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12-27-2007, 10:22 PM   #16
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Thank you for the information.

Bob

12-27-2007, 11:23 PM   #17
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QuoteOriginally posted by tux08902 Quote
Are the DA Limiteds subjected to the 1.5x crop factor limitation? I'm reading reviews and they all refer to an effective field of view. That makes me wonder because I always thought DA lenses were built for digital, and therefore, were not affected by the crop factor.

I think I'll go with the DA Limiteds, though. Having all three would make a nice, light kit for most applications. That's what I'm looking for. I generally do not like to use zooms.
Yes they are still effected by crop factor. Crop factor is a result of a smaller sensor size (APS-C versus Full Frame) which is a body attribute, not a lens one. DA lenses just have a smaller image circle because they don't need to illuminate an entire piece of film. The FA lenses have a full frame sized image circle, but the sensor only detects part of it.

This is my understanding of it, I could be wrong. I reserve the right to be proven incorrect
12-28-2007, 05:29 AM   #18
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Focal length is focal length, whether the lens was designed for the APS-C sensor or 35mm film. DA lenses, are designed to only cover the area of an APS-C sensor. This is called the image circle. A 35mm lens is designed to fill the larger area of a 35mm film frame.

The focal length remains the same, as does the magnification. You can check this by taking your kit lens, setting it to 50mm and take a static test shot. Then mount your M50 and take the same shot from the same place. You will see that the images have identical coverage.
12-28-2007, 06:08 AM   #19
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Read your last post - https://www.pentaxforums.com/forums/general-pentax-photography/16876-crop-fac...at-s-deal.html

QuoteOriginally posted by tux08902 Quote
Are the DA Limiteds subjected to the 1.5x crop factor limitation? I'm reading reviews and they all refer to an effective field of view. That makes me wonder because I always thought DA lenses were built for digital, and therefore, were not affected by the crop factor.

I think I'll go with the DA Limiteds, though. Having all three would make a nice, light kit for most applications. That's what I'm looking for. I generally do not like to use zooms.
Repeat after me = Focal length is Focal length is focal length.....

12-28-2007, 09:51 AM   #20
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Now, I understand. I didn't know that the DAs were still designed against the 35mm standard.
12-28-2007, 10:28 AM   #21
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QuoteOriginally posted by tux08902 Quote
Now, I understand. I didn't know that the DAs were still designed against the 35mm standard.
Well, it's not a 35mm standard. It's a physical property of the lens. ANY lens. I would suggest doing a google search of "focal length" and finding a diagram or something. Focal length is best understood visually.
12-29-2007, 07:29 AM   #22
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While this isn't necessarily technically correct, try the following thought experiment:

Imagine a lens, say 50mm in focal length, that projects a cone of light (containing image) on a film/sensor surface behind it. Imagine there isn't a coverage issue & that other geometrical things are taken care of.

Put a 35mm frame in the center of that cone. Let's say it's a picture of a pair of windows, and you arrange it so the outer window frames are at the edges of the 35mm frame. We'll call that a 'normal' view.

Take the 35mm frame out and put in a medium format 6x6 frame in the same spot. Because the frame is larger, you'll see the wall on either side of the windows, plus more image to the top and bottom as well. You're seeing stuff the 35mm frame cut out. This we'll call a 'wide angle' view, because you're seeing a wider image than 'normal'.

Keeping that 6x6 frame in place, we now check what focal length lens would produce the same view (leaving aside the square format) as the 50mm did with the 35mm frame. It turns out that a 75 or 80 mm lens will make the outer edges of the windows be at the edges of the photograph. So we can say that a 75 or 80 mm lens is 'normal' for the 6x6 format.

Putting the 50mm back in, now put an APS size digital sensor where the 35mm frame used to be. Since the APS is smaller than 35mm, less of the image is captured - the APS frame cuts out a good chunck of each window, top, bottom and sides. The 50 mm lens therefore acts like it is a mild telephoto lens on the APS. Just for fun, put that 75mm lens on the 35mm frame, and you see the same view as with the 50mm on the APS. That's the conversion factor of 1.5x.

The above I hope explains the relation of focal length to film/sensor size. Pop Photo et al periodically run pictures to try to show this. The point there is that focal length is focal length, even if you crop the image. I.e. a true telephoto compresses distance where a shorter lens on a smaller bit of film does not - even if the two show the same amount of image.

If you think about that cone of light, one lens design consideration is how wide that cone gets in order to cover the desired film size without vignetting and with the desired amount of resolution and contrast. The DA lenses are designed for this cone to cover a smaller area than a 35mm lens is - which is one reason that Pentax can make them smaller and lighter. I have my 70mm DA on a film body now, I don't see vignetting in the view finder, but we'll see when I finish the roll of film...
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