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08-18-2008, 08:47 AM   #1
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Pardon my ignorance re focal lengths...

Having jumped into the photo game after the introduction of DSLR, I have no knowledge of the focal difference between using on a DSLR vs film (nor do I probably need to understand it at this point.) However the question I have is when comparing the FA's to the DA's, are we comaparing apples to apples in the focal length? For example when using both FA 77 LTD and the DA 70 LTD on a DSLR is the focal length of the 77 truly only 7 longer than the 70?
Thx in advance...

08-18-2008, 09:07 AM   #2
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first of all the focal length is the focal length ... 77mm is 77mm

because of the predominance of the film days, most focal lengths are compared using a 35mm equivalent, and with an DSLR with an APS-C size sensor (like all pentax dslrs, smaller than a frame of film), there is a crop factor of 1.5 applied to the focal length to get the 35mm equivalent focal length (about 115mm)
08-18-2008, 09:11 AM   #3
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Focal lengths are always the same. The FA77 is a 77mm on any camera. The FA77 is 7mm longer (roughly) than the DA70 on a DSLR. There is no need to get confused here.

However a given focal length will give a different field of view on a DSLR with a sensor smaller than 35mm film. The FA77 gives a smaller FOV on a K100D than on a K1000. That is the "crop factor" (misnomer) people speak of.

It is true that people say the FA77 "acts like" a 77 x 1.5 = 114mm on a DSLR. But that is just shorthand.
08-18-2008, 09:12 AM   #4
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Snap!



08-18-2008, 09:22 AM   #5
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Without being harsh I am staggered at the confusion this presents people. Now compacts are difficult but int he DSLR world it's mind blowingly simple.

A 70 is a 70 and 77 is a 77, that ain't gonna change. If you mount them on an APS DSLR you then multiply by that sensor. For pentax and Nikon it is 1.5. So your 77has a similar FOV to a 116mm lens would on film or a "Full frame" DSLR and your 70 is a 105.
08-18-2008, 09:23 AM   #6
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Thanx muchly all...
08-18-2008, 11:43 AM   #7
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QuoteOriginally posted by Alfisti Quote
Without being harsh I am staggered at the confusion this presents people. Now compacts are difficult but int he DSLR world it's mind blowingly simple.

A 70 is a 70 and 77 is a 77, that ain't gonna change. If you mount them on an APS DSLR you then multiply by that sensor. For pentax and Nikon it is 1.5. So your 77has a similar FOV to a 116mm lens would on film or a "Full frame" DSLR and your 70 is a 105.
Everyone has to learn somehow, so it's one question I never mind answering when asked.

Although Google often does a better job
08-18-2008, 12:02 PM   #8
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QuoteOriginally posted by Alfisti Quote
Without being harsh I am staggered at the confusion this presents people. Now compacts are difficult but int he DSLR world it's mind blowingly simple.
I think the confusion, for the most part, stems from a combination of a general lack of knowledge on what focal length really means and overly technical explanations of the APS-C crop factor. When you start talking about optical power, diffusion and convergence people's brains start to shrivel and their eyes begin to glaze. Then you stack on top of that the APS-C crop factor and multiplying things many folks want to curl up in the corner and weep.

It took me overly long to wrap my brain around the concepts and I'm still not 100% sure I'm right. But here's my understanding of things...

The Focal Length of a lens determines how tight the "beam" of light containing the image is projected onto the film/sensor. Like the difference between a spot and a flood light, but in reverse.

The Crop Factor (1.53:1 or 65.4% for Pentax & Nikon) doesn't really magnify the imagre at all like a teleconverter does. All it does is reduce the amount of the beam of light leaving the backside of the lens and striking the sensor.

For example:
Pretend the lens is a film projector. And lets say your film, or a full-frame 35mm sensor, is a screen and that screen measures 8" by 12". Now adjust the image coming from the lens/projector until it completely covers the 8x12 screen. Now, replace the 8x12 screen with a 5.25" by 7.75" one. Now the projected image overlaps the boundaries of the smaller screen. THAT is the crop factor. Nothing is magnified at all. You mearly don't have the same physical area to catch the light projected back from the lens.

Did I get that right?

08-18-2008, 01:10 PM   #9
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sounds right to me
08-18-2008, 01:23 PM   #10
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QuoteOriginally posted by Venturi Quote
The Crop Factor (1.53:1 or 65.4% for Pentax & Nikon) doesn't really magnify the imagre at all like a teleconverter does. All it does is reduce the amount of the beam of light leaving the backside of the lens and striking the sensor.
Well, it doesn't affect what's coming out of the lens in the least bit. You've still got the same beam coming out. It's just striking a smaller target, as your film projector analogy correctly demonstrates.

QuoteQuote:
Nothing is magnified at all.
True only as long you're just looking at the "screen". Which in the case of your camera, is the sensor. But when's the last time you looked at an image projected onto your sensor? Iin order to see the image, you've got to display it on your computer screen, or print it. And *that's* where the magnification happens. Had you captured the larger image being projected onto the larger "screen" (a frame of 35mm film, or a "full frame" sensor of that size), then the image would need to be magnified less in order to view or print it at a given size. As it is, you are capturing only a smaller portion of the image, so you need to magnify it *more* to view/print it the same size as with film / "full frame" digital.

The end result is indeed basically completely indistinguishable from using a longer lens or a TC on the 35mm film camera. If the image taken by a 50mm lens on film shows a head-and-shoulder portrait, putting that same lens on APS-C digital (eg, all Pentax DSLR's) will produce an image that is just the face, with the shoulders and top of the head not visible. Print that image as 4x6", or view it full screen on your computer. Then go back to your film camera from the same location and try a 75mm lens or a 1.5TC. You'll get essentially the exact same image - face only, with the shoulders and top of head not visible.

The only differences would be in DOF, and that's actually a trickier calculation. In theory, the digital image with the 50mm lens would have more DOF than the 75mm or TC image on film, because it's still only a focal length of 50mm rather than 75mm. But the fact that more magnification is being applied actually shrinks the effective DOF down again (print an image smaller enough and ebverything looks in focus; big enough and nothing does).
08-18-2008, 01:27 PM   #11
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This image is not specifically about cropped sensors, but it demonstrates how a zoom changes focal length and how a smaller sensor decreases FOV.

08-18-2008, 01:41 PM   #12
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Thi sis wher eit gets absurd, why complicate it so much.

In a nutshell a 50 has a similar FOV to what 75 looks lik eon a film camera.

Done.
08-18-2008, 02:54 PM   #13
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Alright folks..let's make this easy.

Focal length of a lens never changes. So..a 50mm on film is the same size (in terms of magnification) as a 50mm on a dslr with a smaller sensor. A 50mm lens will not compress the image any more on a dslr than it would on a film camera, and the same goes for a wide angle lens expanding the image. Focal length is focal length..it doesn't change.

Now for the visual part:

Take a piece of paper and cut a medium sized rectangular hole in it. This is your lens. Hold the paper up to your face about 6" away and look through it. Now....move it another 6" away from you and look again. Yep...everything is still the same size, but this time the edges are cropped further because your field of vision changed. This is an illustrative way of seeing the difference between a film camera and a dslr camera with a smaller sensor.

That's it (well...not all of it, but good enough to understand what's happening).

Class dismissed.

c[_]
08-18-2008, 03:14 PM   #14
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QuoteOriginally posted by Marc Sabatella Quote
Well, it doesn't affect what's coming out of the lens in the least bit. You've still got the same beam coming out. It's just striking a smaller target, as your film projector analogy correctly demonstrates.
Right. I really botched that sentence. The beam coming out of the lens is still the same size but the target is smaller so part of the beam never hits the target (gets cropped).

QuoteOriginally posted by Marc Sabatella Quote
True only as long you're just looking at the "screen". Which in the case of your camera, is the sensor. But when's the last time you looked at an image projected onto your sensor? In order to see the image, you've got to display it on your computer screen, or print it. And *that's* where the magnification happens.
I guess this is where the physics perhaps get muddled in my head a bit?

As I understand things, the captured image at the sensor is of fixed resolution, based on pixel density of the sensor. And whether I make a print at 4x6 or 12x18 the optical characteristics of the image remain constant. Bokeh, DOF, etc all happens between the leading edge of the forward lens element and the trailing edge of the rear lens element (or teleconverter).
Take an image from a 10MP APS-C (15x23mm) sensor and an image from a 10MP FF (23x35mm) sensor cropped to APS-C size. The APS-C image would simply enlarge to 4"x6" better than the cropped FF image due to pixel density (the cropped FF image would be only 6.5MP).
Teleconverters actually physically alter the optics and project back a magnified image of 'infinite' resolution. The teleconverter physically alters the DOF, bokeh, focal length and aperture characteristics of the "beamed" image itself.

QuoteOriginally posted by Marc Sabatella Quote
Had you captured the larger image being projected onto the larger "screen" (a frame of 35mm film, or a "full frame" sensor of that size), then the image would need to be magnified less in order to view or print it at a given size.
Because of pixel density as I outlined above, wouldn't the same sized image (talking field of view here) from a FF image need to be enlarged more than a APS-C image of the same FOV?
Take 2 cameras with identical 50mm prime lenses; one camera is FF the other APC-S. Now take a picture with each of the same scene (the FF will capture have a wider FOV of course). Now crop the FF image to the same FOV as the APS-C shot. The FF sensor having fewer pixels per square inch would be lower resolution therefore would need to be stretched by 1.53x in order to make the same sized print.

I may be picking nits here, but I really do want to fully understand this subject matter.

Thanks
08-18-2008, 03:14 PM   #15
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QuoteOriginally posted by Alfisti Quote
Without being harsh I am staggered at the confusion this presents people. Now compacts are difficult but int he DSLR world it's mind blowingly simple.

A 70 is a 70 and 77 is a 77, that ain't gonna change. If you mount them on an APS DSLR you then multiply by that sensor. For pentax and Nikon it is 1.5. So your 77has a similar FOV to a 116mm lens would on film or a "Full frame" DSLR and your 70 is a 105.
forget all the BS about focal length mnultipliers. Focal lengrh is a physical measurement, and has nothing at all to do with the format of the sensor.

What people should correctly refer to is the "crop factor" i.e. how much out of a 35mm frame does your digital SLR take.

The reason people talk about "focal length factors, is that by taking a crop out of the middle of the 35mm frame, you have to enlarge that cropped section by 1.5 to get the same image size as you get with an image that fills the full 35mm frame.
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