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11-17-2010, 09:12 AM   #1
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dumb f-stop question - all f-stops created equal?

hello. I have shot 35mm and DSLRs for a while. I have read a lot about different size sensors and of course how they affect focal length but never anything about amount of light getting to sensor. Physics would seem to dictate that same aperture to different size sensors would produce different light collecting ability. Anyone thought about this or have an answer? thank you, david

11-17-2010, 09:34 AM   #2
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You see less vignetting on a lens designed for full frame on ASPC (smaller digital sensor), but these full frame lenses simply cast a larger circle of light, so the sensor misses out on the extra light that does not hit the lens. Therefore, at the same aperture, the only difference in light will be the amount of light loss at the corners (vignetting) and should not affect your metering.

The smaller "hole" of the aperture does not change the size of the image circle. but rather how far light needs to bend through the optic to reach the sensor. You do lose light as you do this, but you will notice that the field of view of the image remains the same.
11-17-2010, 10:21 AM   #3
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The Depth of field will change relative to 35mm film it will get longer (more in focus zone) so the shallow depth you would acheive on an 85 f1.8 wide open (if you are lucky enough to own such a beast) would need approximately a 50 f1.2 for the close to the same effect on an apsc sensor ( though with a little wider field of view (75mm equivalent) once again if you are lucky enough to have one.
nothing would replicate the 85 f1.4 (either the A or the FA) though if you could come up with a 50 mm 0.95 Nokton it would be close ( and cost more than a k5)
11-17-2010, 10:24 AM   #4
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QuoteOriginally posted by eddie1960 Quote
The Depth of field will change relative to 35mm film it will get longer (more in focus zone) so the shallow depth you would acheive on an 85 f1.8 wide open (if you are lucky enough to own such a beast) would need approximately a 50 f1.2 for the close to the same effect on an apsc sensor ( though with a little wider field of view (75mm equivalent) once again if you are lucky enough to have one.
nothing would replicate the 85 f1.4 (either the A or the FA) though if you could come up with a 50 mm 0.95 Nokton it would be close ( and cost more than a k5)
The question was about light gathering ability. In essence, there is little practical difference between light gathering on a full frame or ASPC sized DSLR at a given aperture, you simply capture a larger circle. The center point of an ASPC sensor should capture the same intensity of light as the center point of a full frame sensor.

11-17-2010, 10:49 AM   #5
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QuoteOriginally posted by paperbag846 Quote
The question was about light gathering ability. In essence, there is little practical difference between light gathering on a full frame or ASPC sized DSLR at a given aperture, you simply capture a larger circle. The center point of an ASPC sensor should capture the same intensity of light as the center point of a full frame sensor.

yep, the light is a mathematical thing and f1.4 is f1.4 no matter what the size of the sensor as it related to the lens size not the camera

I mentioned the DOF issue as it isn't always something people are aware of when they switch over
11-17-2010, 11:01 AM   #6
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there was a discussion about lens equivalence between FF and APS-C sensors before. I can't seem to pinpoint the thread. although the discussion there is very helpful in explaining the differences with concerns to exposure as well as DOF.
11-17-2010, 11:06 AM   #7
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Not a stupid question!

Truth is, I'm not sure, but if you took the same lens and put it on a FF camera and then on an APS-C camera, both sensors should get the same amount of light and require the same shutter speed and ISO. I can't see any reason why they wouldn't get the same amount of light--what they do with the light is another story.

Well, not true because they'd see a different image. The larger sensor would gather more light just by having more surface area, but if you took the APS-C sized center of the larger sensor and compared it to an APS-C sensor, it should be the same, plus or minus some exposure oddities.

Not all sensors are created equal though, nor are all lenses.

Different lenses have different light transmission properties, so sometimes an f/2.8 zoom (more elements) will require a slower shutter speed than a simple f/2.8 prime, for example.

So there are also t-stops which take the f-stop AND light transmission into account to help keep a constant exposure when switching lenses (mostly used for cinematography).

There's a quick article about t-stops vs. f-stops here:
F-Stops Vs. T-Stops Explained - The Photoletariat

Last edited by farfisa; 11-17-2010 at 11:35 AM.
11-17-2010, 11:19 AM   #8
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QuoteOriginally posted by farfisa Quote
Different lenses have different light transmission properties, so sometimes an f/2.8 zoom (more elements) will require a slower shutter speed than a simple f/2.8 prime, for example.
This is true, but it's very hard to factor into metering. It's almost better forgotten, unless you know "OK, this lens needs EV comp +0.7", and just run with that. I prefer to think of it situationally, instead of lens-based... none the less, it is a factor. I find the biggest discrepancy to be a half-stop though. I think the film industry moved over to T stops because those lenses use so many more elements that the discrepancy is often much larger than 0.5 EV. Could be wrong about that.

My 5 element DA 40 is brighter at 2.8 than my 7 element A 28mm at 2.8, but the metering tends to help me out so I don't think about it much.

EDIT: Not a dumb question at all! Camera's are sort of magic boxes, and it's fun to figure out what actually goes on inside...

11-17-2010, 11:25 AM   #9
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Come to think about it, there could be a 1/1.5x wide-converter (as opposed to a tele-converter) that would give the FOV of a FF sensor as well as gain about one stop in exposure with an FF lens? What would happen to DOF with this?
11-17-2010, 11:54 AM   #10
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david

the simple answer as some have pointed out is that there is no impact on F Stop as a function of sensor. This is also true of focal length. Both these terms are simple physical properties of a lens.

Focal length is the distance of the lens to the focusing plane (seneor or film in our case) for a simple lens when focused on an infinitely distant object.

F-Stop is the ratio mathematically of the lens diameter (or aperture diameter) relitive to focal length.

As you can see both, by definition are lens characteristics that don't care what sensor or film is behind them.

What changes, when going from film (or full frame) to ASP-C sensors on digital is the field of view a lens has. Field of view is a function of focal length and sensor / film size, or more normally the "circle diameter" or corner to corner distance of the frame.

Because it is difficult to discuss field of view, people use the "crop factor" or equivelent focal length to discuss comparisons from film to digital, and this confuses things.

With respect to focal length, and digiral format, the real issue aside from the different field of view, is image shake, and hand holding. THere was a rule of thumb for film that 1/focal length was an appropriate minimum aperture to eliminate camera shake, but this is only true for prints of 8 x 10 inch fromthe full frame. Therefore, when considering digital and the ASP-C format, and still blowing the image up to 8 x 10 you need to multiply shutter speed by the crop factor also.

With respect to aperture, again, considering blowing a cropped image from the ASP-C to 8 x 10, depth of field will change similairly, because the deph of field is based upon the overall image magnification, and the cropped sensor starts with a smaller section out of the frame, than a film camera with the same lens would produce.

Otherwise there is no issue. You can understand easily, by discussing only the issue of cropping the center out of the frame, the total relevant issues of switching from film to ASP-C . Illumination does not change because simply cropping the middle out of a frame does nohing to the light per square mm hitting the frame.
11-17-2010, 11:58 AM   #11
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a much better explanation Lowell
11-17-2010, 02:20 PM   #12
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QuoteOriginally posted by paperbag846 Quote
I think the film industry moved over to T stops because those lenses use so many more elements that the discrepancy is often much larger than 0.5 EV. Could be wrong about that.
Movie zooms can be more complex beasts and eat more light, but I think the reason the industry moved to t-stops relates to switching between prime lenses in the days before zooms were popular. Even a half stop difference will be easy to spot in a movie, so lenses were bench tested so that t stops would allow exactly the same amount of light transmission at a given stop.

I seem to recall that movie guys usually tried to stick to one brand of lenses so that they'd all match for color transmission, too. A warmer/cooler color cast would be pretty easy to spot in a movie, too.

Probably the funniest part of all of this, though, is that after all of that effort, film makers would use lenses with completely different renderings for women and men. Ever watch a classic film and the lens used for an actor is a bitingly sharp, well corrected optic and then later in the same scene a lens with boatloads of spherical aberration is used to give a a soft focus "glow" to an actress? I don't know why, but that's always amused me. (EDIT: Probably the first movie I remember seeing this dramatic difference in was Casablanca...check out some of the scenes of Ingrid Bergman for a really dreamy, glowy effect).

Last edited by GhoSStrider; 11-17-2010 at 02:28 PM.
11-17-2010, 03:49 PM   #13
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The whole issue of T-stops was a stop gap (pun intended) measure before in cera metering became popular in the late 1960's. With ttl there is not as much need for t-stops
11-18-2010, 01:31 AM   #14
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QuoteOriginally posted by Pentaxor Quote
there was a discussion about lens equivalence between FF and APS-C sensors before. I can't seem to pinpoint the thread. although the discussion there is very helpful in explaining the differences with concerns to exposure as well as DOF.
There's always this article: Equivalence
11-18-2010, 07:01 PM   #15
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QuoteOriginally posted by jolepp Quote
Come to think about it, there could be a 1/1.5x wide-converter (as opposed to a tele-converter) that would give the FOV of a FF sensor as well as gain about one stop in exposure with an FF lens? What would happen to DOF with this?
Not possible. You can easily see with wide-angle adapters that you screw onto the front of a lens. You can change the AoV, but you will still have a reduction in light passing through the lens.
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