This post is part of a series of posts about image quality. This post is going to be looking at Stitching Images together to potentially raise the quality of an image. You can follow the Image Quality series here.
Focus stacking has become quite popular in recent years, especially since Photoshop CS4 added the auto blend feature. For some, focus stacking is going the way of HDR, meaning it is being used for the sake of being used.
Focus stacking allows a photographer to take a series of shots that have the same exposure with each image focused on a different spot, to extend its depth of field. Understanding how to use focus stacking is first to understand why it can be used.
Why Use Focus Stacking?
The first question is why do we need to use focus stacking when you can extend the depth of field by using a smaller aperture?
Apertures that are large have small numbers such as F1.4 where as a small aperture will have a large number like F22.
This is a good question and the answer may seem strange. One of the basic rules we learn when we first start getting into photography is that; the smaller the aperture the wider the Depth of Field is (area of focus).
This is correct. F22 has a deeper Depth of Field than F1.4. This does not mean that the lens is at its sharpest. Sharpness being, how defined the focused area is.
Each camera and lens combination will have its own sharpest aperture; this is usually 2-5 stops from the widest aperture, commonly F8 or F11.
Focus stacking therefore allows you to keep the sharpness of this aperture with a wider Depth of Field than the aperture allows.
As well as keeping the sharpness of medium aperture, Focus Stacking can also work when you want to use a smaller aperture to keep that bokeh that you love to get from your lens.
On other occasions Focus Stacking can let you capture a scene beyond the maximum Depth of Field of your smallest aperture for example; when you want the extreme background in focus at the same time as the foreground. Focus staking would allow you to take two shots and have one image with both areas sharp.
Situations to Use Focus Stacking.
- When you want to use the sharpest aperture and extend its depth of field.
- Macro, Still Life, Advertising, as you get closer to a subject the shallower the Depth of Field becomes, this can be widened via focus stacking.
- Low light photography, using a large aperture to have a faster shutter speed to freeze the action of a subject and then a smaller aperture to have the whole scene sharp, eliminates the need for flash.
- Landscape to bring two areas into focus that otherwise would not be possible, eg close foreground and very far background.
When shooting for Focus Stacking.
Focus Stacking is not a five min set up, you need to spend some time composing your shot and arranging your scene.
You will need a tripod to make sure that all the images are taken from the same spot and angle. I also use a cable release to stop any inadvertent movements form pressing the shutter. You also need to make sure that the light is going to be consistent while you shoot. When using natural light, this can mean waiting for a cloud to move (either to pass or cover the sun).
When focusing you can do it either manually or change the AF focus to a single point and change the point each time. The benefit of manual focusing is that you can be sure that you got everything you want sharp. I use my LCD screen and zoom in on the area rather than using the view finder.
I also use a Depth of Field calculator to work out how much Depth of Field I will have with my settings. I do this so I can work out how many pictures to take. In the IOS and Android stores there are few of these calculators you can download.
My image was of my paint brushes, with my fan headed brush to be the only brush in focus. My brushes sit on my windowsill so I used this as my location with natural light coming through the window.
Using the depth of field calculator I knew my depth of field was about 0.7 cm. The depth of field I needed was about 4cm. Mathematically speaking I would need 6 shots to create my stacked image. But since you need to overlap the areas in focus I took nine photos. Each shot focused on a different area of the brush moving from front to back. I did this several times as I wanted to make sure I had captured everything.
You can either start in Adobe Bridge or Lightroom, from here you can load your shots into layers.
1) Once your images are placed in layers, Align Layers, Edit- Auto Align Layers
2) Blend layers, Edit –Auto Blend Layers
3) Check the blending, if there are some areas that are soft that should be sharp find the layer that is sharp and then blend it in.
I do this by duplicating the layer placing it on top with a hide all layer mask and then paint in the sharp area with a soft brush with a high opacity.
4) I then collapse the layers into a smart object and edit in Lightroom, though you can now edit in Photoshop as well.
The first set of images stacked look good but there is one area on the bottom of the brush tip that is soft and I couldn’t blend in sharp. This has occurred because I focused too far or didn’t focus one of the ishots accurately enough. Though as I said, I took several batches of images just to be sure.
The second shots blended together better than the first, though was it needed? For these images I used a 50mm lens at F1.8, not the sharpest setting. The rules of Depth of field state the longer the lens and the close you get to the subject the shallower the depth of field will be.
This image is not a stacked image but has a similar composition. This time I shot 135mm lens at F8. The depth of field was shallower at 0.4 cm. With a similar composition all the brushes are in focus which, I didn’t want but the image is much sharper.
Thinking about the depth of field I recomposed the shot pushing the other brushes into the background more. This image is much sharper than the 50mm composition.
Looking at these images the 135mm portrait shot has better image quality than the 50mm photo. In calculating the Depth of Field, 50mm at F8, it would be 0.4cm, the same as the 135mm composition though I would not have got the same blur in the background. Was it worth sacrificing the quality of sharpness for the blur? In my opinion no!
Focus stacking can increase image quality but at times it is better just to swap lens and compose your shot with a sharper aperture.
Don’t be lazy
Focus Stacking is not for all situations and just because you can it is not an excuse to shoot for it all the time at really large apertures. It is a process that can help when the technical limitations of the camera need to be exceeded. Just like HDR this process can be easily over used and similarly you can easily bracket the focus of shots in most modern DSLRS. Though as with HDR you need a deft hand to make these shots work well.
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