kintatsu
Member
... Let us pretend that this is a photograph, or a negative, and that the image was taken with both of 35mm and a 4x5. As you can see in the center The man is visible in both the larger format and the smaller format, but the surrounding area is visible only by the larger format film, however where both images line up, within that 35mm frame, the depth of field would be the same that is what I'm talking about as far as surface area, The depth of field doesn't change depending on the format, it's the perception of the depth of field the changes... If for example you could see clearly 5 feet in front of the man and 5 feet behind the man and that was the only distance that was visible clearly, a total of 10 feet, within the 35mm frame it appears as if almost the entire image is in focus, and only slightly may be in the corner behind him you could see the edge of a mountain that would be may be slightly out of focus, but with the large-format image you see more area, so your perception is that the depth of field is much smaller given that you're looking at much more information within the image, when actually both depth of field are the same within both images, a total of 10 feet... However within the 35mm frame we are only seeing just outside of 10 feet, where with the 4 x 5 image we are seeing much much more information and much much larger field of view and so our perception of the depth of field is that it is smaller in comparison to what we can see but the depth of field itself does not change, it will always be 10 feet (in this given hypothetical example...).
Does this help or make you more confused?
DOF is very complicated. The COC for 35mm is approximately .03mm, while for 4x5 it's approximately .1mm, while 6x7 MF runs about .06. Given that the same image on each format requires a different focal length to get the same image, the DOF on each would be quite different. Also resolving power limits COC. It can get quite complicated. You can use a 35mm film COC when shooting 4x5 and run the math, adjusting your focus accordingly. Therefore, DOF changes with format, focal length, and distance from the lens to the subject. Of course, when referring to DOF, it's in relation to the focal plan, not the image plane. With 35mm or MF, your focal plane is generally parallel with your image plane. With movements it changes things, as not everything is on the same plane.
Given the same focal length in both, as you showed, the DOF would still be different due to COC and lens resolution. With a 100mm lens at 10 feet, your DOF on 35mm is a total of 2.89 ft, on 4x5 it's 12.1 feet, and on 6x7 MF it's 6.15 feet. The hyperfocal distance on 4x5 is 20.8 feet and on 35mm it's 68.7 feet. These numbers come from the http://www.dofmaster.com/dofjs.html website. A great resource, the linked calculator shows it all. Wikipedia has a great breakdown of the formulae, as well.
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