Rodenstock depth of Field Calculator and Sinar Norma

[Justin Coombes]

Can anyone help? I am looking for a ‘how to’ guide to using the Rodenstock depth of Field Calculator (the little plastic dial which they recently stopped making).

Here’s why I need it: I am relatively new to large format photography, having recently got hold of a lovely old Sinar Norma on eBay.

I want to make some accurate depth of field calculations for some night-time landscapes I am working on using the Norma and its Schneider lenses (90mm and 150mm). Some shots need details in focus from about 6 metres to 25 metres away; others from 6 metres to infinity. Despite that, I ideally want to open up the aperture as wide as possible so that I am not forced to take hour-long exposures. I know about the refraction and diffraction, hyperfocal distance and focussing ‘one third in’, but am after some very precise measurements!

The Rodenstock calculator is confusing to use without any accompanying instructions. I have had no luck searching online or contacting Rodenstock themselves. There is also a ‘depth of field scale’ referred to on the monorail clamp for the Sinar, but I don’t have the actual scale itself….

With thanks,

J Coombes

[BradS]

Here's a pragmatic approach that may also work.

Take your camera out to the scene in the daylight. Mount up the lens of choice and open the shutter for preview. Focus and look at the image on the ground glass....focus, stop down, etc...

A good dark cloth and a loupe will make this easier. I use a black "hoodie" sweatshirt and an unloved 50mm lens from my 35mm SLR kit. A pair of reading glasses from the drug store work well too.

Make notes. Compare what you learn by experimentation with the dial thingy...

[Bob Salomon]

Call us at 800 735 4373 after this week and we will send you instructions by mail. If you can't wait look at the dials. Each step is numbered. Just set the dials in the order they are numbered.

It will be at least a year with our present stock level before the Rodenstock DOF/Scheimpflug calculators are no longer available. We have lots of them at this moment.
The factory stopping production does not mean that they are not in the pipeline. When the distributors like us are out of stock the supply will dry up. Not when production stops.

[Leonard Evens]

I think you should learn a bit more about focusing issues. In particular, you should learn about near-far methods of focusing and determining the necessary f-stop as described elsewhere in the lfphoto web site. Also, you should be aware that the 'one third into the scene' rule is pretty much nonsense. Leterally, it applies only to very limited situations, and interpreted more liberally, it doesn't really give you anything useful.

If you are going to use the near far method, you would be advised, as recommened above, to do a dry run in daylight when you can see what you are doing. You can determine where on the rail or bed to set your focus and which f-stop to use.

If you study the near-far method, you will discover that setting the f-stop is based on the focus spread which is the distance on the rail or bed between the positions for the near point and far point. The lfphoto website recommends using the method of Paul Hansma, which balances defocus and diffraction, to set the f-stop based on the focus spread. However, for your purposes, that may be overly conservative, particularly if you are using relatively large apertures where diffraction is not a serious limitation. Instead, you might try using the folloowing rule (for 4 x 5). Take the focus spread in mm, multiply it by ten and divide the result by 2. That should give you the smallest f-number (largest aperture) which is acceptable. You may then want to stop down further to be sure. (The rule I stated is based on an assumed circle of confusion of diameter 0.1 mm in the film plane.)

If you can't do a dry run at the scene in daylight, you may be best off by focusing at the hyperfocal distance. You can calculate that by squaring the focal length in mm, multiplying by ten and dividing by the f-number. The result is in millimeters. Divide it by 1000 to get the result in meters. Mutiply that by 3.28 to get the answer in feet.

You can determine where to place your standards by to focus at the hyperfocal distance by measuring off the distance in daylight, and focusing there. Alternately, you can do it all on your camera as follows. First focus at infinity and mark the position of the standard on the rail or bed. Then shift to increase the distance between the standards by one tenth the f-number measured in mm. (Again this assumes coc of 0.1 mm. More generally, multiply the f-number by the coc you find acceptable.) You may have trouble doing that precisely.

Finally, if you look elsewhere on the lfphoto web site, you will find instructions for putting a scale on the focusing knob which will allow you to set the focus point and determine the f-stop. Again this depends on an assumed choice of coc. If you want to get further into this, try my essay at
www.math.northwestern/~len/photos/pages/dof_essay.pdf

[Jack Flesher]

Get the IB from Bob.

Beyond that, read the condensed instructions right on the calculator. To get started, simply focus on the 25 meter object and mark your standard's position, then separate the standards to focus on the 6 meter object, then measure the amount of change in mm between the two (that's when the mm scale on the side is for). Take that dimension and place it opposite the format (4x5) scale and you then directly read the aperture needed opposite the format. This works regardless of lens focal length. As an aside, for more precise focus I tend to go one aperture smaller than the wheel indicates.

Cheers,

[Justin Coombes]

thanks to all of you -will try each suggestion here and post anything i can add in a few months time JC

[Bob Salomon]

thanks to all of you -will try each suggestion here and post anything i can add in a few months time JC

Glad you are tring all the suggestions but you have not asked us for an instruction sheet for the calculator yet.

[JW Dewdney]

I don't mean to sound flippant at all - but if you're looking for hyperfocal sharpness from foreground to background... all you need to know is focus 1/3 the way into the scene and shoot at f/22... that will vary according to 'how deep' the 'scene' is, of course. But there's really NO substitute for experience. And that's more than enough info to get you going and learning when to back off on the aperture, etc...! Really.

[Steve Kefford]

..... all you need to know is focus 1/3 the way into the scene and shoot at f/22... ......

Sorry, but that is a load of rubbish, even though it is often quoted in the photographic press and some books. E.g. if near object is say 2m, and far object is infinity, where is the point "1/3 way into the scene"? Ok lets try:

2 + 2x(inf - 2)/3. erm the answer is infinity. The hyperfocal point depends upon aperture and lens focal length, which your "formula" ignores.

Your "idea" suggests that dof extends 1/3 in from of the point focussed on, and 2/3 behind. That is just not true.

And this assumes that no tilt/swing has been used. When it is, it makes this 2/3 rubbish even more false.

Steve

[Leonard Evens]

The "one third into the scene rule" doesn't make any sense. I'm sure people who think they are using it are doing something that works, but I've never been able to figure out what it is.

The only way to try to make sense of the rule is to assume it means that you want the far DOF in back of the plane of exact focus to be twice the size of the near DOF in front of the plane of exact focus. It is easy to see that this applies when the distance at which you focus is equal to the focal length plus one third of the hyperfocal distance. Say you are using a 150 mm lens and stop down to f/22. Taking a coc of 0.1 mm for 4 x 5, the hyperfocal distance would be about 10.23 meters, so you would be focusing at about 3.56 meters. You would have everything between 2.64 meters and 5.46 meters in focus. This is far from the typical kind of scene I encounter in my photography. Often, for example, I want everything from infinity down to something in the medium distance to be in focus---for a typical landscape. In that case the far DOF is infinite, and it doesn't make sense to say the point at which you focus should be one third of the way "into the scene". With that same 150 mm lens set at f/22, if you focused at the hyperfocal distance of about 10.3 meters, then everything from infinity down to about 5.2 meters would be in focus. If 5.2 meters wasn't close enough, you would have to stop down further, which would shorten the hyperfocal distance and also bring the near point closer while still keeping the far point at infinity.

Another way to look at it is as follows. It can be shown that the proper place to focus is the harmonic mean of the near and far distances you want in focus. (The harmonic mean is the reciprocal of the average of the reciprocals of the two other distances.) In that case, the one third into the scene rule will apply just when the far distance is twice the near distance.. If that, or something close to it, holds true, then the rule would work in practice. But you wouldn't necessarily be able to make do with f/22. That would depend on how far apart the near and far points were.

I've spent some time puzzling over just what those suggesting this rule have in mind. It is often quoted, and I've even seen it mentioned in one of Ansel Adams's books. So I assume that those using it are doing something sensible, but I'm not sure what it is. It is possible they are talking about it seeming as though one is focusing one third of the way into the scene although in fact one isn't. Perhaps it just means that the near DOF should be smaller to some extent than the far DOF. If anyone can suggest just what one does to apply this rule, perhaps we can make better sense of it.

[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.