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01-11-2011, 02:27 AM   #1
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understanding lenses and cropping

Hi all.
Firstly, thanks for such a great site. I've been lurking here for quite a while, learning up.

Unfortunately (?), there is so much info here, finding what I'm after through a search can also be difficult

So, can someone explain a few things about lenses please?

I had a MZ-50 back in the day, which came with a Sigma 28-80 and a 100-300. Both are a little 'clunky' but did the job.
I got a K-x in 2009 with the DA L 18-55 lens, but kept the Sigma 100-300. I'm now looking at improving on the lenses, or at very least the zoom.

I understand the 35mm lenses are affected by image crop due to sensor size vs film size etc. Thus the 100-300mm is really something like 150-450mm (in basic explanation)?

What isn't clear to me is when I look at a DA L or DA lens, which were made for DSLR, is the "18-55" nomination still relative to a film camera, or is that now including compensation for the sensor.
Thus if I go out and get a 55-300mm, is it 300mm for a film and 450mm for digital, or effectively 200mm for film and 300mm for digital?
(does that make sense?????)

Secondly, the differences between DA and DA L???
DA is metal, DA L plastic parts, DA has this zoom lock thing etc. But is there any actual difference in the glass or mechanism etc? Is a DA going to give a better picture than a DA L?


01-11-2011, 02:58 AM   #2
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Hmm, good question there, I've been wondering about that myself... sadly I don't have a Pentax film camera to test or have more lenses to test about the cropping.

If it were possible to test a DA lens on a film body, maybe.... (I wouldn't dare though, might break some contact points on the digital-only lens >_<) hope one of the more experienced users post here too....
01-11-2011, 03:02 AM - 1 Like   #3
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Focal length is focal length and is the physics of a lens. So a 300mm is a 300mm, regardless if it's designed for film, apsc or whatever.

As you already have a Kx, forget about the crop factor.

DA an DA-L are optically the same as far as I understand it; lack of a lens hood with the DA-L however will affect image quality in a number of situations.

PS I tried to explain the crop factor in this post

Last edited by sterretje; 01-11-2011 at 03:11 AM.
01-11-2011, 03:15 AM - 1 Like   #4
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First of all, the focal length does not change. A 55mm lens is a 55mm lens is a 55mm lens. What does change though, is the field of view. That's what the "crop factor" refers to. When mounted to the K-x, the FOV of the 100-300 would be similar to what a 150-450mm lens would produce on the MZ-50. But there is no actual increase in focal length, just a reduction in the FOV. You could achieve a similar, though not accurate, effect on the MZ-50 by mounting any lens, and then placing a piece of cardboard in front of the lens with a rectangular cutout in the middle. Reduced FOV, but no extra zoom.

Differences between DA and DAL are usually in the build of the lens and not in the optics. Plastic vs. metal, quick shift vs. none, hood vs. none, etc.

You can mount a DA lens on a film camera. It will only shoot wide open (on most, but not all bodies), and many will vignette a lot.

01-11-2011, 03:25 AM   #5
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Field of view vs. crop factor... ah, thanks for that clarification! I thought it's "a bit zoomed" as some sites say. So what it means is that it actually just "looks zoomed" but not really?
01-11-2011, 04:11 AM   #6
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Precisely, it's not "a bit zoomed", but rather "a bit cropped". So the field of view changes, not the focal length.
01-11-2011, 04:32 AM   #7
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Your angle of view changes due to a smaller sensor. Basically, it is like you cropped the photo down and took the central 2/3rds of it. You won't notice so much with longer lenses, but with 28mm lens, which would be wide angle on 35mm, it won't be a particularly wide angle on APS-C, since the edges are cropped off.

Pentax uses the DA designation to mean that a lens was designed for an APS-C sized sensor. Some of them will shoot fine on a 35mm sensor, but the kit lens will have severe vignetting. The DA -L lenses are cheaper versions of the DA lenses, lacking metal mounts and lens hoods and quick shift. Otherwise, they are the same.
01-11-2011, 04:48 AM   #8
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Thanks for those explanations, that makes a lot more sense now.

As for the DAL/DA, I guess I'll happily hang on the DAL 18-55 i now have, and just look at zooms and wide angles.

Thanks all


01-11-2011, 05:16 AM   #9

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FA*200mm f/2.8 - Angle of view (on Full-Frame film body): 12.5 degrees
FA*300mm f/4.5 - Angle of view (on Full-Frame film body): 8.2 degrees

FA*200mm f/2.8 - Angle of view (on cropped-frame DSLR body): 12.5 / 1.5 = 8.3 degrees

The angle of view (also called FOV in common usage) of the 200mm lens on the cropped sensor is roughly equal to the angle of view of the 300mm lens on the full-frame body. However, the Depth Of Field, background compression (perspective distortion) and other factors are still that of a 200mm lens, of course.

Imagine you shoot both wide open - FA*200 f/2.8 on a DSLR (K-5), and FA*300 f/4.5 on a full-frame film SLR (MZ-S) from the same spot. When you print and blow up both images to the same size this is roughly what it will appear like (ignoring the differences between film and digital such as film grain vs. digital noise and so forth) in 35mm terms:

MZ-S: 300mm FOV, f/4.5 DOF, 300mm perspective distortion

K-5: 300mm FOV, ~f/3.3 DOF, 200mm perspective distortion

Even though the K-5 was shot at f/2.8, when the image is blown up to match the un-cropped MZ-S it will appear similar to having the DOF of a 300mm at around f/3.3 in practice (but not quite the f/4.0 some say it should have in theory).

Accordingly, an FA 43mm (not a 28mm lens) is still a "normal" lens (i.e. has no perspective distortion) on a Pentax DSLR - it just has a more restricted angle of view because of the cropped sensor.
01-13-2011, 04:16 AM   #10
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Crap factor is only significant if you're an experienced full-frame photographer transitioning to APS-C or other half-frame cameras. Otherwise, forget you ever heard of it. A lens is a lens is a lens. My old refrain:

Cut a picture from a magazine. Draw a 60x45mm rectangle on it. Inside that, draw a 36x24mm rectangle. Inside that, draw a 24x18mm rectangle. Inside that, draw an 18x12mm rectangle. Those are about the sizes (respectively) of 645, FF, APS-C, and m4/3 frames. THE PICTURE REMAINS THE SAME. Each smaller frame just sees a smaller slice of it.

Any lens of a specific focal length has its own characteristics, no matter what size frame its image is projected upon. Different frame sizes just cut off ('crop') different amounts of the projection. A 400mm lens mounted on an APS-C camera doesn't become a 600mm lens; it's still 400mm, with the edges chopped off. WHACK!

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