Teach me...Im dumb

Ok, vague title, but accurate and honest.

So I recently purchased a new lens and it go me thinking about the size/wieght of these lenses. Besides the ability to manually chose the aperture size on the SLR lenses, why are they so much larger than a comparable zoom on a point and shoot? Why does it take a huge lens to get in the 200-400mm on SLR but you can get the same zoom on a P&S and have the whole camera weigh maybe a pound?

The lenses on a P&S are actually only 5mm-50mm or so generally. Their sensors are tiny, so the 35mm equiv FoV (field of view) is maybe 28-280mm, for example.

because they have bigger and better glass, and the faster the lens the bigger the glass.

very valid question to ask, particularly in view of the small size yet fast apertures on the Olympus OM lens series! I suspect some size is accounted for by the AF motor, and some by the auto aperture stopdown control

OM's were in a different league altogether. It still brings a tear to my eye when I imagine a beautiful OM2 in my hand, the silky smooth but firm and precise movement of the focus ring and the gentle way the two halves of the split image would magically align themselves. These were among the best cameras ever designed and made.

phreeky wrote in post #8761768
The lenses on a P&S are actually only 5mm-50mm or so generally. Their sensors are tiny, so the 35mm equiv FoV (field of view) is maybe 28-280mm, for example.

I know you are just guesstimating, but lets carry that logic another step. If they can create a lens down to 5mm and still charge sub $200 for the P&S, why does it cost 3-4 times that much to get a 5mm (or close to 5mm) lens on a SLR?

shlurpee wrote in post #8764422
I know you are just guesstimating, but lets carry that logic another step. If they can create a lens down to 5mm and still charge sub $200 for the P&S, why does it cost 3-4 times that much to get a 5mm (or close to 5mm) lens on a SLR?

The answer to this, and your original question is sensor size. Obviously much more glass is required to cover a sensor that is 10-20x the size of a compact camera.

shlurpee wrote in post #8764422
I know you are just guesstimating, but lets carry that logic another step. If they can create a lens down to 5mm and still charge sub $200 for the P&S, why does it cost 3-4 times that much to get a 5mm (or close to 5mm) lens on a SLR?

Because in a P&S the 5mm lens only has to cover a very small sensor a short distance from the sensor with no reflex mirror, but on an SLR it has to cover 15x22mm or 24x36mm sensor and do so from a distance that permits the reflex mirror to swing up and out of the way without hitting the lens, even though the FL is a shorter distance.

It's all about gathering light and physics of it. If you have a 50mm lens, to make an F/2 lens the opening (front element) has to be 25mm. If you have a 400mm lens, to make an f/2 lens the front element has to be 200mm diameter - at a theoretical minimum.

On a point and shoot, because the sensor is smaller, the zoom is usually 5 to 50mm and f/8 at the 50mm end. So the lens has to be 10mm in diameter. To get the same magnification on a large sensor, you'd need a 250-250mm zoom and if you want to be able to handhold it, the front element needs to be about 60mm, which would give you f/5ish at the very best.

Basically you want the lens to soak up as much light as possible and to do that the front hole of the lens needs to be larger, because a larger hole lets in more light.

So it boils down to image sensors, makes sense. But why can a 1D use the same lens as a smaller sensor camera like mine (XSI), bigger sensor should require bigger lens right? So are the lenses built to work on the full frame sensor cameras and for the crop cameras they still work but are, in a sense, over kill?

shlurpee wrote in post #8764612
So it boils down to image sensors, makes sense. But why can a 1D use the same lens as a smaller sensor camera like mine (XSI), bigger sensor should require bigger lens right? So are the lenses built to work on the full frame sensor cameras and for the crop cameras they still work but are, in a sense, over kill?

Any lens that covers a larger format can be used on a smaller format. APS-C has the benefit of being able to use lenses designed for the larger 135 film format (same size as the 'FF' dSLR) The lenses built to work on the full frame sensor cameras which can be used for the crop cameras are, in a sense, over kill. In theory the performance of a 400mm lens could be increased to provide higher lens resolution and cover only the smaller sensor of the APS-C camera. The Olympus 4/3 format does that, in fact.

So I guess what you are all saying is I can't dissect my P&S, superglue the lens to my XSI and become a billionaire. Bummer, back to the drawing board.

shlurpee wrote in post #8764654
So I guess what you are all saying is I can't dissect my P&S, superglue the lens to my XSI and become a billionaire. Bummer, back to the drawing board.

You can dissect your P&S, superglue the lens to your XSI, but you won't become a billionaire and you won't be smarter than a fifth grader!

shlurpee wrote in post #8764612
So it boils down to image sensors, makes sense. But why can a 1D use the same lens as a smaller sensor camera like mine (XSI), bigger sensor should require bigger lens right? So are the lenses built to work on the full frame sensor cameras and for the crop cameras they still work but are, in a sense, over kill?

In the sense that you are carrying a lens capable of creating an image larger than a cropped sensor camera can record, yes, they are overkill. If you want to conserve on lens weight and size., the EF-S series is designed to fit the smaller sensor cameras (Except the cropped camera I happen to own, the 10d) Even if I had XXD cameras that would accept the EF-S, I think I would still stay away from the series and buy the lenses that will fit ALL of the canon lineup including the full frame models. If I want to carry less weight, I would be well advised to go on a diet.

shlurpee wrote in post #8764612
So it boils down to image sensors, makes sense. But why can a 1D use the same lens as a smaller sensor camera like mine (XSI), bigger sensor should require bigger lens right? So are the lenses built to work on the full frame sensor cameras and for the crop cameras they still work but are, in a sense, over kill?

EF lenses cover a FF image size. But EF-S lenses cover an APS-C image size.

Obviously you can use a larger lens on a smaller image, but not the other way around. Because the sensor is smaller, there is a crop factor. This results in the XSI image appearing closer than the FF image taken with the same lens.