07-28-2014 02:30 PM - edited 07-29-2014 04:08 AM
Bob from Boston,
" If that's wrong, I'd be interested to hear why it is."
As a matter of fact like a leopard, a lens can not change it's 'spots'. So to speak. A 50mm f1.8 lens remains a 50mm f1.8 lens no matter what body you bolt it on. Doesn't matter if it is a crop, a medium or a FF. It can not change the way it was born.
Indesputably! But it doesn't operate the way it did on the two different bodies. Think of it this way. Imagine the lens is a pipe running through a dam. The lake is filled with water which act like photons, being held back by a hatch or shutter. On the other side of the dam is an array of buckets just waiting to be filled with photons. (Obviously the array of buckets is an anology for the camera sensor.)
I actually have two arrays of buckets. The first has buckets organized side-by-side to represent the organization of sensor cells on a 35mm frame. The second has the same number of buckets organized side-by-side to represent the organization of sensor cells on a APS-C frame. Of course, these buckets are smaller in size so that they can fit within the smaller area of the APS-C frame.
If you're following me, I'll let you open the hatch and let the water, or photons, in to fill the first array.
What happened? The bigger array captured nearly all the water that came through the pipe.
Now the second array, the one representing the APS-C array, is set to capture the water from the same distance. This time the array captured fewer photons because many of the photons never fell into the smaller area of buckets.
The same pipe is used in both tests and the only variation was the area of the array.
A larger sensor will naturally absorb more light because of size, but the lens has no idea what sensor is behind it.
There is a mathematical formula for aperture. If you have a 500mm lens with a 125mm front objective element, the f-stop will be, roughly f/4, for instance. That can not change. Changing the size of the sensor, crop to FF, doesn't change that fact.
Assuming the same pixel pitch, each pixel will receive the same amount of light, regardless of the size of the sensor.
Check your math. Pixel pitch is the distance from center-to-center of the cells. If the pixel pitch of a 35mm frame is 100 (easy numbers for explanation purposes only) and has 1,000 cells horizontally then a APS-C sensor with the same pixel pitch of 100 can not contain as many cells. The original 1,000 won't fit. Only by making the pixel pitch smaller can you fit the same number in. Smaller cells can not gather as much light.
A properly exposed photo at f1.8 and 1/1000 at ISO 100 from that 50mm f1.8 lens will remain properly exposed regardless of whether the sensor is a crop, FF or medium format. It will receive the same amount of light, no matter what size the sensor is.
It will receive the same intensity of light but certainly not the same amount. It can't receive the same amount because the array area is not as big as that found on a full frame camera. Much of the light is missed as it is projected outside the area around the smaller APS-C sensor.
Now the effective or apparent focal length does appear to change, not the lenses actual focal length (that can not change) but what is seen by the sensor.
True but we're here to take pictures and my goal is to put the "right" photons into the buckets. I can' t put the right photons into a smaller array unless I move closer to the subject or change my focal length. I don't like waste, so unless I can't move close enough, I'll frame for only those photons I want.
A Canon Rebel crop sensor records a smaller portion of the total image that the lens is able to deliver to the sensor, when viewed at the same print size, the image appears larger. But your lens focal length does not get longer.
Now this is where it gets interesting. The focal length does not change but the ability to control the depth of field changes depending on the size of the sensor behind it. Going back to the anology of the dam with a pipe sticking through it, let's add the capability to change the length of the pipe. As the length of pipe gets shorter, it allows water around the sides of its lake-side opening to rush through and spray out the other side. Lengthen the pipe and the water acts in a much more controlled fashion. We like this because our ability to select the right photons is made easier when we can adjust the length of the pipe. Now if I can change size of the pipe, its length, and how long I keep the hatch open, I have much more flexibility to select the right photons. It doesn't matter if I am using a APS-C or full frame sensor, as long as I'm willing to move around to line-up the photons I want, I'll be okay.
But the behavior of the camera varies depending on which photons you want to capture. Let's say I want to capture the photons coming off a chapel at some distance (more than a mile). I don't want all the other surrounding photons so I pick a long pipe, err lens, and I make it narrow, stopped down to f/16, and I let the photons flood into my camera. They arrive in a nice stream of nearly parrallel photons after being organized by the nice long pipe I chose and because I didn't let too many in at a time.
This sort of behavior, the organization of distant photons into a nice nearly parallel stream, is something that most camera lenses do pretty well. The trouble occures on the other end of the specturm. When the photons I want are immediately in front of me, such as when I'm shooting a portrait. I can choose to capture them with a very wide and reasonably short pipe. Assuming that the camera lens can organize the light to create a clear image of the subject in front of me, I can capture a nearly instanteous record of the light where all the light surrounding the subject is diffused. I can do this because my camera receives a huge package of light in one cycle, organizes that which falls within its focal range, in this case the maximum aperture, and can not resolve the light coming from the surrounding areas uniformly.
When the shutter opens and I get the spray, only a large array of buckets is going to capture all the photons I want. The smaller array will receive the same spray but only capture the spray in the "middle". If only I could change something.
It turns out that there is a limit to how close I can get to my subject. And that distance is limited not by factors on the array side but on the pipe side. (Plus the camera's ability to open and close the hatch.) Making a lens that has a very wide opening in relation to its length is extremely difficult. Because of this limitation, photographers will have to rely on being able to chose the right sensor size for the job. (Full frame gear may offer greater control but that comes at a price in the form of additional size and weight. Not all jobs require the full capabilities of full frame, or beyond, and it is great to be able to shed a few pounds every now and then.)
If the limitation on a lens is f/2.8, then the FF array will capture more light than the smaller array and with greater control of that light. If manufacturers of cameras with APS-C sensors can make a lens with a very large opening in relation to its length, they might be able to catch up with the FF sensor in term of capturing the same organization of light. But this is not likely for practical reasons. The benefit of such a lens is uncertain because it would be extremely expensive, pressing the limits of optical technology and such, and that cost would be inconsistent with the lower cost of smaller sensor cameras such as those with an APS-C format sensor. These may not be entirely irreconcilable interests but only time will tell. For the time being, FF dominates the professional world of photography because of the support manufacturers provide to the full frame format and the control it allows.
It is the field of view or angle of acceptance that makes everything look like the lens has a field of view like one that is 1.6 times as long.
If it was compared to a FF sensor that is.
But in the end all of this is just numbers because you get exactly what you see in the cameras view finder. It is better to just ignor it and shoot away!