Mountainlander:
I've posted this elsewhere (photo.net) and thought it might be a little helpful.
Sorry for the long discussion, but I do hope that you and others will find it useful. There's more to come.
There are really only three factors governing the subject of depth of field for 35mm cameras specifically and other film formats generally.
Hopefully, the following discussion will boil them down for you, but before we get into that topic, there are a few basics you will need to know.
1. First of all the eye doesn't see so well - especially under available or poor lighting conditions. The general consensus is that the human eye can see a circle as a dot, if that circle has a diameter of l/100 of an inch or less on an 8 X 12 (or sometimes 7 X 10) enlargement held approximate ten inches away.
That means that in order for the circle of confusion (it's correct name) to be see as a dot on the above standards, that circle of confusion has to be 1/800 of an inch in diameter on the negative.
Thus - the need for critical focusing = especially when shooting at wide aperatures under available light conditions. The problem of critical focusing under such conditions has been solved in various ways - i.e. the use of rangefinder cameras or using high aperature lenses on SLR cameras, i.e. a 50mm F/1.4 lens versus a 50mm F/2.0 lens.
Obviously, if you're slightly out of focus with a 35mm F/2.0 lens on a 35mm SLR camera - which can easily happen especially under low light conditions - you may start off with a large "circle of confusion" on the negative or slide. The more you enlarge an unsharp image (negative or slide), the greater the unsharpness will be noted in the final enlargement or projected image.
For that reason, discussions of depth of field cannot be separated from the issue(s) of critical focusing - again and especially under available light conditions.
If you wish to check this out, you can simply put on a wide angle lens on your SLR, turn off the lights in a room to make it semi-dark - like in the late evening and then walk about 12 - 15 away from the subject or point of focus. With an SLR camera, you may find it very difficult to be certain of accurate focus under these conditions. For example, you may find that you're actually focused at infinity when in reality the subject is only 12 feet away.
Why? For one thing, our eyes don't see to well; for another, a wide angle lens on a 35mm SLR camera will make the image appear smaller and/or further away, thus making the "details" more difficult to see. Under such conditions a split image focusing screen can be very helpful. Depending upon the type of rangefinder camera, a rangefinder camera will be able to focus both accurately and rapidly under nearly all available light conditions using lenses from 21mm or lower to up to either a 90mm focal length or a 135mm focal length. An old Leica M-3 would be a good example.
To give you another example, my 90mm Summicron F/2.0 focused at the closest distance (about 3.5 feet) and using the lens wide open (F/2.0) has exactly 3/4 of an inch of depth of field - which is less than the distance from the tip of most people's noses to their eyes. So what's the point of focus to be and how accurate is it going to be? are useful questions to ponder and/or resolve.
Now that you have, hopefully, understood this discussion - let's go onward to the two important factors regarding depth of field and then we'll tackle the third and most important factor.
2. From a technical stand point, depth of field is dependent upon only two factors (1) the aperture of the lens and (2) the ratio of reproduction. Or to put it in another way, all lenses will exhibit exactly the same depth of field if they are used at the same aperture and at the same ratio of reproduction.
Verification of this basic factor can be found in some of the older and better books on photography - namely - most editions of The Pentax Way or The Leica Way, etc. In the back of these books, you find a series of depth of field tables, especially for macro work or photomacragraphy.
In short, a 50mm macro lens and a 100mm macro lens set at F/4.0 and at a reproduction ratio of 1:2 will have exactly the same depth of field.
Obviously, this may bring up - in your mind - the topic of what is meant by the "ratio of reproduction".
The ratio of reproduction represented by the 1:2 - above - indicates - first - your film size, or 24mm X 36mm or approximately 1 X 1.5 inches. The second number (2) indicates that the area that your lens is covering - in this case it is twice as large as your negative or 2 X 3 (1 x 1.5) X 2. Therefore, a reproduction ratio of 1:10 would indicate that your lens is covering an area of 10 x 15 inches. And so on and so forth.
Therefore, you should now realize that the only reason that wide angle lenses "appear" to have a greater depth of field to most people is that they are simply standing in the same place, but changing their normal lens (50mm - for example) and putting a 28mm lens on the camera. Obviously, you have automatically changed the ratio of reproduction by simply including a far wider area with the 28mm lens.
However, if you would change back to the 50mm lens and then walk backwards to include the same area of coverage as your 28mm lens, you would end up with exactly the same depth of field.
Obviously, you should now understand the statement made in the third sentence.
One of the most unfortunate aspects of photography is that you (and many others) will encounter a host of explanations for various kinds of photographic techniques, procedures, etc. and equally unfortunate much of it will amount to little more than "verbage" or outright "BS".
One good example that I well remember came from reading Ansel Adams explanation of his famous photograph of the moon rise taken somewhere in the southwest - New Mexico, I believe. Sorry, I can't remember the exact title given to this wonderful photograph.
In reading his discussion of how this photograph was taken, Adams goes through all sorts of language contortions about the exposure value of moonlight, how he calculated the "exposure" of this moonlight, etc., etc., etc. And then his finishes the discussion by stating that he made one or several "calculated safety shot(s)."
When I originally read his discussion, I laughed out loud at his "calculated safety shot(s). What a "BS'er" I thought and still do. Why can't this man use the same term that everyone else does?
A rather highbrow term for the simple matter of "bracketing."
Obviously, Ansel Adams was a good photographer, but he was also, IMHO, rather pretentious and verbose. Somewhere there is another example of Ansel's verbosity, you'll find it in a discussion between Ansel Adams and Peter Stockpole who wanted to join the F/64 group and who was fired for using a "miniature camera" on the job. The "miniature" camera in question was an early screw mount Leica. Sorry, I can't remember the title of the book, but I think it was in a book written by Peter Stockpole about his photographic experiences and/or working on photographing the building of the Golden Gate brige.
At the beginning of this discussion, I indicated that there were three factors concerning depth of field and that the third factor was the most important.
Well, what is the third factor?
Aesthetics - pure and simple.
At this point, you must learn to merely keep in mind the basic principles that I've discussed (clearly, I hope) above and concentrate more on what you wish to convey artistically.
Years ago, when I was at New Salem State Park - near Springfield, IL - the park had an interesting event going on - during the early winter.
One of the events being "exhibited" were a small group of women quilting in one of the log cabins. I approach one of the women and began discussing whether or not she had read the book or seen the play of nearly the same name: The Quilters: Women and Domestic Art. (I highly recommend both reading the book and/or seeing the play.) Mostly, because I wanted to convey the information to her and secondly it was a means of breaking the "ice" when photographing people.
While I was discussing the book and the play, I took an incident light meter reading and set my shutter speed and lens opening accordingly. I can't remember the shutter speed, but the aperture was set at F/2.0 since I was using Kodachrome 64 slide film, my Leica M-4 and the 50mm Summicron.
I deliberately focused on the women's eyeglass frame, composed and tripped the shutter. Turn out superb!! Again IMHO!! ;>)
Generally, most people might have focused on her eyelashes or her eyeball, but I chose the eyeglass frames instead because I wanted them to be in sharp focus. And more importantly, because I deliberately wanted to soften the lines in her somewhat aged skin, i.e. soften the wrinkles in other words. In my head, I already knew that the depth of field was going to be rather, if not very, shallow.
In short, I simply made quick use of my technical knowledge and previous experiences and use them to determined what kind of aesthetic results I wanted.
Obviously, now that you know something more about depth of field, you are going to have to do some experimenting with different focal lengths and/or apertures to see what kind of results you will obtain and then use the results to narrow them down to what you wish to obtain. It may take some time and practice, but that's the fun and sometimes fustrating part of photography.
My best wishes in your endeavors and I do hope this not so brief discussion clears things up.
Bill
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