Depth of Field Control

HarveyMushmanHarveyMushman .Registered Users Posts: 550 Major grins
edited November 26, 2009 in Technique
Why do some lenses offer better control of depth of field than others? For instance, my Nikkor AF-S 18-70 f/3.5-4.5 performs very poorly in the this regard, while my humble little 50/1.8 is excellent. About the only way I can blur the background with the zoom is to shoot wide-open from the absolute minimum focus-distance from the subject, and even then the effect is mild. Shooting portraits, even wide-open at 70mm, is hopeless. Yet the fixed AF 50mm offers plenty of depth-of-field control. Educate me, please.
Tim
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Comments

  • Antonio CorreiaAntonio Correia Always learning Setubal - PortugalRegistered Users Posts: 6,172 Major grins
    edited August 29, 2006
    Why do some lenses offer better control of depth of field than others? For instance, my Nikkor AF-S 18-70 f/3.5-4.5 performs very poorly in the this regard, while my humble little 50/1.8 is excellent. About the only way I can blur the background with the zoom is to shoot wide-open from the absolute minimum focus-distance from the subject, and even then the effect is mild. Shooting portraits, even wide-open at 70mm, is hopeless. Yet the fixed AF 50mm offers plenty of depth-of-field control. Educate me, please.
    I don't know the explanation but my guess is that there is some connection between the number of blades in the lens and the aperture.
    This connection is reponsible for the DOF and bokeh.

    Allow me to suggest a visit to
    http://www.dgrin.com/showthread.php?t=11015
    or even post there.

    As you will read there is some experts in there who will know to answer your question.

    thumb.gif
    All the best ! ... António Correia - Facebook
  • wxwaxwxwax Immoderator Registered Users Posts: 15,471 Major grins
    edited August 29, 2006
    Why do some lenses offer better control of depth of field than others? For instance, my Nikkor AF-S 18-70 f/3.5-4.5 performs very poorly in the this regard, while my humble little 50/1.8 is excellent. About the only way I can blur the background with the zoom is to shoot wide-open from the absolute minimum focus-distance from the subject, and even then the effect is mild. Shooting portraits, even wide-open at 70mm, is hopeless. Yet the fixed AF 50mm offers plenty of depth-of-field control. Educate me, please.

    At 70mm your max aperture is f4.5, which isn't small, but isn't all that big, either.

    Your 50mm can get a lot wider, almost four stops bigger. That's a lot.

    We have folks here who'd be happy to go into the math. The room spins when I try it.
    Sid.
    Catapultam habeo. Nisi pecuniam omnem mihi dabis, ad caput tuum saxum immane mittam
    http://www.mcneel.com/users/jb/foghorn/ill_shut_up.au
  • ian408ian408 More wag. Less Bark. Administrators Posts: 21,598 moderator
    edited August 30, 2006
    A calculator.
    An explanation (which is pretty good).
    Moderator Journeys/Sports/Big Picture :: Need some help with dgrin?
  • HarveyMushmanHarveyMushman . Registered Users Posts: 550 Major grins
    edited August 30, 2006
    Thanks much. thumb.gif That's the first time I've seen difraction explained. Makes sense!

    Like Sid said, the difference between f/1.8 and f/4.5 is significant. I guess it has appeared to me that the difference in DoF between the two lenses is noticeable even when using the same or similar apertures. Is that possible, or am I imagining things? I need to try some comparative field tests before seeking everyone's wisdom.
    Tim
  • wxwaxwxwax Immoderator Registered Users Posts: 15,471 Major grins
    edited August 30, 2006
    With both lenses at 50mm and the same aperture, you might notice a difference in the quality of the background blur, but you shouldn't notice much difference in the amount of blur.

    The quality has something to do with spherical abberrations and also blade quantity and design. (Damn room's spinning again.) Since you're shooting Nikon, you might find this interesting, as it discusses Nikon lenses towards the end.

    Here's an interesting description, too.
    Sid.
    Catapultam habeo. Nisi pecuniam omnem mihi dabis, ad caput tuum saxum immane mittam
    http://www.mcneel.com/users/jb/foghorn/ill_shut_up.au
  • pathfinderpathfinder Drive By Digital Shooter western IndianaSuper Moderators Posts: 14,614 moderator
    edited August 30, 2006
    The other factor is that depth of field is a factor of lens focal length and the distance from the film plane, as well as the aperture.

    The depth of field decreases as the subject is nearer the film plane and the 50mm probably focuses much closer than the wide to mid range zoom. That and the much larger aperture of the 50mm lens.

    At 17mm do not expect a narrow DOF, almost everything will be infocus at f 5.6 or smaller. At 70mm DOF will be less, but not as shallow as a 50mm lens at f2.8

    DOF also is a function of sensor size - APS sensors will intrinsically have more DOF than a full frame 35mm sensor, independent of focallength, aperture, or distance to the film plane.
    Pathfinder - www.pathfinder.smugmug.com

    Moderator of the Technique Forum and Finishing School on Dgrin
  • SwartzySwartzy Right Brained Scientist Registered Users Posts: 3,293 Major grins
    edited September 7, 2006
    You know, I've been trying to wrap my head around this
    for the last couple days. Ah, yes, sensor size. Stands to reason. Might be another reason to buy prime lenses that are real fast. I had great success with my film cameras but it's a little trickier with the digital...then again, it might be the user...Laughing.gif. Great thread!
    Swartzy:
    NAPP Member | Canon Shooter
    Weddings/Portraits and anything else that catches my eye.
    www.daveswartz.com
    Model Mayhem site http://www.modelmayhem.com/686552
  • digismiledigismile Major grins Registered Users Posts: 955 Major grins
    edited September 17, 2006
    If you want to know more about DOF, check out this link. You can verify that a 70mm @ f2.8 @ 7 feet distance has the same total DOF as a 50mm @ f2.8 at 5 feet (.32 feet total, using a Canon 20D).

    So, this means that a photo taken with each lens that is visually identical in the view finder, should give pretty much identical results. You will have some differences based on the differences in quality in the glass, but the outcome should be basically the same.
  • mmmattmmmatt Big Grimace Registered Users Posts: 1,347 Major grins
    edited May 14, 2009
    ian408 wrote:
    A calculator.
    An explanation (which is pretty good).
    I know this is an OLD thread but I was wondering about this calculator...

    When I change the camera from full frame to crop sensor (5d to 40d) it gives different measurements for dof, hyperfocal etc... that isn't the case as far as I know. That would be like saying that the center of the lens has different focusing properties than the outside. As far as I know the only difference between different sensor sizes is the field of view and not the focusing properties. Somebody please correct me if I'm wrong about this. Can the difference be that the distance between the film plane and the lens are different? I don't think that is possible.

    I'm basing this all on my experience with a view camera... The image circle has to be in focus on the film plane and as far as I know the distance from the film plane is solely what determines the focus. I'm confused as to how there would be any difference regardless of the size of the sensor. A 50mm lens is in focus at x ft. when the lens is x mm from the sensor regardless of sensor size. If the lens doesn't produce a large enough image circle then it will vignette but it will still be in focus. I am more inclined to think that there would be differences between lens designs but not between sensor size or camer design.

    Please note that I said "as far as I know" about 10 times here and I would like someone with a little more knowledge to explain this.

    Thanks,
    Matt
    My Smugmug site

    Bodies: Canon 5d mkII, 5d, 40d
    Lenses: 24-70 f2.8L, 70-200 f4.0L, 135 f2L, 85 f1.8, 50 1.8, 100 f2.8 macro, Tamron 28-105 f2.8
    Flash: 2x 580 exII, Canon ST-E2, 2x Pocket Wizard flexTT5, and some lower end studio strobes
  • pathfinderpathfinder Drive By Digital Shooter western IndianaSuper Moderators Posts: 14,614 moderator
    edited May 14, 2009
    mmmatt wrote:
    I know this is an OLD thread but I was wondering about this calculator...

    When I change the camera from full frame to crop sensor (5d to 40d) it gives different measurements for dof, hyperfocal etc... that isn't the case as far as I know.

    The calculator is correct. Smaller sensor cameras do have greater DOF for a given aperture than larger format sizes. The difference between a point and shoot 2/3s size sensor and a medium format camera is quite large. (Calculating depth of field is based upon a specified choice of circles of confusion, and that includes an assumption about how much the image will be magnified for a final print.)



    That would be like saying that the center of the lens has different focusing properties than the outside. As far as I know the only difference between different sensor sizes is the field of view and not the focusing properties. Somebody please correct me if I'm wrong about this. Can the difference be that the distance between the film plane and the lens are different? I don't think that is possible.


    A lens does not bend( refract) rays of light that pass through the center or the lens - the only rays that are bent in first order optics are those that do not pass through the center of the lens. Camera lenses are not first order optics, but complex multilens optics, but the basic statement is still a reasonable approximation.

    I'm basing this all on my experience with a view camera... The image circle has to be in focus on the film plane and as far as I know the distance from the film plane is solely what determines the focus. I'm confused as to how there would be any difference regardless of the size of the sensor. A 50mm lens is in focus at x ft. when the lens is x mm from the sensor regardless of sensor size. If the lens doesn't produce a large enough image circle then it will vignette but it will still be in focus. I am more inclined to think that there would be differences between lens designs but not between sensor size or camer design.

    Please note that I said "as far as I know" about 10 times here and I would like someone with a little more knowledge to explain this.

    Thanks,
    Matt

    Go to Wikipedia to this page http://en.wikipedia.org/wiki/Depth_of_field#DOF_vs._format_size and click on #6 DOF vs Format size, and you will find dof is inversely proportional to format size, like I said above.

    One other factor that people confuse, is that focal length has no effect on DOF IF - I repeat - IF the subject is always the same size on the film/sensor plane. A 300mm lens, and a 24mm lens, if shot at different distances from the subject, so that the subject is the same size on the film plane with each lens, then the DOF is the same. Interesting, isn't it?
    Pathfinder - www.pathfinder.smugmug.com

    Moderator of the Technique Forum and Finishing School on Dgrin
  • mmmattmmmatt Big Grimace Registered Users Posts: 1,347 Major grins
    edited May 14, 2009
    OK pathfinder... I'm still going to argue this. Not in any attempt to be confrontational but to better understand... In the wiki article it says "To a first approximation, DOF is inversely proportional to format size. More precisely, if photographs with the same final-image size are taken in two different camera formats at the same subject distance with the same field of view and f-number, the DOF is, to a first approximation, inversely proportional to the format size."

    This says that if the larger sensor and the smaller sensor are at the same distance from subject, filling the frame the same amount, and the same f-stop, then the dof in the smaller sensor will be greater. Yes it will but in order to have the same field of view, distance from subject and subject size, the smaller sensor needs a shorter focal lenght. Doesn't come close to saying that same focal length = greater dof, it only says that same relative focal length, which is a shorter lens and obviously greater dof.

    Then it goes on to say "For example, a point-and-shoot digital camera with a 1/1.8″ sensor (7.18 mm × 5.32 mm) at a normal focal length and [FONT=Trebuchet MS,Candara,Georgia,Calibri,Corbel,serif]f[/FONT]/2.8 has the same DOF as a 35 mm camera with a normal lens at [FONT=Trebuchet MS,Candara,Georgia,Calibri,Corbel,serif]f[/FONT]/13."

    This again is because the "normal" focal length for a smaller sensor is shorter than that of a larger sensor.
    I'm pretty sure this artical proves my point.

    Then one more debunk of the calculator... the dof listed for the 5d is GREATER than that of the 40d.

    As for my second qouted comment about center of the lens, please forgive. I do remember that now and mispoke. I should have said center of the image circle.

    your last comment is in fact very interesting. isn't light/photography wonderful in that regard? I mean really things are recirprical throughout most of camera theroy. I love this stuff!

    Matt
    My Smugmug site

    Bodies: Canon 5d mkII, 5d, 40d
    Lenses: 24-70 f2.8L, 70-200 f4.0L, 135 f2L, 85 f1.8, 50 1.8, 100 f2.8 macro, Tamron 28-105 f2.8
    Flash: 2x 580 exII, Canon ST-E2, 2x Pocket Wizard flexTT5, and some lower end studio strobes
  • QarikQarik Krazy Korean Registered Users Posts: 4,959 Major grins
    edited May 14, 2009
    full frame vs crops is definitely different DOF. It's just lens physics. The full frame has shallower DOF (about 1 stop I have heard) with the same relative image size on the sensor. Note that you will be standing at a different distance (closer) using a FF vs crop to achieve the same relative image size on both sensors.
    D700, D600
    14-24 24-70 70-200mm (vr2)
    85 and 50 1.4
    45 PC and sb910 x2
    http://www.danielkimphotography.com
  • mmmattmmmatt Big Grimace Registered Users Posts: 1,347 Major grins
    edited May 14, 2009
    Qarik wrote:
    full frame vs crops is definitely different DOF. It's just lens physics. The full frame has shallower DOF (about 1 stop I have heard) with the same relative image size on the sensor. Note that you will be standing at a different distance (closer) using a FF vs crop to achieve the same relative image size on both sensors.
    well than it is distance to subject and not the sensor size that creates more dof. I'm not saying there aren't differences when you make adjustments to either the distance, f-stop, or focal length . I'm just saying that the calculator is wrong, and a 50mm (or whatever lens) when in focus at x feet, with x f-stop equals the same dof regardless of sensor size. The linked calculator doesn't state that and shouldn't be listed in this thread as a usable tool. unless of course I'm confused but at this point I am pretty sure of myself.

    Matt
    My Smugmug site

    Bodies: Canon 5d mkII, 5d, 40d
    Lenses: 24-70 f2.8L, 70-200 f4.0L, 135 f2L, 85 f1.8, 50 1.8, 100 f2.8 macro, Tamron 28-105 f2.8
    Flash: 2x 580 exII, Canon ST-E2, 2x Pocket Wizard flexTT5, and some lower end studio strobes
  • Tim KamppinenTim Kamppinen Major grins Registered Users Posts: 816 Major grins
    edited May 14, 2009
    This gets argued over and over, and every time I walk away convinced that there are two things that control depth of field: aperture and focal distance, i.e. the distance from the subject to sensor/film plane.

    If you shoot with the same lens on a full frame and a DX camera, you will have to stand closer to the subject to get the same composition when using the full frame. The closer you are to your subject, the more shallow the DOF becomes. The farther away you are, the deeper the depth of field becomes (when I say "subject" I mean whatever you're focusing on). That's why I've taken shots with the kit lens from my old D50 at f/3.5 and 18mm but had objects just two feet behind my subject way out of focus, when I was focusing about 6 inches from the front element. Short focal distance = narrow DOF.

    DOF is a distance that can be measured, giving you a range of acceptable sharpness to work in. How blurry a background is perceived as depends not only on depth of field but the ratio between the camera to subject distance and the subject to background distance. People often think that telephoto lenses have more shallow depth of field, but they don't. The truth is that at a given aperture and focal distance, every lens has the same depth of field. The reason a telephoto lens will give you a background that appears more blurred than a wide angle lens at the same distance and aperture is because in the telephoto shot the background appears to be much larger. It has the same amount of blurring as the wide angle shot, but in the wide angle shot you see much more of the background and it takes up much less of the frame, making it look sharper simply because the details are smaller so it's not as obvious that they are blurry. So the background will indeed be less "recognizeable" but it's not actually blurred more and the DOF is not actually any shallower.
  • pathfinderpathfinder Drive By Digital Shooter western IndianaSuper Moderators Posts: 14,614 moderator
    edited May 14, 2009
    Matt,

    I am not a wizard with the math about DOF.

    What I do know, is that if I want more DOF when shooting macro, the easiest way is to use a point and shoot at its smallest aperture which is usually f8. Using a full frame DSLR I will have less DOF for the same subject size on the image plane even at f11 or f16 if I am trying to have the image the same size in the final print.

    As Tim says, there are really only two variables for a given camera that affect DOF - aperture, and subject to film plane distance. Despite what lots of folks think, focal length has almost no effect on DOF when used normally.

    Whether you shoot Janey with a 28mm lens or a 200mm lens, - IF, and only IF, Janey's head is the same size on the sensor plane for both lenses, the DOF will be the same for both lenses.
    Pathfinder - www.pathfinder.smugmug.com

    Moderator of the Technique Forum and Finishing School on Dgrin
  • mmmattmmmatt Big Grimace Registered Users Posts: 1,347 Major grins
    edited May 15, 2009
    Ok.. I see what you guys are saying, but what I'm concerned with is distance to subject and not size of subject on the film plane. Tim, I think is saying that even though short lenses appear to have more dof they don't, it is just perception. I'm good with perception as that is what we see!

    I just want to know that with a given lens at a given distance what f-stop I need to maintain to get acceptable focus throughout the depth of my comp. I'm trying to put together a memorizable list so I can make easy estimates in the field for shooting groups. I will do it the hard way by going to the canon site and put it up here so others can use it if need be.

    thanks for all the input guys. I now need a glass of Burbon! Anyone for a couple fingers of Knob Creek?

    Matt
    My Smugmug site

    Bodies: Canon 5d mkII, 5d, 40d
    Lenses: 24-70 f2.8L, 70-200 f4.0L, 135 f2L, 85 f1.8, 50 1.8, 100 f2.8 macro, Tamron 28-105 f2.8
    Flash: 2x 580 exII, Canon ST-E2, 2x Pocket Wizard flexTT5, and some lower end studio strobes
  • Tim KamppinenTim Kamppinen Major grins Registered Users Posts: 816 Major grins
    edited May 15, 2009
    mmmatt wrote:
    Tim, I think is saying that even though short lenses appear to have more dof they don't, it is just perception.

    Yeah. Pathfinder, if you take a shot with the exact same framing at 28mm and 200mm, the 28mm shot will have a much shallower depth of field. It has to because the only way to do that is to get much closer to your subject than you would with the 200mm lens. The 200mm shot may still have a background that is less recognizable, however, because even though it is less out of focus, it's also magnified so much that all you see are abstract shapes that you can't make sense of.

    However, if you took a shot at the same distance from the subject with each lens, the 28mm shot would have a background that appeared to be much sharper than the shot with the 200mm. If you cropped out a section from the 28mm shot to match the field of view in the 200mm shot, however, and put them side by side, you would see that the amount of blurring on the background of each is identical. That's because the depth of field is the same at equal focal distances. It only appears to be more blurred with long lenses because you're magnifying the background and thus making it more abstract.

    I remember seeing a website that had example photos to illustrate this, but I don't remember where it was and it's late... might have been at luminous landscape though. I'll see if I can find it tomorrow.
  • QarikQarik Krazy Korean Registered Users Posts: 4,959 Major grins
    edited May 15, 2009
    mmmatt wrote:
    well than it is distance to subject and not the sensor size that creates more dof. I'm not saying there aren't differences when you make adjustments to either the distance, f-stop, or focal length . I'm just saying that the calculator is wrong, and a 50mm (or whatever lens) when in focus at x feet, with x f-stop equals the same dof regardless of sensor size. The linked calculator doesn't state that and shouldn't be listed in this thread as a usable tool. unless of course I'm confused but at this point I am pretty sure of myself.

    Matt

    Well..it is what it is. It only makes sense to compare images that are framed the same. The only way to do this is for the full frame picture is to move closer then the crop picture. Well..why did you move closer? because it is full frame. *shrugs* in that sense.a full frame has shallower DOF.
    D700, D600
    14-24 24-70 70-200mm (vr2)
    85 and 50 1.4
    45 PC and sb910 x2
    http://www.danielkimphotography.com
  • brocotbrocot Major grins Registered Users Posts: 164 Major grins
    edited May 15, 2009
    D7200/Nikon AF-S DX Nikkon 10-24mm/10-24dx/105mm prime/Nikon 200-500sb900/
    Hello, :thumb Mi Smug :
    http://erikgodderis.smugmug.com/
    http://www.godderis.be
  • Tim KamppinenTim Kamppinen Major grins Registered Users Posts: 816 Major grins
    edited May 15, 2009
    Qarik wrote:
    Well..it is what it is. It only makes sense to compare images that are framed the same. The only way to do this is for the full frame picture is to move closer then the crop picture. Well..why did you move closer? because it is full frame. *shrugs* in that sense.a full frame has shallower DOF.

    Actually, to get exactly the same shot on full frame you'd have to use a longer lens to match the framing. If you move closer the perspective distortion will be different between the two shots. If you do it this way, (with a longer lens that matches the effective focal length of the DX lens) the shots will be identical in framing, perspective, and DOF.
  • QarikQarik Krazy Korean Registered Users Posts: 4,959 Major grins
    edited May 15, 2009
    Actually, to get exactly the same shot on full frame you'd have to use a longer lens to match the framing. If you move closer the perspective distortion will be different between the two shots. If you do it this way, (with a longer lens that matches the effective focal length of the DX lens) the shots will be identical in framing, perspective, and DOF.

    agreed...so now distance is the same but you have different zoom length on the full frame vs the crop. So you hold X contant and are varying Y. SO now zoom length has to be different because you are on full frame. No matter how you slice it DOF is a function of zoom, distance, aperture, and whether you are full frame or crop.

    The DOF calculator work bottom line and must include all factor to be accurate.
    D700, D600
    14-24 24-70 70-200mm (vr2)
    85 and 50 1.4
    45 PC and sb910 x2
    http://www.danielkimphotography.com
  • Tim KamppinenTim Kamppinen Major grins Registered Users Posts: 816 Major grins
    edited May 15, 2009
    Qarik wrote:
    agreed...so now distance is the same but you have different zoom length on the full frame vs the crop. So you hold X contant and are varying Y. SO now zoom length has to be different because you are on full frame. No matter how you slice it DOF is a function of zoom, distance, aperture, and whether you are full frame or crop.

    The DOF calculator work bottom line and must include all factor to be accurate.

    No, it's a function of distance and aperture. Focal length only has an effect on the apparent blurring of the background. Depth of field, however, is simply a range of distances where you will have acceptable focus. For instance, at a given aperture and distance, the DOF might be 2.5 ft. It will be 2.5 ft whether you're using a 28mm lens or a 300mm lens. The background will be magnified in the 300mm shot, however, making it look more abstract than in the 28mm shot. It is not, however, more out of focus.

    Here's the website I was talking about earlier. You can see this effect in action with the sample pics:

    http://www.luminous-landscape.com/tutorials/dof2.shtml
  • pathfinderpathfinder Drive By Digital Shooter western IndianaSuper Moderators Posts: 14,614 moderator
    edited May 15, 2009
    Tim,

    Thanks for finding that link to Michael Reichman's take on this subject.

    I saw an article in Pop Photo or one of the photography magazines in the 1980s or 1990s with a similar set of pictures, demonstrating quite clearly, just like Michael's thread, that the DOF is not a function of focal length, as long as the subject is kept the same size on the image plane.

    However, we rarely try to keep the subject the same size as we change focal length. We usually change focal length to create a change in size in the subject in some manner
    Pathfinder - www.pathfinder.smugmug.com

    Moderator of the Technique Forum and Finishing School on Dgrin
  • QarikQarik Krazy Korean Registered Users Posts: 4,959 Major grins
    edited May 15, 2009
    No, it's a function of distance and aperture. Focal length only has an effect on the apparent blurring of the background. Depth of field, however, is simply a range of distances where you will have acceptable focus. For instance, at a given aperture and distance, the DOF might be 2.5 ft. It will be 2.5 ft whether you're using a 28mm lens or a 300mm lens. The background will be magnified in the 300mm shot, however, making it look more abstract than in the 28mm shot. It is not, however, more out of focus.

    Here's the website I was talking about earlier. You can see this effect in action with the sample pics:

    http://www.luminous-landscape.com/tutorials/dof2.shtml

    Yes I have seen that before with debating with pathfinder on this subject. heh

    Again I agree with everything you say..except distance to achieve the same framing is dependent on what? Zoom length and sensor size. The DOF calculators are accurate in that respect.

    Let me ask you..what is my DOF going to be if I am standing 10 feet from some subject with sperture at f/8? Answer is..well depend on zoom and sensor size. *shrug*

    From a phyics equation POV:

    Let f be the lens focal length, N be the lens f-number, and c be the circle of confusion for a given image format. The hyperfocal distance H is given by
    c4e9780064012b8891ebc70edc5f3fb7.png
    Moderate-to-large distances

    Let s be the distance at which the camera is focused (the “subject distance”). When s is large in comparison with the lens focal length, the distance DN from the camera to the near limit of DOF and the distance DF from the camera to the far limit of DOF are
    a1015da206f3ca83e0b4013dcf1fb1d1.png and
    3dd5632cdad928d3b9c2373a4278e164.pngNote: that H depends on the zoom and circle of confusion, which is is also determined by sensor size.
    D700, D600
    14-24 24-70 70-200mm (vr2)
    85 and 50 1.4
    45 PC and sb910 x2
    http://www.danielkimphotography.com
  • Tim KamppinenTim Kamppinen Major grins Registered Users Posts: 816 Major grins
    edited May 15, 2009
    So, when I posted that article I didn't re-read it, because I thought I remembered what it contained well enough. After reading pathfinder's last comment I was confused by his comment about the subject staying the same size in the frame, which is the opposite of what I've been saying.

    I re-read the article.

    I'm an idiot.

    I would like to publicly recant most of what I've been saying in this thread. I was foolishly giving erroneous explanations based on my memory of how this stuff works, which was clearly flawed. A simple 2 minute test with my 18-200mm lens clearly showed me that at the same camera-subject distance, focal length DOES make a huge difference in depth of field, even when the images are cropped to the same size. I was able to zoom in on the LCD and see fine detail in the 18mm shot which simply was not there, being totally blurred away, in the 200mm shot. So I was very wrong about that whole "telephotos magnify the background and that's why they look more out of focus..." bit.

    Sorry for all the confusing posts. I clearly do not understand this topic anywhere nearly as well as I thought I did. I guess I should check to see if what I'm posting has any basis in reality from now on.
  • QarikQarik Krazy Korean Registered Users Posts: 4,959 Major grins
    edited May 15, 2009
    that was an excellent recant! lol thumb.gif

    I have thought long and hard about this subject..it akept me awake at nights a fwe times! It does take while to really grasp some of the 2ndary concepts.

    I was just thinkiing of better way to say what I was trying to say..just because you can achieve the same DOF using different zooms with just aperture and distance doesn't mean that DOF is ONLY a function of aperture and distance.
    D700, D600
    14-24 24-70 70-200mm (vr2)
    85 and 50 1.4
    45 PC and sb910 x2
    http://www.danielkimphotography.com
  • pathfinderpathfinder Drive By Digital Shooter western IndianaSuper Moderators Posts: 14,614 moderator
    edited May 15, 2009
    I was about to distance myself from this discussion, because my tendency is to avoid confrontation, rather than rant about it.

    People confuse subject to image plane with magnification as the following link explains- http://cornicello.blogspot.com/2008/02/even-more-on-depth-of-field.html


    I do know, despite what Qarik and I have argued about in the past, that if the subject stays the same on the image plain, then the focal length does not have any effect on DOF - but of course for that to happen the lens to subject distance changes greatly depending on focal length. If the subject to image plane distance does not change, then the magnification changes greatly, and the DOF does very greatly as well.

    I hope this will finally put this thread to bed.thumb.gif
    Pathfinder - www.pathfinder.smugmug.com

    Moderator of the Technique Forum and Finishing School on Dgrin
  • Tim KamppinenTim Kamppinen Major grins Registered Users Posts: 816 Major grins
    edited May 15, 2009
    So basically what this all boils down to is that yes, taken as a single variable, ceteris paribus, longer focal length=shallower depth of field. Likewise, longer focal distance = greater depth of field, etc. However, if you keep the framing of the subject the same in each shot, the change in focal length and focal distance effectively cancels each other out and the depth of field is equivalent. It may not appear to be at first glance, however, because of the change in perspective which increases or decreases the size of the background depending on focal length (if the framing remains constant, i.e. you move forward or backward as needed).

    And yes, I am shamelessly attempting to reclaim some measure of my lost dignity through the use of obscure Latin phrases.
  • mmmattmmmatt Big Grimace Registered Users Posts: 1,347 Major grins
    edited May 16, 2009
    So basically what this all boils down to is that yes, taken as a single variable, ceteris paribus, longer focal length=shallower depth of field. Likewise, longer focal distance = greater depth of field, etc. However, if you keep the framing of the subject the same in each shot, the change in focal length and focal distance effectively cancels each other out and the depth of field is equivalent. It may not appear to be at first glance, however, because of the change in perspective which increases or decreases the size of the background depending on focal length (if the framing remains constant, i.e. you move forward or backward as needed).

    And yes, I am shamelessly attempting to reclaim some measure of my lost dignity through the use of obscure Latin phrases.
    or... with a given lens at a given distance and f-stop, depth of field is the same regardless of sensor size. If you change distance to subject, f-stop, or focal length, then your dof changes. Man that took a long time!

    matt
    My Smugmug site

    Bodies: Canon 5d mkII, 5d, 40d
    Lenses: 24-70 f2.8L, 70-200 f4.0L, 135 f2L, 85 f1.8, 50 1.8, 100 f2.8 macro, Tamron 28-105 f2.8
    Flash: 2x 580 exII, Canon ST-E2, 2x Pocket Wizard flexTT5, and some lower end studio strobes
  • kofakofa Big grins ZürichRegistered Users Posts: 60 Big grins
    edited November 24, 2009
    mmmatt wrote:
    When I change the camera from full frame to crop sensor (5d to 40d) it gives different measurements for dof, hyperfocal etc... that isn't the case as far as I know. That would be like saying that the center of the lens has different focusing properties than the outside. As far as I know the only difference between different sensor sizes is the field of view and not the focusing properties.
    Matt, I think the difference lies in 'circle of confusion'.
    http://dofmaster.com/digital_coc.html
    http://en.wikipedia.org/wiki/Circle_of_confusion

    You'll see CoC change on the DOF calculator page as you switch between cameras. On a smaller sensor, the same circle that the lens maps each point of the scene to will represent a larger portion of the frame, thus the physical point will be more blurred in the resulting image (the same physical diameter will be relatively larger). It's as if you cropped your full-frame image and upscaled the crop to the original resolution - dots that used to be sharp-edged pixels become tiny circles with soft edges - no longer tack-sharp; small circles (soft dots) will become more blurred etc.

    Check for example the APC-size Nikons: CoC is 0.02 mm, while for a full-frame sensor, it's 0.03 mm.

    This could also explain why some lenses are very sharp on full-frame bodies, but not on APC-size sensors: they are unable to produce small enough circles, meaning light from a single point in the image will hit more than a single (small) pixel on the APC sensor, while it may only cover a single (larger) pixel on the full-frame sensor.

    This is what I think :-) If I'm wrong, I probably also lie within the 'circle of confusion' headscratch.gif

    Kofa
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