Rodenstock depth of Field Calculator and Sinar Norma

[Jack Flesher]

Guys, let's chill out a bit!

First off, the 1/3-in-front/2/3-behind does work pretty well for subjects at normal shooting distances (those at or beyond 50x the lens focal). It simply means that the toal range of DoF will extend about 2x as far behind the exact PoF as it does in front of it. No it isn't exact, but certainly close enough for practical application. However, I agree it is a bit problematic to simplify that to the point where we say, "focus there and use f22 and all will be well..."

CoC, format size and ultimate image size still come into play for DoF calculations -- f8 might be adequate for a 65mm lens with some images, while f32 and a 300mm lens may not be enough for others. (Use a 450 on 4x5 for a landscape and you'll quickly learn how true this is.) In any case, adding some movements will almost always help, and isn't that why we shoot view cameras to begin with?

So back to the OP topic, I would say the little calculator is a great learning tool if nothing else. I used mine a lot of the time early on and found it helped me gain understanding of how beneficial camera movements are. It also helped me gain an intuitive feel for how much I moved the standards between near and far focus points related to the amount of DoF I'd need to make the image work. And while I still carry it, I hardly use it at all now. (The exception is for closer-in images where I still find it quite useful.)

Cheers,

[Bob Salomon]

" everything from infinity down to about 5.2 meters would be in focus."

No it woudn't.

The point focused on would be critically sharp. The rest would be perceived as being sharp. The more you enlarge the image the less the apparent sharp area will be.

[Steve Kefford]

Guys, let's chill out a bit!

First off, the 1/3-in-front/2/3-behind does work pretty well for subjects at normal shooting distances (those at or beyond 50x the lens focal). It simply means that the toal range of DoF will extend about 2x as far behind the exact PoF as it does in front of it......

That is just not true. One of the key points about hyperfocal focusssing is that the point of focus is such that DOF behind is infinity. If we focus at this point, then the 2/3 rule means that the near point in focus is infinity/2 in front of the point of focus. This is not true. Considering the 50x focal length, the aperture and focal length are so key to this, that any generalisation that does not take account of these just has to be false.

Whilst application of the 2/3 rule might produce acceptable results in some circumstances, this does not prove it is valid. If we are going to close down more than we have to, use a fairly wide angle lens, don't have anything too close to us, and like looking at small pics, then the 2/3 rule works fine. However in these circumstances, we could just as well use a fixed focus lens!

Steve

[Leonard Evens]

" everything from infinity down to about 5.2 meters would be in focus."

No it woudn't.

The point focused on would be critically sharp. The rest would be perceived as being sharp. The more you enlarge the image the less the apparent sharp area will be.

Depth of field is calculated based on a choice of a specific acceptable circle of confusion. The assumption is that if the blur circle is smaller than that, under specified circumstances, then it can't be distinguished from a point. That is the sense in which I meant "in focus".

If you want to split hairs, in fact nothing would ever be in focus, even for a perfect lens. Diffraction would mean that even in the plane of "exact focus", the image would be slightly blurry, and that would show up under sufficient magnification. In addition, for any real lens, lens aberrations would limit the sharpness of the image.

As to the other point, thanks Steve for agreeing with me. In fact, to quantify it, the ratio of the far depth of field to the near depth of field, under ideal conditions, is the same as the ratio of the far distance to the near distance. So, for example, if you want everything from 5 meters to 50 meters to be in focus, then you should focus at 2 x 5 x 50/(5 + 50) ~ 9.09 meters. In that case the near DOF will be about 4.09 meters and the far DOF would be about 40.9 meters, the latter being ten times the former. It would be a mistake to assume the far DOF were only twice the near DOF.

One reason why the one third rule might seem plausible can be further illustrated by this same example. Suppose one focused instead at 20 meters which would be one third of the way into the scene. Suppose also that one was using a 150 mm lens. Then the bellows extension for focusing at the correct 9.09 meters would be about 152.25 mm and that for focusing at 20 meters would be about 151.13 mm. the difference is only about 1.1 mm on the rail. If one is focusing carefully, that is easily noticeble, particularly if using a loupe. But if one is a trifle sloppy and stops down a bit to compensate, the difference might not be great enough to matter. So it might be a rough and ready way to focus without having to fiddle too much and without using a loupe. Even in the case where one wants the back DOF to extend to infinity, it might not make a lot of diffrence provided one chose some relatively close in distance to be "effective infinity" and then focused one third of the way to that. If in addition, if you used the one third rule just to get started and then adjusted the focus on the basis of what you saw on the gg, you could very well end up in pretty much the right place.

Personally, I use the near far method for focusing, which is described in detail on the lfphoto web page. One focuses half way between the near and far points on the rail. That also involves a small error, but except in extreme circustances, it really is too small to notice even if one uses a high powered loupe. It is easy to use, as old or older than the "one-thrd" rule, and more accurate.