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03-22-2009, 05:58 AM   #1
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Lens for isolating subject

Here is my hypothesis:


To isolate a subject, a narrow depth of field (DOF) is required.

For a given camera, DOF is dependent upon

1 - the magnification of subject
2 - the aperture

Now, #1 depends on subject distance and focal length. For a given subject, it should not matter if I use a 50mm close up, or a 300mm at a distance, as long as the subject is the same size in the viewfinder. This means the 55mm f1.4 will have a narrower DOF than the 77mm f1.8 wide open with the same subject size. (The 77mm will require more camera to subject distance.)

Therefore, a shallow DOF requires a wide aperture. The best lens for isolating a subject therefore is the one with the widest usable aperture. This rules out the 50mm as it is very soft until stopped down to f2.8 or so. The candidate lenses therefore would be the 77mm f1.8 or the new 55mm f1.4.

Is this the right reasoning, or am I missing something?

03-22-2009, 06:06 AM   #2
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In fact, if the subject image size remains the same, then at any given aperture all lenses will give the same depth of field.

if you don't believe me, check this out...

DOF2

the only thing the aperture changes really is the quality/character of the OOF areas, a f/1.2 will diffuse things more than an f/2 aperture would

also your subject to background ratio affects the amount of blurring too. a 35mm lens includes too much background for things to get really oof at hyperfocal distances so to achieve isolation you need to get closer.

Last edited by Digitalis; 03-22-2009 at 06:11 AM.
03-22-2009, 06:34 AM   #3
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QuoteOriginally posted by jfsavage Quote
Here is my hypothesis:


To isolate a subject, a narrow depth of field (DOF) is required.

For a given camera, DOF is dependent upon

1 - the magnification of subject
2 - the aperture

Now, #1 depends on subject distance and focal length. For a given subject, it should not matter if I use a 50mm close up, or a 300mm at a distance, as long as the subject is the same size in the viewfinder. This means the 55mm f1.4 will have a narrower DOF than the 77mm f1.8 wide open with the same subject size. (The 77mm will require more camera to subject distance.)

Therefore, a shallow DOF requires a wide aperture. The best lens for isolating a subject therefore is the one with the widest usable aperture. This rules out the 50mm as it is very soft until stopped down to f2.8 or so. The candidate lenses therefore would be the 77mm f1.8 or the new 55mm f1.4.

Is this the right reasoning, or am I missing something?
Your pretty much there. Not all 50s are the same. The K and A 50/1.4 are both pretty good wide open. The new 55/1.4 is quite good wide open, but really isn't hitting it's stride until 2.8 or so as well, at which point it is spectacular.
03-22-2009, 07:01 AM   #4
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QuoteOriginally posted by Digitalis Quote
In fact, if the subject image size remains the same, then at any given aperture all lenses will give the same depth of field.

if you don't believe me, check this out...

DOF2

the only thing the aperture changes really is the quality/character of the OOF areas, a f/1.2 will diffuse things more than an f/2 aperture would

also your subject to background ratio affects the amount of blurring too. a 35mm lens includes too much background for things to get really oof at hyperfocal distances so to achieve isolation you need to get closer.
The focal length used (or equivalently distance to the subject at constant mag) also changes perspective, hence the apparent SIZE of things in the background. This can have a huge effect on perceived isolation. Here's a further interpretation of what the link you referenced has to say:


Here's the same image comparison without the distracting inset:


Notice the isolation impact caused by the bunny in the foreground. The shorter lens makes the bunny appear larger, hence more out of focus & (in my mind) less distracting. If there were no discernible background in the photos, I think the shorter focal length would better isolate the subject, while if the bunny were not in the photos, the longer lens might better isolate the subject.

Here's another example from the web - note the top & bottom images on the left are the same f-number hence same DOF, but the change in focal length makes a big difference in perceived isolation:



The apparent SIZE of things in the background/foreground as well as the sharpness of their edges makes a big difference in what attention we give them when viewing a photo.

QuoteQuote:
...lens includes too much background for things to get really oof at hyperfocal distances so to achieve isolation you need to get closer.
This is not always true because of the psychological effect of the size of oof items in the background/foreground; In the examples I showed, getting farther from the subject increased the isolation by simplifying the background and making it appear fuzzier (at least in my view.)


The above discussion involves MY personal interpretation of images. I have no idea how you might perceive the relative isolation of the subjects in the photos shown above & would like to hear what you think.

Dave


Last edited by newarts; 03-22-2009 at 07:43 AM.
03-22-2009, 08:15 AM   #5
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It should also be mentioned that a small DOF can also work against you. If you are trying for shots of moving targets, or similar, a small "miss" on focus can wreck your entire shot. A stationary target is not as much of a problem, becuase you have time to check and really nail the focus.
03-22-2009, 10:39 AM   #6
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Effects of aperture on depth of field

As a demonstration set of images of the effects of aperture on depth of field for my photo students, I took these four images that are in the photo gallery on site. I used my K10D and the DA* 50-135/2.8 at 1 meter and four apertures at 2 stop intervals.

135mm @ f/2.8


135mm @ f/5.6



135mm @ f/11


135mm @ f/22
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