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05-05-2007, 05:31 PM   #1
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Crop Factor question... 35mm, DA and dslr

I know that the small image sensor on the dslr gives a simulated "built-in-zoom" effect because ot crops the edges of light let in by 35mm lenses. So, if you shoot 50mm lens on a dslr you "see" a 75mm image, that is it's like you zoomed into 75mm instead of the 50mm the lens actually was.

But, with my DA 18-55... isn't it designed for my sensor? Will every DA lens be designed for my smaller sensor? If I see a DA 200mm, it will "see" 200mm on my camera, right?

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05-05-2007, 05:43 PM   #2
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Your 18-55mm lens goes from 18mm to 55mm. A DA 200mm is a 200mm lens. The so-called "crop-factor" DOES NOT change the focal length of the lens. It is ONLY a reference for folks who are used to using 35mm SLR cameras.

When they say a lens is "designed for the smaller sensor", all they mean is the image circle is smaller then that of lenses originally designed for 35mm film cameras (so it would possibly vignette on a 35mm film camera)

So unless you have experience with 35mm Film camera, IGNORE the "crop-factor" and just remember that ~28mm is "normal" and anything less than that is wide, and anything longer than that is telephoto.

In closing, a 50mm will ALWAYS be a 50mm lens It just happens to be "normal" on a 35mm film camera, and short telephoto on a APS-C camera (and wide on a medium format )
05-05-2007, 05:56 PM   #3
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QuoteOriginally posted by mediaslinky Quote
I know that the small image sensor on the dslr gives a simulated "built-in-zoom" effect because ot crops the edges of light let in by 35mm lenses. So, if you shoot 50mm lens on a dslr you "see" a 75mm image, that is it's like you zoomed into 75mm instead of the 50mm the lens actually was.

But, with my DA 18-55... isn't it designed for my sensor? Will every DA lens be designed for my smaller sensor? If I see a DA 200mm, it will "see" 200mm on my camera, right?

slinky
The focal length of the lens does not change with camera format. But the field of view changes with a given focal length depending on the size of the film frame or imaging sensor. The sensor in the Pentax digital SLR is about 18x24mm, or roughtly 2/3 the dimensions of the 35mm film frame (roughly 24x36mm). So you see 2/3 of the field of view. A 6x7 medium format camera (like the Pentax 67) has a film area that is roughly double the dimensions of the 35mm film frame (at 60x70mm).

So, a 200mm lens gives an approximate (diagonal) field of view of about 8 degrees on the digital, 12 degrees on the 35mm film camera, and 24 degrees on the 6x7 film camera. The lens is still a 200mm lens, whether it is a DA lens with a small image circle, a FA lens with a bigger image circle, or a 6x7 lens with a larger image circle. A DA 200mm, an FA 200mm, and a Pentax 67 200mm lens (with adapter) all give exactly the same field of view on the digital SLR.

The "35mm equivalence" is just a point of reference for photographers who are used to 35mm film cameras and associate a particular focal length with the associated field of view. To get the 12 degree field of view that comes from a 200mm lens on a 35mm film camera, you need a 130mm lens for the digital SLR, and a 400mm len for the 6x7 film camera. That is the only point of the "35mm equivalens." Unless you have a lot of 35mm experience, it is a meaningless reference.
05-06-2007, 08:30 AM   #4
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Yes, thank you, I know that focal length doesn't change, and tried to explain that with my ''light'' comment in the original post.

Is that correct? A smaller sensor will not catch all the light a 35mm designed lens allows in, resulting in a cropped image (hence the title)?

Perhaps a clearer way to ask my question... Do DA lenses show numbers based on a small sensor, or based on a 35mm reference?

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05-06-2007, 08:59 AM   #5
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There are no "numbers based on a small sensor". What's your frame of reference? 35mm? 645? 6x7? 4x5 sheet film?

The 150mm lens is a wide normal on my 4x5 view camera, a short telephoto on my RB67, and a long telephoto on the 35mm and APS-C cameras. But it's still a 150mm lens, no matter what you mount it one.

The image circle of the lens determines what format it's usable on. A 6x7 lens can be mounted on a 35mm or APS-C camera and work fine, a lens made for a 35mm camera will work fine on APS-C but will not cover the whole frame on a 6x7 camera.
05-06-2007, 10:42 AM   #6
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OK, so reading into the information you've all said... There are some fundamental areas of information that I lack.

Is focal length determined by lens distance from sensor? or from some other point?

Other questions related to the original post:
Can a DA lens be used on 35mm camera? Are they exclusively designed for dslrs and smaller sensors? If so, how are they different?

If presented with a DA 200mm and an old manual 200mm, would the image I captured on a digital camera be the same for both lenses? Forget focal length, bokeh, etc... Say you are on a tripod, shooting a picket fence, would both images have the same number of pickets?

It's funny, I thought I was asking a very simple question...
slinky
05-06-2007, 10:43 AM   #7
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Pentax does not specify focal lengths of their lenses depending on the format they are made for. If you don't use 35mm, it would be best to forget about the crop factor. As Gary and egordon99 said, it's just a frame of reference for people who are used to the 35mm format.

If you set your 18-55 at 28,then compare with your 28 prime, the image will cover the same area(if the manufacturer of the prime was honest about the focal length; it may vary within 2-3mm).
05-06-2007, 10:46 AM   #8
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Thanks, that's what I needed to know, just had a hard time asking.

05-06-2007, 11:01 AM   #9
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QuoteOriginally posted by mediaslinky Quote
OK, so reading into the information you've all said... There are some fundamental areas of information that I lack.

Is focal length determined by lens distance from sensor? or from some other point?

Other questions related to the original post:
Can a DA lens be used on 35mm camera? Are they exclusively designed for dslrs and smaller sensors? If so, how are they different?

If presented with a DA 200mm and an old manual 200mm, would the image I captured on a digital camera be the same for both lenses? Forget focal length, bokeh, etc... Say you are on a tripod, shooting a picket fence, would both images have the same number of pickets?

It's funny, I thought I was asking a very simple question...
slinky
For your first question:
Photographic lens - Wikipedia, the free encyclopedia

DA lenses can be mounted on a 35mm camera, but most of them will not cover the entire 35mm frame.

Experiment with your 28 and 50mm primes, compare the field of view with the same focal length setting on your kit lens.

Finally, if you don't shoot film, all of this is irrelevant and unnecessarily confusing.
05-06-2007, 11:05 AM   #10
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Our letters crossed in the mail, it looks like.
05-06-2007, 11:13 AM   #11
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I've been so long away from 35mm that the crop factor conversions have been largely meaningless to me in using my K100D. But the other day I took out a recently acquired 35mm 1956 rangefinder with a 45mm fixed lens and caught myself doing the conversion in the other direction trying to figure out what 45mm would be on something I'm used to. (Answer: 30mm)
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