Does size effect DOF?

[chad_goldman]

<p>In the past I have attempted Macro using a teleconverter or Close-up lens. I'm now saving up for a Macro lens. I had the Nikon 105 VR in my sights, but am now thinking of getting the 85 instead. The main purpose of this lens would be macro and Portraits.<br>

My question is, is DOF affected by the size of the lens? With all things being equal, focus distance, f-stop, will a 105 give me a greater DOF than an 85?<br>

Thanks<br>

Chad</p>

[sheldonnalos]

<p>It depends...</p>

<p>If you are shooting portraits at medium distance, the lenses will have roughly equal DOF at equal apertures. The longer lens will give you less background in the frame and the appearance of a more blurred out background, but in terms of actual DOF (eye in focus, ear out of focus, etc) the lenses will have the same DOF.</p>

<p>For macro, its pretty much the same story. Same DOF at the same aperture, but less background in your shot with a longer focal length.</p>

<p>I'm sure there are those who will give a more technically correct answer, but this is a good rough guide.</p>

[steven_f1]

<p>You can use this <a href="http://www.dofmaster.com/dofjs.html">depth of field calculator</a> to determine the depth of field for any given situation. </p>

<p>The focal length of the lens is the primary factor in determining depth of field. Other important factors are the focus distance and <a href="http://en.wikipedia.org/wiki/Circle_of_confusion">circle of confusion (CoC)</a>. The further away you get from a subject the greater depth of field. Circle of confusion relates to how blurry a spot gets as it gets closer to the edge of the depth of field and what size print you are making. The end result is that a small 4x6" image will appear to have a greater depth of field than the depth of field of a 20X30" print of the same image. The spot has to get very blurry before you will see the blurr in the 4X6" while you will notice the blurr much earlier on the larger print. COC is also applies to the digital sensor which is why depth of field calculator has a camera selection. </p>

<p><a href="http://www.photo.net/photodb/user?user_id=1008200">Sheldon </a> post may sound wrong after reading my post but there is one situation where it is correct. The 85mm lens has a larger field of view than the 105mm. If you take one picture with an 85mm at F10 and then put on the 105mm lens (set to F10) you would have to move the camera away from the subject ot get the same field of view. In this situation the depth of field will be the same because the focus distance is different. </p>

<p>Bob Atkins has a <a href="../learn/optics/dofdigital/">detailed post </a>about this in photo.net.</p>

[steven_f1]

<p>Sorry double post.</p>

 

[sheldonnalos]

<p>Yes, I should have qualified my earlier comments. I was assuming that comparison shots would be equally framed between the two lenses - ie. you would back up when shooting with the 105 vs the 85.</p>

[chad_goldman]

<p>Thanks guys, this clears some things up for me. Steven, thanks for the links, I'll be sure to check them out as well!<br>

Chad</p>

[martin-s]

<p>Between the 105 and 85mm differences will be rather subtle.<br>

If you scroll down on <a href="http://www.the-digital-picture.com/Reviews/Canon-EF-100mm-f-2.8-USM-Macro-Lens-Review.aspx">this page</a> you can see a comparision between 180/100/60mm. While the main subject is equal in size and sharpness, the background becomes increasingly blurry the longer the focal length is.</p>

[brian_reischl]

<p>The most in-depth article I've found on this is here: <a href="http://toothwalker.org/optics/dof.html">http://toothwalker.org/optics/dof.html</a></p>

<p>The short version is that as the magnification increases, the focal length matters less. By the time you get to 1:1 magnification, the focal length is irrelevant. </p>

[craig_meddaugh]

<p>Subject magnification determines the DOF, so if you have the same framing the DOF is equal no matter what lens you use. However, longer lenses capture less of the background due to their narrower field of view. This gives the illusion of a smaller DOF and better "bokeh". I personally prefer to shoot the longest lens I can get away with for these reasons. </p>

[dcstep]

<p>Chad, why don't you try Extension Tubes with your current lenses before going to a macro lens? A teleconverter is not going to shorten the focus distance like an ET will.</p>

<p>If you need a dual purpose macro/portrait lens, then consider a new lens, but you might be totally happy with ETs for macro, depending on the magnification you're trying to achieve.</p>