DoF questions

yerlem · Jun 18, 2012

I understand the f number is a whole number. I think about it the "normal way" (smaller, more open) because that's how I learned (and also because there is a picture of a diaphragme on the screen that opens ald closes) .
My point was that it makes no difference what way you think about it, as long as you get it right, and I'm sure for some people is easier to think in fractions.

AaronLLockhart · Jun 18, 2012

Since we are on the subject, the "f-number" is not and does not represent a whole number. By this logic, changing the denominator of a fraction makes that denominator a whole number; which it does not. The only reason you don't see the fraction form written in the camera is due to the fact that it changes with the focal length of the lens. It's still a fraction, and you are simply changing the denominator of that fraction when adjusting f-stops.

So, defending the fraction, it's not a whole number and never will be.

You would not write "16" in your composition log for your photos. You would write "f/16," which is a fraction.

fjrabon · Jun 18, 2012

If there has ever been a more pointless, less fruitful argument that happens on here every other month, I don't know what it is. At least not one that is ostensibly about photography and not who is crazy and creating a conspiracy.

It is EXTREMELY important that you understand that the "f/#" notation COMES FROM fractions. After that, who really gives a crap how you view 'f-numbers'? If you understand why they are what they are (fractions), then it doesn't really matter if you look at that as a denominator in a fraction or a whole number.

EDL · Jun 18, 2012

From wikipedia:

In optics, the f-number (sometimes called focal ratio, f-ratio, f-stop, or relative aperture[SUP][1][/SUP]) of an optical system is the ratio of the lens's focal length to the diameter of the entrance pupil[SUP][2][/SUP]. It is a dimensionless number that is a quantitative measure of lens speed, an important concept in photography.

The f-number is given by

where

is the focal length, and

is the diameter of the entrance pupil (effective aperture). It is customary to write f-numbers preceded by f/[SUP][3][/SUP], which forms a mathematical expression of the entrance pupil diameter in terms of f (a symbol denoting the focal length), and the f-number. For example, if the focal length is 16 mm and the entrance pupil diameter is 1 mm, the f-number is 16 and the aperture would be expressed as f/16. Ignoring differences in lens transmission efficiency, the greater the f-number, the darker the image projected by the lens. The amount of light transmitted from each object in the lens's field of view to each unit of area of the film (or sensor) decreases with the square of the f-number. Doubling the f-number decreases the exposure by a factor of four. To maintain the same exposure when doubling the f-number, the exposure time would need to be four times as long.
The pupil diameter is proportional to the diameter of the aperture stop of the system. In a camera, this is typically the diaphragm aperture, which can be adjusted to vary the size of the pupil, and hence the amount of light that reaches the film or image sensor. The common assumption in photography that the pupil diameter is equal to the aperture diameter is not correct for many types of camera lens, because of the magnifying effect of lens elements in front of the aperture.
A 100 mm focal length lens with an aperture setting of f/4 will have a pupil diameter of 25 mm. A 200 mm focal length lens with a setting of f/4 will have a pupil diameter of 50 mm. The 200 mm lens's f/4 opening is larger than that of the 100 mm lens but both will produce the same illuminance in the focal plane when imaging an object of a given luminance.
In other types of optical system, such as telescopes and binoculars, the same principle holds: the greater the focal ratio, the fainter the images created (measuring brightness per unit area of the image).

tcphotography · Jun 18, 2012

AaronLLockhart said:
Since we are on the subject, the "f-number" is not and does not represent a whole number. By this logic, changing the denominator of a fraction makes that denominator a whole number; which it does not. The only reason you don't see the fraction form written in the camera is due to the fact that it changes with the focal length of the lens. It's still a fraction, and you are simply changing the denominator of that fraction when adjusting f-stops.

So, defending the fraction, it's not a whole number and never will be.

You would not write "16" in your composition log for your photos. You would write "f/16," which is a fraction.

Actually I do write 16 for f stop. It's my log, so who cares? As long ad YOU know what you write. That's the point being made here.

Do you (or anyone) understand how the entire system works? Yes, then move on! Professionals note things different ways. Do what works best for you! It's all opinion.