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06-13-2014, 07:09 PM   #1
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Confused

All along while reading the various FF related arguments I've been assuming that given the same f-stop/shutter speed/ISO, all formats will produce images of equal brightness. Reading comments from other members leads me to believe that I am not the only one under that impression.

I am slowly reading Joseph James' Equivalence essay, and I am puzzled by the following statement:

The same total light falling on the larger sensor will result in a lower exposure than the smaller sensor (the same total light over a larger area results in a lower density of light on the sensor).

Can anyone explain, please? Is there any relationship between what this means ant my opening statement above?


06-13-2014, 07:23 PM   #2
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It's total bunk - your original assumptions are correct.

The light intensity reaching the sensor (any size) is solely a function of the lens and its distance from the sensor. Sensor size is irrelevant.

"The same total light over a larger area" - again, nonsense. The light reaching a 16mm x 24mm area does not change AT ALL when you switch in a larger sensor. The larger sensor capture more of the lens' projected image circle, that's all. Exposure is exactly the same. That same 16mm x 24mm area in the center does not magically get brighter or dimmer. These guys are bending math and physics to their whim and not even thinking sensibly. Reminds me of statisticians.
06-13-2014, 07:41 PM   #3
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This statement isn't valid, as sensor size doesn't affect light distribution. Think of it this way: if you put an APS-C lens like a 12mm on a full-frame camera, even though you'll get black corners, will the exposure be the same as on an APS-C camera? Of course it will.

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06-13-2014, 07:49 PM   #4
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QuoteOriginally posted by OregonJim Quote
It's total bunk - your original assumptions are correct.
What he said. Sensor size does not gate photo flux. It should be enough to remember that the FF cameras from Nikon feature backward compatibility to the DX series lenses by simply applying a crop (ignoring the sites outside the APS-C boundaries) without having to throttle the sensor gain by a factor of 1.5.


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06-13-2014, 07:56 PM   #5
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One thing I have not heard mentioned in this FF debate is resolving power.

A larger sensor makes it possible to create larger photosites without reducing the number of megapixels. This improves ISO range, which the FF fanboys make a big deal of.

What they fail to mention, however, is the tradeoff in resolving power. The FF sensor loses sharpness because the pixels are bigger. Did I miss this discussion somewhere, or did they conveniently gloss over it?
06-13-2014, 08:04 PM   #6
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No sooner than I hit the "enter" key that I remember that media size does matter in at least one case as pertains to amount of light available for the exposure.

There is a light fall-off related to the incident angle light striking the media. The fall-off is equivalent to cos^4 theta where theta is the angle between the center of the lens and a particular point on the media. With symmetrical wide angle lenses in large format photography, this is a serious concern and may result in a difference of 2-3 stops between center and edge of the lens image circle. This is different than vignette and cannot be addressed by optical design per se. As a result, many large format wide angle lenses require a radial gradient neutral-density filter to provide compensation for the drop-off. These filters are very $$$ and are often a dedicated match to the lens in question.

With 4x5 film light fall-off becomes an issue at or near 90mm and gets worse as you go wider. The rear surface of a 65mm lens* on 4x5 is often less than a centimeter from the surface of the film for an image circle of about 17 cm. The angle of incidence is very shallow. The same physics apply to the retrofocus lenses used on SLR cameras but the angles are steep enough to not be a problem. It certainly applies to ultra wide-angle rangefinder lenses and has been a topic of discussion in regards to Leica M lenses adapted to mirrorless cameras. See the paragraphs under "Center Filter" on the filter information page at largeformatphotography.info:

http://www.largeformatphotography.info/filters.html


Steve

* Equivalent to a 16mm symmetrical ultra-wide in Leica M mount

Last edited by stevebrot; 06-13-2014 at 08:31 PM.
06-13-2014, 08:25 PM   #7
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QuoteOriginally posted by bxf Quote
I am slowly reading Joseph James' Equivalence essay, and I am puzzled by the following statement:

The same total light falling on the larger sensor will result in a lower exposure than the smaller sensor (the same total light over a larger area results in a lower density of light on the sensor).
The simple explanation is that it is BS. A more detailed explanation is: It's BS.
A given lighting condition requires a given exposure. Sensor size,whether film or digital, has nothing to do with exposure. If f2.8 @ 1/125 second @ ISO 100 is the proper exposure for a particular scene with a Micro 4/3 camera, it's also the proper exposure for an 8x10 view camera, and for everything in between.
06-13-2014, 08:32 PM   #8
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QuoteOriginally posted by stevebrot Quote
There is a light fall-off related to the incident angle light striking the media. The fall-off is equivalent to cos^4 theta where theta is the angle between the center of the lens and a particular point on the media
Hmm... I wonder if that applies as much in the digital realm. The silver halide particles are relatively flat, but the photosites on a sensor are more dimensional. I wonder if they 'catch' light at shallower incidence angles...

06-13-2014, 08:49 PM   #9
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QuoteOriginally posted by OregonJim Quote
Hmm... I wonder if that applies as much in the digital realm. The silver halide particles are relatively flat, but the photosites on a sensor are more dimensional. I wonder if they 'catch' light at shallower incidence angles...
The light fall-off is a matter of physics and applies regardless of media. It is why the low angle of the sun in winter results in less light.

Google "15mm Heliar light fall-off". There has been a lot of discussion in regards to adapting that lens to the various mirrorless cameras. Problems include light fall-off as well as serious image degradation towards the edges. The light fall-off was a "feature" on M-mount and LTM film cameras, but apparently is worse on the FF Sony sensors. The bad edge performance (smearing) is a digital phenomenon with that lens and not evident on film. Again, this is not an issue with SLR lenses due to the distance of the rear element from the media. The angle for my 16mm Zenitar at infinity is about the same as for my K 55/1.8.


Steve

Last edited by stevebrot; 06-13-2014 at 09:00 PM.
06-13-2014, 09:08 PM   #10
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I suspect the article the OP was reading (I haven't read it) was talking not about the same lens, but an *equivalent* lens for the FF format in order to make the same image as on APS-C. Then you would have the "same total light" spread over a larger area, which is why an f/2.8 lens on FF is equivalent to a f1.8ish lens on APS-C, right? (The APS-C lens would also be equivalently wider to achieve the same FOV.) And therefore this is an advantage for FF because the equivalent for APS-C of a fast lens used on FF often does not exist or is more expensive. (The equivalent of 50mm f/1.4 on FF for APS-C would need to be approximately 33mm f/0.9 and there is no such thing.)

Correct me if I'm wrong...
06-13-2014, 09:41 PM   #11
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QuoteOriginally posted by vonBaloney Quote
Correct me if I'm wrong...
You are wrong.

QuoteOriginally posted by vonBaloney Quote
Then you would have the "same total light" spread over a larger area, which is why an f/2.8 lens on FF is equivalent to a f1.8ish lens on APS-C, right?
f/2.8 is f/2.8 is f/2.8. Tell me this: Have you ever seen a light meter with a setting for media size? No. An exposure of ISO 100, 1/125, f/2.8 produces the same exposure on 110 film, 4x5 sheet film, 35mm film, tiny 1/1.7 P&S sensor, and everything else. Media size has never had any bearing on exposure. Media size has never had any bearing on the ratio of focal length to iris diameter, i.e. f-stop.
06-13-2014, 09:51 PM   #12
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QuoteOriginally posted by OregonJim Quote
One thing I have not heard mentioned in this FF debate is resolving power.

A larger sensor makes it possible to create larger photosites without reducing the number of megapixels. This improves ISO range, which the FF fanboys make a big deal of.

What they fail to mention, however, is the tradeoff in resolving power. The FF sensor loses sharpness because the pixels are bigger. Did I miss this discussion somewhere, or did they conveniently gloss over it?
FF is much more sharp at the same number of megapixels, and with lower cost lenses to boot.
06-13-2014, 09:55 PM   #13
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QuoteOriginally posted by ElJamoquio Quote
FF is much more sharp at the same number of megapixels, and with lower cost lenses to boot.
Do you have a source to back that up? I just explained why it's not as sharp as a smaller sensor.
06-13-2014, 10:28 PM   #14
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QuoteOriginally posted by OregonJim:
You are wrong.

f/2.8 is f/2.8 is f/2.8. Tell me this: Have you ever seen a light meter with a setting for media size? No. An exposure of ISO 100, 1/125, f/2.8 produces the same exposure on 110 film, 4x5 sheet film, 35mm film, tiny 1/1.7 P&S sensor, and everything else. Media size has never had any bearing on exposure. Media size has never had any bearing on the ratio of focal length to iris diameter, i.e. f-stop.
Well, I guess ISO would be the other variable -- ISO would be higher on FF (but with less nolse for same value so equivalent). In other words, in order to make the SAME picture with the larger format, you'd use slower f-stop (and a different focal length and a different ISO). The assertion is not being made that f/2.8 is not f/2.8 is not f/2.8 -- just talking about different things.

QuoteOriginally posted by OregonJim:
Do you have a source to back that up? I just explained why it's not as sharp as a smaller sensor.
I don't think you explained it, you just asserted it that if you have a larger sensor with LESS pixels, than the smaller sensor with MORE pixels would be sharper. (Not necessarily true, depends on the difference -- likely scenario is they'd be about the same.) ElJamoquio pointed out that if you have the SAME number of pixels with the larger sensor, then the larger sensor will be sharper. Which is true, all else being equal. You don't gain sharpness by shrinking the sensor and keeping the MP constant...
06-13-2014, 10:32 PM   #15
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QuoteOriginally posted by vonBaloney Quote
I don't think you explained it, you just asserted it that if you have a larger sensor with LESS pixels, than the smaller sensor with MORE pixels would be sharper. (Not necessarily true, depends on the difference -- likely scenario is they'd be about the same.) ElJamoquio pointed out that if you have the SAME number of pixels with the larger sensor, then the larger sensor will be sharper.
That's not what I said. Please re-read. I asserted that the larger sensor, having LARGER pixels (not less), will have less resolving power than the smaller sensor.

QuoteOriginally posted by vonBaloney Quote
You don't gain sharpness by shrinking the sensor and keeping the MP constant..
Of course you do.

Compare it to LCD screens: a 10", 640x480 LCD will be much sharper than a 30", 640x480 LCD at the same viewing distance.

Or printed images - a 4"x6" print is much sharper than an 11"x14" with the same number of pixels (because they are smaller).

Last edited by OregonJim; 06-13-2014 at 10:39 PM.
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