Does DOF vary from lens to lens?

[sarah_michelle_larsen]

<p>Im just curious how much the different lenses vary in DOF if I shoot at the same apperture.<br>

Lets say I have 50mm fixed lens and shoot it at 1.4 on a full frame camera.<br>

I the take the same lens and put it on a crop camera. Is the DOF exactely the same on both pictures?<br>

And what about a 35mm lens compared to the 50mm both on the same camera and on the same DOF?<br>

And last.. what about a 50mm Nikon lens compared to a 50mm leica lens, both on fullframe houses?<br>

Etc etc.<br>

Thanks. </p>

[paddler4]

<p>You will get a lot of different answers to this. As far as I can tell from both reading and testing my lenses, the answer is that if the field of view is the same (that is, you would be farther from the subject with a long lens, so that the subject fills the same fraction of the image), the answer is that the effect of focal length on DOF is very minor and generally negligible, except that long focal lengths tend to shift the in-focus area to the front of the image. </p>

<p>For a technical explanation of DOF and test results that show what I wrote, see http://www.cambridgeincolour.com/tutorials/depth-of-field.htm. For a more practical explanation that also explains the difference between DOF and background blur, see http://toothwalker.org/optics/dof.html.</p>

[kevin_keller]

<p>The simple answer is that DOF is controlled by 3 factors. Focal lenth, distance to subject, and apeture. The size of the sensor will not change the DOF unless you make a change in one of the three factors. Depth of field is a property of the physical characteristics of the lens.</p>

[mike dixon]

<p><em>The simple answer is that DOF is controlled by 3 factors. Focal lenth, distance to subject, and apeture.</em></p>

<p>Simple, but incorrect. Depth of field is also based on various assumptions about the degree of enlargement and the viewing distance of the final image. Smaller formats require greater enlargement for the same print size, so the same focal length and aperture will have a different DOF depending on whether it used on a APS camera, a 35mm camera, or a medium format camera.</p>

[mike dixon]

<p>To clarify: In answer to the question in your title, no DOF doesn't vary from lens to lens if the they are the same focal length, but DOF does vary between formats even when using the same focal length.</p>

[kevin_keller]

<p>So Mike you are saying that if you place two cameras one with 35mm format sensor and one with aps size sensor at the same distance from a subject and use the same lens on each one you will have different DOF on each one due to different sensor size.</p>

[mike dixon]

<p>Yes. If you have an 8x10 print made from each camera, the print made from the APS capture will have less DOF because of the greater degree of enlargement.</p>

[kevin_keller]

<p>Would that be a perceived DOF and not accual DOF. The image from the aps will be a little softer overall compared to the 35mm. Also to change that situation you would have to change Fstop Distance or focal lenth on the camera with the aps.</p>

 

[kevin_keller]

<p> If you use the same lens on different size formats you are capturing more of the image with the larger format. You will see more of the effect of DOF in that image. The image that captures a smaller area of the cirlce will still have the same DOF as the other image you are just seeing less of the effect. But in either case the only way to change the DOF is to change Fstop, distance to subject, or focal length.</p>

[mike dixon]

<p>Depth of Field is a perceptual concept--there isn't "perceived" vs. "actual" DOF. The DOF markings on a lens (and the results you get from a DOF calculator) are based on assumptions about print size (and, consequently, degree of enlargement), viewing distance, and what will be perceived as "sharp enough." Change one of those "perceptual" variables, and DOF numbers change. For example, the DOF on my Leitz 50mm lens when focused at 10 feet extends to 25 feet at f16; my Contax and Canon 50mm lenses when focused at 10 feet have DOF that extends almost to infinity at f16. A 50mm lens for a Hasselblad focused at 10 feet and stopped down to f16 will be different than those. Which is the "actual" DOF?</p>

<p>How clearly an out of focus object will be resolved on a piece of film or a sensor may be constant for the same focal length, subject distance, and aperture, but that isn't Depth of Field.</p>

[alec_myers]

<p><strong>And last.. what about a 50mm Nikon lens compared to a 50mm leica lens, both on fullframe houses?</strong></p>

<p>As a matter of fact, depth of field *does* vary slightly lens-to-lens, depending on the lens design. For more information, see the wikipedia entry (which I have no reason to suppose isn't authoritative) http://en.wikipedia.org/wiki/Depth_of_field and look at the section at the end under Asymmetrical lenses. Most formulae derived for dof assume symmetrical lens structure which is not true in practice, and clearly can vary one design to another. However as the wikipedia page points out:</p>

 

<blockquote>

<p>Except for close-up and macro photography, the effect of lens asymmetry is minimal</p>

 

</blockquote>

<p>So for practical purposes I think it's ok to assume that Nikon, Canon, makes no noticeable difference.</p>

[sarah_michelle_larsen]

<p>Thank you. Very interesting reading.</p>

[Alan Marcus]

<p >Depth of Field:</p>

<p >We focus on a specific distance to obtain a sharp image. Are objects at all other distance will be out of focus? Practical experience reveals that a span exists before and behind the point focused upon wherein objects appear acceptably sharp. This is the span is depth of field. </p>

<p > </p>

<p >What determines what is in focus and what is fuzzy is the size of tiny almost indiscernible image circles projected by the lens onto the film or digital chip. If we examine the image with a magnifier we find a jumble of tiny circles of light. We might call these circles image points but they appear indistinct so they are called circles of confusion. If we could make perfect lens, these points of light would be so tiny we would not be able to measure their size. Sorry to report, every lens ever made has unsolved aberrations so we get indistinct circles not points. </p>

<p > </p>

<p >Now it’s the size of these circles that cause us to declare an image sharp or unsharp. Cartoons in the funny papers appear unsharp because the dots of ink that make them up are discernable. A giant print (enlargement) will seem unsharp if viewed at close range because these circles will be visible. A print viewed at standard reading distance appears sharp because the circles tiny enough to appear as points, not circles. </p>

<p > </p>

<p >The resolving power of the human eye is such that a point is discerned if a disc is viewed at a distance of 3000 diameters. Thus a 10 mm disc viewed at 30 meters appears as a point, we can’t perceive it is a disc. Now this is a demanding standard. In practice on a camera we use 1/1000 of the focal length. Thus for a 50mm lens the circle must be 5/100mm. </p>

<p > </p>

<p >Factors that govern the span of depth of field:</p>

<ol type="1">

<li >Distance the print or display is from viewer. Most depth of field tables are computed using 1/1000 of the focal length as the permissible circle size. This works out to print or display made using circles that are about 1/100 of an inch in diameter. This size circle is viewed as a point from 10 inches. Another way to say this is the circle appears to be 3.4 minutes of arc from a typical viewing distance. Typical viewing distance is about the same as the diagonal measure of the print or display screen. For an 8x10 this works out to 13 inches. For a 20 x 30 over mantel the typical viewing distance is about 36 inches. You see, the degree of enlargement to make the presentation must be taken into account. </li>

<li >Focal length affects depth of field. Since the image is made-up of clusters of circles of confusion. A short lens (wide-angle) yields tiny images meaning objects appear tiny thus the circles of confusion are rendered tiny. This phenomenon is the foundation as to why wide-angles confer tremendous depth of field. </li>

<li >Distance affects depth of field. The further you are from an object, the smaller its image. Points making up a distant object must cover an extensive area. Distant image points will be rendered small and sharp in focus. Whereas close object are rendered larger, image points fore and aft of the point of focus will also be rendered large and perceived as fuzzy. </li>

<li >Aperture (f/number) affects depth of field. The camera lens is a converging lens. The light rays transverse and are turned inward. The size of the aperture opening has a profound effect on the size of the circle of confusion size. Thus tiny apertures yield superior depth of field. As the aperture size approaches very tiny openings like f/16 – f/22 – f/32, a higher percentage of light rays will be choked off as they are blocked by the blades of the iris. Light rays that hit the iris a glancing blow are not cleanly cutoff. These close encounters produce misdirected rays that intermingle with the image forming rays. This phenomenon is called diffraction. Diffraction causes rings that surround the circles of confusion. Now appear so confused we call them Airy disc in honor of the scientists who first identified them. Thus small apertures grant the greatest depth of field but at a price of reduced acuity. </li>

<li >The full frame size sensor measures 24mm by 36mm. The compact sensor (APS-C) measures 16mm by 24mm. It is 66% of the full frame size. Another way to say this is the inverse; the full frame is 150% larger than the APS-C. We often write 150% as a decimal fraction and call it the crop factor the value is 1.5 meaning if you use lenses designed for full frame on an APS-C the smaller sensor only sees the central portion of the image. Thus the top, bottom and both sides are lopped off.</li>

</ol>

<p > Another way to explain: You set up a slide show in the auditorium. The screen measures 100 inches by 150 inches. You adjust your projector to fill the screen and proceed. In pops the History Professor, lays claim on the screen and takes it away. Lucky you, another screen is in the closet but its smaller 66 inches by 100 inches. No time to readjust, you place it in the same spot as the previous screen. Sorry to report, using the smaller screen you chop off the top, bottom and both sized of your slides. The crop factor is 1.5.</p>

<p > </p>

<p >Let’s see what difference full frame vs. APS-C as to depth of field using a reliable table.</p>

<p > </p>

<p >Full frame with 35mm lens f/11 focus on object 6 feet 6 inches away. </p>

<p >Near = 2 feet 8 inches ~ Far = 4 feet 4 inches</p>

<p > </p>

<p >APS-C with 35mm lens f/11 focus on object 6 feet 6 inches away.</p>

<p >Near = 4 feet 10 inches ~ Far = 7 feet 11 inches</p>

<p > </p>

<p >As you can see, the APS-C yields a shallower depth of field due to the fact that an image falling on a smaller sensor requires higher magnification to produce a print or fill a display. </p>

<p > </p>

<p >Foot note: APS-C stands for Advanced Photo System. Seems all the major camera makers formed a consortium in 1996 to introduce a new film camera format. The film was to be modeled after motion picture film with a transparent magnetic overcoat used for the sound track. The coat was perfect for a hybrid camera, part silver chemical part digital. This was to be the vanguard digital camera scheme. It was small; the film was 24mm wide. This size with a format ratio of 1.5 was called APS Classic. </p>

[jim_momary]

<p>Alan wouldn't the far limit for 35mm on full frame be something more like 14 feet?<br>

Jim</p>

[Alan Marcus]

<p>A tip of the hat to Jim!<br>

Boy did I make a mistake<br>

This is the correction<br>

Thanks Jim<br>

Full frame 35mm f/11 focused 6 feet 6 inches<br>

Near = 4feet 4 inches ~ Far 12 feet 8 inchesFar</p>

[remco-jan.woldhuis]

<p>@Mike: I don't agree that the DOF depends on the enlargement of the image. The effect you describe is the perception of the viewer. The DOF does not change by printing on a larger format. An out-of-focus background will not become more in-focus when printed on a smaller size. We only think it's more in focus because we see less detail.</p>

<p> </p>