understanding lens on a Pentax DSLR

February 6, 2008 7:45 PM

Lens 101 for new Pentax K100d DSLR owner.

I was lucky enough to get a new camera for Christmas, and it replaces point and shoot compacts.
I need to understand a bit about lenses for it, and hope askme can teach me.
My last SLR was before eBay existed, and the listings there confuse me.
Basically, I would like a wide, fast lens for portraits with big depth of field.
My camera came with a f3.5-5.6 18-50mm, and it is nice, but could use more depth of field.
So, how to get this inexpensively?
I would be happy with an old manual focus lens, but I gather the smaller sensor in the DSLR means a film lens is not as "wide" as it would be with a film camera. Does this also impact the aperature?
Are f1.4 or f2.0 all going to cost the earth?
Also, can you help me decode eBay listings to understand which mountings are compatible?
I saw this but could use a bit more clarity.

posted by bystander to sports, hobbies, & recreation (13 comments total)

Let me see if I can take on your questions here.

Your lens is an okay starter lens, but kit lenses are generally not so good (Canon's is awful). If you want more depth of field, you need good light - that is, if you want more things in focus. Your aperture should not go below, say, 5.6 if you want better depth of field; the wider the aperture (smaller number is wider) the more narrow your plane of focus is and the blurrier your background will be.

Go to a used camera shop and ask around, Pentax has the benefit of fitting every lens they've ever made onto their current cameras, so you can just pick a good one in decent condition (be prepared to spend at least $150) that opens a little wider if possible. Any lens will have a "cropped field of view" which multiplies its millimeters by some factor, I think in your case about 1.3x. It doesn't affect the light you get, and in fact reduces vignetting (the darker areas in the corners of some pictures).

Alternatively, you can get a nicer, faster prime lens - 1.4 will be expensive, but I have a (Canon) 50mm 1.8 lens that costs $75. There's surely an equivalent in the Pentax world, again go to the camera store and tell the most geeky-looking camera guy there what you're looking to spend and what you need (a good prime/fixed lens and a decent zoom is all you really need). The K100d is a great camera and you should invest a little in a good lens or two. Bring your camera to the store to find out exactly how lenses feel and look.

Hope this was helpful!
posted by BlackLeotardFront at 8:34 PM on February 6

I'm guessing you're actually looking for less depth of field if you're asking about a faster lens. Short of a pinhole camera, you're not going to get more depth of field than F/22 or F/32 or whatever your lens goes to.

Less depth of field will allow you to blur out the foreground and background, and is the hallmark of pro (typically expensive) lenses that are "fast."

Fast prime (non-zoom) lenses are much less expensive (and typically higher quality) than fast zoom lenses, so I'd look for a prime lens. A fast (F/1.8 or so) 50mm lens (which would be like a 75mm on your DSLR) should be about $100 or even less. Fast wide angle and telephoto lenses will be more expensive - in the $350+ range. Fast zoom telephoto is the most expensive. A 70-300mm F/2.8 is about $1000 in the Nikkor line. Not entirely sure about Pentax prices, though.
posted by mamessner at 8:57 PM on February 6

The smaller sensor on the DSLR only affects the field of view, not the minimum f-stop. I think you have things backwards though -- lower f-stop (like you're talking about) == larger aperture == less depth of field, which is not what you seem to want. The lens you currently have should be able to achieve plenty of DOF.

There's plenty of good vintage Pentax glass out there - the SMC Takumar series from the 70s and 80s is pretty excellent, although they're all manual focus. I believe you will need an adaptor that Pentax sells to convert the screw-mount lenses to modern Pentax mount, however.
posted by neckro23 at 9:30 PM on February 6

mamessner has it right. A 50mm, f2 or faster would make a pretty ok portrait lens. I'm not sure what's commonly available in pentax lenses over 50mm. I had a 135mm prime, K mount for a while. It would be very long for your camera, but they are pretty cheap.

28mm would be a great great walk around lens for this camera. Find the fastest one you can. I think they are commonly available around f3.5, see if you can get something faster though.

Pentax lenses are really nice, and generally pretty cheap.
posted by sully75 at 9:33 PM on February 6

The Photo.Net Pentax Digital SLR Cameras and Lenses page is a pretty good overview of what's available, along with some rough guidelines.

Personally, I'm lusting for this this Pentax SMCP-FA 50mm f/1.4 Lens, under $200. Go for it, and let us know how it works!
posted by IvyMike at 9:57 PM on February 6

DaShiv on his experience with the Pentax system.
posted by heeeraldo at 10:38 PM on February 6

Sorry, my mistake. I do want less depth of field (blurry background). Its been a long while since I had a camera that did anything but point and shoot.
All helpful answers, and IvyMike gets some extra thanks for the specific recommendation.
Any other advice also welcome.
posted by bystander at 2:06 AM on February 7

Blurry backgrounds come with lower f-stops but for true separation say in a portraits, it helps to have a bit of a longer lens (50mm+ at least) along with the fast aperature.

Ok some more links:
First stop would be to lurk on dpreview forums for a while, or do searches on different lenses that you see mentioned.

There are also Manual Focus Forum and
m42 lenses (need cheap adaptor's that you can pick up on ebay)
posted by stratastar at 6:16 AM on February 7

that pentax 50mm f/1.4 lens is AMAZING. combine the speed of f1.4 with the low-noise-at-high-iso k100d and you have a wonderful match for parties and other low-light photography.

i am also a big fan of the tamron 18-200mm lens. great for travel photography and events.
posted by kneelconqueso at 10:51 AM on February 7

Listen to IvyMike and get the FA 50mm f/1.4. I highly, highly recommend it.

I have a K100D and the 50mm used to be my walkaround prime. It has very narrow depth of field, beautiful dreamy bokeh, and is extremely sharp. Take a look at this cabeza taco I took a picture of. I swear that the taco wasn't nearly this tasty-looking in real life - it's that good! The bokeh and dreamy look work extremely well for portraits. Here's one from our company Christmas party.

If there's any criticism of the lens, the autofocus can be a bit slow at times and it doesn't do as well wide open (ie. landscapes, wide angle shots). But at $200, it's hands-down the best deal in Pentax glass.

On the K100D, the 50mm becomes 75mm which is just about the perfect portrait length.
posted by junesix at 11:03 AM on February 7

Alternatively, if you'd like to have something a bit more versatile that compliments your kit lens and is cheap, I recommend looking at the other "kit" lens, the Pentax 50-200mm f/4-5.6. It's a compact telephoto that has great color rendition and sharpness. If you shoot at the long end, you can still get some great portrait shots with it with nice bokeh. Price is also ~$200.

Since upgrading to Limited lenses, I've sold the kit lens but kept the 50-200mm because it's such a great zoom and right now there's little that performs better in the tele end without spending huge chunks of cash.
posted by junesix at 11:17 AM on February 7

There are two mounts on eBay that are commonly called a Pentax mount. The first is the one that your (and my) K100D uses, the K mount. The other one is the M42 Universal mount. You can buy an adapter to use an M42 mount lens on your K100D, but be sure to to some research before you buy. Some third-party M42 to K mount adapters will not allow focusing to infinity.

The crop factor on the K100D is 1.5x. On a 35mm film camera, when you look through a 50mm lens, your field of view is about the same as it would be if you weren't looking through the camera at all. If you take the same lens, and put it on your K100D, the lens makes things look bigger by 1.5 times. This means that less of the scene will fit in the circle of the lens, a reduced field of view. The same view you would see if you put a 75mm lens on a film camera. If you want the same "normal" field of view for the K100D, you need to use about a 31mm lens. A 28mm lens is the easiest lens to find in the normal range for your camera.

The crop factor doesn't affect the aperture. The aperture number is a ratio of the diameter of the hole light passes though to the length of the lens, so it stays the same.

I also started with a 18-50 f3.5-5.6 lens, a Sigma. The second lens I bought was a 80-200 f4 manual zoom. There are a few things you have to do to use a manual lens on a K100D. You can find them in this post on www.dpreview.com.

There are a few places online that you can buy used Pentax equipment besides eBay. I've bought from www.keh.com, and been very happy with them. B&H and Adorama have pretty good reputations, though I have never bought from them. In any case, it is a good idea to check eBay prices with these stores. I've seen a lot of lenses go for more on eBay than they would go for on an online store.

There's a lot of information about lenses on the internet. I would definitely do some research before bidding on a lens. The 80-200 lens I bought is a Sears brand. A little checking let me know that it was manufactured by Tokina, and is a decent lens. I paid $30 for it including shipping, and have been happy with it.

If you have questions on Pentax equipment, www.pentaxforums.com is a good place to go. A google site search on the Pentax-Discuss Mail List Archives can also lead to some good information.
posted by Quonab at 1:10 PM on February 7

Since you mentioned you don't mind manual focusing, I think you should keep an eye on the Marketplace at pentaxforums.com. M 50mm f/1.7 and A 50mm f/1.7 lenses are frequently sold for $60 or less direct from other owners. The difference between the two is that in the A version, you can set the aperture from the dial on the K100D in A(perture) mode. In the M version, you have to set the aperture on the A ring of the lens. Of course, you'll need to manually focus with both types. For now, I recommend avoiding the M42 screwmount lenses as they require you to purchase a separate M42-K mount adapter.

A quick scan of the last few listings show a few A and M 50mm f/1.7 that sold for $45-60. Doesn't get better than that for a cheap but super fast lens.
posted by junesix at 7:15 PM on February 7