depth of field

[ian_whitehead1]

I have just bought a 75mm lens, so near and far distances will be

more critical than ever, do the depth of field tables that are found

on the net refer the focus distance to the subject as from film plane

or from lens? and with these are there any specific recommendations

on which one to use?

Thanks.

[ellis_vener_photography]

Wow, depth of feld tables who' of thought! ! That's why I thought view cameras had groundglass plates carefully installed to co-incide with the film plane: so you could check your focus. I guess I was wrong, and now I must swich to focusing by some table based on a 70 year old relatively arbitrarily arrived at determination of what is acceptable sharpness. To think of all that money I wasted on loupes and Type 55 Polaroid. Shiver me timbers.<P>In other words: Do the work Ian: there are no shortcuts if you want quality.<P>By the way it is subject to lens nodal point.

[ian_whitehead1]

point taken!!!!

[david_kaufman1]

Despite the previous respondent's scoffing, I find depth of field tables extremely useful while in the field and using wide angle lenses to cover wide angle subjects at close distances. I frequently photograph architectural subjects, e.g., store fronts, at close distances, where I have receeding vertical planes that I need to have in focus at a distance range of 4 to fifteen feet. Using a 90mm lens on 4x5 I find it is usually preferable to work with depth of field instead of swings to bring eveything into sharp focus. With wide angle lenses and these subjects one is often trying to judge sharpness in the corners and edges of the ground glass with a high powered loupe, which is not easy to do. Depth of field tables work perfectly well in these situations. For the distances and situation described above, distance from the front of the lens is adequate, but you have to use depth-of-field tables that have small enough circles of confusion for the degree of sharpness and enlargement required. Also be aware that the apparent sharpness requirement for objects close to the lens is slightly lower than for objects farther away from the lens, which gives you a clue as to how to handle situations where you are pushing the boundaries of focus. Swinging the lens board is always a good option but it depends what elements crossing the plane of sharp focus you need to keep in focus. In the situation described above using a depth of field table is often much faster than using swings and making repeated adjustments. As to which tables to use, retrieve a few formulae from different sources and work out some tables for the lens, then go out in the real world and test them out. But a smaller circle of confusion will give better results.

[leonard_evens]

Depth of field tables use the lens to subject distance,or more precisely the distance from the front nodal point of the lens to the subject. (The lens to film distance is measured from the rear nodal point.) For most lenses used in large format photography, both nodal points are pretty close to the center of the lens. But for some wide angle designs---reverse telephoto---the nodal point may not even be in the lens. It is fairly easy to determine the location of the nodal points if you know the focal length, but the standard depth of field tables may not apply if you are close enough to the subject for any of this to be important. You can find exact formulas for the front and rear depth of field at

 

www.photo.net/learn/optics/lensTutorial

 

which are expressed in terms of magnification (also called scale of reproduction), and these are much more accurate for closeups. If you are close enough, the depth of field in front of the exact focus and the depth of field in back of the exact focus are both given pretty accurately by the formula

 

Nc(1 + M)/M^2

 

where M is the magnification. Here N is the f-number and c is the diameter of the largest acceptable circle of confusion. It is up to you to choose that and depends on how much you are going to enlarge the image and how critically you are going to view it. (Depth of field tables have a built in choice for c chosen by whoever prepared the table.) For 4 x 5 film, typical values might range from 0.1 to 0.05 mm. Note that the depth of field does not depend directly on the focal length (but it does indirectly because M will).

 

You can measure the magnification M directly in various ways, one of which is described at

 

www.salzgeber.at/disc

 

If you know the subject distance, the magnification can also be calculated as follows. Divide the subject distance by the focal length, subtract one, and take the reciprocal.

 

You can also use alternate methods based on distances measured along the rail as described in

 

www.largeformatphotography.info

 

or

 

math.northwestern.edu/~len/photos/pages/dof_essay.pdf

[leonard_evens]

I have to agree with David. When you stop down, the image gets dimmer. Depending on how good your vision is in dim light, you may not be able to check much of anything when you stop down to f/22 or beyond. I admit I am probably an extreme case, but everything looks in focus to me on the gg when I stop down to f/16 or beyond. But I really doubt that there are many people out there who can tell if critical parts of a scene are really in focus at f/32, particularly if they are going to be making large prints which will be viewed very closely. Fortunately, with wide angle lenses you don't have to stop down so much, but if you are pushing things and part of your subject is very close to the lens, you may need to. Also, what he says about circle of confusion is much to the point. There are some calculators you can find on the web which allow you to set it, but usually it is assumed to be something, and you don't know what that is.

 

Myself I start with what I see on the gg, but I may modify it based on the focus spread method described in the large format web page I referred to. To my mind, this combines the best of the visual and the computational approaches.

[david_kaufman1]

Here are the formulas I use to create depth of field tables which I find very accurate at non-close up distance, i.e., magnifications of 1/10 and smaller.

First compute the Hyperfocal for a given lens and circle of confusion, then the distance formulas.

Hyperfocal = F squared/(f*d) where F = focal length in mm, f = reciprocal of the f-stop (e.g. 16 in f/16) and d = circle of confusion in mm. I use .05 for my circle of confusion in 4 x 5 work to be enlarged to 16 x 20 or beyond.

Then:

Near limit = H * u (distance)/ (H + (u-F))

Far limit = H * u (distance)/ (H - (u-F))

The results are in millimetres which can be translated into feet by dividing by (25.4*12).

The tables generated by the formulas seem to work very well.

[joseph_dickerson]

Ian,

 

Try to find a copy of the Kodak Professional Data Guide. It's published/distributed by Silver Pixel Press (Tiffen). It has a lot of valuable info including DoF calculators for medium and large format cameras.

 

Joe Dickerson

[philip_sweeney]

Ian: the shortest lens I use is a f6.8 90mm. And I have had numerous focus problems using it: low light and vertical devices outside of the depth of field. I almost always use this lens outdoors. I would not suggest anyone carry tables around but I think it is good to understand HFD at various f-stops and selected CoC. I have responded to similiar discussions before on this subject. I use movements on all other lenses. However with the 90mm: if there are no vertical devices, I'll use the movements. But at night and with the problems I have outlined I have pencil marks on my wood field camera for prefocus at about 12.2 feet (CoC = 0.1mm) for f22 which yields a focus from about 6.5' to inf. Standards level. I always use f22 or f32. With my F1 in the same situation I have a small wooden dowel (cut to the appropriate length), and after I level the camera I use the dowel as a gage between the two standards, set f22 and shoot. For the night work I had to. Incidentally a CoC of 0.108mm is good, 0.1 is better, 0.07 is tight and allows for very big enlargements.

 

I would think prefocusing for shorter lenses could be helpful.

[ken_schroeder]

I use the groundglass for focusing the 210 and the 135. I think the 75 would be much easier to focus by scale. Most press cameras had scales attached to the beds. Download Leonard Evens' posting mentioned earlier. Calculate the hyperfocal distance for f22. Then find where the focusing rails are positioned for focusing at that distance. Use math and or loupe on the groundglass. Do this very carefully once, and mark the spot. From then on just set the camera to that spot for landscapes. The old press photographers had three distances before rangefinders, "here, there and yonder." I think the 75 is an excellent candidate for careful simplicity.

[dave schlick]

the joke is on the ground glass oonly scoffers.. there is much much more to focus than the simple point of focus that the ground glass gives.. for instace one can focus from 15 feet to infinity using the right f stop and focus distance with some lenses.. with testeing one learns that this is not a perfect distance to work on and is streaching the limits of some large format situations.. but 25 feet to infinity gives some pretty fantastic results in some win win situations....(agian depending on the lense..) dave..

[jerry2]

If your goal is contact prints, or small enlargements, <3x, I would say gg DOF checking is possible....however, even if you rely on the gg for DOF checks, you still need to define the point of exact focus. This is where tables come in handy.

 

If you plan to make big enlargements >5x, its just not possible to check DOF on the gg. The limitation is the amount of magnificaiton a gg can deliver. Forgetting the low light issues with stopping down, the finest grain gg can rarely hold resolution of 3x.

 

If your in a rush to shoot, well then you take your chances...but if you have time to set up, its so simple to determine your near and far point, glance at chart to find point of exact focus, then use a laser rangefinder to focus at that exact distance. This also can be done in a less precise way using standard movements on the rail.... this system is explained well on the LF home page.

[brian_ellis3]

Don't take the point so well or so quickly Ian. Your question was a good one and shouldn't be dismissed. There's a big difference between using the ground glass to focus on something and using the ground glass to determine the aperture needed to produce the desired depth of field. I photograph a lot in the early morning and in the evening, when the light is so dim that I can barely see the image on the ground glass, forget about being able to tell anything about depth of field when I stop down to the taking aperture. That's when I use the table that I carry around in my back pocket.

 

We know that only one plane in a scene can be in focus, everything in front of and behind the plane of focus is by definition out of foucs. Everything in the plane of focus is represented by points on the film. Everything in front of and behind the plane of focus is represented by circles. The question is how small the circles have to be in order to be perceived as points (i.e. to appear to be in focus even though they really arent't) when the negative is enlarged. That depends on several variables - the enlargement size, the viewing distance, and what different people consider to be an acceptable degree of "sharpness." Anyone who constructs a depth of field table has to make certain assumptions about those variables. So tables are very useful for determining depth of field if the assumptions made by the person who constructed the tables correspond to your way of working. But if for example a table assumes an 8x10 print and you make 11x14 prints you'll have to modify the table - usually by stopping down an extra stop or two - in order for it to work.

 

You see these tables all over the place and I don't know of one specific one that is better than another. Just find one in a book or magazine article and try it to see how it works for you.

[emil_ems1]

Ian,

 

The solution to your problem is not so much a depth of field TABLE, but rather a depth of field SCALE. Linhof is selling such a scale and others as well. I derived mine from Linhof's depth of field table and drew it on a small piece of paper (for the 4x5 and 6x9 format respectively). I use the scale in the following way:

 

1) I focus on the object farthest away I wish to keep in focus and mark the position of the lens on the bottom of the camera by tape.

 

2) I focus on the object nearest to me I wish to keep in focus. I then apply the scale to the distance between 1) and 2) (as seen on the bottom of the camera) to find out the minimum lens opening which would keep both positions in focus.

 

3) I put the final focus at exactly half of the distance between 1) and 2).

 

With this approach you don't need any distance scale on your camera and it works very quickly, I assure you. I apply it all the time on my Toyo Field 4x5. In practical terms, I focus 1) by sliding the lens board, keeping the focusing sled in alignment with the bed underneath. I focus 2) by turning the focusing knob. The distance between 1) and 2) can then be easily be observed by the distance the focusing sled has moved away from its alignment with the bed underneath.

[ellis_vener_photography]

<I>f you plan to make big enlargements >5x, its just not possible to check DOF on the gg.</I> How big is big? a 5x magnification means a 20 x 35 for a 4"x5' negative/transparency. I do that pretty regularly here is my current method<P>

1.) I use a Rodenstock depth of field calculator.<P>

2.) Roughly (very roughly) determine the angle the base of the camera tilted.<P>3.) is tilted.<P>3.) set the calculator for the format I am using. If I want to be ultra cautious, I choose the next smaller format. This is important because it takes into account the Circle of Confusion (limit of perceived sharpness) for that particular format.<P>4.) .) Set the magnification ratio. Magnification ratio refers to the object size to size on film. in the field this is usually 1:20 or 1: infinity. <P>5.) Focus to the far point for my composition. Focus to the near point and measure the distance either my film (rear) or front (lens) standard as moved. <P>6. Use this measurement on the calculator to determine optimum aperture for this 'focus spread".<P>7.) Back the standard half of that distance traveled and set the aperture to the indicated aperture. 8.) If there is time, check this with Polaroid Type 55 and inspect the Type 55 negative to see if I did everything right or forgot something. 9.) Use film.<P>

Excluding step 8 (Polaroid T55 negative) this takes me all of maybe 45 seconds. It works for me.

[brian_ellis3]

Wait a minute. In your first message you dismissed Ian's question about tables and said you just used the ground glass. Now you tell us you use a Rodentstock depth of field calculator, you measure focus spreads, you figure out the magnification, and then after you determine the focus spread you look at something that tells you what aperture to use in order to have the near and far appear to be in focus. That thing you look at to determine the aperture sounds suspiciously like a depth of field table. Somebody has made certain assumptions about enlargement size, viewing distance, and acceptable sharpness and has constructed a table that tells you what aperture to use if you want the near and the far to appear to be in focus, based on the focus spread you've calculated. In any case, the system you now describe sounds like an excellent system but it sure is a lot different than just looking in the ground glass.

[bob_salomon]

The Rodenstock calculator computes both DOF for 35mm to 8x10" cameras as well as Scheimpflug by measurements on the ground glass of the near and far points and measuring the bellows extension between focus on the near and far points - with a level or an inclined camera.

The only factors you need to operate the calculator - which has a list price of $26.00 - for DOF is the magnification ratio from 1:1 to infinity in 9 steps, the film size (the smaller the selected film size the smaller the COC) and the rough angle of inclination of the camera.

 

After focusing on the far and near points and entering the values the calculatorgive the required aperture and any required exposure correction.

 

So if the camera is level, the reproduction scale is 1:20, the format is 4x5 and the extension difference is 6mm from the far to near point the required aperture is f32.

 

If the film format was set to 6x9 cm and the above conditions met the required aperture would be 45 + 1/3rd.

 

If the angle of the camera was changed to 40° and the format set to 4x5" and the extension difference remained 6mm the required aperture would be 16 +2/3.

 

it is simple. It is fast and it is convenient. It is also plastic for rough handling under all conditions.

[ellis_vener_photography]

Did I mention it only costs $35.00 US? <P>And when I am forced to work really fast I do rely on my groundglass: I focus on what I absolutely know I want to have in focus and then (based on experience) stop down accordingly. If i am shooting a landscape where what I want is near "infinity' for that lens I focus onthat and stop down minimally (usually to f/16. I do not try to do do some sort of "hyperfocal zone focus setting" with infinity as the distant point because what I want to be in focus is at or very near the infinity distance. If Iused a hyperfocal zone focus focal pointmidway between that and a hypothetical near point, what I really want to be in focus will be sort of in focus and I rather have an out of focus foreground or background then a semi focused subject.<P>

There is also a lot to said for "focusing on the far and tilting for the near" approach when that is appropriate.

[bob_salomon]

"Did I mention it only costs $35.00 US"

 

I am sure a lot of people would be happy to sell them for $35.00 but the list price is $26.00.

[jerry2]

Brian Ellis wrote..... You see these tables all over the place and I don't know of one specific one that is better than another. Just find one in a book or magazine article and try it to see how it works for you.

 

Brian, I wish I had the energy to type such a thorough explanation as you did in your post, very well done. I also agree that the chart or method should fit the needs of the task, not the visa versa.

 

I also agree with your position on Ellis's second post. Although I know Ellis surely understands all this, sometimes his posts seems as if they are contradicting themselves, but in reality, he is just presenting two different methods to use under different sets of circumstances. For newbies, this is sometimes confusing.

 

Ian, as many of these posters have suggested, the only "right" or most useful answer to your question revolves around exactly what you are trying to accomplish. For example, if your goal is super big enlargements, and you have the time to set up, I would reccomend either the Rodenstock Calc. method or my laser rangefinder / DOF table method, both are eqaully as good. Under such requirements the gg itself is not a sufficeint tool to meet your needs.

 

On the other end of the spectrum is..... if you are making contact prints or very small enlargements, say <3x, in such case, Ellis's first method is more than satisfactory.

 

So your approach to DOF, like most things in photography should be to work backwards from your desired end goal. This criteria determines the method (s) that will work best for you...and there certainy is no reason why you only need one method.

 

An alternative to DOF charts is a small programmable calc. these are very easy to use and quite convienent... there is several people listed on the LF homepage that offer free downloads in into programmable calculators or Palm Pilots etc. so all the work is done for ya.... (Bob Wheeler is one name that comes to mind) this is nice if your criteria is constantly changing, such as enlargement factor, lens fl's, etc. Otherwise you can be carrying many DOF charts which can be cumbersome at times.... The benefit of the Rodenstock cacl. is the distances of the near, far, and point of exact focus are figured out via the movement of the standards when focussed at each distance, so it accomplishes some of the leg work for you in the mathematics... hope this helps....

[ken_schroeder]

Ian, I suggest downloading Leonard's posting. A little study time and a few minutes with a calculator will privide you with the imformation you need. AND, you will have learned the skill to do this yourself. Do the math and write it down on a 3x5 card. With this card in hand, you can decide whether to purchase a ready made or put the money toward film.

[ken_schroeder]

another posting mentioned the depth of field tables on schneideroptics.com it's worth a look.