Michael Glantzis

Hi all, found the answer I was looking for. Since DOF = (2x distance² x fstop x Acceptable CoC) / (focal length²), focal length has a greater effect on the DoF than the CoC. Also, solving for CoC, focal length has a greater effect on projected CoC than the size of the entrance pupil. Basically I was confused because I assumed that the size of the entrance pupil directly varied with the size of the projected CoC by a linear constant, however the constant is not linear but rather a more complex relationship between things.

Wishing I could've posted this in the "Cameras Systems and Formats" forum, but there really isn't a way to post about "formats" in general, so I'm posting it here. Basically I'm trying to figure out why large format systems have a shallower depth of field than smaller formats. This seems to be the common wisdom amongst everyone I talk to, and DoF calculators online seem to agree. I know that large format imagers require proportionally longer lenses to maintain the same framing, and that longer lenses at the same stop will have a larger entrance pupil which projects a larger circle of confusion on the imager. What I'm not understanding though is why this would lead to a shallower depth of field since a larger imager has to be scaled less in order to be viewed in its intended format. For example, if I had a 100mm diagonal imager area and I were shooting on a 50mm lens at an f2, I would have a 25mm entrance pupil. If I then moved to a 50mm diagonal imager, I would need to shoot with a 25mm lens to maintain the same framing, and if I were still shooting at an f2, I would have a 12.5mm entrance pupil. When viewing the final picture, the image from the 50mm diagonal imager would need to be scaled twice as much as the image from the 100mm imager. This leads me to expect that the circle of confusion projected by the 12.5mm entrance pupil will ultimately match the circle of confusion projected by the 25mm entrance pupil since the smaller imager is being scaled by an additional factor of 2. But everyone seems to disagree with this! My only blind spot here is that I don't know the math behind how the size of the entrance pupil relates to the size of the projected circle of confusion on the imager. I've been assuming that they are related by a constant factor, but perhaps my answer lies in the math behind this relationship. Would honestly love to find this out. I've been arguing with my coworkers on set about this for years now, and I honestly need one less reason to be as obnoxious as I already am.

Thanks for the response. I agree that would be an interesting experiment. Perhaps some day when I have access to more filters I'll give it a go haha. In the meantime, I suppose I'll have to stick to what's available. We're really fortunate to have so much testing footage available to us, and I'm sure I'll develop more of an intuition around these things as I get more experience shooting. Once again, thanks for the response. Definitely interested to see if anyone else has thoughts on this.

Hi All, I've been finding myself watching a lot of filter test videos lately, and was wondering if anyone had a more precise method to classify the differences between the types of diffusion filters. Something like Tiffen's "Triangle of Diffusion" interests me, however I find it troublesome that this model presents halation, contrast reduction, and resolution reduction as mutually exclusive. Each of these qualities is a scale in and of itself, and adding to one does not necessarily take away from another. Additionally, I've noticed some qualities of diffusion filters that are not represented on the triangle of diffusion, or are sub-categories of those that are. These qualities include: Darkening of the Highlights Brightening of the Shadows Changes in Saturation Color of Halation Color Rendering Warming/Cooling Atmospheric Effects And I'm sure there are more distinguishing qualities that I'm not including. I was wondering if any information is available that describes the behavior of filters in a more comprehensive way. Of course, the artistic impression of a filter is ultimately going to be subjective, but I do think it is extremely helpful when developing an artistic intuition to understand precisely how a piece of optics affects an image. Ultimately, when I'm telling my colleague why I prefer Glimmerglass over Black Pro Mist, I want to talk less about Glimmerglass's "dream like quality", and be able to point to specific, concrete, variables that are changing within the image. While I've been spending some time evaluating these filters myself, I'm fairly new to the field, and would love to hear the approaches of people who have been doing it longer than me. Excited to hear what people think.