Depth of field different lens but same f-stop?

[george_ferguson]

<p>Given the same f-stop, is the depth of field different for a 50mm prime versus a 100mm prime lens? To clarify, if I take a picture with a my camera equipped with a 50mm prime lense versus equipped with a 100mm prime lens but have the same f-stop, shutter speed, and ISO setting then will my depth of field for the picture be the same or different. If so, how much and how is that determined?</p>

[James G. Dainis]

The depth of field will be different. You can see how much by using this on line depth of field calculator:

 

http://www.dofmaster.com/dofjs.html

[Ray House]

<p>The effects of DOF are dependent upon lens focal length, distance from camera and aperture.</p>

[Wayne Melia]

<p><a href="http://www.film-and-video.com/dofmyth.htm">http://www.film-and-video.com/dofmyth.htm</a></p>

[steve_levine]

<p>If the F stop is the same, and the subject is the same size in the frame, the DoF <em>will be the same</em> with any focal length lens. The reason? The distance between <em>lens and subject </em>changes in the above example. And this effects DoF more than either focal length or F stop.</p>

[William Michael]

<p>Don’t forget, <strong><em>camera format</em></strong></p>

<p><em>“The effects of DOF are dependent upon lens focal length, distance from camera and aperture”. . . <strong>and the camera format.</strong></em></p>

<p>Similarly:</p>

<p><em>“If the F stop is the same, and the subject is the same size in the frame”<strong>, and the cameras are the same format</strong>, “the DoF will be the same with any focal length lens.” </em><br>

Also note that this applies to most subject distances provided they are NOT macro: it wanders fractionally as the subject gets further away, but the difference is insignificant for practical purposes. </p>

<p>IMO, the point Steve Levine makes, is the most powerful axiom a Photographer can use in the field: and very few know it; fewer use it.</p>

<p>WW</p>

[Matt Laur]

<p>Guys, we have to assume that George understands that when he takes off his (for example) 50mm lens and swaps it for a 105mm, that the composition is going to change change dramatically. He didn't ask if the DoF was going to stay the same in a scenario where he also picks up his gear and backs away another twenty (or 100?) feet from his subject in order to re-compose and keep the subject the same size in the frame. Which, of course, completely changes perspective.</p>

[daverhaas]

<p>Correct me if I'm wrong, but different lenses also exhibit differing DOF characteristics - depending upon the quality of the lens, elements and other manufactoring "tricks"</p>

<p>meaning - a 50mm f1.8 from nikon will have a different dof than a 50mm f1.8 canon.</p>

[dan_south]

<p>David, in the case you mentioned (Canon 50 f/1.8 versus Nikon 50 f/1.8), the depth of field would be the same (given the same aperture and distance to subject), but the perceived look of the blurred background might have a different quality. That said, even the in focus area will have a different quality depending on sharpness, distortion, and other optical qualities.</p>

[William Michael]

<blockquote>

<p>"Guys, we have to assume that George understands that when he takes off his (for example) 50mm lens and swaps it for a 105mm, that the composition is going to change change dramatically . . . etc</p>

</blockquote>

<p>It would be good if George came back and confirmed his understanding of the different points being made, or ask for clarifcation if not.</p>

<p>WW</p>

<p> </p>

[steve_levine]

<p>This might be a good time for George, and everyone else to visit an online DoF calculator. It is quite interesting to jump around between formats, apertures, focal lengths and focal distances. And to see how DoF and "hyperfocal distances" change.</p>

<p>As for differing brands of the same speed and focal length lenses having different DoF. This isn't so. DoF is a product of physical laws. Yes, some glass will have different "circles of confusion" (the subject of a future discussion). And "acceptably sharp" can change, since "acceptably" sounds very subjective to me.</p>

<p>Most experienced photographers learn to add DoF "wiggle room" ,when shooting at hyperfocal distances. Or as the guru said, "Infinity isn't always infinite".</p>

[dbcooper]

<blockquote>

<p>IMO, the point Steve Levine makes, is the most powerful axiom a Photographer can use in the field: and very few know it; fewer use it.</p>

</blockquote>

<p>+1 Ansel Adams made this point, with example photos, in his book 'The Camera'.</p>

 

<blockquote>

<p>Guys, we have to assume that George understands that when he takes off his (for example) 50mm lens and swaps it for a 105mm, that the composition is going to change change dramatically.</p>

</blockquote>

<p>Why do we have to assume that? Doing so could give him a wrong or misleading answer.</p>

<blockquote>

<p>He didn't ask if the DoF was going to stay the same in a scenario where he also picks up his gear and backs away another twenty (or 100?) feet from his subject in order to re-compose and keep the subject the same size in the frame.</p>

</blockquote>

<p>OTOH, he didn't say he wouldn't change camera position either. The correct answer to the OP's question as-asked, without elaborating, would be 'maybe' or 'it depends', which would be unhelpful. A good answer needs qualification or explanation, especially since the subject (i.e. DoF vs different focal length lenses) is generally misunderstood.</p>

<p>@George Ferguson: you might want to give the book I mentioned a read - it's worth it. As mentioned, the sensor size has an effect on DoF (google 'circles of confusion'...smaller sensors use smaller CoC in DoF calculations), but not a relative one if you're using the same camera to compare DoF of lenses. The ISO and shutter speed have no effect on DoF in and of themselves, but if either or both causes a change in aperture, <em>that</em> would certainly affect DoF. 'Acceptable sharpness' is a subjective term, which is why you should test your lenses on your camera at different apertures and focal distances. Drudgery, yes, but you'll know for sure how your gear will act.</p>

<p> </p>