"Crop-ability" of images?

Started 3 months ago | Discussions thread
bobn2
bobn2 Forum Pro • Posts: 69,811
Re: Exposure vs Exposure
3

john isaacs wrote:

bobn2 wrote:

john isaacs wrote:

bobn2 wrote:

Iliah Borg wrote:

john isaacs wrote:

He can talk about Lux all he wants. No one cares.

If no one cares, why would Kodak mention lux 70 times on 24 pages of their "Basic Photographic Sensitometry Workbook" brochure?

I think in this case, 'he can talk about Lux all he wants' is shorthand for 'I'm incapable of following the discussion and don't want to admit to it,' and the statement 'No one cares' should have been prefixed with the words 'I hope'.

His definition is useless.

Not very clever, all those people like Hurter and Driffield, ISO, Kodak, Fujifilm, Ilford, Agfa and all the manufacturers that used and designed their products around that definition of exposure. Or the photographers like Adams that based their practice around it. Of course, it's to be expected, without the help of all those You Tube videos and Web explainers to put them right. I mean, what were they to do? They went to college and learned from people who actually know what they were talking about, and got their qualifications by satisfying examiners that they had learned it all properly. What a useless way of doing it. All that really matters is how many likes you get for your post. That tells you what's right and wrong.

And none of them had even heard of the 'Exposure Triangle', so really, how were they ever going to understand anything?

His insistence is futile.

Not quite as futile as someone who continues an argument in which their every contribution is yet another eloquent demonstration that they haven't a clue what they are talking about. It must be a lack of self-awareness.

This was all done in the film days, where the film was the sensor and the sensitivity was generally fixed. And so Shutter Speed and Aperture were the only parameters you had to deal with (although you might get involved with the film ASA if you needed to push the sensitivity for low light...but oh the noise).

Whatever. The definition of exposure has not changed.

What happened with digital was that base sensor sensitivity was easily modified by a gain adjustment (ISO),

ISO is not a 'gain adjustment', at least according to ISO, who should know. Moreover a gain adjustment would not and does not change sensor sensitivity.

and could be modified on a per image basis. So shutter speed and aperture could be set within less rigid constraints, and ISO adjusted to increase the brightness of the image, avoid crushing blacks, and reduce noise. And, the product of the camera was not an exposed film, but a processed image. So exposure on the sensor became an intermediate step, and not a final product, of the camera.

The exposure at the sensor (or film) was never, ever the 'final product', so all that discussion is one big red herring,

As a result, the term "Exposure" became synonymous with the image file.

What? In whose world it the term 'exposure' synonymous with the image file? This is something that you have made up.

But ISO could now vary with each image (Exposure) and hence ISO became an important parameter of the Exposure.

This is dissembling. We were talkiing about what is the definition of 'exposure', not 'The Exposure'

And while none of those people you mention had heard of the "Exposure Triangle", they had heard of film speed ASA or DIN, and used it to set their exposure parameters. And the corresponding value of ISO is still used both in camera and in exposure meters to set exposure parameters. Including ISO.

An exposure index is not the exposure triangle. Another red herring.

Now you can claim forever that ISO is not an exposure parameter, but in practice it is. And if you want to derive anything about scene luminance from an image, you need to know what the ISO and Exposure Compensation values were set to, along with Shutter Speed and Aperture. And, of course, you need to know what the actual ISO of the camera is, and you need to know the actual transmissivity of the lens (including the effect of any filters that were used).

The statement that you jumped in on was not about whether or not ISO is an exposure parameter (though it isn't), it was about whether, if the Nikon shoots at f/8 and ISO 3200 and 1/500. The M1X shoots at f/4 so the DOF is the same, and ISO 800 and 1/500, the exposure is the same, and it isn't, at least by the standard photographic definition of exposure, which you seem to be determined to supplant. Another red herring.

And of course, you have to close with another personal attack.

Look back, your post, that I was relying to would have been considered a personal attack by many. All I did was to describe a behaviour pattern. If you want to pick up the cap and wear it, up to you.

Beats me.

Well beaten, I would say. In the future, if you want to go through an extended dispute, it's probably a good idea to ensure that you know what you're talking about, and not come up with gems like there is no standard for ISO. The clue is right there in the name. It doesn't take a lot of checking to avoid a blooper like that.

and then john isaacs wrote:

bobn2 wrote:

john isaacs wrote:

Iliah Borg wrote:

john isaacs wrote:

"f/2.8 is f/2.8" is true.

"50/2.8 is 200/2.8" isn't always true.

It is never true. What is true is 50/17.86 is 200/71.44.

But you actually knew that, right?

The statement was 'f/2.8 = f/2.8'. If you're talking about the f-number always being the same, then the statement would have been '2.8=2.8' and is of course always true. However, the value of 'f/2.8' depends on what is the value of 'f', and in the example that Iliah gave, the 'f' was different.

It's why it's a silly statement. '2.8=2.8' is a tautology, as is 'f/2.8=f/2.8' if the value of 'f' is the same. However, the people that recite the little mantra do so in situations when the 'f' on the left hand side is different from the 'f' on the right hand side. Using the same signifier for two different things is almost guaranteed to cause confusion, but we've been there before, haven't we?

Well, apparenly someone didn't know it. f/N is not an equation,

No, it's a formula, in which 'f' signifies the focal length, '/' signifies the division operation and the number below is the f-number. It isn't an equation because an equation has to have an equality operator. So f/2.8 is a formula. f/2.8=f/2.8 is an equation.

it is a nomenclature to express the f-number of an aperture.

Sorry, you are wrong. You don't need any 'nomenclature' (I think you meant 'notation') to express the f-number, since it's just a number. You can just write it as a number, as in 'the f-number is 2.8', like 'pi is 3.14159265359'. It is a formula which give the aperture from the f-number. You find the aperture by dividing the focal length by the f-number, which is just what the formula says. That isn't a co-incidence.

The f-number of an aperture is the ratio of the focal length and diameter of the entrance pupil. Also written as N = f/D where in this case f is the focal length. This can cause confusion because the nomenclature looks like and equation.

It's not confusion, the standard notation (not nomenclature) is a formula. Once again., you cry confusion when you are the one that is confused.

And so f/2.8 = 50/17.86 = 200/71.44 (for the 50mm and 200mm lenses in question).

Or, how about 50/2.8 = 17.86, or 200/2.8 = 71.44. Interesting that, isn't it? It has to do with knowing how division works.

So to speak of the "f" of "f/2.8" as having any value which could be different or the same is erroneous; it is a nomenclature.

It's not erroneous. That is exactly how the notation (not nomenclature) was developed.

But the "f" of focal length can be different (for different lenses), and yet the f-number can be the same.

But the aperture will be different.

So, the mantra is correct.

Only if the 'f' is the same on both sides of the equation.

Where the error occurs is assuming that depth of field is only dependent upon f-number, and not on actual focal length as well. And thus the fallacy of equivalence; you cannot speak of equivalent focal lengths for field of view without also speaking of equivalent aperture for depth of field.

That's a different argument, and it's your understanding that is at fault, not equivalence. However, it was a revealing statement, because refusal to accept the standard definitions for things like exposure, ISO and f-number on this forum often arise because if you accept them the notion of equivalence follows. Rather than accept it, people would rather chgange the definitions.

I would give this up if I were you. Best to actually know something before laying down the law on it.

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