DOF of MF lenses compare to SLR

[alifatemi]

about to shift from Slr to MF, I like to know if slr lenses have the same DOF with same aperture compare to MF lens and same aperture? because most MF lens have smaller aperture and don't have 1.8 or 1.4 so in case I need very narrow DOF, can a MF lens with f2.8 give me a very narrow DOF? I am going to get Phase one iq280 FYI. Tnx.

[jerome_m]

A MF lens with 2.8 will give you roughly the depth of field of a 24x36 lens of the same field of view with f/2.0: a 80mm f/2.8 lens for a iq280 will be about the same as a 60mm f/2.0 lens for a full frame DSLR.

[JoeKitchen]

Depth of field of a lens is dependent on the focal length.  Longer focal lengths give less depth of field.  So, if you shoot with a 24mm on a Canon full frame and now have to shoot with a 35mm on a Phase Back to get the same field of view, you will see noticeably less DoF on the 35mm.  This, of course, gets worse as the focal length increases.  

The main this to keep in mind, especially if you want to use a tech camera, is that the lenses are considerably sharper (and they have no [Moderator Edit: aberration or distortion?]).  IMO, if you focus correctly, the areas out of focus on the MF will be sharper than the areas in focus on the SLRs.  

However, I am speaking from my experience with tach cameras; I have never used a handheld MF system.  

[Doug Peterson]

The visual effect of shallow DOF (isolation of the subject, a feeling of dimensionality, or however you wish to describe it) is more pronounced when the plane of focus is SHARP.

With an 85/1.2 on a Canon I've found I can't get a sharp plane of focus until at least f/2 (arguably even f/4). Use of f/1.2 can still come in handy for it's light gathering (e.g. on a dark reception dance floor) but I rarely use it below f/2 if I have enough light.

With a 110LS/2.8 on a Phase body, even with the higher resolution of a 60mp back I find the plane of focus is sharp even wide open.

So the pure aperture number, and the effect of sensor size, are not the only consideration.

[jerome_m]

The visual effect of shallow DOF (isolation of the subject, a feeling of dimensionality, or however you wish to describe it) is more pronounced when the plane of focus is SHARP.

With an 85/1.2 on a Canon I've found I can't get a sharp plane of focus until at least f/2 (arguably even f/4). Use of f/1.2 can still come in handy for it's light gathering (e.g. on a dark reception dance floor) but I rarely use it below f/2 if I have enough light.

Now, this is quite an interesting observation, but the explanation you give to it is only part of the story. The reason why the plane of focus is not sharp on the 85/1.2 or similar lenses is mainly spherical aberration and that aberration actually increases the thickness of the zone of "near focus", thereby influencing bokeh. A quite interesting phenomenon.

Another point which I don't see mentioned here: users of MF usually print bigger. After all, this is the whole point of higher IQ, isn't it? On a bigger print, DOF does not need to be as shallow.

[paul_jones]

The visual effect of shallow DOF (isolation of the subject, a feeling of dimensionality, or however you wish to describe it) is more pronounced when the plane of focus is SHARP.

With an 85/1.2 on a Canon I've found I can't get a sharp plane of focus until at least f/2 (arguably even f/4). Use of f/1.2 can still come in handy for it's light gathering (e.g. on a dark reception dance floor) but I rarely use it below f/2 if I have enough light.

With a 110LS/2.8 on a Phase body, even with the higher resolution of a 60mp back I find the plane of focus is sharp even wide open.

So the pure aperture number, and the effect of sensor size, are not the only consideration.

are you sure doug??  i shoot at f1.2 on a canon 85mm almost exclusively and its sharp as a tack when you can actually nail it (not quite so easy). sure its got a fair bit of CA if the contrast is high.
i love the 85mm f1.2, its has a beautiful quality of blur- not unlike a cooke super speed cinema lens (i've basically the same stuff side by side one of these). the blur still looks nice till about f1.6- after that it looks reasonably normal.

about the comparison- i have tested the contax f2 fully open alongside the canon f1.2 50mm. the blur quality is very similar, maybe nicer on the contax- but the amount is the same. but the contax is a fair bit sharper in the center- and of cause you have a fair bit more res. the contax has CA open wide like the canon 50mm f1.2.
using the 140mm at 2.8 on the contax doesn't blur as nice or as much as a 85mm f1.2 on the canon imo. but its ok.

I've tested these things with my gear many times (contax and canon), and its clear to me that medium format doesn't increase the amount of focus drop if as much as fast 35mm lenses.

its been a long time since ive used it, but i had a mamiya 80mm f1.9 that i was convinced had a really nice focus drop off- but i havent had a chance to do a side by side test with the canon.

paul

[gerald.d]

Slight diversion, but for those who missed it, Canon 85/1.2 on IQ180:

http://www.luminous-landscape.com/forum/index.php?topic=82361.0

Kind regards,

Gerald.

[hjulenissen]

I find the write-up below valuable for comparing systems on a generic/theoretic basis. Whenever possible, theoretic considerations should be complemented with hands-on experiments with the tools in question.

http://www.josephjamesphotography.com/equivalence/

[ondebanks]

about the comparison- i have tested the contax f2 fully open alongside the canon f1.2 50mm. the blur quality is very similar, maybe nicer on the contax- but the amount is the same.

And so it should be. Divide the focal length by the taking aperture stop. 80mm/2 = 40mm entrance pupil. 50mm/1.2 = 42mm entrance pupil. Practically the same.

That's really all there is to comparing theoretical DOF across different lenses, without changing the camera-subject distance - how big are the entrance pupils?

using the 140mm at 2.8 on the contax doesn't blur as nice or as much as a 85mm f1.2 on the canon imo. but its ok.

Again, this is entirely predictable. 85mm/1.2 = 71mm; 140mm/2.8 = 50mm. Big difference in favour of the Canon lens. If you shot with a Mamiya 200/2.8 APO (71mm pupil) at the same distance, you'd have a proper match to the 85/1.2.

its been a long time since ive used it, but i had a mamiya 80mm f1.9 that i was convinced had a really nice focus drop off- but i havent had a chance to do a side by side test with the canon.

Indeed - the Mamiya 80/1.9 (42mm entrance pupil) would be an even closer match to the Canon 50/1.2.

Now in practice, two things:

1) As Jerome has noted, aberrations, principally spherical, tweak the actual focus falloff, yielding small differences between lenses of the same entrance pupil.

2) To get the subject framing you want for the sensor format you have, you probably won't maintain the same camera-subject distance. Even using the same lens on two different formats means changing that distance to deliver the same framing. E.g. switching the 50/1.2 from a full frame DSLR to an APS-C DSLR means stepping back from the subject, and losing a lot of that shallow-DOF magic.

Ray

[gerald.d]

So the Canon 85/1.2 on MF is equivalent to a 54mm f/0.75 on 36x24 then?

And so it should be. Divide the focal length by the taking aperture stop. 80mm/2 = 40mm entrance pupil. 50mm/1.2 = 42mm entrance pupil. Practically the same.

That's really all there is to comparing theoretical DOF across different lenses, without changing the camera-subject distance - how big are the entrance pupils?

Again, this is entirely predictable. 85mm/1.2 = 71mm; 140mm/2.8 = 50mm. Big difference in favour of the Canon lens. If you shot with a Mamiya 200/2.8 APO (71mm pupil) at the same distance, you'd have a proper match to the 85/1.2.

Indeed - the Mamiya 80/1.9 (42mm entrance pupil) would be an even closer match to the Canon 50/1.2.

Now in practice, two things:

1) As Jerome has noted, aberrations, principally spherical, tweak the actual focus falloff, yielding small differences between lenses of the same entrance pupil.

2) To get the subject framing you want for the sensor format you have, you probably won't maintain the same camera-subject distance. Even using the same lens on two different formats means changing that distance to deliver the same framing. E.g. switching the 50/1.2 from a full frame DSLR to an APS-C DSLR means stepping back from the subject, and losing a lot of that shallow-DOF magic.

Ray

[MarkoRepse]

So the Canon 85/1.2 on MF is equivalent to a 54mm f/0.75 on 36x24 then?

Yup.

Another thing which was not mentioned is that this calculation stops being accurate at (very) close distances. The larger format enters into "macro" territory sooner so it has even less DOF than a smaller format with the same entrance pupil. It depends on format size so lets just say this effect becomes noticeable at distances of maybe 1m or less. At close distances you need a proper DOF calculator to do comparisons. Thats also why tight head shots on 8x10 look like nothing else (among other things).

[eronald]

Doug,

You are breaking my heart - next you will be accusing Mother Teresa of taking time out for a lunch break

 I strongly suggest you check out the camera you are using, or maybe you have a sample issue. DOF on this lens is probably around 2mm (!) wide open at min focus distance, which leaves little space for error. BTW, I'm not sure this thing doesn't have a curved plane of focus.

Edmund

With an 85/1.2 on a Canon I've found I can't get a sharp plane of focus until at least f/2 (arguably even f/4). Use of f/1.2 can still come in handy for it's light gathering (e.g. on a dark reception dance floor) but I rarely use it below f/2 if I have enough light.