View Full Version : Depth of Field with Nikkor 300M---??

John Kasaian
8-Aug-2004, 08:48
Now that I've given it some thought, I'm really curious about this: I've got a 300 M aboard a Gowland 8x10 aerial camera---it is the lens this particular model camera was designed to use. Aerial cameras, this one anyway, is shot wide open(f9) at the fastest speed allowed by whatever natural lighting there is(1/400, maybe 1/250) and is fixed focus at infinity. Everything from about 50' on is in focus and extremely sharp as one would expect from a Nikkor lens. My question is, why? On other lenses shooting wide open seems to yield a fairly shallow field of focus while with the Nikkor, everything to infinity is in focus (for example, when I shoot my Crown Graphic at infinity(with a 127 Ektar, I have to stop down to f32 or f45 to get everything into focus!)

8-Aug-2004, 09:04
It's that Nikon magic! Greater depth of field than ordinary lenses with half the focal length!

Seriously, are you sure you're comparing apples with apples here? All lenses are bound by the same physical principles. But I wouldn't discount perceived sharpness on the ground glass, either. f/9 is a lot brighter than f/45. Have you compared them on film?

Brian Ellis
8-Aug-2004, 09:41
What are you looking at for your comparisons and how did you make the comparisons? Depth of field is a murky subject,there are a lot of variables that enter into it and different ways of looking at it, and people tend to get downright murderous when discussing it. I wouldn't think it would be of concern with aerial photography because of the usual (I assume) great distance from the subject (if you double your distance from the subject depth of field increases by four times, if you triple your distance from the subject it increases by nine times, etc. etc. i.e. depth of field is proportional to the square of the distance. But then all I know about arial photography is that I'm not about to hang myself out the door of an airplane a thousand feet above the ground and try to make a photograph.

Apart from that, how are you comparing the different lenses? When determining the effect of lens focal length alone on depth of field you have to keep everything else affecting depth of field identical, i.e. you make the same photograph of the same subject with the same camera from the same distance at the same aperture then enlarge or project the resulting negatives or slides to the same size and view them from the same distance under the same lighting. Then you get into questions about the different angles of view of the different lenses. Do you keep the angle of view the same in each comparison (i.e. do you crop the prints made from the wider lens to encompass only the area covered by the longer lens), which of course results in different magnficiation factors which in turn changes the size of the circles of confusion in the prints which in turn changes the perceived "sharpness" of the prints, which in turn can drive you crazy.

My suggestion is to not worry about why you perceive these differences, it can hurt your brain.

Leonard Evens
8-Aug-2004, 09:45
Choosing a maximum allowable coc of diameter 0.2 mm for 8 x 10, the hyperfocal distance of a 300 mm lens at f/9 is about 164 feet. If you focus at that distance, then everything from about 82 feet to infinity will be in focus. If you focus at infinity then everything from 164 feet to infinity should be in focus.

For a 127 mm lens, wiith a coc of diameter 0.1 mm for 4 x5, the hyperfocal distance at f/32 is about 16 feet.

These calculations are based on the prinicples of geometric optics. Any real lens will have deficiencies which will cause it to depart from those principles, and stopping down will often minimize those. With a good 127 mm lens for 4 x 5, you shouldn't have to stop down to f/32 to get everything from 50 feet to infinity in focus. So we can presume your Ektar is not doing all that well.

Why the Nikor appears to do better than theory predicts is not clear. I can come up with an explantion or two, but without being there and testing it myself, I am just guessing. One possible explanation is that you are used to such poor performance with your Crown Graphic that you are in effect using a very large coc in judging focus. Doubling the coc halves the hyperfocal distance. In addition, you may not actually be focusing at infinity but somewhat short of infinity. For example, if you focus at the hyperfocal distance itself, theory predicts that everything from half that distance to infinity will be adequately in focus.

One explantion I reject, unless someone can come up with a explanation using detailed reasoning from optics, is that your lens is doing better than theory predicts. It is not that physical theory is always right, but about simple things like this, it is close enough for all practical purposes.

Ralph Barker
8-Aug-2004, 10:36
Although I generally adhere to the idea that physics applies consistently to all lenses, I suspect that what you are considering to be DOF, John, is "sharpness" instead. The Nikkor M may simply be designed to be sharper at maximum aperture than some other LF lenses - just as Leica lenses on 35mm perform better wide open than most.

tim atherton
8-Aug-2004, 11:01
"or perhaps using a faster lens and Tech Pan film"

Didin't I just read somewhere in the last couple of days (I'm thinking on here?) that according to Ctein, Kodak has told him they are discontinuing Tech Pan film?

Jim Rice
8-Aug-2004, 12:59
I tend to believe Ralph.

John Kasaian
8-Aug-2004, 13:18
Thanks! I think I understand...uhh...I think I do.

Michael S. Briggs
8-Aug-2004, 23:32
My hypothesis is close to the opposite of Ralph's. I suspect that the sharpness of the 300 mm Nikkor-M lens isn't very good wide-open. There is really one one plane of best focus -- depth of field is determined by comparing the best focus with images of objects at other distances and deciding when the sharpness is sufficiently worse that the other objects should be considered to out of focus. If the object that lens is focused on is somewhat soft due to abberations in the lens, then the viewer's standards for deciding for deciding whether other objects are in focus will be relaxed and the apparent depth of field will be increased. In other words, large blur circles from abberations for the plane of best focus will lead the observer of a print to use a large circle of confusion in judging sharpness and thus a larger depth of field.

The usual equations for depth of field are very fundamental and almost more geometry than physics. The main physical principle used is that light rays travel in straight lines. The "art" in this approach is deciding what value to use for the size of the acceptable circle of confusion. The actual image also has effects from the lens, such as imperfect imaging known as abberations, and diffraction. Perhaps in this case the abberations of the wide-open lens cause the appropriate size of the circle of confusion to be larger.

I made a quick study of the imaging quality of the baby brother of the lens in question, the 105 f3.5 Nikkor-M: http://www.largeformatphotography.info/lfforum/topic/494534.html (http://www.largeformatphotography.info/lfforum/topic/494534.html#373881).
At least for that Tessar (which does have a wider maximum aperture), the image quality was distinctly better at f16 than f5.6. Presumedly the difference from f3.5 to f16 is even greater.

Some experiments might help decide the issue. Using a tripod and objects at a wide range of distances, what is the depth of field found to be at f22? Does it agree with theory? Comparing the f9 image and an image made at f22, does the f9 image appear sharp? What distance is in best focus?