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11-03-2011, 09:05 AM   #1
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Conversion of SLR lens to DSLR lens with regards to wide angle

Hey guys, obviously, I'm new here. I'm the proud owner of a K20D with your typical kit lens (18-55 f/3.5-5.8) as well as a SMC Pentax-M 50mm f/1.7.

I have a problem. Ever since I bought the 50mm, I've realized the value Pentax has in the fact that it's so backwards compatible with lenses. The 50mm never leaves my camera. However, I've now decided that I need a wide angle lens. I want to go the cheap-yet-high- quality older lens route that I did with my 50mm prime. My problem is that I'm having trouble making sense of the math.

I've heard the conversion factor for Film to a Digital APSC sensor is roughly 1.5 So while my 50mm is actually a 50mm on a film SLR, it is roughly a 75mm prime on my DSLR.

Now I'm looking for a wide angle, with the largest angle of view that I can get for under roughly $200. So I find these cheap f/2.8 28mm SLR manual focus lenses on Ebay.


So an SLR 28mm would actually be a 28mm*1.5= 42mm lens.

That means that in reality, buying a 28mm film lens is not all that wide angle? Are there any solid manual focus wide angle lenses that would be much cheaper than their autofocus alternative, or would I be better off saving up?

I really appreciate any advice I can get on this subject. Thanks!


11-03-2011, 09:45 AM   #2
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The thing to keep in mind is that it is a "crop factor" and not a "magnification factor". So the lenses look just the same as the do on film (in terms of compressing or expanding space with long or short focal lengths respectively) and they are just being cropped. As far as wide-angle, nothing above a 24mm "looks" wide-angle to me -- 28mm on a DSLR is basically the new normal FOV-wise.

Last edited by vonBaloney; 11-03-2011 at 09:57 AM.
11-03-2011, 09:47 AM - 1 Like   #3
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That's the problem using old lenses on "crop sensor" digital. Telephoto is no problem but shorter lenses are needed for wide angle. The problem is that the really short wide angles weren't so popular with film (too wide for most people), so there aren't a lot on the market. That drives up the price since they are more popular now due to the crop sensor cameras. I have a 24 f.8 Pentax (about like a 35 on film), and you find some 20 f4 Pentax manual lenses, but they are rare and expensive.
New lenses in 15 or 21mm length are ideal, but tend to be abour 3x your price target.
11-03-2011, 09:59 AM   #4
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The kit lens is much better at 18 than it is at 55, so there's that.

11-03-2011, 10:04 AM - 1 Like   #5
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Your math is right, for APC sensors (such as your K20D) you've got the multiplier. You already have a wide-angle lens (18-55), either you want a better one like a prime 15mm, or you want even wider (~10) which are actually super wide lens. I don't think there are old lenses in those focal distances, so as I see you have a few options:
- Get a used lens with those distances - those are newer models and more expensive than 10+ year glass, but cheaper than brand new ones. This forum has great lens database to help decide which one, and a great marketplace as well
- Use panorama software to stitch up your photos to create a large angle one. It's an extra step, but on the other hand you don't get distortions that wide angle lenses have. And it's the cheapest alternative.
- Get new glass - Expensive options, but if you can't find what your looking for in the used market, this could be your destination.

Hope this helps
11-03-2011, 10:39 AM   #6
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please excuse this rant

THERE IS NO SUCH THING AS CROP FACTOR

As the title suggests please excuse my shouting the above, but also please take note.

Lenses are defined by 2 things and 2 things only.

Focal length, and aperture.

Focal length is fundamentally the magnification of the lens. For a simple lens it is the distance from the focal plane for an infinitely distant subject. For complex optics like a camera lens with many elements it is the magnification of the lens when compared to a simple lens of the same focal length.

Aperture is the ratio of focal length divided by diameter.

Nowhere in these two definitions is there anything ralated to the size of the sensor/image capturing mechanism.

"crop factor" was a way to define the field of view of the lens relitive to a 35mm film camera, in order to make the transition from film easier.

For the OP who has never shot film, forget you ever heard the term crop factor, as you don't need to know anything about it.

The 50mm MF prime you bought has the same field of view as the kit lens when set to 50mm because focal length is focal length and that is all there is to it.

your kit lens at 18mm is relitively wide, but getting old primes of that focal length can be expensive, almost as expensive as new AF primes. the price break starts at about 24mm. Below 24mm things get ex*****ve in a hurry, at 24mm and above, you can get legacy lenses and third party lenses for less than $50 that are good performers.
11-03-2011, 10:46 AM   #7
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Thanks for the quick responses! That cleared 100% of the questions up that I had. I'm not too fond of my 18-55, but it definitely helps that I'll mainly be using it as a wide angle from now on. Also, it's good to know that older wide angle film lenses aren't so much an option, so I can stop wasting my time looking for those. I'll probably just start saving up for a higher quality but used wide angle lens, and the 18-55 will have to do until then. Thanks!
11-03-2011, 10:47 AM   #8
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QuoteQuote:

That means that in reality, buying a 28mm film lens is not all that wide angle?
That is true. Crop factor is term many people (and manufacturer's) use while some can't get over it. If you never had a a 35mm film camera, it will make less sense, but it's not a difficult concept in itself. The various sizes of sensors in cameras make the same lens "appear" to have a different focal length. In reality the focal length is always the same - but the angle of view changes.

11-03-2011, 10:49 AM   #9
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QuoteOriginally posted by Lowell Goudge Quote
THERE IS NO SUCH THING AS CROP FACTOR
Yes there is, and it is just what you said -- relative to 35mm. The 35mm part is often left unsaid though.

It is funny though -- I think the lens properties of expanding or compressing space (what's the word for that?) is much more important to their "look" than their FOV (and in that sense something around a 50mm is the still the most "normal"), but no one ever talks about that in terms of the complications that the crop factor brings up when comparing to film. (You might find you now like a 28-35mm as your "normal" lens, but you're also getting that space expansion which no one seems to mention.)
11-03-2011, 10:56 AM   #10
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Good information above. The terms GOOD CHEAP OLD WIDEANGLE are mutually exclusive. In the days when 135/FF rules, 24mm was ultrawide and anything wider was exotic and pretty rare and costly. Now, the DA18-55 is the best cheapest widest lens available. To go wider, the lowest-cost option is the manual Zenitar 16/2.8 in PK-M mount, slightly fishy on our dSLRs. Next are newer AF UWAs (ultra wide angles): the Tamron 10-24; either Sigma 10-20; the DA10-17 fisheye (FE); the Sigma 8-16; the DA12-24. Those are roughly in ascending order of cost.

No, 28mm is not wide angle on APS-C; it's precisely normal on my K20D. APS-C sensors vary slightly in size, so on some cameras, 30-31mm are 'normal', the diagonal of the sensor. 24mm starts to look wide, with AOV/FOV (angle/field of view) equivalent to 36mm on film. Even 24s are fairly uncommon and costly, unlike 28s which are common as dirt and about as cheap. My favorite there is the Vivitar-Kiron 24/2 in PK-M mount, now probably under US$150.

A bit wider is the Tokina-made 21/2.8 in M42 or other mounts, variously badged as Vivitar, Soligor, Lentar, whatever, now probably a bit under US$100. Its FOV-equivalent is ~31mm. Various other Japanese- and Russian-made lenses around 20mm exist, mostly in M42 screwmount, with names like Vivitar and Mir. Search eBay for (21mm,20mm,19mm) LENS -Canon -Minolta -Sony -hood -chart* -reflect* -ring -case -bag and see what pops up. Be sure to sit down before reading the prices. EDIT: Search string fixed.

Again, the crap.factor (I prefer format.faktor) means a smaller frame (film or digital) sees less of what a lens projects. Take a 135/FF frame, chop off the edges, and you've got a 135/HF or APS-C frame. Moving a lens to a different doesn't change DOF, proportion, any of that, but our USAGE will change those qualities because we're likely to position ourselves differently.
_______________________________________

HOW TO GO WIDE:
  1. Spend money; look for a wide old manual prime. Or,
  2. Spend more money; look for a new AF UWA or FE, something that at least covers 16mm. Or,
  3. Cheat; shoot vertical (portrait mode) 'scape shots with a 28mm prime and stitch them together into undistorted panos. Or,
  4. Buy nothing; just use your DA18-55 at its widest, and fix its shortcomings in PP.

Last edited by RioRico; 11-03-2011 at 03:04 PM.
11-03-2011, 10:57 AM   #11
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The kit lens is a good focal length simulator. For lengths that aren't marked on it, the camera knows what focal length it's set to. You can analyze the EXIF data from previous shots to see if you favor a particular length.

For some reason, 28mm on APS-C starts to look wide to me. That saves me some money on wide angle. I am pretty happy with a Sigma Super-Wide II 24mm f2.8 manual focus, or sometimes a Kiron 24mm f2 for 24mm, a Pentax-F 28mm f2.8 at 28mm, and a Pentax DA 16-45mm f4 to fill in everywhere else. You could maybe get the Sigma or Kiron 24mm and a Pentax-A 28mm f2.8 both for $200 total. The zoom is about $230 used. The zoom is better than the kit lens and the cheapest/best way to get to 16mm.

There are other old lenses wider than 28mm, but I haven't tried many of them. I think the kit lens is going to be hard to beat when you consider IQ, price and features. With some cameras or software you can even correct some of its flaws.

If I had some extra cash, I might get the DA 15/4 and DA 21/3.2 just to see if I like them.
11-03-2011, 11:00 AM   #12
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for a WA field of view you are better off with a new lens. the first lens I bought back when the ds was released was the DA14 because i knew i had nothing that would be properly WA on my digital.
As mentioned though there is plenty of legacy glass that gives excellent performance at the wide normal to tele FOV for very little cash. Try an M28 3.5 or 2.8 (the 3.5 will cost a little more but is a sharper lens) or any one of the numerous Taks or Ks
My 28 3.5 is amongst my favourite lenses and cost about $14, and most of the time i prefer using it to the m50 1.7 (which i also like a lot, but prefer my 55 1.8 tak and 50 1.4 tak)
The one benefit is your reach seems greater on the tele's so a 200 f4.0 you pick up for $50-100 will act like a 300 4.0 would have on film. when film was the only game in town that 300 would have cost you an arm and a leg
So you lose at one end but gain on the other
11-03-2011, 11:05 AM   #13
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Also if you don't want to spend as much as the AF WA lenses are (zoom or prime) there is the Rokinon 14 2.8 for less than 400. I've heard good things about it but never looked at it since I had the DA
If it is as good as the others (85 1.4, 35 1.4, 8mm 3.5 then it's worth a look (heck so is the 8 if you want that fishy look )
11-03-2011, 11:08 AM   #14
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An oddball option -- an old Nikkor non-ai lens. I actually picked up a old 24/2.8 Nikkor-N lens a couple of days ago for $60 (expect $80+ on ebay) -- at just the right rotation it does mount on the Pentax, but doesn't click in. It won't work on all Pentax bodies [i.e. film bodies], but does work on my K-7 & K10D just barely -- also got a Nikkor-S.C 50/1.4 and that fits easier. (See the threads about Nikon lenses on Pentax.)
11-03-2011, 12:20 PM   #15
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QuoteOriginally posted by vonBaloney Quote
An oddball option -- an old Nikkor non-ai lens. I actually picked up a old 24/2.8 Nikkor-N lens a couple of days ago for $60 (expect $80+ on ebay) -- at just the right rotation it does mount on the Pentax, but doesn't click in. It won't work on all Pentax bodies [i.e. film bodies], but does work on my K-7 & K10D just barely -- also got a Nikkor-S.C 50/1.4 and that fits easier. (See the threads about Nikon lenses on Pentax.)
I'll recapitulate some of that: Notice when I presented an eBay search string above, I didn't have it exclude Nikon, Olympus, Yashica. NI, OM and C/Y lenses (and Exakta mount) are readily adapted to PK. The Nikon pre-AI don't *need* modification, but can still stand a simple cut which I'll describe below (see Lock-On).

* Nikon: pre-AI fits right on. Some sort, I don't know the name, has a rim around the aperture ring. Unscrew the base and add 1mm spacers around the screws, then reassemble. AI lenses have aperture-ring protrusions I call "nub and arc" that can be cut off with a dremel. All these sorts will then press-fit in a PK mount.

* Olympus OM and Contax-Yashica C/Y: The lens bayonet flags are slightly too thick to fit under the Pentax mount lugs. The solution is to use a dremel to taper those flags slightly.

* Exakta mount: These have a bayonet about 38mm across, and an index screw, and sometime a hanging stop-down button. If it has that button, forget it. Else remove (and save!) the index screw, and glue an ultra-cheap M39-M42 adapter ring around it. While that's drying, grind off (using 100-grit sandpaper taped to a ceramic tile) about 0.5mm from the wide side of a cheap close M42-PK adapter. Assembled, the lens will focus to infinity.

* Lock-on: None of the above locks tightly into the PK mount. I use a dremel to cut a slot about 1mm+ wide and deep and 5-6mm long for the Pentax locking pin to engage. That works with NI, OM and CY. Many Exakta-mount lens bases are too narrow for this. Just twist them down and hope. I haven't lost any yet!

I have seen OM and Nikon mount 24's go cheap on eBay lately, like under US$20. Forget about Canon, Sony, Minolta, Konica, Leica, X-Fujinon, Praktica-Pentacon bayonet, other mounts; they aren't readily adaptable. NI, OM, CY, EX are IT!
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