DOF - OK, I'm confused!
I'm confused about DOF when different image size AND equivalent focal length is concerned. I'd like to confirm my understanding.
As a multi-format shooter, this is important to me.
Let's say I have a 35mm film format camera with 50mm lens.
Let's say I have a 645 film camera with 80mm lens.
They are said to be about the same when it comes to viewing angles - correct?
If I set an aperture to f/2.8 for example, I will have LESS DOF on 645 camera than 35mm camera. At least that's true according to online DOF calculator at www.dofmaster.com
So... to generalize this, for equivalent focal length lens between different image size cameras, larger the image size, tighter DOF for a given aperture size. I have MORE control over DOF (ie. being able to obtain narrower DOF) on MF cameras than 35mm cameras.
Going extreme, if I have a pocket digi cam (sorry guys) with a tiny sensor, even if I go down to f/2.8 (if I could), I have a huge DOF that it is virtually impossible to have anything defocused.
If I have a 8x10 for example, LF camera, if I wanted to keep everything in focus, I really have to stop down to f/64 or something.
Am I correct on this?
Please keep this discussion simple. Please try not to add to my confusion. I really want to understand this.... if someone wants to discuss fine details on this, please start your own thread.
It all depends on how well you understand the "Circle of Confusion"
I dont think circle of CONFUSION will confuse this guy any less
Originally Posted by tkamiya
I actually understand CoC at some preliminary level. But I want to grasp this practical DOF issue before digging into details. That's why I asked for simplified discussions first. Thanks.
Your understanding is reasonably correct, although there are a couple of wrinkles to be aware of.
1) The focal length equivalents are a bit of a problem, because the aspect ratios of the negatives/slides differ so much. A comparison between 2 1/4 x 1 5/8 vs. 4 x 5 would take that concern out of the question;
2) To measure the similarities or differences accurately, you need to go so far as to actually print the negatives (or project the slides). And the viewing distance can factor in as well; and
3) The tiny sensor on the digicam will involve you in a further set of complexities - the Bayer array on the sensor, and the equivalent array on the video monitor you use for display. If you don't use a video monitor to display the results, then you get into real concerns about print size if the digicam doesn't have a particularly high resolution sensor.
1) yes, I know.... roughly equivalent is what I meant. I am aware of aspect ratio issue.
3) bad example. I should have said 110 film maybe.
Concerning 2.... My understanding is that definition of DOF itself relies on final print size and the viewing distance. So to aid my understanding, I was assuming one final print size and identical viewing distance. Why is it that bigger the film size, smaller the DOF? Why does this even matter when I'll be printing to the same size paper to compare? It became further confusing when I started to factor in equivalent focal length size adjusted to the film image size....
Can you help me explore the topic 2?
The way I learned it, and this is very simple, is that the dof is pretty much dependent on the actual focal length of the lens rather than the format size. In other words, on a 28mm lens, you have generally pretty deep depth of field. If you are using a small format camera and that represents a normal or tele lens, you still have a deep depth of field. A 90mm lens will have generally a pretty shallow depth of field, if you are using a 4X5, that is a wide angle and you have a shallow depth of field, if you are using a 35mm, it is a portrait length and you have a fairly shallow depth of field. I am sure that it is more complicated than that, but it works for me to understand the general issue.
The reason that viewing distance and the nature of the printing process matters is that, the entire process itself must be taken into account when you are examining whether something is "sharp". This includes the camera lens, film flatness, the resolution, contrast and acutance capacities of the film (and development) itself, and the qualities of the enlargement equipment. In the case of smaller formats like 35mm, there is a much greater chance of running up against a limiting factor like film grain.
If your viewing distance is limited to a distance that doesn't reveal grain on an enlargement from 35mm, you may be too far away to detect a lack of sharpness that would otherwise reveal itself.
From Wikipedia, http://en.wikipedia.org/wiki/Depth_of_field:
“Same picture” for both formatsFor the common “same picture” comparison, i.e., the same camera position and angle of view, DOF is, to a first approximation, inversely proportional to format size (Stroebel 1976, 139). More precisely, if photographs with the same final-image size are taken in two different camera formats at the same subject distance with the same angle of view and f-number, the DOF is, to a first approximation, inversely proportional to the format size. Though commonly used when comparing formats, the approximation is valid only when the subject distance is large in comparison with the focal length of the larger format and small in comparison with the hyperfocal distance of the smaller format.
"To maintain the same angle of view, the lens focal lengths must be in proportion to the format sizes. Assuming, for purposes of comparison, that the 4×5 format is four times the size of 35 mm format, if a 4×5 camera used a 300 mm lens, a 35 mm camera would need a 75 mm lens for the same field of view. For the same f-number, the image made with the 35 mm camera would have four times the DOF of the image made with the 4×5 camera."
Simply speaking: you are correct in all points.
A bit more complicated: The equation for DoF has four variables (subject distance u, focal length f, circle of confusion c, and aperture setting N). Only c is format dependent, and all but f are linear (see attached equation).
The picture comparison illustrates the increase in DoF with smaller formats.
Picture 1 shows that a 24mm lens has the same viewing angle with a small format as an 85mm lens with a larger format. Picture 2 shows that, if enlarged to the same image size, the smaller format has a DoF advantage at the same aperture setting, due to the shorter focal length.
Hope this helps.