Who is this guy Northrop... or whatever?
It seems to me that he's being paid to "bend" the numbers against aps-c formats (or any other crop format).
But there are things that not even with a million dollars in sponsorship, can change: (Some physics laws!)
1) Lens speed (max aperture) is a mathematical formula given by the focal length divided by the effective max diameter. In real world situations, this max aperture is compensated for glass light transmission and lens to film (sensor) distance, giving a more standarized f/stop effective value.
The area on which the image projected by the lens (full frame, aps, micro 4/3 or whatever) has nothing to do.
Lens effective aperture has to do with LIGHT TRANSMISSION, not with picture format.
The reason that you should multiply the f/stop by the magnification in a teleconverter,
is because the projected image is ENLARGED (blown, etc) from the lens towards the film (sensor). Its the same principle of having a garden hose with a stream then press a finger on the spout to create a "fanning" effect. The same amount of water is released, BUT LESS WATER FALLS ON A GIVEN AREA AT THE SAME TIME. OTOH, how come we have to consider extension tubes for light loss, even though we are not changing the light path? One simple reason: YOU ARE INCREASING THE DISTANCE, THUS, LOOSING LIGHT THAT FALLS OVER A GIVEN AREA. You also loose light. (Inverse Law of Squares: For any given light, the intensity drops to 1/4th for every distance doubling. In this case, since distance from lens to sensor is increased, then light loss (intensity over a certain area) drops accordingly.
http://hyperphysics.phy-astr.gsu.edu/hbase/vision/isql.html
2) Speaking about f/stop and "crap" factor, what matters is NOT the light gathering ability of THE WHOLE SENSOR. Of course, a larger sensor will have a larger light gathering ability,
BUT ONLY AS A WHOLE. What matters, is how much light falls into a given area of the sensor, which for measuring purposes, should be equal. Think of it as rain (precipitation) measurements. Worldwide, the measurement of fallen rain is in mm per square meter. One millimeter per square meter equals one liter of water. This is given ALWAYS as how much water falls PER SQUARE METER along a given amount of time.
Read carefully the following to questions (stratements)
Lets compare a football field with your back yard.... under the rain!
1) (Question as Northrop??? states we should get a statistical answer that matters)
How much water rained over the football field and how much water rained over your back yard yesterday?
Obviously, the answer would be something like "The football field gathered about 2 times more water than my backyard", and this answer would be right for certain purposes, but none of them are related to what matters (in photography)
2) (Right question to evaluate the fallen rain over a certain time frame and over a certain area. Statistically valid for our purposes)
How much rain PER SQUARE METER rained over the football field and how much rain PER SQUARE METER fell over your backyard yesterday?
If the football field and your house are close together, there is only one possible answer: IT RAINED THE SAME AMOUNT OF WATER PER SQUARE METER OVER BOTH PLACES.
In shorter words: The only way to honestly compare exposure through a given lens and a certain aperture, is to compare similar situations. As I said before: THE SENSOR SIZE HAS NO PART here. What matters is the effective light transmission of the lens (f/stop) and the time frame during the actual exposure (shutter speed). PERIOD.
3) One last thing: Please, stop saying and arguing that depth of field comes from the sensor (negative) size and lens at a given aperture. Again, the sensor size has nothing to do here. Depth of field is from MAGNIFICATION, within a specific focal length and aperture, but taking into account the sensor/film size to determine the maximum size of the "circles of confusion" that draw the line between whats in focus and what is not. Pretty complicated right? No, just think like this: For a smaller sensor you need a shorter focal length to produce the SAME MAGNIFICATION (over the sensor). Using a shorter focal length produces SMALLER circles of confusion, thus the apparent sharpness gap increases accordingly. That is why the same picture taken with your full frame camera (at 40 mm f4 for example) has a visible and measurable depth of field compared to the same picture shot with your smartphone, who for
the same magnification, is probably using a 3 mm focal length and f/4, giving you an almost "infinite" depth of field. Again, the sensor size has nothing to do. IS MAGNIFICATION WHAT MATTERS!
And for Gods sake! If you lived on the East Coast (USA) and moved to the west... when someone asks you "What time is it?, do you answer... "Ehhh... its three o'clock but its really 7 o'clock!" I don't think so. (Only broadcasting companies that show the same programs to the whole country at the same time, state the "east" scheduled time plus one or two other time zones. But that is just because they are doing it at all the time zones AT THE SAME TIME).
So, if you live in California (or Oregon, or Washington state), set your watch for western time zone and forget about the east. Just switch back any time to take a trip back to the east coast.
Last edited by rburgoss; 10-09-2014 at 08:53 AM.