Real CoC (circle of confusion)

[stefaan_ellebaut]

<p>Hi,<br>

I'd like to make DoF calculator table for my Holga but therefor I need the Circel of Confusion for this camera. Normal 6x6 is around 0,045 but I read here (http://www.flickr.com/groups/86881528@N00/discuss/72157613069887918/) it is 0,053. Anyone has other sources were I can find this?<br>

Thx,<br>

Stefaan</p>

[nickc1]

<p>There is no fixed value for circle of confusion that anyone can give to you, only accepted values.<br>

An optical system will only produce a focussed image at a single distance - there is no, theoretically at least, such thing as depth of field. In practice the human eye is unable to distinguish between a sharp image and one very very very nearly sharp - hence the depth of field that appears to produce a sharp image. This will depend upon the resolving power of the lens, the ability of the film or sensor to record a sharp image, the amount of enlargement if any, the viewing distance & angle and the ability of the eye to distinguish between sharp and not so sharp. In practice this is the size of a splodge or circle of light that the viewer cannot tell from a point of light (which is an an optical concept only)<br>

<br />The circle of confusion is a measure of the diameter of this circle - the larger the value the more chance there is that a particularly acute set of circumstances may reveal it to be a circle and not a point.<br>

It would not usually be practical to measure the circle of confusion for all of the variables, so instead one of a number of formulae is used. These include d/1730 and d/1500 where d is the diagonal size of the film or sensor, and f/1720 where f is the focal length of the lens. All of these will produce a value that is a suitable approximation so the eye can interpret an image as having depth of field.<br /><br />I suspect that the different figures you have are from these different approximations, and if you put your figures into these formulae you will see that any one is suitable and produces little difference in depth of field, and because this is usually found to be the case, the formula d/1500 is the one most often used.<br />It might be necessary to adjust the values in your case, as all of these approximations assume a lens of 'standard' resolving power is used - with a plastic lensed Holga the approximation may no longer hold true.<br /><br />Finally, as a point to think over - these are only approximations, you may wonder why when using the d/1500 formula you would end up with different CoC, and hence different depths of field for 120 film and 35mm film in a Holga, even though the optical system has not changed - but if you use the f/1720 you would not - Go figure!</p>

[jim_jones3]

<p>Good answer, Nick. Long ago I spent hours wrestling with the formulae for depth of field, circle of confusion, and hyperfocal distance, and eventually came to an understanding that worked for me. Making this understanding comprehensible to anyone else would be impossible. Doing the math yourself may well be worth the effort. Also, these subjects are all too often treated as science. The art [?] of Holga photography may be too remote from science for DOF charts to be applicable.</p>

[stefaan_ellebaut]

<p>Thx for the responses Nick & Jim. I know the Holga requires a bit unpredictability but I'm a bit of a control freak, even with these subjects :D<br>

The reason I ask is, I'm planning a strobist shoot outdoors. So it would come in handy to have a hyperfocal distance or a rangefinder card so I can focus more on handling the strobes and directing the models :)</p>

[sarah_fox]

<p>Stefaan, an alternative approach to dealing with DoF tables and CoC numbers is to use Harold Merklinger's object field method, which determines DoF with respect to physical aperture size vs. focal distance vs. distance from the objects you're photographing. It's very much worth studying and learning, as it will make DoF estimations very intuitive. You will no longer need charts/tables and/or any calculator. Moreover, it will give you a way to estimate degree of blur, which is a concept completely absent from conventional DoF computations. You can find the articles online here:</p>

<p><a href="http://jimdoty.com/learn/dof/dof_merk/dof_merk.html">http://jimdoty.com/learn/dof/dof_merk/dof_merk.html</a></p>

<p>I will only add that I don't agree 100% with Merklinger's approach. I think his infinite focus approach is silly (beyond a useful conceptual exercise), and he totally ignores the importance of visual angle. However, what he lays out is a very practical and useful approach to aperture selection.</p>

[JDMvW]

<p>CoC (and for that matter DoF) are largely psychological factors, not physical ones. It's only that most people share the same general perception that make tables work at all. That's why Nick's first sentence is really correct.</p>

<p>If you are a control freak, by the way, you <em>are</em> missing something critical about the whole Holga experience. ;)</p>

 

[stefaan_ellebaut]

<p>Hi Sarah, <br />Looks like an interesting link you posted. Althoug I won't be able to go through it all by the shoot I have planned on Saturday :-) I'll sure read it when I have a bit more spare time.<br>

JDM,<br />Correct, I know it's largely a non-physical aspect but I was wondering if Holga-experienced photographers have an idea on what CoC come close to a 'standard issue' Holga 120, as far as that even exists :D<br />I know I'm missing some critical stuff about the whole Holga experience this way, but my question is only asked for the strobist shoot I'm doing this weekend. I've done some 'freestyling' without thinking about any settings or technical stuff this summer and liked the results already (see http://www.focaldesign.be/v2/2011/09/08/oostende-op-zn-holgas/)</p>

<p> </p>

[rob_piontek]

<p>I suggest you actually try calibrating/checking your own holga to see what distance the symbols actually correspond to. Mine focuses much closer than what I would have thought based on the symbols. </p>

[stefaan_ellebaut]

<p>How do I check this the best? As it isn't watching through the lens... What is the best setup/working method to check this?</p>

[richardsperry]

I think you take pictures at the printed icon places. Then look at the pictures to see what's in focus at those marks.

 

Honestly, it's a Holga, stop worrying so much about it. Go out and shoot it. Run a dozen rolls through it. The only thing

you need to worry about is remembering to take the cap off.