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03-22-2010, 02:36 PM   #1
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focal length

Just want to clarify something.

a Pentax SMC-M 50mm lens = 75mm on APS-C sensor...correct?

does that mean the DA series that is APS-C specific is 1.5x the focal length of the lens or are they actually the proper focal lengthe (ie. Pentax DA 35mm = 35mm or 52.5mm?)

Thanks

03-22-2010, 02:40 PM   #2
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See the article on focal length is the Articles section for a technical explanation.

The focal length of a DA 50mm is 50mm on an APS-C sensor; focal length never changes. However, it is equivalent to a 75mm lens on film.
03-22-2010, 03:10 PM   #3
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let me try again....

a pentax M 35mm on a FF/35mm camera...

does it have the same FOV as the new DA 35 on an APS-C sensor?

I guess what I'm asking is wether or not the DA series lenses fall into the same crop factor catagory as older pentax film lenses or has that been accounted for in the production of the new series?
~Nick
03-22-2010, 03:19 PM   #4
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Good question. I guess I've been wanting to ask the same thing. I've been picking up alot of primes locally and the 50mm is like a mini zoom lens!

03-22-2010, 03:52 PM   #5
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QuoteOriginally posted by landscaped1 Quote
let me try again....

a pentax M 35mm on a FF/35mm camera...

does it have the same FOV as the new DA 35 on an APS-C sensor?

I guess what I'm asking is wether or not the DA series lenses fall into the same crop factor catagory as older pentax film lenses or has that been accounted for in the production of the new series?
~Nick
A 35mm lens on a full frame camera will give you a wider view (larger field of view), than the same 35mm lens mounted on a APS-C camera.

A lens marked with having 35mm focal length has... 35mm focal length, whether it be an old M42 screw mount type, a FA or DA lens.

It would be plain stupid for any maker to "account" for a crop factor by falsifying the focal length of the lens, as the fl is a fixed physical property. The only thing a maker sensbly could do, is to give several angles of view (usually stated across the sensor's diagonal) for different sensor sizes.

Ofcourse some makers are that stupid and will mark lenses on their point-and-shoot cameras with 35mm-equivalent focal lengthes. But luckily until now, no maker of a DSLR adopted that bad habit.

Ben
03-22-2010, 04:32 PM   #6
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QuoteOriginally posted by landscaped1 Quote
a pentax M 35mm on a FF/35mm camera...

does it have the same FOV as the new DA 35 on an APS-C sensor?
yes, both lenses have exactly the same FoV on APS-C sensor - they both are 35mm lenses, despite the image circle they're covering.
03-22-2010, 05:12 PM   #7
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If you had a 50mm lens on a 35mm film camera, the circle of light that the lens casts onto the film was big enough to cover the whole rectangle of film. The 50mm had depth of field characteristics (such as depth of focus at a given f-stop) that is different than lenses that had other focal lengths. If you put that same lens on an APS-C dSLR, that same image circle and lens characteristics are projected onto the back of the camera. The APS-C sensor is smaller than a 35mm frame, and so you get a crop of the image instead of the whole picture. Since the sensor is 1.5 times smaller than the ff, you get a crop factor of 1.5. So, everything is the same, its just like you cropped a 35mm negative by 1.5. You don't get more reach and its not the same at any other focal length except 50mm.

The DA lenses are the same as FA lenses. A fa and da 50mm would produce the same lens characteristics (same dof at a given f-stop). The DA lens, however, doesn't need to project a light circle that fits a whole 35mm frame. The circle just fits on an APS-C sensor. So, you can't use it on film because it doesn't project enough image to cover the film. There isn't any math to do. 50mm = 50mm. The difference is, its like cropping the negative a bit.
03-22-2010, 05:14 PM   #8
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Ok, so let me get this straight. 50 mm is 50mm.

I put the lens on my super pentax and it will be wider in the viefinder, I put it into my Kx and it will be zoomed in due to the crop factor.

03-22-2010, 05:18 PM   #9
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Yes, the same lens will give a different appearance through the viewfinder of a new dSLR Vs a film camera.
03-22-2010, 05:20 PM   #10
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QuoteOriginally posted by The Kurly One Quote
Ok, so let me get this straight. 50 mm is 50mm.

I put the lens on my super pentax and it will be wider in the viefinder, I put it into my Kx and it will be zoomed in due to the crop factor.
yes, 50mm is 50mm. It won't be zoomed in more on the crop sensor. It will look the same, but with cropped edges.

Lets say you take a picture with a K1000 film camera and a 50mm lens. You develope and enlarge a print to 8 x 10 and plan on framing it. The frame is 1.5in thick and covers up the up the edges of the picture making it a 6.5 x 8.5. That is what a crop sensor does. Doesn't zoom in or change the lens characteristics in any way.
03-22-2010, 05:21 PM   #11
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So as an intereting sidenote, have any of you put your DA or DAL lenses onto film cameras? Happy with the results? or should I start a new thread instead of hijacking the OP's thread?
03-22-2010, 07:07 PM   #12
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I have put DA lenses on my 35/FF (film) ZX-M -- and immediately removed them, because of severe vignetting.

For crop factor: Here's a 80mm lens. I put it on a 6x6cm medium-format camera, like a Hasselblad. (Yeah, I wish.) It's a "normal" lens; the diagonal of a 6x6cm (actually 56x56mm) frame is 79.2mm, close enough. Now I put that lens on a 35/FF camera, whose 36x24mm frame has a 43.3mm diagonal. It looks like it was taken with a short tele. Now I put it on an APS-C camera like a Kx or K7, whose 16.7x25.1mm frame has a 30mm diagonal. It looks like it was taken with a long tele. But it's still the same lens.

That lens projects the same image in the same image circle. But the 35/FF and APS-C frames are smaller, so they see less of the image. Try this: cut out a picture from a magazine. Cut it so it's 56x56mm square. Note where the center is. Now draw a 36x24mm rectangle centered on it. That's the 35/FF frame. Now draw a 17x25mm rectangle centered on it. That's the APS-C frame. Same picture, same perspective, same sharpness -- but the smaller frames see less of it.

All the "crop factor" does is measure HOW MUCH LESS is seen. An 80mm lens on an APS-C camera sees as much of the picture as a 120mm lens would on a 35/FF camera. An 80mm lens, no matter what camera body it's on, is still an 80mm lens.
03-22-2010, 09:48 PM   #13
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QuoteQuote:
Thht lens projects the same image in the same image circle. But the 35/FF and APS-C frames are smaller, so they see less of the image. Try this: cut out a picture from a magazine. Cut it so it's 56x56mm square. Note where the center is. Now draw a 36x24mm rectangle centered on it. That's the 35/FF frame. Now draw a 17x25mm rectangle centered on it. That's the APS-C frame. Same picture, same perspective, same sharpness -- but the smaller frames see less of it.

All the "crop factor" does is measure HOW MUCH LESS is seen. An 80mm lens on an APS-C camera sees as much of the picture as a 120mm lens would on a 35/FF camera. An 80mm lens, no matter what camera body it's on, is still an 80mm lens.
The focal length being the focal length makes sense. The "crop factor" and FOV is what I was having trouble wrapping my head around.

This makes sense to me now. Thanks to everyone.

last question then. Looking through the viewfinder of the K20 what do I see? Do I see the full cast of light from the lens or the image that the sensor is going to create?
03-22-2010, 10:24 PM   #14
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The latter. A camera wouldn't be very useful if the viewfinder didn't show you something resembling the actual image.
03-23-2010, 05:15 AM   #15
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QuoteOriginally posted by The Kurly One Quote
So as an intereting sidenote, have any of you put your DA or DAL lenses onto film cameras? Happy with the results? or should I start a new thread instead of hijacking the OP's thread?
There is already a reference thread about DA lenses on film cameras. Just search for it.

Ben
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