I've decided I need/want a lens that has a thinner possible DOF than my Asahi Pentax 50mm F1.4 can give me. I'd prefer if it was as tack sharp wide open as my Asahi is, but I'm willing to sacrifice if I have to. I'm willing to spend some decent cash to get this kind of lens.

I know of two options so far:
1. One of those 50mm F1.2s
2. The Pentax 77 F1.8 (The 70mm f2.4 is not an option - the DOF won't be as thin)

But what else is there? I'm looking for the shallowest possible DOF I can get for a Pentax camera. What are some obscure/obvious lens choices which I have not thought of?

Any ideas?

Isn't there an oldschool 135 F2 that can be adapted to Kmount? I forget.

Shallow? The A* 135mm f/1.8 will give you a pretty damn thin DOF. The FA* 85mm f/1.4 is also a nice combo of fast and long - and will autofocus on newer bodies. I only really know Pentax lenses, so there may be some 3rd party stuff you can adapt that'll be even better.

On a given camera (sensor size), actual DOF depends only on aperture. A longer lens of the same aperture requires you to move backwards to get the same framing.

Note that actual DOF is different from perceived background blur, which does indeed increase with longer focal lengths.

AFAIK, the Zeiss Arri 150T/1.3 (F/1.15), which is available now in "nice-price combos" with one of the shorter T/1.3s from dealers for the quite reasonable price of 55.000Us$ (+tax). You need an ARRI-PK adapter though, which will cost you about 4-500$. Maybe the adapter isn't a deal-breaker in this case though... :-)

Dof is a function of shooting distance and front pupil size (diameter of the aperture when viewed from the front). And then you have to factor in the "scale", or the "picture angle" depending on how you want to visualize this parameter. The combination of those two things give you amount of background blur.

A 300F/2.8 will give you almost exactly the same "dof" as a 50F/2.8 would if you frame the subject the same (to "fill the frame"?). Background separation will be very much bigger with the 300F/2.8 though.

Do you want "more background separation" or "less DoF"?
Background separation - longer lens. Less DoF - smaller F/nunber.

A large factor is the distance to the subject. What fl do you need?

Alright, something needs to be cleared up here. I'm sure you guys can help. I suppose that I already knew that technically, a 50mm F2.8 and a 200mm F2.8 should have the same measurable depth of field.

But the bokeh at 200mm F2.8 will be many times thicker than the bokeh at 50mm F2.8. Why is that? Take a look at my example:

You're on the right track but not comparing apples to apples. It's not about the closest focus of one lens compared with the closest focus of the other. It's about shooting the same subject composition with both lenses--e.g., filling the frame with someone's face. When you do that with a 50 and a 200, you have to move much farther back with the 200, and you'll wind up with the same DOF (say 1 foot) at a given aperture. HowEVER, because the field of view is so much more narrow with the 200, you're going to see much less background stuff in the corners of the image. If the 50mm shot of the person's face shows a whole tree in each corner of the photo, the 200mm shot with the same subject composition will show just some leaves in each corner. You're smearing out those leaves into the same space that was occupied with entire trees in the 50mm shot. That's what is making the background blur more pronounced.

In that gif, the 50mm and 135mm example are from the same lens, for what it's worth. The branch in the 50mm image is perfectly overlayed and in line with the branch at 135mm, it's just cropped in. I still don't really understand your explanation. Could you go grab a camera, attach a zoom lens, manual focus on something close and watch the bokeh as you zoom in and out? The bokeh does get thicker - it does morph. I'm definitely not imagining that.

Anyway, I've come across the Pentax SMC 85mm F1.4, and it looks fantastic, with a price to match. I suppose I can afford the $800 to get it, but something has made the decision a bit complicated:

I also just learned that Vivitar makes an 85mm F1.4 too, for Pentax, and for $300. I also see alot of rave reviews from people about it.(Not just Ken Rockwell, by the way) This lens also appears to be sold under the names Polar, Bower, Rokinon, Phoenix, and Samyang.

Should I go for a knockoff? They seem like excellent quality lenses. If so, which knockoff should I go for? Should I stick with the legendary A*85mm F1.4 Pentax instead?

I don't know!

Never said you were imagining it. Of course, "thicker" is subjective, but yes, background blur seems greater with longer lenses for the reason I stated: You're taking a much smaller thing (a leaf instead of a whole tree) and smearing it out across the same amount of space in the photo.

No experience with the 85s you mention. I've heard good things about the Vivitar. I assume it's MF, if that matters.

I'd say go for the Pentax A 85/1.4.

Old school screwmount S-M-C Takumar 135/2.5 has narrow DOF wide open; interestingly that's what's on my camera right now!

The DOF formulae and 4 sentences describing effects of changing different parameters is at http://www.isorainbow.com/tech/depth.html

Tongue-in-cheek, for shallowest DOF mount the camera on a microscope!

Oi! I went and actually browsed ebay for the price of the Pentax, and the cheapest price was $1800.

Maybe the Vivitar is the way to go. It sounds like it will give me everything I need and for cheaper than I was willing to pay... It's got a reasonable sharpness but terrible CA (Though I'm not bothered by that, really).

Hmm..

You might like one of the screwmount Super- or S-M-C Takumar 85/1.9 or the better S-M-C 85/1.8. On ebaY going average prices in 2009 are respectively $215, $230, and $405, in USD.

The Vivitar/Samyang 85/1.4 or Zeiss ZK 85/1.4 are probably the best easy to find options.

DubiousDrewski wrote:
Oi! I went and actually browsed ebay for the price of the Pentax, and the cheapest price was $1800.

Maybe the Vivitar is the way to go. It sounds like it will give me everything I need and for cheaper than I was willing to pay... It's got a reasonable sharpness but terrible CA (Though I'm not bothered by that, really).

Hmm..

The Samyang is definitely an option. If you find the Samyang CA terrible I guess you haven't checked the multitude more expensive Pentax for CA yet? I don't know what word to use for it. "Not good", maybe?

siriusdogstar wrote:
I'd say go for the Pentax A 85/1.4.




I used to own that lens. I was a big Pentax fan back in the day. The Pentax A* 85/1.4 is a rock star lens. Wish I had never sold it. The value of good condition samples is way, way high though. I saw some tests matching this lens against the Pentax A* 135/1.8 and the newer "Limited" 77mm offerings. The A* 85/1.4 is still top of the heap in my book. Somewhere there is a test matching the A* 85/1.4 against the EF 85/1.2 and it did very well there as well.

You might want to have a look at this. I don't own it but I've seen some pics that suggest the DOF is pretty thin. It's a sweet piece of glass for a reasonable price...

I guess my last post didn't make much sense since I never attached the "link" to what I was talking about. Try this...

http://photozone.de/nikon--nikkor-aps-c-lens-tests/360-voigtlander-nokton-58mm-f14-sl-ii

How about Nikon glass with the glassless adapter? You won't get infinity focus, but if you are looking for background separation you won't be focusing at infinity either. With the longer glass you might be able to focus pretty far (though I really have no idea if it would be far enough). I imagine the 200/2 would do very well at blurring the background.

The easy way to think about the great background blur with longer FLs with the same DOF is that the angle of view is significantly narrower with longer focal lengths...thus, while your subject is framed the same, because of the narrower cone, the background captured is much less, and as Greg stated, that smaller area captured in the background is then spread out over the remainder of the photo.

You see, that is the explanation that I've understood for a long time. (Until now) But I recently paid closer attention to the bokeh as I zoomed, and more is happening than what you describe. If what you and Greg Feldman described were true, then the 135mm and the cropped 50mm image in my Gif above would be identical, but they're not.

The background is surely stretched to fit the narrower field of view, yes, and this likely is the cause of some of the effect, but something is happening even beyond this. Take a look for yourselves! I swear, I'm not crazy!