[Paul Ewins]

The f-stop is primarily a measure of theoretical light transmission, not of DOF, so saying a 50/1.2 is the equivalent of a 75/1.8 is misleading. Fast lenses were made to transmit more light, not to give thinner DOF and a 55/1.4 APSC will require the same exposure as a 50/1.4 on FF or an 85/1.4 on FF which is what most people need to know. Adding in a poorly understood secondary characteristic of the lens design is not in anyway helpful. If you are the sort of person who understands the way DOF works in practice then you are unlikely to need to be told some sort of equivalence factor. If you don't understand it then the equivalence won't help.

[gazonk]

The f-stop is primarily a measure of theoretical light transmission, not of DOF, so saying a 50/1.2 is the equivalent of a 75/1.8 is misleading.

That depends on whether you're considering light transmission purely for exposure purposes, or if you're interested in the actual number of photons hitting your sensor or film i.e. in the picture quality. In the first case, you're interested in light transmission per sensor area, in the second case you're interested in the total light transmission to the sensor.

[Rondec]

The f-stop is primarily a measure of theoretical light transmission, not of DOF, so saying a 50/1.2 is the equivalent of a 75/1.8 is misleading. Fast lenses were made to transmit more light, not to give thinner DOF and a 55/1.4 APSC will require the same exposure as a 50/1.4 on FF or an 85/1.4 on FF which is what most people need to know. Adding in a poorly understood secondary characteristic of the lens design is not in anyway helpful. If you are the sort of person who understands the way DOF works in practice then you are unlikely to need to be told some sort of equivalence factor. If you don't understand it then the equivalence won't help.

Well, the issue with equivalence is that it doesn't just apply to the aperture and focal length, it also applies to iso. This is what people forget when they are looking at the equation.

[ElJamoquio]

That depends on whether you're considering light transmission purely for exposure purposes, or if you're interested in the actual number of photons hitting your sensor or film i.e. in the picture quality. In the first case, you're interested in light transmission per sensor area, in the second case you're interested in the total light transmission to the sensor.

...and in the first case you mention, 'actual' aperture/iso/shutter succeeds, but so does equivalent aperture/iso/shutter.

In the second case you mention, only equivalent aperture/iso/shutter succeeds.

In the third case, you don't mention, where you care about DOF/AOV/SNR, only equivalent aperture/iso/shutter succeeds.

Hence the beauty.

[Digitalis]

saying a 50/1.2 is the equivalent of a 75/1.8 is misleading

In the context of the lens being used on an APS-C sensor it is accurate. It is still a 50mm lens, and it still transmits all the light an f/1.2 lens does the only thing that has changed is the size of the recording medium behind the lens. For photographers like me I can switch formats faster than you can change a roll of film, equivalency is a useful tool. I use APS-C format all the way to 8X10 format, and I did a brief stint of work with a 20"X16" Ultra large format view camera - developing those massive negatives was really fun.

I really do not get why people are so against Falk's idea of equivalency, it is hardly a new concept*. I have used a similar albeit not so rigorously mathematically calculated method with my students for years. It is nice to have a member like Falk who cares so much about photography, to be so thorough in his ideas. As it happens I teach equivalency to my students and as I do with all my students i'm just an instructor - they can use what I teach them and use it, or ignore it.

*my great grandfathers notebooks contains evidence that he was thinking along similar lines - sadly most of those journals are in a form of German shorthand that is practically undecipherable.

[Aristophanes]

The f-stop is primarily a measure of theoretical light transmission

No. Light transmission is a T-stop.

F-stop is purely the relationship between FL and effective optical diameter.

[falconeye]

No. Light transmission is a T-stop.
F-stop is purely the relationship between FL and effective optical diameter.

You are 100% right.
But I think I understand what Paul Ewins tried to say. And he has a point, wether you call it T-stop or F-stop (cf. below).

The f-stop is primarily a measure of theoretical light transmission, not of DOF, so saying a 50/1.2 is the equivalent of a 75/1.8 is misleading. Fast lenses were made to transmit more light, not to give thinner DOF...

Paul Ewins makes a valid point and he basically uses T-stop and F-stop like synonyms. Which is ok in our context.

A wider lens' main purpose is to allow shorter exposure times or less sensitive film.

Therefore, one easily discards equivalence as it seems to apply to depth of field only. Which is a secondary concern as Paul writes corrrectly.

All my work and postings are to make knowledgeable photographers like Paul understand that equivalence applies to the primary concern of light gathering capability foremost (and then to every other concern up to diffraction as well).

This is also why equivalence is per camera, not lens.

So, the correct sentence would have to read:

saying a 50/1.2/ISO100 (APSC) is the (35mm-)equivalent of a 75/1.8/ISO200 is misleading.

And all of a sudden, one can see that it is not.

Both cameras capture an identical total amount of light, read, have equal speed. And therefore produce images of equal noise (noise is a property of light, not camera, one may have to know too).

We may discuss endlessly. But every single photographer who understands this is not a depth of field discussion makes me think that it was worth it.

[Kunzite]

For what purpose do you want to know the total light gathering?
You're saying noise (as one of the reasons); but there are different types of noise, I disagree that it's only "a property of light" - and IMO people would rather visually evaluate the noise by looking at image samples, taken at different settings. A quantitative measurement - for a photographer at least - is mostly useless.

I can't see how such a system would be useful for an ILC, except for lab measurements/reviews. It's not like I don't understand something, I simply view it differently.

[falconeye]

It's not like I don't understand something, I simply view it differently.

In all due respect, I beg to differ, Sir.

Maybe, the reason for our different views of this topic does lie indeed in differing understandings of the nature of light. And what creates noise in an image (or the bulk of it). And I say understanding, not opinion. There is no room for subjectivity here, its all simple physics.

[bxf]

In all due respect, I beg to differ, Sir.

... simple physics.

oxymoron.

[Uluru]

Gosh, guys, can you please cease fire...
Pentax is preparing to launch a K50, which means they are well and alive and we can put this thread into its retirement.
Where is that champagne?!

[normhead]

Gosh, guys, can you please t-stop or f-stop...

Don't make me call you an Aperture...

[Paul Ewins]

I'll repeat this bit:

If you are the sort of person who understands the way DOF works in practice then you are unlikely to need to be told some sort of equivalence factor. If you don't understand it then the equivalence won't help.

The days of people using hyperfocal distances are long gone as are the DOF scales on lenses. How many people use DOF preview, assuming that their camera is capable of doing so? The only time they will need to know the difference is if they want to achieve extremely narrow DOF in which case simply telling them to buy a full frame camera and a fast 50 or 85 will be enough. In all other cases they should be able to achieve the desired DOF by altering the aperture until the preview looks right.

[derekkite]

(noise is a property of light, not camera, one may have to know too).

Could you explain how noise is a property of light? Maybe I'm going at it backwards but I thought noise was a factor of signal to noise, ie. the sensor designs produce noise in the process of reading the photon loading, and the more light the less apparent noise.

As for the 35mm equivalent, this stuff always comes down to marketing and convention. Digital cameras, particularly in the professional and expert hobbyist market has had to get better than film. My Olympus E-420 had a 4/3 sensor and at iso 400 would produce a grainy result similar (but not as nice) as film. The digital slr's and larger format bodies were for a long time attempting to match the quality of film. This goal was met a while ago, but up to that transition stage photographers were thinking in terms of film, many times using film lenses, so the numbers, settings and the like were aimed at that group who thought in those terms.

I've shot film but very little and long ago, and my experience and thinking is in the digital realm. The capabilities, such as very nice clean shots at 3200 iso that I get with my K5, HDR capabilities etc. are beyond what film could even dream of. I was shooting with a friend who had long experience with film, and he was amazed at the speed that I could shoot multiple exposures, and yet I who have quite a bit of computer experience chafe at any delay that occurs in any digital device.

There is an elegance to the f-stop model. Half the light either by aperture, shutter speed or sensitivity of the sensor. It is a working language, and part of the learning curve of a body and lens combination is getting the intuitive feel for what that body and lens does with a specific light level. If there is an elegant working language that would tell me those things, I suspect that it will be adopted.

[derekkite]

I'll repeat this bit:

The days of people using hyperfocal distances are long gone as are the DOF scales on lenses. How many people use DOF preview, assuming that their camera is capable of doing so? The only time they will need to know the difference is if they want to achieve extremely narrow DOF in which case simply telling them to buy a full frame camera and a fast 50 or 85 will be enough. In all other cases they should be able to achieve the desired DOF by altering the aperture until the preview looks right.

Or dig out their DOF calculator on their Iphone.

Something to consider is the ease and very low cost of shooting a couple hundred frames in different settings and conditions. One would have to be wealthy to do that with medium or large format film, and moderately well off with 35mm film.