[jsherman999]

I will remind you once again that Joseph James' "equivalence" does not include noise.
It's all in here:
Equivalence
Search for "MYTHS AND COMMON MISUNDERSTANDINGS", the "assuming "equivalence" is based on equal noise" section.

Noise have to be measured and observed.

Yes, I know. Equivalence cannot encompass noise as a parameter, because too much is dependent on read noise (vs. photon shot noise) and sensor efficiency. However, a direct application of equivalence is assessing what kit to put together to minimize noise while keeping sensor efficiency in mind. Joseph James himself uses it this way, and has explained scenarios in those terms to people in forums.

For example, given sensors of roughly equal efficiency (same gen) as measured by DxOMark or sensorsgen or some similar entity, what combo has the potential to give you less noise?

50mm f/1.8 FF
35mm f/1.4 aps-c
25mm f/0.95 m43

You can't use equivalence alone to answer that, (unless you just go with a very theoretical answer based on "sensors of equal efficiency",) However you can apply equivalence in combination with the externally-sourced data to come to a pretty accurate conclusion.

In other words, image noise is dependent on A) Total Amount of Light Falling on the sensor and B) the sensor efficiency. Equivalence helps you determine A.


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---------- Post added 06-01-14 at 08:08 PM ----------

I sense some irritation, Jay.

On this ^^ point you would be correct

I do get irritated when people waffle on a question they just don't want to answer. C'mon, it's like:

Q: "Which car has more HP, A or B?"
A: "I wouldn't buy a car based on power alone."
Q: "I know, neither would I, but I'm wondering because I've seen some conflicting into. I understand you know which has more ponies?"
A: "I'm not sure why you care, it's shouldn't be important to you. Have you looked at the interiors?"
Q: "Yes, please, just.... which has more HP?"
A: "I think it's also important to consider gas mileage, don't you?"
Q: "Yes, but that's not what I'm asking at this particular moment. It's a very simple question. Horsepower?"
A: "In my experience, a buyer gets more out of having a good knowledge of the safety records..."
...etc...


For the sake of argument - say someone heard this statement and really, really wanted to know what the actual situation was - how would you respond to it technically? :


"I don't know what's so great about your 77mm f/1.8 Limited - I have a 75mm f/1.8 for my Olympus m/43, same FL and just as fast, so it gives me the same shutter speed and noise performance on my camera."


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Last edited by jsherman999; 06-01-2014 at 07:18 PM.

[Kunzite]

For example, given sensors of roughly equal efficiency (same gen) as measured by DxOMark or sensorsgen or some similar entity, what combo has the potential to give you less noise?

50mm f/1.8 FF
35mm f/1.4 aps-c
25mm f/0.95 m43

50mm f/1.8 on a "full frame" camera

[Rondec]

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---------- Post added 06-01-14 at 08:08 PM ----------



On this ^^ point you would be correct

I do get irritated when people waffle on a question they just don't want to answer. C'mon, it's like:

Q: "Which car has more HP, A or B?"
A: "I wouldn't buy a car based on power alone."
Q: "I know, neither would I, but I'm wondering because I've seen some conflicting into. I understand you know which has more ponies?"
A: "I'm not sure why you care, it's shouldn't be important to you. Have you looked at the interiors?"
Q: "Yes, please, just.... which has more HP?"
A: "I think it's also important to consider gas mileage, don't you?"
Q: "Yes, but that's not what I'm asking at this particular moment. It's a very simple question. Horsepower?"
A: "In my experience, a buyer gets more out of having a good knowledge of the safety records..."
...etc...


For the sake of argument - say someone heard this statement and really, really wanted to know what the actual situation was - how would you respond to it technically? :


"I don't know what's so great about your 77mm f/1.8 Limited - I have a 75mm f/1.8 for my Olympus m/43, same FL and just as fast, so it gives me the same shutter speed and noise performance on my camera."


.

The issue for me is that to be a Pentaxian means that you aren't pursuing the fastest lenses. Which will give you better performance, a DA 15 limited on a K5 II, or the Nikon 24-85 variable aperture lens on a D600? Well, the answer is clearly the 24-85. And yet, I would prefer the DA 15 because if what it is and how it renders photos and its flare resistance. Obviously equivalence does tell you which lens is faster and that should be a factor when choosing what lens/camera body to purchase. But if it is the only factor, then we need to give up on Pentax, even if they come out with a full frame camera, because their fastest lenses (the FA limiteds) are a half stop slower than Nikon's fastest lenses.

[TzalamChadash]

I watched the video. It was very enlightening.
Am I understanding it correctlypp that -as an example - the tamron 28-75/2.8 lens I'm interested in for my k-50 is not really 2.8 for my camera as it is a full frame lens. And that the tamron 17-50/2.8 is really 2.8 because it was designed for APS-C? So if I was looking for a trey faster lens - for my camera - I should prefer the latter designed for apsc cameras?

[Kunzite]

The video served its purpose to confuse you. Those lenses are equally fast.

[ElJamoquio]

I watched the video. It was very enlightening.
Am I understanding it correctlypp that -as an example - the tamron 28-75/2.8 lens I'm interested in for my k-50 is not really 2.8 for my camera as it is a full frame lens. And that the tamron 17-50/2.8 is really 2.8 because it was designed for APS-C? So if I was looking for a trey faster lens - for my camera - I should prefer the latter designed for apsc cameras?

Close! But not quite.

The most poignant use of equivalence is when comparing between different sensor sizes.

Let's say you're starting with a K-50 + the 17-50 F/2.8.

Going to a full frame camera and a 28-75 would be the equivalent of a 18-50mm F/1.8 on your K-50.
Going to a APS-C camera and a 28-75 would be the equivalent of a 28-75 F/2.8 on your K-50.

I don't know what you're looking to do - have lower noise? Smaller DOF?

The reason many of us are excited about FF is basically because of this - a 18-50mm F/1.8 lens for ~$300-500. The Sigma is going for $900 I think?

[Rondec]

I watched the video. It was very enlightening.
Am I understanding it correctlypp that -as an example - the tamron 28-75/2.8 lens I'm interested in for my k-50 is not really 2.8 for my camera as it is a full frame lens. And that the tamron 17-50/2.8 is really 2.8 because it was designed for APS-C? So if I was looking for a trey faster lens - for my camera - I should prefer the latter designed for apsc cameras?

No. A 17-50 f2.8 lens will give you the same exposure on APS-C as a 28-75 f2.8 would give you on full frame, but with a stop more depth of field. The whole "designed for APS-C" factor doesn't mean much except that the image circle is smaller. That is to say, if you mounted it on a full frame camera, you would have dark borders on your image. On the other hand, when you mount a full frame lens on a crop camera, it just crops the middle part of the image.

The whole thing is confusing and not easy to understand...

[OregonJim]

I do get irritated when people waffle on a question they just don't want to answer.

...and I get irritated seeing all the time people waste on this whole pointless debate instead of learning how to take better pictures!

[jsherman999]

No. A 17-50 f2.8 lens will give you the same exposure on APS-C as a 28-75 f2.8 would give you on full frame, but with a stop more depth of field.

And about a stop more noise.

[TzalamChadash]

What would be the fastest possible aperture of the 28-75/2.8 FF lens when mounted on a pentax APSC camera? Is it 2.8? Or 4.2 after multiplying the crop factor?
That's where I'm confused.

I am looking for an affordable fast indoor low light zoom lens.
I have a k/50 which of course is APSC

[Kunzite]

f/2.8, of course.
It won't change with "crop factor", unless the lens would physically change when mounting on different cameras - or even when cropping in Photoshop.

[TzalamChadash]

Ok. Understood.
So why is there a preference for dedicated APSC lenses?

[Kunzite]

There is no reason to prefer APS-C lenses just because they are APS-C. However:
- they might have a more suitable focal range (e.g. 17-50 is a wide-normal zoom on APS-C, while a 28-75 is a normal-short tele zoom)
- they might be cheaper and lighter than if they were made to cover a larger format

[Aksel]

f/2.8, of course.
It won't change with "crop factor",...

2.8 or 4 - these are numbers. But FF and APS-C are different systems, and that means also that we get little-bit different pictures with different systems.

The DOF will be like 4 on FF, but the relation of the near and far objects in the photos will be the same on APS-C and FF.

[Kunzite]

The f-number is the focal length divided by the diameter of the entrance pupil, and it's a measure of lens speed. It stays the same regardless if the lens is mounted on a camera (with an unspecified format) or not.

While it's one of the factors determining the DOF, it's not a measure of DOF; it is incorrect to say that a lens' set aperture had changed by activating the crop mode in a D800.
What's really changing between formats is the circle of confusion, which due to the higher enlargement (necessary in order to get the same output size from the same format) has to be smaller. Not the aperture!

Perspective depends only on the camera position.