Depth of Field question

[kevin_penczak]

<p>I am looking to achieve the most shallow DOF. How do i know which will give me a smaller DOF, say when comparing the 135mm 2.8 vs 85mm 1.8 vs 50mm 1.4. I know the longer the lens the smaller DOF, but also the smaller f/stop the smaller DOF. At some point these two factors would seem to cancel each other out. Working distance is not a problem, I can use the 135mm and back up to get about the same field of view as the 50mm from closer. Is there some sort of formula or general rule to figuring out DOF when comparing lens? Thank you in advance!</p>

[Matt Laur]

<p>Actually, you can simulate combinations of camera bodies and specific lens recipes (focal length, apertures) as used on subjects at various distances, using <strong><a href="http://www.dofmaster.com/dofjs.html">this DoF calculator</a></strong>. Quite handy.</p>

[zigzag]

<p>Read "Understanding Depth of Field" here: http://www.cambridgeincolour.com/tutorials.htm , which also has a DoF calculator. The turorials on this Cambridge in Colour site may cover a range of other questions too.</p>

[jeff_conrad]

<p>As in so many cases, it depends ...</p>

<p>At moderate distances, i.e., much less than hyperfocal but not in the macro range, all lenses give approximately the same DoF when set to the same <em>f</em> -number and the magnification is kept constant (i.e., the subject size in the image remains constant). So if you have a fixed subject size, the 50 mm <em>f</em> /1.4 will give the shallowest DoF. There's obviously quite a difference in perspective, so you'd need to decide how important perspective is.</p>

<p>Another issue is the background blur; for the same magnification and distance from the subject to the background, the 135 mm <em>f</em> /2.8 will give a slightly greater background blur. There's yet another twist; the magnification of the background also increases with focal length, and the ratio of background blur to background magnification is independent of focal length. For example, if the background included a sign that read <strong>Photography of Bridges Strictly Prohibited</strong> , it would be equally legible with the 50 mm and 135 mm lenses set to the same aperture. Here again, the larger maximum aperture of the 50 mm lens would permit making the background less recognizable.</p>

<p>These concepts are explained in detail in the Wikipedia article <a href="http://en.wikipedia.org/wiki/Depth_of_field">Depth of field</a> .</p>

[Michael R Freeman]

<p>For the <strong>same image size</strong> (magnification), the faster lens will always give the shallowest depth of field, regardless of focal length. So if you fill the frame with say a flower using an 135/2.8, and then shoot the same photo with a 50/1.4 by <strong>moving</strong> closer such that the flower fills the same space in the frame as with the 135/2.8, then the 50/1.4 will have the shallowest depth of field.</p>

[kevin_penczak]

<p>So shooting a full body portrait at 2.8 with a 15mm fish and a 200mm lens will give the same depth of field is the magnification is kept constant (i.e., the subject size in the image remains constant)?<br>

the DOF caculator is hard to use because i dont know how to compare the lens, 10 feet for a 24mm lens and 10 feet for a 200mm have very different view of fields</p>

[jeff_conrad]

<p>To be honest, I've never thought about it for a fisheye, but at least on axis, the answer should be yes. With a 14 mm rectilinear lens, the yes would be unqualified. Of course given the considerable difference in perspective, I'm not sure the comparison is apt. For example, if you shot in portrait orientation and filled a 36 mm frame with about 6 ft of subject height, magnification would be about 1/48. The object distance with the 14 mm lens would be 686 mm and that with the 200 mm lens would be 9.8 m.</p>

<p>What exactly are you trying to do? It's tough to give useful answers in the abstract.</p>

[alec_myers]

<blockquote>

<p>So if you fill the frame with say a flower using an 135/2.8, and then shoot the same photo with a 50/1.4 by <strong>moving</strong> closer such that the flower fills the same space in the frame as with the 135/2.8, then the 50/1.4 will have the shallowest depth of field.</p>

</blockquote>

<p>Actually, there is a very useful rule of thumb which is this: <strong>as long as you are far from the hyperfocal distance(s)</strong><strong>, the depth of field depends only on the size of the object in focus and the f-stop.</strong> Not the focal length of the lens, or your distance from the subject. You can verify this rule with some simple maths, and the DoFmaster website.</p>

<p>So make your plant (pot) fill the frame with both lenses and at f/2.8 they have the same depth of field. Open the 50mm up to f/1.4 and it will have the narrower dof.</p>

<p><a href="../photodb/user?user_id=533353">Michael R. Freeman</a> (above) is correct, except he forgot to add the rider about being far from the hyperfocal distance. The rule won't work for you example with the fisheye, because the hyperfocal distance even for very wide apertures is very short and the rule breaks down in this region.</p>

<p>Jeff Conrad also states it correctly.</p>

[alec_myers]

<blockquote>

<p>the DOF caculator is hard to use because i dont know how to compare the lens, 10 feet for a 24mm lens and 10 feet for a 200mm have very different view of fields</p>

 

</blockquote>

<p>A couple of diagrams, and thoughts about "similar triangles" will show you that the subject distance (for fixed magnification) varies in proportion to the focal length of the lens. You need to be four times further away from the subject with a 200mm lens as you do with a 50mm lens (outside of the macro regime).</p>

<p>If your subject is at 10 feet for a 24mm lens, then it would need to be at 200/24 * 10 = 83.3 feet for a 200mm lens. However the hyperfocal distance for a 24mm lens (using a 35 micron CoC) is only 20 feet at f/2.8. The depth of field for the <strong>24mm lens</strong> in this example is <strong>14.2 feet. </strong> </p>

<p>The <strong>200mm lens</strong> at 83.3 feet has a DoF of <strong>10.4</strong> <strong>feet </strong>only. (Because the 24mm lens is nearing the hyperfocal distance at this magnification the DoF is greater.) Also the distribution of DoF around the focal plane is different, rougly 50/50 for the 200mm lens, but 25/75 for the 24mm lens - the rear limit of acceptable focus is moving away towards infinity, again, because the hyperfocal distance isn't much further than the focal plane. </p>

<p>Compare with the <strong>50mm lens</strong>, though, which would focus at 20.8 feet on the same subject for the same magnification. The DoF (f/2.8, 35 micron CoC again) is <strong>11 feet</strong>, distributed roughly 40/60 about the focal plane. These figures are very close to (and in photographic terms, indistinguishable from) what you get with the 200mm lens. The hyperfocal distance (same stats) for the 50mm lens is 83 ft, and we are well inside that.</p>

<p>Move to a <strong>100mm lens</strong> at 41.6 feet (again, in proportion) and the hyperfocal distance is 330ft, the deption of field is almost identical to the 200mm lens, this time it's <strong>10.5 feet</strong>, and 44/65 distribution.</p>

<p> </p>