Trying to explain focus stacking

This post references my last article on stacking.

When you take a photograph, whether you make the decision or the camera, there are essentially only three settings involved: aperture, shutter speed, sensor sensitivity.

Aperture:

how wide an aperture you use is denoted by its F stop number. Perhaps counterintuitively a small number means a big aperture (hole for the light to come through and hit the sensor). A bigger number means a smaller aperture. The important aspect is that big apertures create shallow depth of field (DoF) = how much of the image is in focus in front of and behind the chosen focus point. Small apertures give increasing DoF as the aperture narrows.

Shutter speed:

How long the aperture remains open to allow the light to hit the sensor.

Sensor sensitivity:

is like the old ISO (or ASA) of film. Film had grain and a low ISO number meant fine grain. Sensors have digital noise and this is a very unscientific equivalent of grain.

In focus stacking the critical aspect is DoF. When I shot the flowers I wanted only the flowers in focus, not all the distractions around them. To get them in focus from front to back needed a small aperture. Probably something like F16 or F22. The problem then was that all the distractions are also sharply in focus. So how did I achieve the end result?

I selected a large aperture (F5) that gave me a shallow DoF. So when I took my picture only part of the flower could be in focus. However providing the flowers did not move I could and indeed did take a series of photographs with each frame focussed on a different part of the flower. Usually I try to start with the point furthest away and work forwards but it does not matter. In this case I took 15 frames. Each one looks odd in isolation because only a thin segment of the subject is sharp.

The next step is to import or transfer the 15 files into a software programme that will select the sharpest part of each frame. The 15 sharp portions are blended so that they form a single in-focus subject whilst leaving everything else out of focus. I used a programme called Helicon Focus. Photoshop will do it. In some cases PS is better but Helicon Focus is very, very fast compared to PS. I then saved the blended frame into Adobe Lightroom.

Shutter speed has little relevance here. I want as little grain or noise as possible so I set the ISO to as low a number as possible. I used ISO 200.  The shutter speed in effect is the balancing item to give me the correct exposure. If the breeze had been blowing the flowers I might have tried a higher shutter speed and allowed the ISO to be the balancing item. More likely I would have given up.

There is no formula for how many frames you need. In a studio you might shoot 100. With something that itself has a shallow plane you might use 3 or 4. For a landscape you could use a high F number – maybe 11 or even 16 and just shoot 3 to make sure the frame is sharp from front to back. It is test and learn.

Finally I used PS to remove two tiny blemishes on one of the petals and added a vignette to help focus the viewer’s eye on the flowers. I did some additional burning (darkening) of any remaining lighter areas. The whole processing time was less than five minutes.

There are people out there who are far more expert than I and if they wish to add or correct my explanation please leave a comment.

ADDENDUM:

Here is an example of a single frame – I’m not sure whether the crop is identical – it is for illustration purposes only. You can easily see that only one leaf – and everything that falls within the same plane of focus – is sharp. The software will identify that and the sharpest areas of the other 14 frames to generate the final image.

Back to the stack

A while ago now I posted some articles on my learning curve for macro photography. One such post was this one .

It dealt with the concept of stacking, using multiple images each with restricted depth of field around a different plane of focus and combining them to create one shot with sharpness only on the key subject. The rest of the image is rendered pleasantly (hopefully) out of focus. It is quite common in studio work, less so in the field as even the slightest breeze can ruin the sequence. All the images have to align perfectly.

Well here is today’s effort. Shot in the field of course 🙂

This is 15 photographs blended in Helicon Focus. It can be done in Photoshop but PS takes far too long for me. I have zero patience and if I can get the job done in 30 seconds I see no reason to faff about whilst Photoshop allows me to watch paint dry.

Oxalis corymbosa

I think it turned out ok but I’d like feedback.

NB: Following feedback I have substituted a slightly cropped version to take out a distracting leaf.

Stacking the grasshopper

If you are hoping for some obscure Japanese game show then I am sorry but you have come to the wrong place. There are indeed some decidedly odd games shows in Japan but I think they are not suitable for the audience I try to reach. Stacking the grasshopper is a rather weak follow-on from yesterday’s tale of the gropper’s outing to Shatin. I was checking the plants on the balcony this morning to see if there were any nice lepidoptera larvae to photograph but all I could find was a grasshopper. I wish I could assert that it was the same adventurous explorer, who hitched a ride on the Audi but even the most basic of ID skills tells me it is not.

Be that as it may I wanted to try to get a decent shot and I decided to try out my stacking software again. So all the images were shot at F3.5, etc. etc.  You know the routine by now. Here is my final output courtesy of those awfully nice people at Helicon Focus.

Xenocatantops brachycerus

Not too bad. I didn’t get the right hand antenna despite thinking that I had. None of the source images gave me anything good enough to copy in. Other than that though it has succeeded reasonably well. Maybe 7/10 for technical expertise and about 5/10 for interest and aesthetic appeal. I am a very generous marker when assessing my own work. Did I ever mention that I was offered fried grasshoppers in Buenos Aires once? It was over ten years ago so I honestly don’t remember whether I ate them. I suspect I opted for half a hundredweight of prime beef. I have eaten snails though.

Now this is the Photoshop CS4 effort.

Look carefully at Mr. Gropper’s rear end. See the ghosting? Yup. So do I and I don’t like it. And PS has “image align”. Now in fairness my first effort with HF also had similar ghosting:

This could of course be down to bad technique on my part but as I looked very carefully through Live View I realized that the grasshopper was breathing in and out quite visibly. This is barely visible to the naked eye. Bit when magnified by 5x or 10x it looks like an earthquake. And it manifested itself in the rear section of the abdomen. Shocking…… breathing grasshopper. Or something like that.

So I redid the HF version and retouched it by using a ‘clean’ image to sharpen up both this aspect and the near antenna, which also moved slightly. The HF effort took about 3 or 4 minutes to run the original stack and then maybe 5 minutes of fiddling to do the touch up. PS CS4 on the other hand took about half an hour to process the 14 images and still failed. The recommendation is to ‘clone out any ghosting’. Well I’m sorry but I just don’t have the time to spare. Maybe CS5 or 6 has a very quick way of doing this. Conceivably CS4 has too but if so, this user isn’t sufficiently advanced to do that and I can do without spending half an evening agonizing over the final image. I’ll take the HF 5 minute job. As John Cleese asked of Michael Palin, “is this a 5 minute argument or the full half hour?”

So there we are. HF wins again on time and faithfulness. And after going through this exercise I think I shall try to sell this to a Japanese TV channel as a game show. I think it may catch on.