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View Full Version : Theoretical Field of View vs. Reality

Larry Huppert
3-Nov-1999, 23:39
I printed out a reference sheet of the various fields of view for the different lenses I use in large format (using Rui Salgueiro's angle calculator found at ht tp://www.mat.uc.pt/~rps/photos/angles.html). For the 4x5 format, I used an imag e area of 97mm Wide x 120mm Long (approximate image area of a Fidelity film hold er).

Although the calculator gives me 4 decimal places of accuracy, when I compare wh at I'm seeing on the ground glass with the theoretical value, they differ by a l arge amount (~5 degrees). The Palm Pilot PCAM program generates similar numbers for field of view. I'm measuring the field of view with a very accurate compas s.

Can anyone explain why I'm seeing such a difference? In some situations, I'd li ke to use tables such as these as a visualization tool.

fw
4-Nov-1999, 03:33
The actual focal length of a lense may vary from its indicated focal length - for example, the focal length of a lense badged as 90mm may actually be about 86mm, depending on the manufacturer. Have you factored this into your calculations?

Larry Huppert
4-Nov-1999, 07:39
I have not factored that into the calculations. However, I believe what I'm seeing is beyond this difference. For example, on the Schneider SA 75mm 5.6, the data sheet lists the effective focal length as 75.7mm. My observation for this lens would equate to a focal length closer to 82mm.

M Heal
4-Nov-1999, 08:45
Have you accounted for the fact that the lens moves further away from the film as you focus on objects closer than infinity? The length you should plug into the angle-of-view calculator is not the focal length of the lens, but the distance of the from lens to film necessary to obtain focus. The difference can be quite substantial, especially when you use the camera indoors. You can calculate the correct value as follows.

length = (focal length) x (1 + <sup>1</sup>/<sub>magnification</sub>)

For example, if you focus your 75mm lens on an object approximately 8 meters away, your magnification will be about <sup>1</sup>/<sub>10</sub>. Thus, the length you should plug into the calculator is 75 x 1.1 = 82.5 mm.

M Heal
4-Nov-1999, 08:54
Oh, dear. I mean focus your 75mm lens on an object 0.9 meters from the film plane.

Larry Huppert
4-Nov-1999, 16:54
In my case, I was viewing something about 11 ft away (3.3 meters). The magnification is 0.023, so by your formula, the observed focal length would be about 77.4 mm. Via the angle calculator, that give me a horizontal angle of view of ~75.6 degrees. That is still about 3 degrees off from my measurement. Could I be making a mistake with some other factor?

Alan Gibson
5-Nov-1999, 06:36
Larry: What are you actually measuring with your compass? I would think you are looking at your ground glass screen, identifying objects at the edges, and finding the angle subtended at the camera by those objects. If that is so, then you should use the dimensions of the GGS, rather than the film holder, in your calculations.

A question I have often wondered is how much the actual focal length varies between examples of a lens. For example, a 'Schneider SA 47 XL' is said by Schneider (if I remember correctly) to be 48.1mm focal length, but I don't know what the tolerance is (+/- 0.1mm? +/- 1mm?), nor whether my lens falls within the tolerance.

You might try the reverse calculation, to see from the observations and calculations what your focal length calculates to be.

Ellis Vener
5-Nov-1999, 09:45
What I am trying to figure out is why you are wanting to make an estimate so pre cise. It is an estimate after all, and 5 degrees over a field of view (so let's say it is 2.5 degrees of leeway on either side of the film) is for me, except in ver y rare occasions, "close enough for goverment work." the one time this was reall y a factor for me was in choosing a fisheye lens and matching camera for making a perfect image of a rotunda and was calculating based on the known diminsions of the buildings first level (height to ceiling, less minimum height necessary for camera operation), the diameter of room, and the actual angle of coverage of the lens across the actual width of the short side of the format. </ P> Good luck in your experiments. I will be interested in seeing the differences be tween what you see on the groundglass vs. what is actually covered on the film.

Larry Huppert
5-Nov-1999, 10:41
Alan:

Your correct - I am measuring the angle between objects I identify on the edges of the ground glass. The GG is slightly smaller than the film area. When I plug in these figures for my "75mm" lens, I get an angle of ~74.2. I'm getting closer yet to the measured figure of 72 degrees (using a Suunto Tandem compass). Maybe now I've gotten to a point where it's good enough for gov't work. I wasn't willing to accept being off 4 or 5 degrees.

Ellis:

You ask a good question - why am I so concerned about this precision? I guess I'm just mad! Actually, my interest in this comes from my increasing use of Palm Pilot tools such as PCAM and Vade Mecum. For interior architectural shooting, I'll routinely calculate either the plane of focus or the aperture for a given desired depth of field (or know before setting up if the shot is possible at all). It's like using the Rodenstock calculator, but better. I'm finding that that the 3 or 4 minutes spent on these calculations after I pre-visualize a shot, is time well spent. Maybe as my experience level grows, I'll use these tools less. My concern with the angle of view came from a mistake I made planning a recent architectural exterior shoot. It was a highly cost sensitive job, and I had a strong desire to shoot using roll film, if possible. The front elevation had trees fairly close to the house, but I previsualized a single location just out of the view of these trees where the angle of view should have worked with a 6x9 back, a 47mm lens and a bit of shift. Needless to say, it fell 1 or 2 degrees shy of working, and it was impossible to move back any further. I learned two things - don't play it so close to the margin of error, and use the best possible data if you are going to use these tools (hence this question). For table top work, I'm finding the use of Vade Mecum incredible. In less than 5 minutes I can calculate and set tilt and swing. The tools required are basically some simple carpenter's tools like a good angle finder. So far, I'm finding little or no fine tuning is required at the ground glass.

Ellis Vener
5-Nov-1999, 14:36
Larry, good answer and a problem I have run into. What is "Vade Mecum"?

Larry Huppert
5-Nov-1999, 15:52