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Forum: Pentax SLR Lens Discussion
07-17-2015, 06:19 AM
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You're right that this is basically an argument over semantics. However, the problem with calling it "additional light" is that it's not the amount of light that makes the difference; it's where the light travels. You can have less light because of environmental changes in a picture at f/1.4 and ISO 400 than you do at f/2 and ISO 200, and the picture with less light will still have a shallower depth of field because the light that you do have is distributed a different way.
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Forum: Pentax SLR Lens Discussion
07-17-2015, 05:48 AM
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Yes, I understood your thinking, which is why I was explaining that this was a miscommunication/misunderstanding. I was just stating what was meant by dtmateojr in his original post. There was no point in arguing back and forth about the physics when the actual issue was just one of semantics.
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Forum: Pentax SLR Lens Discussion
07-16-2015, 02:05 PM
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That's not what the original statement was about. It was about what I said in my previous post. That's why I said that perhaps you misunderstood dtmateojr when he first used the term, "projected image size." I'm guessing that Fogel70 misunderstood as well. Incidentally, just in case there is any question, it doesn't matter what system a lens was made for, at the same focal length, the projected image size will be the same (at least as long as it's mounted at the correct registration distance for the lens of course).
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Forum: Pentax SLR Lens Discussion
07-16-2015, 01:31 PM
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That doesn't matter. Focal length is the only thing that affects projected image size (that is, the only thing related to optics; changing distance to subject will of course change the projected image size of the subject). Perhaps you misunderstand what he meant when he said "projected image size." The projected image size is the absolute size of the image when it hits the sensor regardless of the sensor size, so smaller or larger sensors will capture a smaller or larger part of that image, not a smaller or larger version of the image. If the image circle of the lens is smaller or larger it will project a smaller or larger part of the image, not a smaller or larger image.
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Forum: Pentax SLR Lens Discussion
07-15-2015, 08:00 AM
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Some of what I said may have been a bit simplified, but it was not wrong. Your statements don't even really mean anything when it comes to light gathering. You're trying to illustrate equivalence, which has its uses as far as comparisons go, but is very misleading when you try to talk about it in terms of absolutes. To state that the Pentax Q 01 lens gives you the same noise levels as a full frame lens at f10.64 as though that is an absolute is entirely misleading. The noise levels depend very much on the sensor used as well as other factors.
If you were theoretically to have a sensor of the same technology with the same pixel density on full frame as the Q, and used the same focal length (8.5mm), and the same f-stop, then the noise levels would be the same in any given area of the sensor (there is no camera and lens combination that meets these criteria of course). Of course the picture from such a camera would cover a much greater angle of view and contain a much greater number of pixels, and if printed at the same dimensions would have the noise reduced in size along with everything else, but if cropped to the same angle of view as the Q, would have the same amount of noise. It's not switching to a full frame sensor that causes the difference, but the other changes you make to get a similar picture (changing the size of the pixels, changing the focal length, etc.). However, you will never find an exact equivalent. There are combinations that may be compared and relevant in certain situations, but they are not absolutes (that is, they may apply quite well under certain conditions, but changing conditions will make them not apply).
Your final statement may be generally useful as a comparison, but again, it is not absolute.
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Forum: Pentax SLR Lens Discussion
07-14-2015, 02:48 PM
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My take on this is that "equivalency" is never perfect. There are too many variables involved, and thus not really any such thing as equivalency. I tend to agree with Lowell. It's about learning to use your equipment to get what you want, and learning when you need to add another piece of equipment because what you have won't quite give you what you want.
This is not to say that comparisons can't be useful (notice how I said "comparisons" rather than "equivalency" ;) ). If the original poster is still paying any attention, then I would explain it a bit differently:
When you use the Tamron 17-50mm 2.8 on your K-5 your angle of view will be similar to 25 - 75 mm on a 35mm sized sensor.
Your aperture as far as light gathering ability goes will (theoretically, because some lenses have truer aperture ratings than others) be similar at f2.8 as any other lens that is f2.8. That is, the same ISO to f-stop ratio will get the proper exposure.
Your aperture as far as depth of field goes will be different because depth of field varies with focal length as well as aperture and you are shooting at a different focal length. Wider aperture gives less depth of field, and longer focal length gives less depth of field. So when you shoot at a shorter focal length as with APS-C, then you would need a wider aperture to get the same depth of field.
ISO/noise ratio is governed by pixel size. There are two types of noise involved with digital cameras: shot noise and read noise. Shot noise is almost completely dependent on area. Read noise is dependent on the number of pixels (more pixels mean more read noise). When signal is low (high ISO settings) then the fact that you get more read noise per pixel is the deciding factor and larger pixels give you less noise overall. When signal is high (low ISO settings) then the greater signal more than compensates for increased read noise and you actually get less noise with more pixels (because you have plenty of light to go around). Of course, you'll generally notice noise more when there is less signal, at high ISO settings, so for practical purposes you get less noise with larger pixels.
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