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Selective Focus


 

What is Selective Focus anyway. It is a method of controlling the focus of a photo, so that only the main subject is in focus. This is done by controlling the Depth-of-Field of the lens, so that only a short range of distances are in focus. This is often very desireable in portraiture, as it blurs the background, which directs attention to the intended subject in the photo.

Consider the two photos below:

                

 

Most people would agree that the photo to the left is not as pleasing as the photo to the right. The background of the left photo is interfering with the main subject, so much so that it is hard to pick it out. In contrast, in the right photo, you clearly see the intended subject as the background is blurry.

 


 

So how do you limit the depth-of-field (shallow depth-of-field)? Selective focus is controlled by one (or a combination of) three things:

Focal Length Increasing the focal length of the lens will decrease the Depth-of-Field.
Decreasing the focal length of the lens will increase the Depth-of-Field.
ApertureIncreasing the aperture (lower f/stop) will decrease the Depth-of-Field.
Decreasing the aperture (higher f/stop) will increase the Depth-of-Field
DistanceDecreasing the distance of the subject to the lens will decrease the Depth-of-Field.
Increasing the distance of the subject to the lens will increase the Depth-of-Field.

If you can conceptualize the above drawing, you should be able to see that when you focus on your subject, an area in front of, and behind the focus point will be in focus. This is called the Depth-of-Field (or depth of focus). Depending on the setup of your camera, the Depth-of-Field will either be very wide, or very narrow. To achieve selective focus and get that desired background, we want a narrow Depth-of-Field.

However, should you be taking photos of a landscape or scenery, you would of course want a very wide Depth-of-Field so that you could capture those majestic mountains in the background and the forest in the foreground, and have everything in focus. So in reality, selective focus - or managing the Depth-of-Field is a creative element, and highly dependant on what you are wanting to achieve.

 

Editorial Comment

You will find on the internet folks claiming that DoF is affected by the sensor size, and others that claim it doesn't (including even one particularly rude loudmouth on YouTube). The simple truth... they are BOTH wrong.

The one factor that mostly determines a shallow DoF is focal length. And a larger sensor allows a longer focal length, which results in a shallower DoF. So while sensor size by itself does not contribute to a shallower DoF, it does allow use of longer focal lengths which does result in a shallower DoF.

So sensor size INDIRECTLY contributes to the DoF. This would to most seem just a matter of semantics - however, the arguments on the internet have become so abrasive - especially since both sides are not 100% accurate - that it has gone beyond semantics, but affects their basic understanding of the concept.

 


Setting up your camera.

From the discussion above, you should realize that the three controls you have to controlling the Depth-of-Field is the lens Focal Length, Aperture, and Focus Point (the distance your subject is from the camera). To assist in determining where to begin, consider using a DoF calculator. If you have an iPhone or Android device, there are many free DoF calculator apps that will help in this regard.

The app I use is called "Simple DoF" by Indie Film Gear. It is a free app, and can be downloaded from iTunes.

To use the calculator, you first have to enter your camera type. The sensor size of your camera will affect the Depth-of-Field (smaller sensors have a wider Depth-of-Field), so the accuracy is dependant on selecting the proper camera. If your camera is not listed, then simply chose a camera that has the same sensor size as yours.

Then simply input the focal length of your lens, the focus point (distance to your subject), and aperture. The calculator will then give you the Depth-of-Field you can expect, as well as the distance in front of, and behind, the focus point that will be in focus.

If you cannot control the aperture of your camrea, try using f/5.6 or f/8. That should get you into the ballpark at least.

 

A Short-Cut to Background Blur.

1. Set the aperture to the largest opening you can (lowest number). While this is not the major factor in obtaining a shallow DoF, you will still want to use the widest aperture you can in all cases - so set it first.

2. Use the longest focal length you can (zoom in). This is especially true for a compact camera, but with a DSLR, the minimum focal length is 50mm - if you have a fast lens, or around 200mm if you have a consumer-grade lens with a modest aperture (variable - f/5.6). The longer the lens focal length youuse - the shallower the DoF will be.

3. Get as close to your subject as you can. The closer you are, the shallower the DoF. Obvously, this is counter to the second step, so you will have to comprimise here. Try different combinations of focal length and focus distance until you get the best results.

4. (optional) While not a direct factor in determining DoF, in marginal situations (i.e. especially with a compact camera) try to compose the scene with the background some distance away. The further the background is, the more out of focus it will be.

 

From the example shown here, if you used a 50mm lens, and an aperture of f/2.8, and your subject was 10ft from you (your focus point), then anything closer than 9.32ft, and further away than 10.78ft will be out of focus. Only the narrow area around your subject will be in focus.

DoF at 50mm


Myths

Myth #1: To obtain a shallow Depth-of-Field, you need a fast lens.

Nothing could be further from the truth. Using fast lens is only one possible method of obtaining selective focus. In fact, of the three factors, focal length trumps all other settings.

Compared to the 50mm drawing above, if we change the focal length to 200mm, and even stop down the aperture to f/5.6 (which is typical of inexpensive telephoto lenses), you can see that given the same 10ft focus point, the DoF is much shallower (data provided by the Simple DoF calculator).

DoF at 200mm

Of course the trick is to keep the focus distance at 10ft, which may not always be possible. However, the concept here is that using a longer/slower lens provides shallower DoF than a shorter/faster lens.

25mm Lens DoF Chart
 
f/1.4
f/2
f/2.8
f/4
f/5.6
10ft
2.79'
4.02'
5.92'
9.1'
15.6'
20ft
11.9'
18.3'
31.5'
78.5'
30ft
29.8'
53.1'
155'
   
50mm Lens DoF Chart
 
f/1.4
f/2
f/2.8
f/4
f/5.6
10ft
0.68'
0.96'
1.36'
1.94'
2.76'
20ft
2.75'
3.91'
5.58'
8.04'
11.8'
30ft
6.24'
8.92'
12.9'
19.1'
29.7'

 

100mm Lens DoF Chart
 
f/1.4
f/2
f/2.8
f/4
f/5.6
10ft
0.17'
0.24'
0.33'
0.47'
0.67'
20ft
0.68'
0.96'
1.36'
1.92'
2.73'
30ft
1.54'
2.17'
3.08'
4.36'
6.2'
   
200mm Lens DoF Chart
 
f/1.4
f/2
f/2.8
f/4
f/5.6
10ft
0.04'
0.06'
0.08'
0.11'
0.16'
20ft
0.17'
0.24'
0.33'
0.47'
0.67'
30ft
0.38'
0.54'
0.76'
1.07'
1.52'

Crop DSLR (1.5x) DoF values

 

From the above charts, you can plainly see that the longer the focal length, the shallower the DoF. Compare a fast 50mm f/1.4 lens to an inexpensive 200mm f/5.6. Even if the 50mm is at 10ft, the 200mm lens, at 20ft still has a comparable DoF. And if you can get the subject properly composed at 10ft, the 200mm lens has a significantly shallower DoF.

This is the reason that you do not need a fast lens for pleasing shallow DoF, if you have a longer focal length. Only if you have a short focal length do you need a fast lens.

 

Examples

Nikon D90 DSLR - 50mm f/1.8
Nikon D90 DSLR - 18-200mm @ 200mm f/5.6

Compare the two photos above. The photo taken with a 200mm lens @ f/5.6 has every bit as much of a blurry background as does the left photo with a fast f/1.8 lens. In fact, if you compare the DoF calculator estimate for each lens, the Depth-of-Field is not a lot different for each setup.

Nikon D90 DSLR - 16mm f/2.8
Nikon D90 DSLR 200mm f/2.8

The effect of the focal length on the lens is no better shown than in these two examples. You can see that the focal length is much more important than aperture. In fact, at 16mm, the aperture has almost no effect at all. And in contrast, at 200mm, combining a fast f/2.8 results in milky smooth "bokeh". This is why some professional portraiture photographers have gone to using 200mm f/2.8 lenses.

 

Myth #2: To obtain a shallow Depth-of-Field, you need a DSLR.

This is somewhat true (see my editorial comment above), but while the ability to obtain a shallow Depth-of-Field in a compact camera is limited, some degree of the effect may be achieved, given the right model of compact camera.

Hopefully by now, we have established that the lens focal length is paramount in achieving shallow Depth-of-Field. This of course presents a huge problem for compact cameras as their small sensor sizes also mean that the focal length of their lenses are short.

For example, most compact cameras have a crop factor of 5.6x. So typical zoom lens on a compact camera may be say 6mm to 60mm, which in DSLR terms, would be equivalent to a 36mm to 336mm zoom lens. Also, since most compact cameras are fully automatic, you do not have any control over the aperture - as limited as it may be.

However, there are a few compact cameras that have manual controls, so you can set the aperture lens... and if they have a focal length long enough, you can obtain some of the blurry background effect.

Canon SX130 Compact Camera - 5mm f/5.6
Canon SX130 Compact Camera - 60mm f/5.6

In the above photos - taken with a Canon SX130 compact camera, you can see that there is no blurry background effect at all at the 5mm focal length (camera zoomed all the way out). But even at f/5.6, with the camera zoomed all the way in (60mm), you can begin to get some of the effect. Of course, this is nowhere near the superior blur that can be achieved with a DSLR, at least some compact cameras can begin to get there.

In the photo taken at 60mm, I had to backup a bit to compose the photo. As you should realize by now that increasing the distance also increases the Depth-of-Field, some experinentation of different focal lengths vs. distances from the subject may achieve different results.

Myth #3: DoF is also influenced by the distance to the background.

DoF - Depth of Field - is a measurement in distance of the in-focus area; front-to-back. The background distance is not part of this equation. However, the background distance can be a contributing factor. So while it is not directly a part of determining DoF, it is still influenced by the DoF.

Simply put, the the further an object is from the DoF area, the less in-focus it will be. So especially in marginal conditions, furthering the background distance can improve the effect.

Again, this is seemingly just semantics, but there are many neophyte photographers out there - especially on the internet - that claim professional status, that these points-of-order have to be maintained. Everyone with a DSLR claims to be an expert, while the truth is, we are al learning more and more each day. If you have not had experience with fast telephoto lenses, you will have no idea how they behave, and so on.


Conclusion

Selective focus is a great technique to separate the foreground interest from the background. It is especially dramatic when done for portraiture, but it can also help "clarify" any photo (see the stacking stone example above).

While it is certainly easier to do selective focus with a DSLR, and the results can be more dramatic, the above examples prove that selective focus can be done with a compact camera. Just remember to use the longest focal length you can, and get your subject as close as it can be (with the background as far away as you can get it). Those combinations will provide some degree of background blur.

So to recap, to get the maximum effect - use the longest focal length you have; that is - zoom in as much as you can. This is true whether you are using a DSLR or compact camera.

Next, get as close as you can to your subject. Obvously, this goes hand-in-hand with the above, so you may have to use the combination of the two that seems to work best. If you have a bridge camera with a super-zoom, start with the equivalent of 200mm, which is around 35mm for most compact cameras.

Finally, use the largest aperture you can. This may not be possible with many compact cameras, but in a DSLR, it will be possible.

Remember:

The longer the focal length, the shallower the Depth-of-Field.

The closer the focus point, the shallower the Depth-of-Field.

The larger the aperture, the shallower the Depth-of-Field.

 

(Another) Editorial Comment

There are those that claim it is a fast aperture that is required for obtaining shallow DoF. Certainly the aperure has some bearing on this capability, but it is only one of three things required.

In fact, I'd argue that a fast aperture has less effect on DoF than either focal length or focus distance, and here is why:

If you have ever used an ultra wide DSLR lens, such as a Tokina 11-16mm f/2.8, you know that at 11mm and f/2.8, it is all but impossible to obtain anything less than infinity DoF. This actually helps the concept of landscape photography - as you want everything in focus. The only time you can reduce the DoF at all is if you close-focus the lens.

Or, if you use the same 11-16mm lens at 16mm, you will find that you can focus a bit further and still see less than infinity DoF.

And finally, if you use a lens like a 70-200mm f/2.8 at 200mm and f/2.8, you will find a very shallow DoF (just inches) even up to around 20ft away.

These real world experiences strongly suggest that focal length is the major influence on Dof, followed by focus point, and then aperture. And that you can use a consumer-grade telephoto lens at 200mm and f/5.6 and still get a good effect also suggests aperture is not the major influence. Also, if you compare the above photos, the effect of a 50mm DSLR lens at f/1.8 is not much different than a 200mm lens at f/2.6 should dispel this myth once and for all.

And one peek at the DoF charts above will confirm this. It's my opinon that those that believe aperture is the major influence have never used fast lenses, but rather chase myths.