What’s the difference between entrance pupil and exit pupil?
The camera pupil in general seems to be semi-circular, but the final photo is always rectangular. Are there any cameras that capture the entire input of photons? Are there any pros (other than additional angle) and any cons (such as perhaps greater blur near the edges that normally get cut off?). Does the final rectangle to input ratio in digital cameras compare favorably or unfavorably to analog cameras?
Any interesting tidbits related to the OP that I didn’t happen to think of?
About the last part of #2 (Does the final rectangle to input ratio in digital cameras compare favorably or unfavorably to analog cameras?): Crop factor
Basically, because the sensor sizes on digital cameras are different, their resulting fields of view are different too. If you take a photo with a film camera and a digital camera – both with the same lens – the digital one will generally result in a narrower field of view unless it has a “full-frame” sensor.
There is some feature in your optical system which restricts the angular extent of rays entering the system. This is the aperture and its edge is the aperture stop. The definition of the entrance pupil is “the image of the aperture stop in object space”(that is, as you’d see it looking through the input side of your lens), while the exit pupil is the image of the aperture stop in image space (as you’d see it looking through the back of the lens). In a simple, single-element lens with no mount or anything, the aperture stop is simply the edge of the lens, and the entrance and exit pupil are all that same thing, and located in the same place – the edge of the lens itself.
In more complex lens systems, especially multiple-component lenses, the aperture stop might be somewhere inside the assembly, and the entrance and exit pupil in generally won’t be in the same place as it, or each other. They might not even be within the physical lens itself. If you’ve got a good camera lens with an adjustable aperture, this is generally the case.
2.) I don’t know what you mean by the aperture being “semi-circular”. To me that’s a half-circle. They generally try to make camera irises pretty close to circular, but they’re often polygonal (and when you see lens flares they have the shape of the aperture – sometimes being pentagonal or hexagonal).
3.) Your question about “the entire input of photons” is worded oddly – designers try to capture an appropriate number of photons (the primary purpose of the aperture is to restrict the number of incoming photons, after all), but once past the stop, they try to get the maximum number of those entering photons to the correct spot on the film or CCD plane. That’s why they anti-reflection coat the lenses and do extensive design work, and minimize aberrations.
Your question really seems to be "does anyone make a camera that tries to capture the entire image made available by the lens – which ought to be circular, since the lens system has circular symmetry. Generally not, for whatever reason. Fisheye cameras and old point-and-shoot cameras sometimes have circular images that are completely captured, but only by using a piece of film that’s larger than the image diameter, so you inevitably have unexposed portions of film, and the part of the image you do have covers a smaller area to fit on that film, and this limits the resolution determined by the film grain. I suspect that film cameras used rectangular images (and effectively “threw away” part of the potential image) so that you got a
I’m not quite sure why you would want to make a circular image unless there is some sense of “waste” involved. As it is, most cameras produce an image that is actually larger (covers a greater field of view) than what you see in the viewfinder.
If you want a circular image on 35mm film or “full frame” DSLR, you could try a fisheye lens such as the 8mm Peleng:
Another way is to mount a third-party lens intended for APS (“crop”) digital cameras on a “full frame” digital camera or 35mm film camera.
Another common situation where you get a circle is with large format cameras. If you use a lens intended for 6x6 cm on a 4x5 camera, you will get a circular image about 7mm across.
As for pros/cons, the edges of the image circle that a lens produces typically are less corrected, less sharp, and less bright than the center. In some artistic contexts this can be desirable but generally you will get a more pleasing image using the center of the image circle rather than the edges.
Correct, but I think irrelevant. You normally don’t use the same lens for a digital camera as you do for a film camera. While the light sensor on a digital camera is usually smaller than 35mm, the common lenses are a shorter focal length so the resulting image is the same. On my Nikon film camera I used a 70-210 zoom. On my digital SLR I use a 40-150. Digital camera lenses are often listed with the actual focal length, and their 35mm equivalent.
example
A more important difference is that the shorter focal length gives you a greater depth of field, which is not always desired.
Really? I know that Canon or Nikon or somebody did release a “made for digital” lens, which if I remember correctly was basically a cheaper version of its film counterpart because it didn’t need the full focal length.
However, I thought most lenses were still designed with 35mm in mind, meaning digital cameras have to apply the conversion or use the 35mm equivalent. Are you saying this is no longer the case?
Can you clarify this question? I’m not sure what it means exactly. There are certain lenses meant for digital cameras (in Nikon parlance, they are “DX” lenses, for Canon, it’s “EF-S” for the 1.6x crop sensor.) They can be very good glass, but they don’t throw the same size image (given the crop) as a full-frame sensor, so if you use them on a 35mm film camera or a full-frame camera, you will see very obvious vignetting along the edges.
Definte “popularity.” For the crop sensors, yes they are popular lenses. For full-frame sensors, they are useless. There is some spectacular (and comparatively affordable) glass in the DX or EF-S series. It depends on the photographer. I’ve only used full-frame digital cameras from 2008, so those lenses are (next to) useless to me. If I were using a Nikon D300 or a Canon 7D or whatnot, I would certainly consider the crop-sensor lenses. I prefer the full-frame lenses because of their flexibility.
You may be understanding how they work fine–I gather you have the basic idea from your posts. Basically, the lenses designed for crop sensor lenses are cheaper than their 35mm counterparts because they don’t have to throw as large an image on the sensor (as it’s smaller than 35mm). So there’s not as much glass involved in the manufacturing, so far as I understand. This doesn’t mean necessarily they’re any worse optically than the 35mm lenses, at any rate. It just means that when you use it on a 35mm format, you’ll clip on the corners because the circle of the image being focused on your sensor doesn’t cover the whole sensor. Here’s an example of what happens when you use a crop sensor lens on a full-frame camera. I believe that’s the 12-24mm f/4 lens there, as I’ve used it on a full-frame camera myself for the same effect. That’s at the 12mm end of things. When you zoom in (at about 20mm or so), you can actually get past the vignetting and get a reasonable image for a full-frame camera.
Thanks for checking in. I do understand that part; my error was in assuming that the OP talked about switching lenses between a digital SLR and a film one. S/he never mentioned that and why I got that idea in my head, I have no idea… irrelevant, as RedSwinglineOne said; it just took me a while to realize it.
At the professional level, which I know very little about, I think some digital cameras are designed to work with standard 35mm film lenses so the professional can use the stable of lenses they already have. At the mid and lower level DSLR’s most or all the lenses are digital camera specific, hence my comment that comparing digital and film lenses on the same camera is not usually an issue.
A bit of googling has shown me that there are people using older lenses on newer digital cameras. I went straight from a fully manual SLR to my digital SLR, so the idea of mixing old lenses with digital bodies seemed like more work that it is worth, but people are doing it.
There are a few exceptions, but, for the most part ,as long as they have the same mount, Canon and Nikon lenses manufactured for film cameras will work with mid-level and lower-level dSLRs. So, an Nikon D3000 will work with 35mm AI lenses manufactured in the late 70s onward, and a Canon T2i will work with EF-mount lenses from the late 80s onward. There will just be a “crop factor” to take into account. So, for non-full frame Nikon digital cameras, the crop is 1.5x, and for Canon it’s either 1.3x or 1.6x (depending on the camera. The 1-series has the 1.3x crop, the rest have 1.6x crop.) So, effectively your 50mm becomes a 75mm on Nikon and and 80mm on Canon.
Conversely, there are lenses made especially for these crop sensors. They are a bit cheaper because they don’t have to have as much glass, because they’re focusing a smaller image than their 35mm counterparts. But when you use these lenses (DX for Nikon, and EF-S in Canon parlance) on 35mm or full-frame cameras, you will get the sort of vignetting I linked to above, because the lenses are not throwing as large an image to the sensor/film.
Is this true for the fully manual type lenses? When I think of 35mm film I think of manual focus, manual aperture. I kind of skipped the whole autofucus, autoexposure film era. I (perhaps wrongly) assumed a digital camera might have a problem with those, or can you just put the camera on Manual Mode?
I have a couple old Nikon MF-only lenses (like this 55mm macro from 1979) and they work fine on every Nikon digital camera I’ve had (D70, D200, D1, D3).
