DOF calculation

[Anausagi]

I have a question about DEPTH OF FIELD. I'm using 8x10 with 240mm lens. Is it possible to calculate the NEAR LIMIT where I can get focus when I shoot something with infinity at f/45. In another words, I like to know the distance from nearest point where I can focus to infinity at f/45 with my 240mm lens. For example, at f/45 I can get focus from 20 meter to infinity.

[Sean Billy Bob Boy yates]

What you are inquiring about is the Hyperfocal Distance, that distance at which everything between half the HD and infinity appear accebtably sharp, for a given lens at a given aperture.

Ellis and others more technuically astute than I have answered this question before, and I imagine you could find it by looking through the LF Q&A Forum under lenses and the homepage itself.

Short answer, At F/45, focused at 16ft. (+/-) you're D.O.F. should extend from about 8 ft. to infinity, for a 240mm lens, for a circle of confusion derived by dividing the focal length of a normal lens for this format (usually stated as 300, sometimes 305 and even 350 depending on who you ask) by 1000.

You might get from 5.5 to infinity, when focused at 11 feet.

This answer was arrived at by using the dials found in Kodaks Professional Photo Guide, available at the better bookstores (B&N, Borders, etc.)

Now, long answer, you can calculate your HFD with the following formula, assuming you agree on a C. of C. based on F/1000 or maybe F/1720. If that's too sloppy for you you could use this formula to determine your C. of C.

c=(vXD)/(1000XS) where v = film format, D = print viewing distance, and S = print size.

So, to get the HFD, you could calculate it with:

HFD = F (squared)/(f X c) when F= focal length, f = aperture and c = circle of confusion.

When calculating keep all your measurements in the same system and length to avoid confusion, i.e. FL= 240mm or 9 1/2 inch.

I am by no means an optical wizard and I imagine others will have additions, ammendments and or corrections to all this. Bear in mind that I passed high school algerba by the skin of my slide rule.

[Sean Billy Bob Boy yates]

An' mah spelin ain't nun tew gud eethur!

[Erik Ryberg]

At infinity your "disk" of confusion (not circle) will be the actual diameter of the aperature. Objects will resolve to that degree; ie things ten feet away will be "seen" by the film in "disks" the size of your aperature, as will objects one inch and one mile away. At a mile with your lens and that aperature, that's pretty good; at six inches, it's pretty bad. How bad it is at ten feet or thirty feet is a personal judgment. Take a look at Merklinger's "Ins and Outs of Focus." At f45 you should get pretty good resolution at thirty feet or so with that focal length.

The best thing to do is run some tests and see for yourself. I strongly advise you ignore standard DOF advice; to see why run a few tests. If you accept the DOF advice and elect not to focus at infinity objects further away than your focus point will very quickly go blurry, regardless the dozens of authorities who claim otherwise. Just try it: take your 240 mm lens and focus at 16 feet or 11 feet or whatever you like and develop the picture. Observe the horrendously blurry background. This is a problem that is a lot worse with 8x10 than smaller formats. (Now that I look at it again, this may be why you phrased your question the way you did; perhaps you've already noticed this problem.)

[Sean Billy Bob Boy yates]

My Mistake, the Depth of Field Mayven I was thinking of was Alan Gibson of the Philosophy of Photography Page.

Also, Schneider's website has some tables to look at for a 240mm but it ends at F/32.

www.schneideroptics.com/large/depth/8x10/240depth.htm

[Alan Gibson]

Taking the conventional formulae, Sean is right, focusing at 16 feet will make everything from 8 feet to infinity 'resonably' in focus. Put this another way, if you focus at infinity (as the question says), everything from 16 feet onwards will be 'reasonable'.

But I don't like the conventional formulae for modern films and large formats, the assumtions about 'acceptable' circle of confusion are simply wrong, in addition to the factors mentioned by Erik, and I would go with 'focus at infinity, and objects down to about 30 feet will be reasonable'.

It is, of course, much better to tilt your lens or film, and get everything exactly in focus.

[Gary Frost]

I agree the accepted circle of confusion is rather generous and will not produce the sharpest results at the distance limits. But that is not a good reason to just focus on infinity and let DOF beyond inf go to "waste". My simple solution is to select a smaller CoC to work with. (ie. .05mm instead of .1mm for 4X5) DOF is limited enough with long lenses used in large format, use what little you have.

http://www.syspac.com/~gillettm/DOF.html

Try this link for a DOF calculator. By selecting different formats, it will select different CoC. (go to formula used...) So you can select format of 6X4.5 for your 4X5. if I recall, will give you .05mm CoC.

[Erik Ryberg]

Okay, I'm going to settle this question once and for all. According to the link Gary provided (very impressive, by the way) I can focus my 10" lens at 23 feet at f32 and be "in focus" from 11.5 feet to infinity. With 8x10 format.

Using a CoC of .15 mm (half what the calculator uses) I can focus at 45 feet and be in focus from 22 feet to infinity. This is Gary's recommendation.

I'm going to do just that this weekend, and make a third exposure at infinity, and compare. My hunch is that the 45 foot focus will be a lot better, but the overall picture (I'll do some kind of typical landscape type scene) will be much superior at infinity. I realize "superior" is a generalization and some images require the closer subjects to be in better focus, but I think most landscapes suffer quite a bit when the distance is blurry.

Erik

[Gary Frost]

Eric, I think your assumptions are correct. At f/32 and CoC of .15mm, I don't doubt that the lens will resolve better at the distance focused than at the extremes of the CoC. Yes the infinite focus will be better. The tradoff will (obviously) be in the foreground, esp from 42-22 feet. I have Merklingers books and agree with his points on many things. But I think the advise to just focus on infinity and let everything else fall where it may (assuming you want infinity sharp) is taking what he is saying too literally... Yes, infinity focus will be sharper than the hyperfocal. If a CoC of .15mm is still too large to get infinity as sharp as you want, try .075, etc. (The referenced calculator is IMO too generous with the CoC for various formats.) Obviously a big factor here is how big/sharp you want the final print. If you were contact printing, it would all be a wash.... So for your test, try inf. focus hyperfocal for .15mm Coc and .075mm. See which has acceptable sharpness at infinity, and use that CoC.

[Gary Frost]

I might add here to put things in perspective, I don't use DOF tables in the field. I use a 10x loupe for fine focus and find that I can just resolve on the ground glass to about .05mm, the CoC I most use. If you are working smaller than f/22 & in dim light it gets difficult to see though. Try this. Select an aperture, say f/16 or 22. Focus carefully on infinity with the lens wide open, then stop down. Observe on the ground glass at infinity while you slowly focus closer. Stop as soon as you start to detect the slightest blur & back off till sharp again. Now open up the lens and see what distance you are focused to. For me it is always somewhere very near hyperfocal for .05mm. Granted this is mainly due to grain in the GG, etc, but for pragmatic use, is handy. Another handy method I have used well is to check the distance the focus is racked from near to far objects you want sharp. For a CoC of .05mm, if you move the lens 2.2mm, f/22 is required, if you move the lens 3.2mm, f/32 is required, etc. If you accept a CoC of .1mm you can open up 2 more stops: 2.2mm, f/11 is required, etc. This is a handy technique because it remains constant regardless of lens focal length. I estimate this mm focus distance by how far I must turn the focus knob. (ie. 3.5mm is 1/4 turn) Anyway, just thought I would add some practical techniques to the theoretical discussion.

[Henry Stanley]

For additonal reference, refer to DOF in the Technique archives (about 1/2 way down) where Ron Shaw wrote:

An article in Photo Techniques (Mar/Apr 96) on view camera focus used this method. Basically, you tape or glue a mm scale on your bed, and then focus on the nearest object of interest, read the position on the mm scale, then focus on the farthest object of interest, and note its position on the mm scale, and then, using this focus spread (in mm), refer to a chart for optimum f stop to use, based on line pairs per mm resolution. I photocopied the chart and taped it to the back of my camera for reference. I hope this helps.

-- Ron Shaw (shaw9@llnl.gov), August 28, 1998.

...and Alan Gibson did some fancy typewriter diagramming as a follow up.

I have copies of the articles referenced by Ron and they are useful, practical, flexible and well-founded. I don't have to take my calculator out nearly as often now. --Henry

[Robert A. Zeichner]

The suggestion about putting a mm scale on the focusing bed and calculating the focus spread is one I have found to be extremely useful. You can first make all your adjustments of movements and then calculate the spread. The resulting answer will determine the aperture needed to achieve the desired depth of field. The reason this is a more practical method is that once a rear tilt, for example, is implemented, objects that you consider to be at infinity may no longer be optically furthest from the groundglass. With the focus spread method of DOF calculation, you simply focus on the extremes, make note of the spread, look at the table of apertures (I taped a little cheat sheet to the back of my camera bag ID tag), set my lens and make the exposure. The article in Phototechniques also has the actual formula for anyone wanting to calculate the intermediate values.

[martin tai]

One of the strength of LF camera is movement, and with it comes a complete new s et of rules for Depth of field. The convention way of plane of exact focus, with two paral lel planes of near limit and far limit definining the depth of field no long app lies. Here Scheimplug fist rule and Scheimplug Hinge rule reigns.

Harold Merklin ger's other book"FOCUSING THE VIEWO CAMERA" has extensive formular and tables of depth of field for view camera. It is a must read for LF photographer If you are not well versed in math, you may skip the mathematical formula and go direct to the Chapter 7 Tables.