[chucuyuco]

here goes anyway! So I'm looking at adding a lens to upgrade from my 18-55 kti. I want a diverse lens that doesn't break the bank and delivers well. I like what I hear about the Tamron 28-75 (though I wish it went a little wider). I read somewhere that it's made for full frame sensor cameras? Does that mean it's not a 28-75 on my K-x? I'm not referring to what would it be on an old 35mm but rather is it different on full frame vs. aps-c?

Thanks!

[drei3]

It's going to be 28-75mm on your K-x. It's the same on FF and APS-C. The image will just be cropped compared to FF.

I suggest the 17-50, I believe it's the same price.

[Adam]

The only dumb thing about your question was the thread title, so I changed it

It's still a 28-75mm (focal length never changes), but the image seen by the K-x will be zoomed in by about 1.5x compared to the same lens on a full-frame camera. This crop factor essentially means that, for instance, a 28mm on APS-C would deliver the same field of view as a 42mm on full-frame.

[chucuyuco]

Thanks Adam. So, does that mean that a 28mm shot on my current 18-55 would be the same as a 28mm shot on the 28-75?

Drei3, why the 17-50? Is it just where I want preference? on the wide or zoom end? I'm thinking I will be using it for portraits too and I tend to like a little more length for that.

[Mareket]

Yeah, 28mm on the kit lens will be the same as on the 28-75. Well, it might be ever so slightly different but not anything hugely noticeable.

[Lowell Goudge]

Thanks Adam. So, does that mean that a 28mm shot on my current 18-55 would be the same as a 28mm shot on the 28-75?

yes focal length is focal length

Drei3, why the 17-50? Is it just where I want preference? on the wide or zoom end? I'm thinking I will be using it for portraits too and I tend to like a little more length for that.

the 28-75 is good for portraits, as it is a little longer, as you say.

[chucuyuco]

Ok, I think I'm going to do it! I just had my first paid gig- shooting local murals to be displayed in a real estate office, shot with my kit and the 35/2.4 and I'm going to use the money for this new lens in hopes that it brings me some more bacon!

[RioRico]

Whatever you've heard about equivalents and crap.factors, forget it. A lens' focal length remains the same no matter where it is. Lenses do not stretch nor shrink when placed on different cameras. The frames (film or digital) of different formats see different amounts of the project image, that's all. If you're not a long-time 135/FF shooter who's transitioning to APS-C cameras, forget all about it.

My favorite example: Cut a picture from a magazine. Draw a 60x45mm rectangle on it. Draw a 36x24mm rectangle inside that. Draw a 24x18mm rectangle inside that. Those are about the sizes of 645, 135/FF, and APS-C frames. Each rectangle sees a different portion of the picture, but the picture hasn't changed.
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Here's how format equivalents really work: The 'normal' focal length of a format is its frame's diagonal. For 135/FF that's 43mm. For our APS-C cameras it's around 28-30mm depending on the exact sensor size; for my K20D it's 28mm. That ratio is 43/28 or about 1.5. That's the crap.factor.

The AOV (angle of view) of a lens is measured on the frame diagonal. A certain lens on APS-C will have the same AOV as a lens of about 1.5x on FF. So a 50mm lens on APS-C will see the same portion of what's in front of you as will a 75mm lens on FF. Other characteristics such as DOF (depth of field) and perspective will differ, so we can only really talk about AOV equivalence.

Back to kit lenses: An 18-55mm lens on APS-C sees about the same AOV as a 28-80mm zoom on FF. It's pretty close: 18x1.5= 27 and 55x1.5= 82.5. If you use a 28-80mm lens on APS-C, its AOV is about the same as a 40-120mm on FF. Are you confused yet? So am I. But that's how the math works.

As I said, these numbers don't mean much unless you're familiar with lenses on FF cameras. Otherwise, mox nix (it doesn't matter). Just learn what various lenses do on the camera that you have.
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And to answer your lens queries: The Tamron 28-80 is superb but maybe too long for your needs. The Tamron 17-50 is highly regarded as a kit.lens replacement. But you can get very good results with your DA18-55 with simple technique:

1) Use a tripod.
2) Use shutter delay.
3) Stop-down the lens to f/8.
4) Shoot between 21mm and 50mm.

A decent tripod usually costs rather less than a superb lens. Cheers!

[chucuyuco]

Thanks, good explanation Rio. I think you misunderstood my needs, I was able to get good results from the DA18-55 for the mural job, I was just saying I wanted to use the money from that job for a new lens (though the new lens won't be for architecture/landscape type stuff, more general + portraits). My $20 tripod does the trick for now but I can tell it's pretty much a piece of garbage!

[RioRico]

Thanks, good explanation Rio. I think you misunderstood my needs, I was able to get good results from the DA18-55 for the mural job, I was just saying I wanted to use the money from that job for a new lens (though the new lens won't be for architecture/landscape type stuff, more general + portraits).

Ah, sorry if I didn't read closely enough. Then the 28-75 should be splendid for you! In a slightly narrower range but a real deal, you might look at the F35-70. It's the smallest zoom Pentax ever made, agile (VERY fast AF), sharp as a bag of primes, and generally available for about US$50, less if sold with an attached AF film body like the SF-1. But the Tamron 28-75/2.8 is indeed a whiz-bang. Have fun!