CoC factor for 11x14?

<p>Hi All,<br>

I'm in process of building (well, more in the design stage) and i'm trying to figure out some of the core math. Can someone point me to a CoC formula (for dummies) or throw me a bone as to what a good CoC factor would be for an 11x14? I'm making a ULF tintype and trying to see if i want/need to go this big or if if i should go larger. <br>

I'm assuming that essentially the short side is the predominant factor so if an 8x10 is .22 and a 16x20 is a .44, the 11" side should fall proportionally between the 8@.22 and the 16@.44, which i'm extrapolating at around .303 for 11"<br>

Thanks ahead of time!<br>

--PatrickD<br>

btw, these are the ones i've been using so far:</p>

<table width="248" border="0" cellspacing="0" cellpadding="0"><colgroup><col width="142" /> <col width="106" /> </colgroup>

<tbody>

<tr>

<td width="142" height="20">Film Size</td>

<td width="106">CoC (mm)</td>

</tr>

<tr>

<td height="20">8x10</td>

<td align="right">0.22</td>

</tr>

<tr>

<td height="20">5x7</td>

<td align="right">0.15</td>

</tr>

<tr>

<td height="20">4x5</td>

<td align="right">0.11</td>

</tr>

<tr>

<td height="20">16x20</td>

<td align="right">0.44</td>

</tr>

<tr>

<td height="20"> </td>

<td align="right"> </td>

</tr>

</tbody>

</table>

 

<p>Why are you even looking at CoC's? Are you trying to get shallow DOF? Magnification and lens choice are going to be far bigger factors in the image than a theoretical CoC based on format size. Have you shot tintypes in smaller formats before? What exactly are you looking to take pictures of and what are you trying to accomplish? </p>

<p>It's based on the linear dimension, unsurprisingly: just take the diagonal. From your data the factor is about 1479, where everything is in mm. So for sides x and y, c = sqrt(x^2 + y^2)/1479, about. Remember x and y are in mm. So for 8x10 we get c = sqrt((8*25.4)^2+(10*25.4)^2)/1479 = 0.22. For 11x14 you get about 0.31.</p>

<p> </p>

<p>Hi Sheldon, i agree that the magnification and lens size are most critical. I've not shot anything larger than 4x5 (film, not wetplate). I'm haven't shot wetplate before but have been putting in some of the bookworm stuff...trust me, i know that does not qualify as practical experience. :) I am *trying* to build a camera from scratch. When i delve into something i want to know down to the atomic level as to what is going on. It's more intellectual curiosity than practical application...BUT i do want to get a good idea as to the operating limitations, such as DoF vs Magnification vs Bellows extension. I would hate to build a camera that has too little bellows or invest in something with too much, that's what's driving this.<br>

Thanks!</p>

<p>Hi Tim. THE DIAGONAL!!! of course! I'm an idiot. That makes perfect sense. THANK YOU THANK YOU THANK YOU!</p>

<p>Interesting thought experiment but perhaps disconnected from the more important questions involved in choosing what format size to shoot, especially with tintypes.</p>

<p>Hi Sheldon. I think i know what you're saying. To clarify, i'm working on a multi-format wetplate camera that'll will have film holders that support *up to* 11x14 AND down to 4x5. I'm not a fan of unitasker hardware, camera or otherwise. I'm still in the head scratching phase of this so not sure how practical/rational this idea is. I got a process lens (360mm, f/9) that i've been using for a camera obscura to get a better understanding of the mechanics/physics. But yes, it is indeed a fun thought experiment and i've learned a helluva lot in the exploration.</p>

 

<p>I'm a bit confused here as well.</p>

<p>In the first post, you state you are making a tintype camera. Based on that statement, I assume you are not going to enlarge anything. (It is possible to enlarge wet plate, glass negatives, but not common.) Since tintypes are not clear but on metal, they are pretty hard to enlarge.</p>

<p>If you are not enlarging, then there is no reason to have any COC calculations. All images will be viewed as originals. The only figure you need is a constant, the size of a point appears as a circle at X size. The figure will not vary from 4x5 to 8x10 as all images would be viewed at the same distance. Perhaps 11x14 would be a LARGER figure, because it is viewed from a greater viewing distance. </p>

<p>One definition of circle of confusion I heard was a group of photographers sitting around a camp fire discussing depth of field. </p>

<p> </p>

<p>Allen. Maybe i've not been clear (self-taught newbie to darkroom/film)...i like your CoC definition. I'm not "enlarging" as in a darkroom technique, but my understanding is that based on subject distance/bellows extension, there will be some proportional magnification/reduction that occurs. I wanted the CoC because that's a key factor in estimating the DoF when shooting. I understand it's not a definite, but was thinking that it should be helpful in understanding the relative relationship between sizes. <br>

All that being said, i now see a flaw in my logic/understanding. Doing the math, shooting for 11x14 @ f9.0 and the subject is 6ft from the camera, i calculate a DoF of about 4.5 inches...the bellows needs to be just shy of 18". Assuming i only swap out the film for a 4x5 format and keep everything else the same, the dof should be the same, but mathematically the DoF is 1.5". Sure, the coverage area will be different (smaller) but the DoF should be identical, shouldn't it? I'm using the formulas from DoFMaster...is there a different formula when the film plane is adjustable (large format) as opposed to Fixed (SLR)?</p>

The CoC size is a (rather arbitrarily) measure for how big a blob a point may be before it is recognizable as a blob and not a point when magnified (!). The usual DoF formulae assume that people watch prints of a given size, so when you change to a different format film, the degree of enlargement changes too, with the smaller format being enlarged more than a larger format to arrive at the same size final image. More magnification when using smaller formats means that the CoC has to be smaller if it is supposed to be still too small to be recognized as a blob instead of a point.<br>When you don't enlarge the image, view both 11x14" and 4x5" as they come out of the camera, there is no reason to use a different CoC size in the formula, and DoF will indeed be the same.<br>Unless you will be viewing the 11x14" from further away because it is too large to view at the same viewing distance as the 4x5". The the larger viewing distance means a smaller magnification, and the larger image will have more DoF than the smaller.<br><br>DoF and CoC are very diffuse notions, and i think you are troubling yourself with considerations that are not important. You will chose your lens on the 11x14" according to the scene you want to capture. The bellows extension and subject distance will be set by what focal length it is you need to capture the scene on that format.<br>You can use different apertures to play with whatever DoF there is. But you don't get much of a choice: it is what it is.

<p>Ok, so i'm getting further and further down this rabbit hole. It seems like there are two similar but distinct uses for CoC here. When you focus on something, technically everything but an infinitely small point (or plane) is in focus, and everything else is out of focus progressively as you move away from the focus point. By my understanding the CoC is "simply" the demarcation point that we can visually start to discern that something is out of focus. To Q.G.'s point, the further back you stand from an object, the less likely you are detect whether it's a blob or point...this is what i'll call the "in camera" CoC. The other is in regards to the print such that if you print something larger, the easier it is to detect the lack of sharpness/focus (much like standing closer to the subject). That being said, if while shooting a portrait and focused on the eyes, the depth of field can be such that the ears and nose are out of focus. True focus is an infinitely small point, the zone that is in focus is the DoF, the CoC "factor" determines where that front/back focus boundaries are. So if i'm shooting with 11x14 plates and the person's head takes up the entire frame, when i downsize to to 4x5 (with everything else remaining the same), it crops out to maybe just the person's nose and eyes (you get the point). However, the focus and the DoF should be the same...BUUUUT when i run the numbers based on all the formulas that i've found, the 4x5 DoF is in fact shallower. THERE is my confusion...so far....i think. (I also think Allen is probably rounding up some wood for my campfire)</p>

That will be because those calculators use formulae that contain assumptions that do not hold in your case, such as that the CoC for 4x5" has to be smaller (hence shallower depth of field). That assumption would make some sense if you were calculating DoF for prints made from 4x5" that need to be enlarged more than same size prints made from 8x10". See the numbers you posted in your first post.<br><br>But you are not enlarging that 4x5" more, so that assumption, the formulae based on that assumption and the calculators using those formulae are incorrect, of no use to you. For your example and purpose the allowable CoC size does not change, even though the format does. So DoF does not change either. (Assuming, of course, the only change is taking the 4x5" crop from a 11x14", with the rest, including viewing distance, remaining the same).

<p>I put "Depth of field Master" into Google and found this site:<br>

http://www.dofmaster.com/dofjs.html<br>

I assume it is the calculator you are using. </p>

<p>If you select a CoC of .030mm in the fist box, "Camera, Film Format or circle of confusion" and then complete the rest of the information for your scene, you should get a DoF calculation that will work for contact prints up to 11x14 (and possibly all contact prints.)</p>

<p>.030 mm is a generally used figure for PRINTS. If a circle appears this size or smaller on a print, it will look like a point on the print to the average person. Since you are producing in camera originals, or contact printing, the same constant should work for you. </p>

 

<p>I have a simpler answer for you. On 8x10 the DOF will be too shallow. On 11x14 the DOF will be WAY too shallow. :-)</p>

<p>I hope you have a WHOLE lot of strobe power. Don't forget that your ISO on tintypes is going to be incredibly low. I got to see a setup of someone who was doing daguerrotype portraits. He was using a beauty dish with a twin head strobe running 9600 watt seconds to make his exposures. </p>

<p>There's a whole additional wrinkle that happens too when you start to shoot portraits with big formats... it starts to become a "macro" shot, since technically you are at 1:1 magnification when you shoot a tight headshot on 11x14. Add in the bellows factor for that, and you'll need another 2 stops of strobe power. You get a change in field of view too as you drift into that macro territory, the lens starts capturing a narrower and narrower field of view as you extend the bellows. </p>

<p>It's a pretty big undertaking to dive right into both tintypes and very large formats at the same time. </p>

<p> </p>

<p>Patrick,</p>

<p>A few practical things for you to consider.</p>

<p>If you are building a camera with a ground glass, you really won't need any DoF or CoC calculations to consider. If the image is sharp on the ground glass, it will be sharp on the tintype. You either stop down or use movements to get the image sharp. </p>

<p>I shoot wetplates and have an interesting holder I picked up on eBay. The holder is 8x10, but it has wooden inserts that nest inside each other. The one holder can be used for 8x10, 5x7, 4x5 and some of the older plate sizes. I don't need different backs or holders. I just select the insert for the size glass I'm using and shoot in my 8x10 camera. It is a lot more convenient than having different backs and holders around. <br>

<br>

Sheldon brings up an interesting point about 11x14 and larger. If you shoot portraits with 11x14 and fill the frame with the face, you are basically shooting 1:1, which is macro. There was a series in View Camera Magazine couple of years ago about choosing lenses and bellow's draw with ULF cameras. I don't remember if Fred at the View Camera Store wrote it or if it was Steve Simmons. You should be able to find the article with some diligent google searching. It is something to consider when thinking about how much bellows you will need. </p>

<p>For chemistry, Bostick and Sullivan have ready to shoot chems available--unless you plan on japanning your own plates. </p>

<p>I got aluminum plates from a local trophy shop. The owner had odd sized pieces left from cutting 20x24 pieces down for name plates. He gave them to me just to clear them out of his store. Unfortunately, the store closed a couple of years ago and lately I've had to buy big sheets and cut them myself. </p>

<p>I did a little testing and found that my collodion mix (landscape #7) usually had a shooting speed equivalent to ISO 1 in bright sunlight. It made it easy to use a standard meter to get the exposure correct.</p>

<p>I read a great deal before starting wet plate. I then took a one day workshop. The workshop was well worth the time. It answered all my questions and I learned a lot of ins-and-outs that don't show up in the books.</p>

<p>There is a wealth of information on wet plates on the collodion forum: http://www.collodion.com</p>

<p>Quinn Jacobsen has a number of videos on the process on the net. <br>

</p>

<p> </p>

<p>Hmm. Patric started out saying that he wants to shoot tintypes and such that won't be enlarged. He's making the final print in camera. Fine, wonderful.</p>

<p>In that case, if he wants the image to pass the standard close scrutiny (viewed from 10 inches) with the usual sharpness criterion (at least 8 lp/mm) then the CoC he uses to calculate DoF can't be larger than .125 mm and he can't shoot at an aperture smaller than f/200 or so. This is independed of format and lens.</p>

<p>After some prodding, Patrick said he wants to shoot wet plate and enlarge. If he still wants the final print to pass the standard close scrutiny with the usual sharpness criterion then he needs 8 * n lp/mm in the negative. n is the enlargement, e.g., 4 to make a 16 x 20 print from a 4x5 neg. Go to it, Patrick.</p>

<p>To further complicate the problem, the required DoF varies with different subjects. Edward Weston sacrificed sharpness in favor of greater DoF in some of his macro-photographs by using tiny diffraction limited apertures. Because of the subjects, form and luminosity were more important than apparently perfect sharpness. Soft focus lenses also complicate DoF calculations. DoF considerations are more art than science.</p>