DOF for same aperture, equal?

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jaifo

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Some folks claim that some lenses seem to have different DOF (not talking about bokeh) at the same aperture. They say that at f2.8, some lenses seem to have a shallower dof that others. Is that possible?
I'm planning to buy a 28 for my M6 and i like he look of shallow dof.
Thank's
 
Depth of feld is largely a function of lens diameter and shooting distance. Short focal length lenses have smaller diameter apertures at the same f/stop resulting in more depth of field. If you shoot a street scene at twenty or more feet away you'll have a lot more depth of field than if you shoot a couple of friends from four feet away. Using a 90mm lens will give you a lot less depth of field than a wide angle lens. The 28 is NOT the lens to get if you like shallow depth of field. A 50mm f/1.4 would be a lot more suitable.
 
Two 28mm lenses from two different camera makers used at the same aperture will give exactly the same depth of field.

There's a handy page here, with a calculator toward the end where you could look up the depth of field for a 28mm lens and see if it's shallow enough for your likes.
 
Actually, this is closely related to bokeh. Mathematically, two lenses may have the same d-o-f but one may look as if it has more -- which is basically what bokeh is about too, the appearance of the out-of-focus areas.

As Al says, wide-angles have more d-o-f at a given distance, because the magnification on the film is less; but if you frame a picture with (say) a 50mm. and then walk forward and frame exactly the same image with (say) a 28mm, d-o-f will be identical too, subject to the qualification noted above.

Cheers,

R.
 
Thank's

As Roger says, two different 28s shot at the same f have the same dof. But also the gradation, the way they come from focus to out-of-focus (very related to bokeh) is something that counts. That's why some users of the Zeiss ZM 28 claim that it has a shallower dof than the elmarit.
 
Mathematically, a 28mm lens would have the same DOF as any other 28mm lens at the same aperture. Differences, if there are any, would occur because not all 28mm lenses *are* 28mm lenses. Most are somewhere in the ballpark. And not all f-stops marked as a given number *are* that number. Again, most are in the vicinity. So yes, it could be possible for two 28mm lenses to have a different DOF for the same aperture, but no, not if they are what they are supposed to be, with regard to focal length and aperture.

Roger is also correct about the 'look' of the DOF, due to lens design, glass used, number of aperture leaves, the moon being in the Seventh House, and etc.
 
The amount of DOF will be identical under the above mentioned conditions (focal length, aperture, film/sensor format). Additionally film resolution and enlargement have to be identical to get the same results.
The depth itself depends on the film/sensor resolution, the required enlargement and the viewing distance of the final image. These variables are the base of the coc (circle of confusion), which, as a standardized calculation, is used to mark the DOF scales on your lenses.
Not the easiest thing to understand, but maybe this spreadsheet helps:

http://www.alpa.ch/index.php?path=news&tablepage=2&detailpage=80

Just my 5 cents.
 
There are some lenses that have adjustable floating elements to show different depth of field at the same focal length and aperture as ordinary lenses (look for Nikon-DC, Minolta-STF, Canon-SF). However, I don't think they're available in shorter focal lengths. I'm only aware of lenses in the 105-135mm range. Another thing: they're SLR lenses only as the effect has to be evaluated through the lens..
 
schaubild said:
These variables are the base of the coc (circle of confusion), which, as a standardized calculation, is used to mark the DOF scales on your lenses.
Click to expand...

Yes, but the trouble is, it's not a standardized calculation, as evidenced by the different-sized d-o-f scales on different cameras. For example, original Contaxes took a much more relaxed view of d-o-f than original Leicas.

The older I get, the more convinced I am that d-o-f markings are at best a rough guide, not least because (as you say) so much depends on final enlargement size.

Cheers,

R.
 
Roger Hicks said:
Yes, but the trouble is, it's not a standardized calculation, as evidenced by the different-sized d-o-f scales on different cameras. For example, original Contaxes took a much more relaxed view of d-o-f than original Leicas.

The older I get, the more convinced I am that d-o-f markings are at best a rough guide, not least because (as you say) so much depends on final enlargement size.

Cheers,

R.
Click to expand...


Indeed, the scales are somewhat outdated, as people tend to print bigger nowadays. It's also a reason why on the spreadsheet several alternative coc calculations are offered 🙂
 
The DoF tables are based on a simple geometric model and an assumed acceptable amount of "somewhat-unsharp-but sharp-enough" generally called circle of confusion. Typical lens markings are for a degree of "acceptable" sharpness that is unacceptable in many cases. So if you want a sharp picture you can use the DoF markings for a wider aperture.

Further, and this is something quite different, the simple geometric diagram for DoF is a simplification of the real situation. The simple diagram shows focus at a point on the film. In reality, it is not a point, but it's close to being one. There is (or maybe was) a page on Erwin Puts's website that shows that there is not a simple point of focus, but a range (the "caustic surface"). (This is not what we normally call DoF.) There is not a single point of perfect focus. There is a point of maximum contrast but less than maximum resolution, and a point of maximum resolution at not maximum contrast, and a small zone in between. The lens designer decides which point will be assigned to the "correct" point as far as the rangefinder (and infinity stop) is concerned. (If you're not using a rangefinder, then you will estimate focus on the groundglass). It is feasible that two lenses might differ substantially in their types of transition between the two points (and beyond into the area of nearly-in-focus), and this could lead to a visible difference in the way the slightly out of focus subject transitions into the more out of focus subject. I hazard a guess that this would be more noticeable in wide aperture lenses.
 
Depth of feld is largely a function of lens diameter and shooting distance
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Isn't depth of focus actually only an attribute of the lens focal length and aperture setting, irrespective of format size?

And I'm not referring to the DOF scale markings on some lenses, which are based on assumptions about how much acceptable image blur is considered sharp, at certain specific viewing distances of the finished print. What I'm assuming the OP is asking about is the visual appearance of selective focus in the image.

On-axis, a 28mm lens is a 28mm lens, regardless of lens design; and they all should exhibit the same DOF quality at the same aperture setting (again, ignoring the subjective qualities of bokeh); where it gets a lot more complex is OFF-AXIS, where one lens design (and even manufacturing tolerances between the same lens model) will vary from one lens to the next.

~Joe
 
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JoeV said:
Isn't depth of focus actually only an attribute of the lens focal length and aperture setting, irrespective of format size?
Click to expand...

Actually, it's really dependent on focus distance and aperture size only. Focused at the same distance, a 50mm lens at f/2 will have the same DOF as a 10mm lens at f/4 (since both have an aperture opening of 25mm).
 
I think we're all agreed that lenses' depth of field, like for like, does not differ.

What we need to consider is:

1 Different designers have different opinions about what is acceptable sharpness.

2 Out of focus parts of the image have different characteristics between lens designs.

3 Depth of field, and focus, are not really always a compelling feature of a photo.
 
JoeV said:
Isn't depth of focus actually only an attribute of the lens focal length and aperture setting, irrespective of format size?

~Joe
Click to expand...
Juts to be a little pedantic here but depth of focus is NOT the same parameter as depth of field. Depth of focus is a lens-designer's concern and relates to how far you can move the FILM (or sensor) and retain acceptably sharp focus - NOT how far you can move the subject and retain it.
 
JoeV said:
Isn't depth of focus actually only an attribute of the lens focal length and aperture setting, irrespective of format size? ...
Click to expand...


You are correct, sort of, in that the same lens projects the same image irrespective of what it is casting the image onto - 35mm film, APS-C sensor, 4x5 film.

However, as I understand it, the whole concept of depth-of-field and circle-of-confusion is based around a set of assumptions about a standard-sized enlargement viewed from a standard distance. The larger the negative/sensor, the less magnification needed to produce the standard size print. Thus, the same lens will be thought to have a different depth-of-field depending on what format you place it in front of.

Wikipedia has a reasonable article on the subject - http://en.wikipedia.org/wiki/Circle_of_confusion
 
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