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Old Apr 17, 2007, 2:46 PM   #1
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Everyone knows a FA-50mm lens is mult by 1.5 to give a focal length of 75mm

Question, for DA lenses say the 18-55, is this already multiplied by 1.5, or is it given in the same spec as the earlier FA, F, A, K, M -series lenses?

If the 50mm has an effective focal length of 75mm... equivalent to what?

Real noob question but I just realized my confusion.
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Old Apr 17, 2007, 3:38 PM   #2
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Hi Gazander,

All DSLR lenses show their actual respective focal lengths. In other words, focal length is focal length. The reference to 35mm equivalent is something that the makers have adopted to give experienced 35mm shooters some perspective on the Field of View that they should expect considering the cropping action of the smaller sensor compared to the 35mm film frame.

The fact that DA lenses are built for the smaller than 35mm frame (the Pentax sensors are about 24x16mm as opposed to about 36x24mm) does nothing to the size of the subject projected on the sensor, but they are somewhat smaller, lighter, and are optically optimized for use with a digital body. The 1.5x "crop factor" is due to the fact that the Pentax DSLR sensor is about 2/3 the size (in each dimension) of the "full frame" 35mm film frame, thus you would have to multiply the focal length by about 1.5 to get the focal length of a lens that would have an equivalent Field of View on a 35mm camera. The fact that the DAs might not be appropriate for use with 36x24mm sensors or film refers to the use of a smaller image circle, so you might get vignetting (dark corners) .

While the subject might appear larger on a crop factored camera, it's only because you've cut out a lot of the background, so it's larger relative to the frame, but it's actually no larger than it would be on a 36x24mm sensored or film frame at the point where it focuses at the back of the lightbox in the camera body.

The reason why this continuously comes up is because it's hard to visualize and explain. I hope I haven't added to the confusion.

Scott
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Old Apr 17, 2007, 4:29 PM   #3
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Scott, That was a good explanation and thanks for the time it took to write it.

Glenn
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Old Apr 17, 2007, 4:39 PM   #4
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Scott

Nicely put, the 1.5 x multiple gets eveyone confused and I am glad this forum knows the difference. There is a difference between focal length and FOV.

I hate seeing adverts for "50-200mm lens for sale" and on a DSLR thats equiv to 75-300mm blah blah blah" Totally misleading. :angry:

Crash
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Old Apr 17, 2007, 4:56 PM   #5
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Crash, absolutely right. The focal length has nothing to do withwhat is behind the lens. It is just for those of us who were used to film so we would have anidea what to expect when we "developed" our photos.

Glenn
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Old Apr 17, 2007, 5:21 PM   #6
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right, there is NO magnification, 0,nil,zip. it's a crop factor not a magnification of the image.
a good example is that if you are using a 100mm lens then the SUBJECT will be the same size at the film/sensor plane no matter if you are using a digital, a 35mm, a 6x7, or even a large 8''x10'' view camera. wow, talk about a crop factor from a 8x10'' view camera. would probably at least a 100x instead of 1.5x from a 35mm sized camera. what scott is saying is that thesubject stays the same size. with the larger format cameras, all you add is more background surrounding the subject.

i've got a 1:1 macro. is the image i get life size or times 1.5 because it's a digital. it's lifesized on the digital the same as on a 35mm but it doesn't have as much background surrounding it.

roy
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Old Apr 17, 2007, 5:22 PM   #7
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YES!!
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Old Apr 17, 2007, 7:02 PM   #8
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just so i understand it,

magnification remains the same but fov changes...?

if 3:2 aspect ratio is maintained, and you print a 35mm and dslr pic, both with the same 50mm lens.... won't the DSLR pic appear more zoomed in?

As for DA vs FA, i think a "standard" lens for 35mm camera is about 43mm, i define this as being able to look, one eye open and one eye in the viewfinder and not notice an appreciable difference in FOV between each eye.

I know the 50mm is NOT like that, not even close on my DL. Since the 40mm is close to the ideal 43mm, will it match the human FOV or will it be out of whack like the 50mm?

By the crop factor the lens to get the "ideal" 43mm is a 28mm lens or thereabout.

Now i just went and confused myself again.
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Old Apr 17, 2007, 7:04 PM   #9
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tom,
thought i'd jump in before you did. i think this thread is one of the most comprehensive that explains it that i've seen.

THERE IS no MAGNIFICATION factor. it's exactly the same size no matter what format you are using. how about one of our math specialists figure out the crop factor of a 8x10'' view camera????

roy
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Old Apr 17, 2007, 7:25 PM   #10
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robar wrote:
Quote:
tom,
thought i'd jump in before you did. i think this thread is one of the most comprehensive that explains it that i've seen.

THERE IS no MAGNIFICATION factor. it's exactly the same size no matter what format you are using. how about one of our math specialists figure out the crop factor of a 8x10'' view camera????

roy
Roughly speaking, a 10"x8" view camera would be a -7 crop horizontly and -8.5 vertically.

So a crop factor of around -7.5x


I think!


Darren
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