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04-15-2008, 03:40 PM   #1
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How to translate from binoculars to Pentax viewfinder/lens combo

What is the equivalent lens on my K100D to a pair of 8x25 binoculars?

Does the viewfinder magnification change the answer? whats the equivalent on the K10/K20?

how do I figure out what the view through 12x36 binoculars will look like in camera terms?

04-15-2008, 04:48 PM   #2
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QuoteOriginally posted by kmccanta Quote
What is the equivalent lens on my K100D to a pair of 8x25 binoculars?

Does the viewfinder magnification change the answer? whats the equivalent on the K10/K20?

how do I figure out what the view through 12x36 binoculars will look like in camera terms?
There are several different calculations involved here. The first is that the binocular magnifies the view by a factor of 8 (in the example given). A lens "magnifies" the view proportionally to the diagonal of the viewfinder frame, which is considered to be 1 X. The diagonal of a 35mm film frame is about 43mm, of the APS-C sensor on a Pentax dSLR, about 28.4 mm. Just to throw something else at the equation, 35mm normal lenses tend to be 50-55mm rather than 43. To get 8X magnification of the distant image, one would need a (28.4 X 8 =) 227.4 mm lens. However, if you are attempting to get an 8X view through the camera viewfinder, you will need to add in the 85% reduction, and use a lens of (227.4 / .85) = 267.5 mm.

Just to throw another wrench into things, the field of view of the binoculars may be wider than the camera lens' field of view. They usually do have a wider field of view, but for a premium, one can get an even wider field of view.

Just some musings while recovering from a migraine headache in the Rocky Mountains.
04-15-2008, 04:48 PM   #3
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QuoteOriginally posted by kmccanta Quote
What is the equivalent lens on my K100D to a pair of 8x25 binoculars?

Does the viewfinder magnification change the answer? whats the equivalent on the K10/K20?

how do I figure out what the view through 12x36 binoculars will look like in camera terms?
In theory, it is 8 * 50 / 1.5 = 270mm and 12 * 50 / 1.5 = 400mm for any APS-C camera and 90-100% viewfinder.

The second figures are apertures a in mm and by dividing f/a you get:

8x25 ~ 270mm f/11
12x36 ~ 400mm f/11

Of course, you know that a viewfinder image will never beat that of good binoculars.
04-16-2008, 11:08 AM   #4
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lets see if I got it...

So, my old SuperProgram has a 43mm diagonal "sensor" and a 0.82 magnification viewfinder
my K100D has a 28.26mm diagonal with 0.85 magnification.

A true normal lens on either would magnify by 1x on the sensor? so in the viewfinder I need slightly longer (more magnification) to compensate.

so on the SuperProgram I need (43*0.82)*(desired magnification=8x) = 282mm ??
on the K100D I need (28.3*0.85)*(desired magnification =8x) = 192mm

I guess I dont get it yet... why are we using the idea that 50mm is normal on the old camera? Is it because of the viewfinder magnification? 50mm*0.82 = 41mm

If so, then shouldn't I ignore the old 50 is normal rule of thumb and just use the diagonal* the viewfinder magnification?

Also, now two more things totally baffle me...

1) if the second number of the binocs specs is the aperture, wouldnt the binoculars have a really dim view? f/11 is really dark in the camera viewfinder (using DOF preview)...

2) where is the field of view in all this? I thought I understood how the field of view changes from the SuperProgram to the K100D... but in practice this seems to equate to magnification. Thats why the 200mm seems like a 300mm on the film camera...

ugh... I am more confused now.

p.s. sorry about the migraines.

04-16-2008, 12:46 PM   #5
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QuoteOriginally posted by kmccanta Quote
So, my old SuperProgram has a 43mm diagonal "sensor" and a 0.82 magnification viewfinder
my K100D has a 28.26mm diagonal with 0.85 magnification.

A true normal lens on either would magnify by 1x on the sensor? so in the viewfinder I need slightly longer (more magnification) to compensate.

so on the SuperProgram I need (43*0.82)*(desired magnification=8x) = 282mm ??
on the K100D I need (28.3*0.85)*(desired magnification =8x) = 192mm

I guess I dont get it yet... why are we using the idea that 50mm is normal on the old camera? Is it because of the viewfinder magnification? 50mm*0.82 = 41mm

If so, then shouldn't I ignore the old 50 is normal rule of thumb and just use the diagonal* the viewfinder magnification?

Also, now two more things totally baffle me...

1) if the second number of the binocs specs is the aperture, wouldnt the binoculars have a really dim view? f/11 is really dark in the camera viewfinder (using DOF preview)...

2) where is the field of view in all this? I thought I understood how the field of view changes from the SuperProgram to the K100D... but in practice this seems to equate to magnification. Thats why the 200mm seems like a 300mm on the film camera...

ugh... I am more confused now.

p.s. sorry about the migraines.
  1. The second number of the binocular is not an f/stop, but the diameter of the objective lens. So an 8X50 binocular has an 8X magnification with a 50mm objective. For reference, the navy specs used to be for 7X50 binoculars, which were usable in quite dim light. If you want easier to carry binoculars that are not as bright, a pair of 8X35 is about half the weight of 8X50's.
  2. Field of view equivalents are just so that those of us who have been using 35mm cameras for a few years (48 for me) can "see" the field of view in their heads. We "know" that a 200mm lens will see this much at 100 meters on a 35mm film camera, but if it is on a dSLR with an APS-C sensor, it will only see what a 300mm lens sees on the film camera.
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