off that would work.  Also, I think using only three shots is the very minimum.  I did a close up test using seven shots and that was not enough.

Overall, this is an excellent technique to add to your skill set.