[Pål Jensen]

[/COLOR]

Only true if assuming that the sensel size is the same of different sensor size. If sensor size was immaterial, then why are not everyone using smartphones?

Sensor size is immaterial when it comes to a lens aperture; aperture is a property of the lens. Hence, the concept of equivalent lenses is meaningless as long as it demands different external factors.
It is comparing apple to oranges

---------- Post added 07-15-15 at 03:30 PM ----------

Answer: Because there is no separation between "light gathering ability" and "DOF" and statements like "It has the light gathering ability of an f/2.8 lens but the DOF of an f/4 lens" are complete and utter nonsense.
.

The 2.8 is not defined by DOF but exposure. What makes it nonsense is not understanding this simple point. DOF doesn't enter its definition and therefore shouldn't be used as marker for equivalency.

The law of reciprocity gives freedom from what parts that make up the exposure you want to change due to changing conditions to achieve the same goal exposurewise. Note that reciprocity does not "care" about DOF or motion blur or image noise; only exposure. This freedom mean that it makes no sense insisting on constant DOF in the image in order to compare lenses. It is possible to do that, sure, but that just an opinion; it makes no sense making into a rule because different subjects and circumstances demand different solutions, and besides, DOF equivalent lenses cross formats only exist in theory - not in real life. No one shoots after the "law of DOF wide open equivalency at moderate distances" anyway either. You do not shoot your FF camera at 200ISO so that it is equivalent to an APS body. It is not mandatory for an APS shooter to choose his/hers lenses from FF DOF wide open and shoot accordingly. Most will use fast lenses in order to use their cameras to their best advantages in low light regardless of format. In this way you maintain the benefit of your larger sensor camera.
The only thing that you can't get away from is the lens speed; at 1.4 at a certain light level and at a certain sensitivity will give you a certain shutterspeed regardless of format. This is the only equivalency there is.....

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Last edited by Pål Jensen; 07-15-2015 at 06:50 AM.

[CFWhitman]

That's wrong (as are further statements you make about pixel size and noise that I won't address).

Many people have this misconception of "aperture for light gathering" vs "aperture for DOF". There is only one aperture with a certain diameter and the latter determines both "light gathering capability" and "DOF".

The fact that there is less DOF is caused by the additional light gathered. The fact that less light is gathered causes more DOF.

It is true that exposure only depends on the f-ratio, but to equate that with "light gathering ability" is 100% misleading. The Pentax-Q 01 lens is stated to have a widest aperture of f/1.9. Now everyone knows is that this doesn't mean you get the shallow DOF that you can get with an FA 43/1.9 on an FF camera. What a lot fewer people know is that while you technically get the same exposure with f/1.9 independently of the format size, the noise levels that you get with the Pentax-Q 01 lens at f/1.9 are the same as with an FF frame lens at f/10.64. Therefore, the focal length of the Pentax-Q 01 lens is FF-equivalent to 47.6mm and its "light gathering" ability is FF-equivalent to "f/10.64".

Using the example of the OP, if you convert the focal length range of the 17-50/2.8 into its FF-equivalent range, you also need to convert its f-stop to its FF-equivalent. The 17-50/2.8 on APS-C hence allows you to take the same pictures as a 26-75/4 on FF.

Some of what I said may have been a bit simplified, but it was not wrong. Your statements don't even really mean anything when it comes to light gathering. You're trying to illustrate equivalence, which has its uses as far as comparisons go, but is very misleading when you try to talk about it in terms of absolutes. To state that the Pentax Q 01 lens gives you the same noise levels as a full frame lens at f10.64 as though that is an absolute is entirely misleading. The noise levels depend very much on the sensor used as well as other factors.

If you were theoretically to have a sensor of the same technology with the same pixel density on full frame as the Q, and used the same focal length (8.5mm), and the same f-stop, then the noise levels would be the same in any given area of the sensor (there is no camera and lens combination that meets these criteria of course). Of course the picture from such a camera would cover a much greater angle of view and contain a much greater number of pixels, and if printed at the same dimensions would have the noise reduced in size along with everything else, but if cropped to the same angle of view as the Q, would have the same amount of noise. It's not switching to a full frame sensor that causes the difference, but the other changes you make to get a similar picture (changing the size of the pixels, changing the focal length, etc.). However, you will never find an exact equivalent. There are combinations that may be compared and relevant in certain situations, but they are not absolutes (that is, they may apply quite well under certain conditions, but changing conditions will make them not apply).

Your final statement may be generally useful as a comparison, but again, it is not absolute.

[clackers]

How can you not see that if you keep pixel size constant that a larger sensor will have more pixels?.

You've mixed up correlation and causation.


You're wrong to say large sensors mean more pixels, or expensive cameras mean more pixels.


Even my phone has more pixels to downsample than either an A7S or Nikon D4.


So it's not the format size, it's about the number of pixels for resolution and pixel pitch for SNR.

OK, so you are challenging the idea that additional light gathered will reduce DOF.

Yes,


If there is a lot of light from the subject falling on the sensor, or very little, the DoF is exactly the same.


The DoF depends firstly on aperture and focal length (as printed on the lens by the manufacturer, not multiplied by 2.25 or any other number) and the subject distance. There is the circle-of-confusion to add fun to proceedings.


Equivalence fanatics do their usual carnival operator's trick of changing a variable when it suits them to say DoF changes - they move the FF camera closer to the subject.

[clackers]

Within the context of additional light from a larger aperture ...

See the post above, Brian.


The DoF difference is not due to additional light from opening up the aperture, it's due to the light (whatever intensity) now following a different path.


Total Light fanatics seem to think otherwise. :-)

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Last edited by clackers; 07-15-2015 at 06:50 PM.

[clackers]

Forget I mentioned it. For some unknown reason I was thinking the registration distance was different for FF and APS-C. I've been thinking a lot about MF lately and I have been preparing to mount an exhibition of my late mother's artwork. Lots of stress.

That's quite okay, Bossa.


Good luck with your mum's exhibition - perhaps you could upload a few sample shots done justice with a macro lens, maybe?

[clackers]

I happen to have a degree in physics and I majored in optics.

There have been some real atrocities against the art and science of optics committed in this thread, Dtmateojr.


Ring the 'Equivalence' bell and the freaks come running from all directions.

[Class A]

You are wrong to say large sensors mean more pixels, or even expensive cameras mean more pixels.

I would have been wrong, if I had said that. But I didn't. I didn't say many other things you attack either. Please read carefully. And apply reasonable interpretations, not some nonsensical fantasies.

If there is a lot of light from the subject falling on the sensor, or very little, the DoF is exactly the same.

I was only talking about an increase in light gathering in terms of opening up the aperture more (or using a faster lens).

It is so obvious that variations in scene illumination won't affect the DOF that I did not explicitly mention the assumption of constant scene illumination and frankly I fail to see how anyone could read your interpretation into what I wrote. Someone knowing one thing or two about optics, such as BrianR, read my statements as they were intended, so I don't think the problem was with how expressed myself.

Equivalence fanatics do their usual parlour trick of changing a variable when it suits them to say DoF changes - they move the FF camera closer to the subject.

I don't know any equivalence fanatics, but those who correctly reference equivalence principles do exactly the opposite of what you are stating. They do not pick a random variable out of context to make a point. This is the tactic of people who are not bound to rational reasoning when they for instance claim that today's APS-C sensors are better than yesterday's FF sensors or that there is no APS-C & FF lens pair where the lenses are exactly equivalent to each other. These statements a) try to refute claims that no one ever made, and b) are besides the point because you cannot make comparisons without assuming some aspects to be invariable (e.g., sensor technology) and equivalence does not require an actual FF-equivalent lens to exist for the respective equivalent parameters of the APS-C lens to make sense.

Finally, no one using equivalence correctly would change the distance between subject and camera as this would change perspective and hence create non-equivalent images.

Again, I'd like to leave it at this so please don't put more wrong statements into my mouth.

There have been some real atrocities against the art and science of optics committed in this thread, Dtmateojr.

Finally, something we can agree on.

I suggest that dtmateoir takes the nonsense we wrote up to his former Physics professor for validation. May I also remind you that you did not point any errors in falconeye's equivalence formulas. It is understandable, because there are no errors, but you continue to assume that the equivalence principle is bogus even though you have not presented one counterargument on the same scientific level as used by falconeye.

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Last edited by Class A; 07-15-2015 at 06:59 PM.

[BrianR]

The DoF difference is not due to additional light from opening up the aperture, it's due to the light (whatever intensity) now following a different path.

Where do you think the light that's following this different path came from? It's exactly the extra light that was hitting the aperture blades before, but is no longer blocked since you've opened the aperture. This additional light is enlarging the blur discs.

[normhead]

Someone needs to look up circles of confusion. That's all I'm going to say about this.

[BrianR]

Someone needs to look up circles of confusion. That's all I'm going to say about this.

If you mean me, go ahead and point out where I'm wrong. I have an open mind.

[bxf]

Someone needs to look up circles of confusion. That's all I'm going to say about this.

My head is already going in circles, resulting in lots of confusion.

Someone pass the Aspirin, please.

[dtmateojr]

I was only talking about an increase in light gathering in terms of opening up the aperture more (or using a faster lens).


I suggest that dtmateoir takes the nonsense we wrote up to his former Physics professor for validation. May I also remind you that you did not point any errors in falconeye's equivalence formulas. It is understandable, because there are no errors, but you continue to assume that the equivalence principle is bogus even though you have not presented one counterargument on the same scientific level as used by falconeye.

That's amusing

In telescopes DoF does not change even if you change apertures but f-stop is f-stop regardless of imaging format used behind the eyepiece.

That practically refutes all your arguments.

[everydaylife]

That's amusing

In telescopes DoF does not change even if you change apertures but f-stop is f-stop regardless of imaging format used behind the eyepiece.

That practically refutes all your arguments.

Isn't diststance from camera to subject relevant re DOF?

What is more interesting to me is what will I gain if I trade in my k5iis for an FF camera. So I read this thread. Apparently the lenses I have, according to what I have read on this thread, will perform slightly better not only in the centre but at the edges also. Is it worth the extra money for that slight gain. I suppose it depends if want to make A2 size or A1 size prints.

[normhead]

Isn't diststance from camera to subject relevant re DOF?

What is more interesting to me is what will I gain if I trade in my k5iis for an FF camera. So I read this thread. Apparently the lenses I have, according to what I have read on this thread, will perform slightly better not only in the centre but at the edges also. Is it worth the extra money for that slight gain. I suppose it depends if want to make A2 size or A1 size prints.

No, it's not that much difference, The gain from APS-c pushing 3000/lw/ph going to FF 36 MP hovering around 4000 w'ph, is somewhere in the order of 35%. That's a long way from enough to go up a paper size at the same resolution.

However, printing at 50 lw/ph which would be approx. 100 DPI, you could go from 60 inches to 80 inches, or printing at 200 DPI, 30 inches to 40 inches.

[osv]

Ahahahaha!!! Gold!

Focal length does not affect projected image size?! Really?

he specifically qualified it as an m4/3 vs. ff scenario.

which apparently went right over your head, lol

---------- Post added 07-16-15 at 11:58 AM ----------

I suggest that dtmateoir takes the nonsense we wrote up to his former Physics professor for validation. May I also remind you that you did not point any errors in falconeye's equivalence formulas. It is understandable, because there are no errors, but you continue to assume that the equivalence principle is bogus even though you have not presented one counterargument on the same scientific level as used by falconeye.

would that really help? he doesn't even know what film speed is.

---------- Post added 07-16-15 at 12:06 PM ----------

Equivalence fanatics do their usual carnival operator's trick of changing a variable when it suits them to say DoF changes - they move the FF camera closer to the subject.

not if changes the perspective... clackers, do you know what perspective is?

"DEFINITION OF EQUIVALENCE
Equivalent photos are photos of a given scene that have the:

Same Perspective

Same Framing

Same DOF, Diffraction, Total Amount of Light on the Sensor

Same Shutter Speed

Same Brightness

Same Display Dimensions"
Equivalence