Calculating depth of field

[Keith Cocker]

Hi,

I wonder if anyone can suggest how I can calculate depth of field with my LF Camera (Toyo Field 45) for any particular lens. I realise I can gain an impression by stopping down the lens and looking at the Screen but sometimes thats just too dim to be of any value. Is there some form of calculation taking into account Aperture, focal length, subject distance, bellows extension etc for example?

Thanks

[eddie]

here are A few i use.

http://www.bobatkins.com/photography/technical/dofcalc.html

http://www.dofmaster.com/dofjs.html

there are many more but these will get you going.

eddie

[Scott Whitford]

Keith,

If you google "depth fo field", you will find several on-line calculators.

Better yet, you can also find the governing equations themselves as well as guidelines for which circle of confusion to use for a given format. Then you can make your own spreadsheet.

Scott

[Keith Cocker]

Thanks Eddie & Scott - thats really helpful. Sorry about the duplicate post!

[Leonard Evens]

Start off by finding the hyperfocal distance. The formula for the hyperfocal distance is

H = f^2/(Nc)

where f is the focal length, N is the f-number and c is the diameter of the maximal acceptable circloe of confusion (all usually abbreviated `coc').

The coc is dependent of the format. For 4 x 5 phtography, a reasonable value is 0.1 mm.
This is based on the assumption that you are viewing an 8 x 10 final print from about 10-12 inches or a larger print proportionately further away. But some people may insis on viewing a large print from close-up instead, and they would choose a smaller coc, e.g. 0.075 - 0.05 mm.

Foir example, for a 150 mm lens at f/22, and using c = 0.1 mm, this yields

H = (150)^2/(22 x 0.1) = 22500/2.2 ~ 10,2227 mm ~ 10.2 meters.

Generally the smaller the hyperfocal distance, the larger the depth of filed. If you focus at the hyperfocal distance, then in principle everything from half that distance to infinity will be in focus.

For distant objects, when focused at distance D, the formulas for near and far depth of field are
in front of focus point : D^2/(H+D)
in back of focus point: D^2/(H-D) unless D is equal to ar greater than H, in which case the back DOf is infinite.

For relatively near objects the (exact formula) is more complicated. (See
photo.net/learn/optics/lensTutorial). Often for that case one expressed the answer in terms of the magnification M, which is the ratio of the image size to the object size.)
In this case the distances in focus in front and behind the point of focus are essentially the same and approximately equal to
Nc(1 + M)/M^2
Thus for relatively close objects, provided you keep the format and image size the same, the depth of field is essentially independent of focal length.

All the aobve rmearks assume that the lens is close to being symmetric. That is almost always the case for large format photography. More generally there is a factor called the pupil magnification which enters into the formulas. To see how refer the the link to Jacobson above.

Having said all that, let me add that you seldom need to use these formulas. Use instead what you observe in terms of focus shift along the rail between near and far points. How to do this deicussed on the large format page lfphoto.info.

[Leonard Evens]

Having given you the formulas for DOF, let me make some comments about using what you see on the ground glass. First note that to get reasonable results, you need to be viewing the gg, proportionately from the same distance as you would view the final print. So, for an 8x 10 camera, you can rely on what you see on the gg from about 10 to 12 inches, but for 4 x5, you would need to be at half that distance or 5 to 6 inches. Unless you are still very young or you are near sighted to just the right degree, you are not going to be able to do this without some aid, such as a 2 X loupe or a strong pair of reading glasses.

If you view the gg from further away, you will be less critical of what you see, which means that when you look at the final print from a normal viewing distance, you are likely to be disappointed. On the other hand if you use a loupe stronger than 2 X, you are in effect being more critical when viewing the gg than you will be when viewing the final print. So you won't have to worry about not being satisfied with what you see on the gg, but you may end up using too small an aperture, and hence too long an exposure time to account for subject motion.

You are certainly right about the gg getting dim. Few of us can see much of anything beyond f/16-22 and many of us would be more comfortable not going further than f/11-16. You can make it easier to see what is on the gg by getting a bright screen---I have a Maxwell screen, which is very bright---and also using a 4 X or higher loupe will help. The loupe, as noted above, may make you stop down more than you absolutely need to, but that is often harmless.

Let me suggest another simple trick. Stop down as far as you can and still be able to judge what is in focus and note the f-number. Gauge as best you can the fraction of the desired DOF that you see on the gg at that f-stop. Divide the viewing f-number by that fraction and use that when taking the picture. That will likely get you pretty close to what you need. This is most useful when you have tilted or swung and you are trying to estimate the DOF on either side of the plane of exact focus. For example, suppose you have tilted and looking at the gg at f/16, you see in focus a vertical strip of about 50 mm in focus above and below the plane of exact focus, but that you actually need 100 mm, i.e., the fraction is 1/2. The rule suggests that an f-number of 16/(1/2) = 32 will suffice.

Of course, with any of these methods, it usually doesn't hurt to stop down a half to a full stop more., and since the formulas all assume perfect optics and perfect focusing, it may be wise to do so to compensate for inevitable errors.

[jon.oman]

http://gogebic-pc.home.comcast.net/~gogebic-pc/dofcalc/win/

http://gogebic-pc.home.comcast.net/~gogebic-pc/dofcalc/pocketpc/