Hyperfocal question

[johncox]

<p>Am I correct in assuming that lens formulation ( lens format) dictates the hyperfocal focusing formula, opposed to the focal length. In other words; a lens for a full frame camera will show more in focus than the same focal length made for a APS-C camera?<br>

If this is the case does anyone have any suggestions as to a good 35mm 645 lens I can adapt to Sony (Nex) Alpha?</p>

 

[dave_s]

<p>(i) Your assumption is incorrect. Any 35mm lens will have the same depth-of-field and hyperfocal characteristics as any other 35mm lens. It has everything to do with physical optics, and nothing to do with lens design. If your pockets are fairly deep, there's an excellent Zeiss 35mm for the Sony, but you probably know that.</p>

<p>(ii) The experts will say that depth of field (and by extension, hyperfocal distance) is only determined by subject magnification. One of them will perhaps pitch in with an explanation of the physics.</p>

[Craig_Cooper11664875449]

<p>Following on from dave's explanation, 3 approaches:</p>

<p>1) Focus is only accurate at the focal plane, perceived DoF is about what is considered visually acceptable. If you look into DoF calculations you find a parameter called "circle of confusion". These "circles" represent a level of being "out of focus" in the image but still remaining visually acceptable as being in focus when viewed. On the sensor/film, regardless of format, they are the same for any lens at a particular setting. However, the more you enlarge an image, the more some of these CoC grow to a point where a viewer perceives that part of the image as "not sharp". The logic is that you would need to enlarge a small format more than a larger one for the same size output, therefore the large format has a greater perceived DoF. If you also cropped the FF image to be the same FoV as the APS-C image - they are both exactly the same for DoF.</p>

<p>2) If you follow on from point #1 and change your distance to subject to achieve the same FoV, the APS-C will have more DoF at a ratio equal to the format multiplier (ie x1.5)</p>

<p>3) The above approach assumes the use of the same lens, which across formats is going to give a different field of view. Therefore, rather than comparing, for example, a 50mm on full frame to a 50mm on APS-C, its more likely you would use a lens that gave a similar FoV across formats for the same subject. Therefore, a 50mm on full frame would be replaced by a 35mm lens on APS-C. And a 35mm lens has more DoF to begin with than a 50mm, regardless of format used - but which is offset by the greater enlargement required. The basic calculation for this would be - on APS-C to FF comparisons - to use the 1.5x format multiplier on the aperture to say that a 35mm lens [at, for example, f/4] on an APS-C sensor would provide the same DoF as a 50mm lens [at f/6] on a full frame sensor.</p>

[Peter_in_PA]

<p>A smaller format has greater apparent depth of field for sure.<br /><br />f1.4 on a µ43 camera, for instance, is equivalent to f2.8 on a full frame FX or 35mm.<br /><br />Mostly because of crop factor.</p>

[Craig_Cooper11664875449]

<blockquote>

<p>Mostly because of crop factor</p>

</blockquote>

<p>The crop factor as of itself has nothing to do with the DoF. It is only due to one of the following:</p>

<ol>

<li>The magnification of any enlargement differences</li>

<li>The change in distance to subject in order to achieve a similar FoV with the same lens</li>

<li>Using a shorter focal length to achieve the same FoV from the same subject distance</li>

</ol>

[steven_clark]

A 35mm lens on full frame should have greater hyperfocal

distance than the same lens on APS-C at the same

settings because the APS-C image is being enlarged more,

magnifying the out of focus effects. Of course the

perspective is different because of the crop factor as

well.

[Craig_Cooper11664875449]

<blockquote>

<p>A 35mm lens on full frame should have greater hyperfocal distance than the same lens on APS-C at the same settings because the APS-C image is being enlarged more, magnifying the out of focus effects.</p>

</blockquote>

<p>Not clear by what you mean by "hyperfocal distance" here. The hyperfocal distance of a particular focal length at a particular aperture is a distance "D" such that (under the assumptions of sufficiently in focus for the ouptut) everything from D/2 to Infinity is in focus. As you say, the increased magnification on the small format is the DoF limiting factor here. But because you need to satisfy "D" such that acceptable focus starts at D/2, the hyperfocal distance "D" must be greater (further away) on the smaller format. If you mean that the total distance of D/2 to infinity is greater on the larger format, for the same output size, then yes. </p>

<blockquote>

<p>Of course the perspective is different because of the crop factor as well.</p>

</blockquote>

<p>Only if the distance from the subject is different...</p>

[pics]

<p>Without trying to delve into the mathematics and circles of confusion, the smaller the sensor size the greater the depth of field. As sensor size increases, depth of field decreases.</p>

<p>From Bob Atkins article on the subject:</p>

<p>http://www.bobatkins.com/photography/technical/digitaldof.html</p>

<p>"So the bottom line - and all you really need to know - is that <em><strong>DOF is inversely proportional to format size</strong></em>...Note also that now you can see one of the reasons large format camera users need tilts and swings to get adequate depth of field. With an 8x10 camera you have about 8.5 times LESS depth of field than you do with 35mm for the same image. This also explains why consumer digicams, some of which have sensors 1/6 the size of 35mm film, have such a large depth of field and one of the reasons why it's almost impossible to get blurred backgrounds when using them....So if you make the same size print and shoot with a lens that gives you the same view and you use the same aperture, if you halve the format size you double the DOF, if you double the format size you halve the DOF....."</p>

[johncox]

<p>From what I understand -and the reason I asked- I had thought that a large circle of confusion equaled very little depth of field in lens design. So my theory was that a lens for larger circle of confusion ( a full frame camera ) would show more in focus on a wide angle lens.<br /> And since smaller sensors magnify the percieved focal length, what would be a wide angle lens on a full frame with a lot of DoF would be a standard on a M43, with the same DoF. Hence I want to use a 645/6x6 lens for street photography.</p>

[Craig_Cooper11664875449]

<p>John,<br>

Firstly decide if you're talking about same focal length or same FoV across formats. Basically, a lens of focal length "X" is the same across any format (image circle to cover the format aside). You are obviously aware of CoC; the effect being that for a given aperture the CoC increase in size as you move away from the focus plane until they "appear" as soft, out of focus, into blur.<br>

Regardless of format, those CoC are "recorded" at exactly the same size, for the same lens, on the media - the sensor/film size does nothing to it. If you cropped the larger image to be the same coverage as the smaller one, both would be exactly the same - DoF, viewing magnification, etc.</p>

<blockquote>

<p>And since smaller sensors magnify the percieved focal length...</p>

</blockquote>

<p>Maybe this statement is where you're getting confused. To repeat, sensor size does NOTHING to the DoF in and of itself. The only thing sensor size does is drive user behaviour to choose a focal length and/or distance from the subject to make the image. Three things are basically at play.</p>

<ol>

<li>You used the same lens but had to move further back on the smaller sensor to get the same FoV, and therefore more DoF</li>

<li>You used a shorter focal length to get the same FoV from the same distance - the shorter focal length at the same distance is giving you a great DoF</li>

<li>You "printed" (viewed) the image at the same size (magnification) from both sensors. The smaller one required a greater enlargement which increased the observed size of the CoC moving more of the image into the perceived out of focus section.</li>

</ol>

[johncox]

<blockquote>

<p>Maybe this statement is where you're getting confused. To repeat, sensor size does NOTHING to the DoF in and of itself. The only thing sensor size does is drive user behaviour to choose a focal length and/or distance from the subject to make the image. Three things are basically at play.</p>

<ol>

<li>You used the same lens but had to move further back on the smaller sensor to get the same FoV, and therefore more DoF</li>

<li>You used a shorter focal length to get the same FoV from the same distance - the shorter focal length at the same distance is giving you a great DoF</li>

<li>You "printed" (viewed) the image at the same size (magnification) from both sensors. The smaller one required a greater enlargement which increased the observed size of the CoC moving more of the image into the perceived out of focus section.</li>

</ol></blockquote>

<p>No that's my Point, a 645 35mm has immense depth of field. About the same as a fish eye. Shouldn't that stay the same on a m43 or nex 7?</p>

[Craig_Cooper11664875449]

<blockquote>

<p>Shouldn't that stay the same on a m43 or nex 7?</p>

</blockquote>

<p>Why does a 645 35mm have good DoF on a 645 camera? Primarily because a 35mm lens generally has good DoF and when used on a 645 you are not enlarging the 645 negative/sensor as much as you would a smaller negative/sensor for the same output, therefore smaller CoC.<br>

For this argument, a 35mm lens is a 35mm lens, thats all there is to it it doesn't matter what format it was made for <em>(that is more about image circle size)</em>. The DoF characteristics will only remain the same across formats if you behave exactly the same:</p>

<ul>

<li>Set same aperture</li>

<li>Are at the same distance to subject <em>(narrower FoV,78 degrees on 645, 38 degrees on Nex 7, 29 degrees on M4/3)</em></li>

<li>Results viewed at the same size (if the 645 image is (56cm x 42cm), then Nex 7 would need to be (23cm x 15cm) and a M4/3 image would need to be (18cm x 12cm) for equivalence</li>

</ul>

<p>The only real value in looking at medium format lenses on small format cameras is for use with i) tilt/shift movement adaptors available or ii) Fotodiox Rhinocam type adaptors that leverage the larger image circle.</p>

[pics]

<p><strong>"To repeat, sensor size does NOTHING to the DoF in and of itself"</strong><br /> <br /> I think this may be a somewhat confusing statement. Yes by itself sensor size doesn't affect depth of field, but adding the equation of lenses (as most photographers do) it certainly does play a role and I <em>think</em> this is what the OP is asking about. (Although from some of his statements I think he may be confusing the terms field of view, focal length and depth of field.) Anyone who has shot a format larger than 35mm knows you have less depth of field if you choose a lens that gives you the same field of view at the same subject to lens distance as its 35mm equivalent and use the same aperture. Lets say I am in a situation where the subject to lens distance is fixed and I use a 50mm lens on 35mm format camera and shoot it at f5.6. If I then go to the 645 format, choose an 80mm lens (which gives roughly the same field of view at the same subject to lens distance) and shoot at f5.6 as well, the depth of field will certainly be shallower. On a large format camera it would be even more shallow.</p>

<p>Yes you can find ways to achieve similar depth of field by adjusting f-stop, lenses, lens to subject distance, tilt-shift etc. but these things are all done to account for the inherent differences in depth of field caused by the different sensor formats.</p>

[jcuknz]

<p>For a given subject matter the DoF remains the same irrespective of what focal length lens is used [kodak reference booklet years ago]</p>

<p>except when the magnification is obtained in editing. The result of this is that it can be better to stay back a bit and enlarge in PP rather than use the lens to achieve the tight framing required. [<em>Stagecoach</em> in CiC forum and proved in a personal experiment]</p>