[flash1100]

Ah, but the reason for the very shallow DOF is how closely the lens is focused - at portrait distances, something like the 85mm 1.4 will have considerably shallower DOF... However, having the SP AF 90mm f/2.8 version of that lens, I will agree that it's a fantastic portrait lens!

Check out this calculator to see how DOF is influenced by focus distance (or other variables).

Another vote for the Tamron 90 / 2.8. Tack sharp and an awesome portrait lens + you get a 1:1 Macro lens.

[virgilr]

heliphoto-

I do understand the effect of subject/focus distance vs. depth of field. Hadn't read through enough of the original posters' intent for the lens to see that they're looking for a portrait lens. Just didn't think someone who was looking to get a wider aperture lens in a ~50mm would be thinking portraiture, but I see now that based on the required "closeness' to subject one would need to be with a 50mm, shallow DOF would be more pronounced.

I'll try and convince the signifigant other to pose for a quick shot at f/2.5 with the 90mm f/2.5 and post it when I can.

[Kerrowdown]

Have you thought about one of those lensbaby jobs, that would do it.

[newarts]

For a constant camera & width of field, depth of field depends ONLY on f-number, so a longer lens really doesn't help for the same composition.

I believe a computational approach might be most successful.

If two images are taken side-by side, it is easy to figure out which pixels are at a constant distance from the lens using simple geometry. I haven't done enough analysis to tell you precisely how thin a slice of the photo can be isolated, but it'll be quite thin I'll bet.

This is a route to realistic computational depth of field reduction.

Dave

[Douglas_of_Sweden]

I've decided I need/want a lens that has a thinner possible DOF than my Asahi 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 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.

[virgilr]

So, as I said, I would post a "portrait" style shot from the Tamron SP 90mm f/2.5 adaptall-2. Here it is. Note that the white wall is about 10" behind my wifes head. Focus was on right eyebrow area.

Shot at f/2.5, 1/250 second. Only PP was a .7 EV exposure bump and a slight warming of the color temperature. Exported from RAW processed file. Put up a fairly large version for detail.

I'll only leave this up for a week or so- that's what we agreed to when I asked her if I could take the shot. Want to help out the community!

[Fixcinater]

That is a great portrait, super sharp!

[Dubious Drewski]

For a constant camera & width of field, depth of field depends ONLY on f-number, so a longer lens really doesn't help for the same composition.

Right. This is a very hard aspect to understand, let alone describe, but I want a longer lens because there's more affecting the bokeh than most people think.

Yes, a 50mm at F1.4 will have the same measurable depth of field as an 85mm at F1.4. But a 50mm and an 85mm will have physically different sized apertures, even if they're both at F1.4

For the 50mm, the aperture would be 50mm/f1.4 = 37mm wide
For the 85mm, the aperture would be 85mm/f1.4 = 61mm wide

This doesn't make the depth of field any narrower, but it makes the bokeh thicker. This extra-thick bokeh is what I'm after, for maximum isolation of my subjects. To test this out for yourself, go grab a constant-aperture zoom lens and zoom in. Watch the bokeh in the background. It does far more than just magnify.

(For anyone following along, but not understanding: any "constant aperture" lens is actually making the aperture bigger as you zoom in, to allow more light to enter)

[Dubious Drewski]

Oh, and I've decided on the Vivitar 85mm F1.4

What an amazing lens, even disregarding the ridiculous cheap price.

[jogiba]

[Dubious Drewski]

Yup, that's the one. What a gem!

[virgilr]

Dubious-

Wondering what you paid for the Vivitar. I was looking for a super takumar 85mm f/1.4, I wasn't able to find anything under $500.00

I paid 80.00 for the Tamron w/ a PK-A adapter.

That's ridiculously cheap to me- and I love the lens to bits!

[Lowell Goudge]

Dubious-

Wondering what you paid for the Vivitar. I was looking for a super takumar 85mm f/1.4, I wasn't able to find anything under $500.00

I paid 80.00 for the Tamron w/ a PK-A adapter.

That's ridiculously cheap to me- and I love the lens to bits!

I paid $350 US in the spring, at B&H they are still dropping in price, I started thinking about them when the samyang came out at $549 and figured $350 was OK

[Marc Sabatella]

Yes, a 50mm at F1.4 will have the same measurable depth of field as an 85mm at F1.4. But a 50mm and an 85mm will have physically different sized apertures, even if they're both at F1.4

For the 50mm, the aperture would be 50mm/f1.4 = 37mm wide
For the 85mm, the aperture would be 85mm/f1.4 = 61mm wide

This doesn't make the depth of field any narrower, but it makes the bokeh thicker.

"Thicker"? I have to assume you're talking about the same phenomenon I'd describe as simply "blurrier". That is, with a longer focal length, you don't have *more* of the picture out of focus for a given DOF, but the parts that *are* OOF are *more* OOF. Often, a shorter focal length will produce a background that's a confusing jumble of non-quite-recognizable shapes, whereas a longer focal length lens might smear all that background confusion away. Although I find you don't *really* starting getting this effect until closer to 200mm.

To test this out for yourself, go grab a constant-aperture zoom lens and zoom in. Watch the bokeh in the background. It does far more than just magnify.

OK, but if you zoom from one position, you aren't actually keeping DOF constant; you're shrinking it. To do that, you need to do back up as you zoom, to keep the subject the same size. Unless I'm missing something, which is always possible...

[virgilr]

"Thicker"? I have to assume you're talking about the same phenomenon I'd describe as simply "blurrier". That is, with a longer focal length, you don't have *more* of the picture out of focus for a given DOF, but the parts that *are* OOF are *more* OOF. Often, a shorter focal length will produce a background that's a confusing jumble of non-quite-recognizable shapes, whereas a longer focal length lens might smear all that background confusion away. Although I find you don't *really* starting getting this effect until closer to 200mm.

Wherein you start to need to look at the "thickness" or blurriness of the bokeh produced by different lenses- which seems to be a bit of an X-factor.