I hear people say that a lense that has, say, a max zoom of 50mm is equivalent to a film lens of like 100mm (this is only an example!) But how, and why is this?
I hear people say that a lense that has, say, a max zoom of 50mm is equivalent to a film lens of like 100mm (this is only an example!) But how, and why is this?
Digital cameras use different size sensors. Some of the top pro cameras (D3, 1Ds, 5D) use the same sensor size as a film camera: 24x36mm
Other cameras use smaller sensors, so there is a crop factor, which piles more pixels into a smaller area size, making the output (screen, print) look larger. Many Nikons use sensors that create a 1.5x crop factor. Canons use slightly smaller sensors which create a 1.6x crop factor.
Point and shoot cameras use even smaller sensors creating even larger crop factors. With smaller sensors comes greater sensitivity and greater noise (grain.) That is why P&S cameras suck in dim/low light. They have to jack up the ISO to get the picture.
Anyway, it varies!
Smaller sensors don't pack more pixels into a smaller area than a larger sensor unless they have more pixels (mb) in which to pack.
Slightly different subject, but related to the full-frame vs. crop sensors size issue: The one thing about the crop factor which I'm not sure of is this: If I'm using a 100mm prime lens on a 35mm SLR camera it will have a certain compression, or flattening of the image, foreground to background. Now, if I put that same lens on my D50, it will be as if it were a 150mm prime lens, so I'm told. The sensor is smaller, and "crops" the image circle cast upon the sensor in the middle, sort of like zooming in to the image. Yet the physical image cast by the lens doesn't change, it's the same image through a 100mm lens, just less of it hits the sensor. In reality, it's not like seeing through a 150mm lens, because the front-to-back compression is still that of a 100mm lens.
The reason I noticed this is that I originally believed that with my 55-200 lens on my D50 that I'd get the "reach" of a 300mm lens (35mm equivalent) on the long end, but what I get is the reach of a 200mm lens, because that's what it is. Cropping into any image doesn't really "zoom" it to a different focal length, whether cropping from a print or a negative or a digital image. Would my DX lens at 200mm become a 300mm lens on my 35mm full frame camera (vignetting notwithstanding?) No. It would still be a 200mm lens (made smaller and lighter so it couldn't cover the full 35mm frame completely with it's image circle.) But, 200mm is still 200mm. At least in my book.
Anyone else care to shed some light on this...?
From an absolute standpoint, a 200mm lens is still a 200mm lens. As such, elements of depth-of-field and similar compression/perception elements don't change. You're essentially correct in saying that all that the 'crop body' dSLR is automatically cropping the image for you, as it is optically configured to subtend a smaller geometric angle. Simplistically, what's being overlooked is that for a standard 200mm film lens on the digital, what's captured by the sensor is the "looks like 300mm" center, but the rest of the light is passing through the lens...its simply not hitting the recording media, so its not being recorded.
This would become much more apparent if someone were create a, say, 200mm EF-S lens and then mount it onto a full-frame camera body (with EF mount): the resulting photo would be fine in the middle, but the outside third would effectively be black (due to no exposure)...it would be something like this image.
-hh
So, basically you're confirming what I'm saying. Thanks for further explaining it, it does help the discussion.
Let's assume the following discussion refers to prime lenses that would work on both full-frame and DX Nikon (or any brand) dSLRs:
To further simplify this illustration, (and I'm thinking aloud here, perhaps trying more to cement my own understanding) let's just hook a 200mm prime lens up to a full-frame digital camera, mounted on a tripod in a stationary position, take a picture, examine the results. Now, let's consider that this same camera can also "crop" the sensor down to a smaller megapixel, DX mode, and now with the same setup other than the smaller sensor recording the image, fire away once more. Examine the results. I'm betting the image doesn't look like it was shot with a 300mm lens (1.5 magnification) but looks exactly like the middle part of the 200mm image from the larger sensor, which is what it is... same apparent telescopic effect, same detail, same resolution (presuming the sensors are equal in that area, which they probably are since it's actually the same sensor, just less of it.)
Now, let's photograph the same scene by attaching a 300mm prime lens to the camera and shooting in full-frame mode, examining the results. Next, attach the 200mm lens and switch the camera to DX mode, examine the results. Compare the images (I really wish I could do this, but I can't so I'm using my imagination here... ) I'm betting the 300mm full-frame image, while covering the same angle of view as the 200mm lens on a "cropped" (DX) sensor, will have a more compressed telescopic effect, with background objects appearing larger than on the 200mm DX combo.
I have found my 55-200mm DX lens doesn't give me what I used to get with my old 100-300mm for film. It takes nice, sharp, clear pictures, but is slower at each focal length, and doesn't give me nearly the telephoto reach. At first I was ignorant and just assumed if I multiplied by 1.5 I'd have my effective 82.5-300mm range, but it just never felt right, and was lacking something... until I thought about it.
The major difference is depth of field. Depth of field depends on focal length and subject distance. If you take a photo with a 200mm lens on a small sensor camera, then take a photo at the same distance (and same aperture) with a full frame camera using a 300mm lens to get the same field of view, the depth of field will be greater on the 200mm/small sensor image. This could be either an advantage or disadvantage, depending on what you're trying to achieve.
I don't quite understand what the problem is and what differences you are seeing. It's certainly true that a 200mm lens remains a 200mm lens - what ever body you put it on. Saying it is a 300mm equivalent is only really a convenient shorthand for describing the reduced angle of view on a smaller sensor body. But the sizes of the objects in the image and their relative sizes will be the same for a 300mm lens on a full frame camera and a 200mm lens on a camera with a smaller sensor (1.5x).
The fact that the small sensor only sees the central part of the image can be thought of as a good thing. This is where the lens will be sharpest.
By your logic, if I took an image from a 50mm lens and cropped it to the equivalent crop of a 6x lens, right in the middle, I'd get the same image, more or less, that I'd get from a 300mm full frame image.