Depth of Field "Loss" in Medium Format

Discussion in 'Medium Format' started by tony_doucet, May 29, 1998.

  1. There have been, among the responses to other questions, comments about "losing" depth of field as you increase negative size. That is, there is less depth of field in a 6x7 negative than in a 35mm. I think that this whole argument is wrong and more than a little confusing. Let me explain. I have always thought that depth of field (DOF) was a zone of sharpness that extends both in front of and behind the point at which a lens is focused. Furthermore, depth of field is dependent on three things only: the distance from the lens to the point of focus; the aperture at which the lens is set, and the focal length of the lens. For example, if three different format cameras are all equipped with a 45mm lens, focused on the same point, and set with the same aperture, then the DOF will be exactly the same in all three photographs. It is totally unaffected by film format. Admittedly, bigger film formats like 6x7 have larger image circles, and will include more "stuff" in the frame than 35mm. Some of the extra "stuff" will be out of focus if it is outside the depth of field, but it is wrong to say that there is any loss of DOF.
     
  2. When people talk about "loss" of depth of field in MF compared to 35mm, they are referring to comparisons between lenses with similar angles of view, not similar focal lenths. It is true that a 45mm lens with have the same depth of field whether used on a 6x7 camera or a 35mm, but that is an irrelevant comparison since, on a 35mm it is a nearly "normal" lens while on 6x7 it is a wide angle. The 45mm wide angle (on 6x7) has substantially less DOF than a 24mm on 35mm, which has a similar angle of view. When you move from 35mm to medium format, as a practical matter, you do, indeed, "lose" substantial depth of field at comparable fields of view.
     
  3. DOF is only depends on aperture (which you can control) and MAGNIFICATION. PERIOD!
     
  4. Tony, You're absolutely correct on all counts. But, I believe that most observations about having "less depth of field with MF than with 35mm" have to do with comparisons between particular angles of view (wide angle, normal, telephoto), rather than comparing specific focal lengths, expressed in mm's.
    As you know, a 45 mm lens is considered normal for 35mm, wide angle for MF, and an ultra-wide angle for LF. As you stated, the same 45mm lens, if used in all three formats, would have the same DOF.
    But when comparing a "normal" 35mm lens, let's say a 45mm focal length, to a "normal" MF lens, let's say a 90mm focal length for 6x7, to a "normal" lens for LF, about a 150mm focal length for 4x5 or about 300mm for 8x10, one would find considerably less DOF as the negative size, or format size, increases, when using a "normal" angle of view for each respective format.
    When I use a 50mm "normal" lens, with my 35mm camera, I enjoy considerable DOF. When I use a 305mm "normal" lens, with my 8x10 camera, I have considerably less DOF. Thus the comments about less DOF as format size increases.
     
  5. Tony, I take a picture with my Nikon and 50 mm lens. Then I take the same picture in medium format: I reach for my Hasselblad with 80 mm normal lens (same view as the Nikon plus 50 mm lens). Now this 80 mm Hasselblad lens has the same DOF as an 80 mm Nikon lens would have, namely significantly less than the 50mm. So when I compare the two pictures (taken at the same aperture) the medium format picture has less DOF than the 35 mm one.
     
  6. Thanks to Tony for the original message, and the additional comments. This issue has confused me and this discussion clears up my confusion.
     
  7. I feel a little silly answering my own posting, but thanks to you all for your responses. I don't disagree with any of the discussions about having to use different focal lengths in different formats to get the same view. But I just don't see this as a depth of field loss or gain. Also, to Chun In Martinez: I know that depth of field is a function of magnification, but magnification is a function of lens-to subject distance and lens focal length, the two properties I included in my original posting. I find it easier to think about DOF in terms of these two things, which can be determined from the lens barrel.
     
  8. well, some semantics here, but it looks as though folks getting down to similar conclusions. My two cents worth - dof oas defined by accetpable circle of confusion is settled by lens focal length, aperture, magnification -- not by film format, as optics doesn't care how much wasted negative space is around a little dot. However, magnification does come back into play when you MUST ALSO INCLUDE ANTICIPATED VIEWING. If both 35 and 6x7 are to be enlarged to only 8x10 (or any other fixed comparison size), then the 6x7 can tolerate a larger circle of confusion, and thus the calculations of dof are not as strenuous as for 35, --- dof of a normal lens MAY only be slightly less than a normal lens on the 35, though one be 50 mm and the other 90. However, if your 35s are enlarged to only 5x7, and your 6x7 to 16x20, the size of that circle of confusion becomes progressively more important for the larger format.
     
  9. Re Lacey's point about the larger circle of confusion compensating for "loss" of depth of field: Maybe someone can help me here, but as I recall, when you run the calculations, even if you are enlarging to the same size print (say 8x10), the larger "COC" factor for the larger format size only compensates for 1/2 of your "loss" of depth of field (keeping aperture and field of view constant). So you "lose" depth of field in going to MF, even if you are not making bigger prints. And, after all, one of the reasons to move from 35 to MF is so you can make bigger prints. The real bottom line here (for me anyway) is that if you are trying to squeeze out the most depth of field (like if you're in love with those dramatic close foreground/infinity background landscapes) moving from 35mm to a larger format (at least without camera movements) has a substantial drawback.
     
  10. I agree with chris, I went to a 6x7 system and was very displeased with the amount of DOF I was getting. I like those foreground, background relationships and they are most definitly lost even when using the widest lens out there for medium format! I may just stick to my 35mm until I'm ready for a 4x5.
     
  11. It also depends on whether your depth of field is limited by available light (that is, you're comparing f/8 to f/8) or it's limited by diffraction (that is, you stop down both cameras until just before diffraction becomes unacceptable) Since the lesser degree of enlargement lets you get away with smaller (numerically higher) apertures before you have a diffraction problem, all formats are equal in situations where you have "unlimited" light (like long exposure landscapes). Finally, this all assumes equivalent film speed. The lesser degree of enlargement for a given print size means that you can use an 800 speed film at 6 x 7 and still get results somewhat sharper than 160 speed film at 35mm. This gives you at least enough bonus depth of field to even things out in light-limited situation. Of course, you eventually run out of films of sufficient speed, and run into tonality problems as well - 800 at 6 x 7 may beat 160 at 35; 800 pushed 3 at 6 x 7 may still be sharper than 800 pushed 1 at 35mm, but the contrast and color are going to be a little strange.
     

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