In the good old days when almost everyone owned a 35mm film camera, the concept of an F-number was extremely useful, giving clear information about low light performance and depth of field, which was directly comparable between different (35mm) cameras. The small number of people using other than 35mm film were generally professionals who knew how to account for different film sizes, so life was simple and everyone was happy.

However, in the age of digital cameras with widely varying sensor sizes, the usefulness of traditional F-numbers is less clear. When sensor size is variable, knowing only the F-number gives no information whatsoever about low light performance or depth of field.

Here is a summary of what F-numbers meant in the past, and what they mean now.

GOOD OLD DAYS – ALMOST EVERYONE HAD A 35mm FILM CAMERA

(a) Low light performance

With a 35mm camera, the smallest F-number attainable by a lens was a direct measure of how much light the lens would capture and deliver to the film. This was great. Photographers of the time quickly gained a “feel” for what an F-number meant in terms of getting a good shot in poor light. F-numbers for different lenses could be compared directly with confidence.

(b) Depth of field

With a 35mm camera, the depth of field depends on the F-number. This was great. Photographers of the time quickly gained a ‘feel’ for what an F-number meant in terms of depth of field. F-numbers for different lenses could be compared directly with confidence.

TODAY – ALMOST EVERONE OWNS A DIGITAL CAMERA

(a) Low light performance

The F-number, on it’s own, tells you nothing. This is a pain, as well as causing much confusion. Regarding the true ‘speed’ of a lens, F-numbers cannot be compared for cameras with different sensor sizes. For example, the lens on my point and shoot camera is F2.0, but it does not gather anywhere near as much light as an F2.8 lens on an SLR style camera.

(b) Depth of field

The F-number, on it’s own, tells you nothing. This is a pain, as well as causing much confusion. Regarding depth of field, F-numbers cannot be compared for cameras with different sensor sizes. For example, F8 gives a completely different depth of field on a point and shoot camera and an SLR.

In summary, F-numbers are at best confusing, and at worst near useless, in the age of digital cameras with their widely differing sensors. The ballgame has changed, but the photographic community has not. The purpose of this thread is to see if there is some other ‘F-parameter’ that avoids these problems.

The obvious solution is to normalize F-numbers with respect to sensor size, as follows :-

Normalized F-number = F-number x Crop Factor

where Crop factor is the sensor size relative to standard 35mm film format.

For brevity, I shall refer to Normalised F-numbers as NF-numbers.

I state without proof that if NF-numbers were used in place of F-numbers, all of the problems described above would instantly disappear. My previous threads explain why this is so. Some examples are the best way to explain this concept.

For lenses on 35mm film cameras, or digital cameras using the same 35mm format, the NF-number will be identical to the F-number, as the crop factor is 1.0

As the majority of digital cameras use sensors smaller than 35mm, the effect will be to normalize the F-number to a higher value. For example:

Typical SLR with APS-C sensor

Crop factor = 1.5

F-number = 2.8 (typical consumer zoom lens)

NF-number = 4.2

The interpretation here is that the F2.8 lens on this SLR , is equivalent to an F4.2 lens on a 35mm camera. That is, the depth of field will be the same, and the all else equal the amount of light gathered by the lens will also be the same. It really does make sense to discard the F-number altogether, and quote the NF-number instead. Here is another example, for a fairly good point-and-shoot camera :-

Canon G2 with 1/1.7” sensor (typical good quality point and shoot)

Crop factor = 4.7

F-number = 2.0

NF-number = 9.4

What this means, is the apparently fast (for a consumer zoom lens) F2.0 lens on the point and shoot is really no such thing, in fact the effective F-number of 9.4 is more than double that of the lens on a typical SLR. (9.4 vs 4.2), even though the point and shoot lens has a lower F-number. For portraits (or whatever) where a poor depth of field is required, we also see how misleading the F2.0 number is – it is actually equivalent to F9.4 on a 35mm camera. Again, it is evident that the NF-number is far more useful than the F-number.

The bottom line is this. If all lenses were specified in terms of NF-numbers, then all of the NF-numbers could be directly compared with each other, just as in the ‘good old days’ when we used 35mm film cameras. That would be exceedingly useful, and eliminate the very considerable present confusion and misunderstanding regarding F-numbers.

However, there are disadvantages as well. An F-number is the ratio of a lenses focal length to aperture diameter, and is therefore a property only of the lens. A NF-number is a property of the lens and the sensor with which it is to be used. For cameras with non-removable lenses there is no problem. However, for removable lenses, the lens NF-numbers will be different depending on the camera and sensor that the lens is used with, so what should be written on the lens? This may not be much of a problem in practice. For example, the new 4/3 lenses are designed very specifically to work with ‘4/3’ cameras having a standard ‘4/3’ size sensor, so there would be little problem here. I guess the key point here is that with removable lenses, NF-numbers would ALWAYS need to be quoted in conjunction with the intended size of sensor. That allows the user to convert back to F-numbers if desired, or recalculate the NF-numbers should the lens be used with a different sensor.

Adoption of NF-numbers would not mean that the concept of F-numbers is redundant. It would still be true, for example, that the cost of making a quality lens depends very much on the F-number, not the NF-number. Even if NF-numbers were adopted, I’m sure many users would still want to know the F-numbers for all sorts of reasons that I haven’t thought of.

In practice, it is incredible unlikely that the photographic industry would ever print NF-numbers on lenses, or in camera specifications, and even less likely that any manufacturer would build a digital camera where the entire menu and parameter display system worked with and displayed NF-numbers rather than F-numbers. I wish they would, but there is just too much inertia for this to change – too many existing products described in terms of F-numbers, and generally just too much familiarity with F-numbers, despite the advent of digital cameras that makes the usefulness of F-numbers appear questionable. I am not asking whether I think industry adoption of NF-numbers will ever occur. It won’t. This is an exercise in free thought. I am asking whether NF-numbers would be a better way of describing lenses, or at least a useful alternative way of describing lenses, not whether such a change will ever happen. Of course, anyone is free now to quickly and easily calculate NF-numbers for lenses/camera of interest – I certainly intend to calculate and think in terms NF-numbers from here on because in my view they are so much more useful, and I sure wish all my cameras displayed NF-numbers, too.

I enjoy ‘what if’ scenarios. If all digital cameras adopted NF-numbers rather than F-numbers, then I bet more people at the lower end of the market would spend more money to get a better camera, as it would be very obvious just how bad (=slow) the lenses on small point and shoot camera really are, whereas at the moment this fact is hidden because the F-numbers for these cameras look OK, on account of the small sensor size. More people would buy SLRs, too.

This thread is closely related to my previous threads, where I was trying to throw around some ideas regarding how best to compare the light gathering ability of different camera, which is distinctly confusing when sensor sizes is not a constant. An excellent start would be if F-numbers were replaced by NF-numbers.

Comments?

Any disadvantages of working with NF-numbers rather than F-numbers? (I bet there are)

What do YOU use F-numbers for, that NF-numbers would not be suitable for?