depth of field. how to increase it with photoshop layers
The concept of Depth of Field can be a bit confusing at first until you get your brain around it.
Hopefully this tutorial should help you do this.
1. Depth of Field is outside, and in front of, the camera lens.
2. It describes an area from a point near the lens to a another point further away. Not side to side.
3. It is not to be confused with Depth of Focus which is a corresponding space at and around the image area inside the camera, determined by the lens.
This image of a Keyboard defines Depth of Field quite well. It was shot at f5.6
In this shot one can clearly see a narrow band of image that is visually sharp. The point of focus was on the letter D and from the letter X to the letter R we see a sharp image. In front of, and beyond this area, we see the image becoming increasingly un-sharp to the point of illegibillity.
What can we do about this? First we can try 'stopping down' the lens aperture. This next shot is at f16
Now we see a greater Depth of Field. That narrow band of image has become deeper and from the letter Z to the letter U we can say is in sharp focus but, it's still not good enough and f16 is our minimum, smallest aperture!
This is the key concept in understanding Depth of Field. A smaller aperture increases Depth of Field but in a specific way. Look at these two graphics below showing how the Camera was tilted down at the subject.
The Red line on the Camera is the image plane axis. The Green line is the lens axis. Note that these are parallel.
The Blue line is at the point of focus, the letter D. Note that this is also parallel to the Camera axis and is at the same point on both graphics.
This graphic represents the top photo shot at f5.6.
The narrow band of sharp focus (letters X to R) are shown as the Yellow line to the Magenta line and this is the Depth of Field. Apart from also being parallel we see that this depth of sharp focus extends further back, beyond the point of focus, than it does forward, in front of the point of focus. As a general rule, one-third of the Depth of Field is in front of the point of focus and two-thirds are beyond it. Grasp this essential concept and all becomes clear. It applies to all lenses all Cameras.
In this graphic, representing the shot at f16, we see a greater Depth of Field (the letter Z to the letter U) even though the point of focus remains the same however, we still see one-third of the Depth of Field in front of the point of focus and two-thirds beyond it. You can shift the whole Depth of Field by re-focussing on a point closer to, or further from, the lens but you can clearly see that we cannot get a shot showing the whole keyboard in focus as the Depth of Field with this lens, at this aperture, at this distance from the subject, is inadequate.
So what can we do about this as our client, Keyboards Inc., wants a shot with all in focus or we won't get paid. Well, Adobe Photoshop has a really neat little trick where one can superimpose images, one upon the other, by using the 'Layers' dialogue. What we need are seperate images that are identical except for the position of the Depth of Field. Because our Depth of Field is limited, even at f16, it will take four shots to perform this trick.
Here are the four shots with a selective enlargement of each.
Placing the point of focus on the Backspace Key the Depth of Field begins at a point in front of this but extends to beyond the length of the whole Keyboard.
Focussing on the letter O places the Depth of Field just beyond half way down the length of the Keyboard. Compare the sharpness of the F10 Key on both the above shots.
This is the central section from the main image above shot at f16.
And this shot was focussed on the CTRL Key placing the Depth of Field over the closest part of the subject to the Camera.
What we are going to do is create a new 'Layer' from one of the images then segregate these sections of each of the other three images and superimpose these Layers, one over the other, to create an all new montage of images that will appear as one. It's worth pointing out here that one cannot perform this trick on any subject other than a still life, having identical lighting on each image, with the Camera in a fixed position (tripod essential) and identical exposures. Do not alter these images i.e. colour/brightness/contrast etc. until the final image is complete.
Don't worry if your Photoshop doesn't look like this screen grab. This is CS3 from a wide-screen monitor but this trick can be done in any version of Photoshop as we are only manipulating Layers. I have my four images open and I shall select the main image, shot at f16 and focussed on the letter D, to create a new Layer upon which I shall superimpose the selected areas of the other images to build our montage.
In effect it's like having four photographic prints on the table, three of which will be sliced up with a knife, and laid over the first. Obviously image alignment is crucial. This is the procedure step by step.
1. Highlight the main image then > Layer > New > Layer.
2. The Layer box opens. Do not engage Previous Layer button nor alter Colour. Mode Normal is good as is Opacity at 100%. > OK
3. Highlight the next image. I have chosen the shot with the foreground area in focus. Right > the Marquee Tool and > Rectangular Marquee.
4. Use this tool as you would the Crop tool. Start outside the image and marquee the selected area.
5.
Engage the Move Tool
6. Click on selected area and Drag it.
7. Drag it across and dump it into the Main image.
8. When it becomes visible pick it up again with the Move tool and gently float it around the area where it needs to slot into place.
What happens next is a really neat trick but I don't know if it performs in earlier versions of Photoshop. In CS3 this selected area, upon coming close to the where it needs to be, suddenly jumps into the exact position as if by magnetism!!!. If in your version this doesn't happen then you must place it manually.
9. So that's the first of three sections superimposed onto the main image. Repeat steps 3 - 8 with the remaining images until you have four images within one. At that point each of these sections can still be moved independently and as you will see, over on the right under the Layers panel, are the three Layers on top of the Background Layer. At this point I close down what's left of the three other images and Maximise the main composite image then Fit the image to the screen so I can see it all in close up. By selecting each Layer from the box on the right I can still move the selected areas if some fine manouvering is needed for alignment.
Having done this and the composite image looks perfect then we can MERGE all the images into one image.
> Layer again from the top Menu and find, down toward the bottom of a long list, MERGE VISIBLE.
> on this and the four images become one allowing you to alter any aspect of it like colour/brightness/contrast/size etc.
Don't forget to Save it as a lot of work has gone into this.
Compare this final result with the images at the top of the page. This is a very useful trick indeed but it must be said that it may not work quite so well on all subjects. When refocusing the lens on another area of an image there may be a very slight shift in Perspective and also Image size. For example, look very closely at the edge of the back blue panel of the Keyboard. Note the slight mis-alignment. Though it's nothing that a bit of retouching in Photoshop can't handle, it is not quite perfect, but it's a bloody useful trick nevertheless.
Text and Images Copyright 2008. Steve Rostron
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