LF Depth of Field

[John Downie]

Okay, this one has been bugging me for a while. LF is said to have relatively little depth of field, compared to 35mm, for example. If I cropped a 24mm x 36mm section from the middle of a 4x5 frame, wouldn't it have the same depth of field as the 35mm frame taken with the same focal length lens?

I await elucidation.

TIA.

[Martin Patek-Strutsky]

Yes

Elucidation? Well, you see...

[N Dhananjay]

Yes - if you used a 300mm lens on 35mm and on 4x5, at the same f stop etc you have the same DOF. Except that you use longer focal lengths in larger formats to achieve the same framing. That is, the 50mm lens that gives you a particular composition in 35mm will require you to use a 150mm lens on 4x5 to achieve the same composition (ignoring aspect ratios etc). And since DOF is inversely proportional to focal length, you end up with less DOF in larger formats.

It is also worth keeping in mind that typically you can sto down further in larger formats (diffraction losses are less of an issue since you re enlarging more), so in practical terms larger formats suffer from a loss of speed rather than a loss of DOF, in practical terms.

Cheers, DJ

[Jim Galli]

Actually 4X5 isn't too bad with 180mm being about a true normal. 8X10 is where the depth of field problem becomes more apparent with 360mm being "normal." So with LF we need the longer lenses to do the same job that your 50 does on a 35mm. It's inherent in lens geometry that the shorter the distance to the film plane the greater the perceived depth of field. That's also one reason why the new digital stuff is so attractive. Since the lenses are looking at a smaller field than ever a normal can be 20mm or so which is always in focus everywhere.

[Dick Roadnight]

If you are not going to enlarge much, then why use Large Format?

If you use LF because you need movements, and have no smaller format camera with adequate movements, you might consider a MF roll film back, and theoretically, if you use a 300mm lens with a 66 MF back on an LF camera you would get the same DOF as you would if you use a MF 300mm on an MF camera, but…

DOF is not clearly defined, and depends what Circle of Confusion (COF) you (or the manufacturer) considers acceptable. When what we now call LF was all there was, LF was frequently enlarged little if at all, and lower res (larger COF) was acceptable.

Even today it is technologically difficult to make a long lens with an image circle to cover 5x4 (with movements) that is as sharp as the best MF lenses of the same focal length. (Having said that; I think that (if you do not have a Leica) the easiest way to get a sharp 24 x 36mm slide is to take a pair of scissors to a Hasselblad slide!)

If the DOF of the LF lens is calculated on the larger COF, it might indicate a greater DOF, with less difference in sharpness over the DOF range, and fewer LPPM – less res/mm^2.

WA and UWA SLR (MF) lenses are retrofocus, so the LF lenses would (tend to have) less distortion, and compare better in terms of sharpness.

[Emmanuel BIGLER]

I agree with DJ. Often comparisons of DOF between formats are not fair because the comparison does not take into account final images of the same quality.

We could even prove that in some circumstances there is more DOF in LF for a given quality !! Lets us compare standard lenses in different formats. Assume that there is no grain, and let us enlarge our final printed images to the same final size. If we scale the sharpness criterion with film format, DOF decreases in LF as an inverse proportion of the focal length or format diagonal. Now let us consider the fact that the best aperture is 5.6-8 in 35 mm, and 16-22 in 4"x5" for standard lenses. In fact the best aperture scales with format. So if we compare images taken at the best aperture, there is no gain in DOF for small formats, and if we re-introduce film grain size, we can say that used at the best aperture LF lenses yields the same DOF with a better quality. Unfortunately, as pointed by DJ, you'll have to use slow speeds on a tripod because the optimum apertures are so small. Stopped down beyond the best aperture, we can even say tha the classical model of geometrical DOF on which conventional comparisons are made is wrong because we are in a diffraction-limited regime. So do not tell me that you are using for the comparison a 35-mm SLR 50mm lens on a 24x36 frame stopped down to f/22, you loose the match by diffraction against MF and LF before even starting ;-);-)

Now if you relax the image quality criterion, you can always consider that there is much more DOF in smaller formats, but the comparison is not fair.

[Jay DeFehr]

Circle of Confusion = CoC, and there are many reasons beyond sharpness to use Large Format.

[Thilo Schmid]

John, DOF is not a matter of lenses. There is no DOF for a lens. It is always "0", because there is only one plane of sharp focus. DOF is a matter of magnification and the resolution limits of the human eye. So it depends on negative size, print size and viewing distance. In your case, the 4x5 and the 35mm negatives will only show the same DOF, if the magnification ratio at print time and the viewing distances are the same.