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03-13-2008, 07:51 PM   #1
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Lenses and DSLR

ok.. so the pentax lenses at pentax.com do I need to multiply them all by 1.5x to get my focal length? How can one truely get a wide angle digital lens?

Taken from pentax.com...
" The image circle in DA-series lenses is designed to perfectly match the 23.5mm x 15.7mm size of the CCD used in PENTAX digital SLRs to optimize camera performance." does that mean the DA 10 - 17mm is actually a 10-17 mm lens on the DSLR? I am confused...



03-13-2008, 08:04 PM   #2
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The 1.5x factor is used only if you want to get the 35mm equivalent of DA lens or if you want to know the actual focal length of non-DA lens when it is mounted on DSLR. So you are right, 10-17 DA lens mounted on DLSR is just that. But if you want to know how that relates to 35mm (that we are all familiar with), then multiply it by 1.5x.

In other words 10-17 on DSLR has pretty much same coverage as 15-26mm on SLR.
03-13-2008, 08:10 PM   #3
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It's often discussed! In my case, I cheated - I searched for "focal length" and any posts by my user name, which made it easier to find. You'd have to be more creative... I'd suggest trying the advanced search if you're willing...

Some answers here:

https://www.pentaxforums.com/forums/pentax-dslr-discussion/19110-lens-conversion-dslr.html

Bottom line: focal length is focal length - nothing but a teleconverter can increase it's magnification (focal length). Magnification requires optics - the size of the sensor used will not change the focal length of a lens. It's primarily a reference to FOV (field of view) or perspective. A 200mm lens on a DSLR (with a 1.5x crop factor) does not give that lens a focal length/resolving power of a 300mm lens. It does give the 200mm lens the effective FOV of a 300mm lens.

Hope that helps?

Cheers,
Marc
03-13-2008, 08:13 PM   #4
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The 10-17mm DA lens is still a 10-17mm perspective whether the projected image is 'shining' onto a digital sensor or film... its just that on our digital sensor with the 1.5x crop factor it has the field of view [FOV], or "coverage angle", of a 15-26mm on a film SLR.

This was the perfect "short & sweet" picture that explained it to me: Focal Length Multiplier: Optical: Glossary: Learn: Digital Photography Review

03-14-2008, 09:37 AM   #5
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so confusing.. so you don't actually get a more "telescopic" view persay? When I look through a 300mm film lens is on a digital body it should "look" like I am closer by 1.5 right? So a 300mm lense will look like a 450mm lense thru the viewfinder, or am I wrong. I thought some of the new digital lenses didn't have "conversion factor" so to speak
03-14-2008, 10:56 AM   #6
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Yes, it is sort of confusing.

If you put older (or any) 200mm lens on DSLR then it actually will be the same as DA 50-200mm (at the long end). But both lenses will act as 300mm lenses when compared to 35mm format. In which case they will cover same view as 300mm lens mounted on SLR (non-digital).

So to make the thing less confusing:
DA 200mm on DLSR = 300mm on SLR

Hope this helps.
03-14-2008, 10:59 AM   #7
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How to make it all clear

If you're a visual learner, try this exercise (with real pencils and paper and a ruler). It may seem a bit silly but I guarantee that if you do this you won't be confused.

Take a piece of paper and put it on the table in front of you with a short edge facing you (the normal letter orientation).

With a ruler, draw a line near the bottom of the page and parallel to the bottom edge, centered from side-to-side and precisely 36mm across. This represents the film in a 35mm camera, or else a "full frame" dSLR sensor.

Measure up from this line exactly 50mm and mark a small dot, again centered side-to-side. This represents the idealized aperture of a f/infinity lens focused at infinity good enough for the illustration.

Now, draw two lines, one from each edge (end point) of your initial line exactly through that dot and extending all the way up to the top of the page. This represents the field of view of a 50mm lens on a film or full-frame dSLR. (Not just metaphorically, but actually just like that.) You're drawing the edges of where light from the outside coming through the lenses hits the sensor anything wider than that is "off film". Anything inside that cone will be captured in your image.

Do the same thing to represent a 75mm telephoto lens using a dot 75mm from your "sensor" line (smaller field of view, clearly...), and one at 25mm to represent wide angle. Now you've got the three different cones for a normal lens, a wide angle lens, and a telephoto lens.

Now, using a different color pen or pencil if you can, mark a 24mm line right on top of your initial line, again centered side-to-side. (Maybe just mark where the endpoints would be.) This represents the typical sensor size used in a 1.5 crop camera (like Pentax, Sony, or Nikon).

Then, draw lines through your existing 50mm, 25mm, and 75mm "lens" dots this time connecting to the new, smaller sensor. See what that does to the field of view of the lens?

All other optical properties of the lenses remain the same, but the field of view is reduced because you're only capturing the middle part. A 50mm lens is still a 50mm lens, but you're capturing only the middle part of the output so it happens to line up with the same field of view one would get from a 75mm lens on a full-frame camera.

[This is essentially a repost of something I wrote somewhere else.]
03-14-2008, 11:06 AM   #8
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QuoteOriginally posted by madisonphotogrl Quote
so confusing.. so you don't actually get a more "telescopic" view persay? When I look through a 300mm film lens is on a digital body it should "look" like I am closer by 1.5 right? So a 300mm lense will look like a 450mm lense thru the viewfinder, or am I wrong. I thought some of the new digital lenses didn't have "conversion factor" so to speak
No. The "sizes" of the foreground OOF area, subject, and background OOF area, will be the same. You just see less of the frame you would see through a SLR viewfinder, in the DSLR viewfinder (and respectively projected onto the film or digital sensor) ... giving you the FOV of a lens 1.5x longer.

Put another way, the crop factor does not change optics, just how much of the view circle through the lens is projected onto a medium that can get exposed and capture and image.

03-14-2008, 11:11 AM   #9
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Hi madisonphotogrl

As Marc said previously:

QuoteQuote:
The size of the sensor used will not change the focal length of a lens.
The concept you need to grasp is that the focal length of any lens remains constant. Period ! Once you accept this fact, the rest should become slightly easier to comprehend. Focal Length and Field of View are two entirely different animals.
To quote Marc yet again:
QuoteQuote:
A 200mm lens on a DSLR (with a 1.5x crop factor) does not give that lens a focal length/resolving power of a 300mm lens. It does give the 200mm lens the effective FOV of a 300mm lens.
Note that it is the effective Field of View (FOV) that changes, NOT the focal length of the lens, which stays the same regardless.
If for example you owned a full-frame (36 x 24mm) DSLR such as one of C***n's 1DS Mk 11's/C***n 5D's or N***n D3's, the confusion you are experiencing would magically disappear ! That is simply because with such cameras, the physical size of their digital full-frame (36 x 24mm - fractionally less in real world terms) sensors are identical to that of an individual frame of 35mm film emulsion (36 x 24mm). The dimensions of the APS-C (Advanced Photo System-*C-type (*Classic)) sensor in the K10D is approx 24 x 16mm. Do you notice a correlation here ? 24 x 16 is precisely two-thirds the size of 36 x 24 ! Hence APS-C sensors are often described as ⅔rds or 2/3rds sensors. So lenses designed to function correctly with the older 35mm film cameras have no problem working properly with full-frame DSLRs (there are certain limitations - i.e. manual operation etc). However, the inconvenient fact is that both C***n & N***n decided to abandon their existing film lens bayonet mounts when they first designed their range of new digital SLR cameras. So as a consequence, most older C***n & N***n lenses don't function with the new digital bodies.....ouch !!!!
Fortunately Pentax had the intelligence & foresight to retain their K-mount lens-bayonet when they brought their digital DSLR cameras to the marketplace, ensuring backwards compatibility for existing film users. But sadly Pentax don't yet produce a full-frame DSLR, which is where the 1.5 x crop-factor rears it's ugly head again. So, if you attach a K-mount lens to the K10D/K20D that was originally designed to work correctly with a Pentax 35mm film camera, something has to give and that is the effective Field of View, or 1.5 x crop factor. Hope this lengthy diatribe helps to clarify matters for you a little......

Best regards
Richard

Last edited by Confused; 03-14-2008 at 03:38 PM.
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