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08-18-2008, 12:31 PM   #31
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QuoteOriginally posted by little laker Quote
I wonder if that's just a Pentax thing.
I shoot my K-Mount 55mm f1.8, along with my screwmount 50mm f1.4 without a problem on my Sigma, but have problems focusing with my K100D.
OH, MAN!

I'm having the same problem with the same camera/lens combo!

I have a K100D, and recently bought a 55mm 1.8 K lens to use for portraits.

The lens has nice bokeh and color reproduction, but I've noticed that most of my images are just a bit soft, and that's not like me.

I was beginning to think there was something wrong with the lens, as this doesn't happen with any of my Pentax DA lenses. It's easier to focus my Lensbaby than it is the 55mm, and Lensbabies are not the easiest lens to focus!

The 55mm will focus accurately, but you have to make sure the focus is absolutely on the mark or you'll get a soft image. I usually have the lens set at f2.8 for the nice bokeh it gives at that aperture.

After reading this thread I don't feel so bad, as now I know it's not just my lens.

I think my solution to this will be to get a DA 70mm f2.4, since I've wanted this lens for sometime, and as previously mentioned, all my Pentax DA lenses focus very quickly and accurately.

As most of you know, if you take too long focusing on someone sitting for a portrait, you can lose the person's interest.

A very informative thread!

Mike


Last edited by Mike Bokeh; 08-18-2008 at 06:57 PM. Reason: misspelled word
08-19-2008, 05:32 PM   #32
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QuoteOriginally posted by Marc Sabatella Quote
An important thing I learned about manual focus with fast lenses is that the focus screen lies. The stock one, anyhow, is not capable of rendering DOF any less than that of f/2.8. Not sure about Katz Eye. But the issue is that things look in focus in the viewfinder but won't be in the picture when shooting wider than f/2.8 or so because the DOF of the picture is narrower.

This fact of focus screens complicates manual focus with fast lenses. Not sure that it by itself completely explains what you are seeing, but it really does take quite a bit of getting used to in order to overcome.
This statement may have solved a quasi-mystery, that I, and others here have been dealing with.

I've been thinking about why my 55mm 1.8 is difficult to focus, but my 100mm f4.0 macro is not. I also remember making a comment in a Lensbaby thread about how hard it was to focus the Lensbaby at f2.0 or F2.8, but much easier at f4.0 or f5.6.

The two Pentax lenses focus at wide-open aperture, with the 55 being more than 2 stops faster than the macro, hence the difficulty in focusing it.

The Lensbaby lens on the other hand, becomes the f stop of whichever apeture ring you have in it while taking a particular shot, therefore the more it's stopped-down, the easier it would be to focus provided that there's sufficient light.

Does anyone agree/disagree with this?

I think I'll do an aperture test with my Lensbaby soon, using all apertures from f2.0 (wide-open) to f22, and use both the wide-angle and tele lens attachments, and see if my images are soft at the wider apertures.

This is the most thought-provoking thread I've come across here.

Mike
08-20-2008, 01:54 PM   #33
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QuoteOriginally posted by Mike Bokeh Quote
This statement may have solved a quasi-mystery, that I, and others here have been dealing with.
Drove me crazy until I finally read enough to and experimented enough to figure out exactly what was going on. Indeed, now that I know what to look for, it is *very* easy to verify. Shoot a piece of newspaper at an angle with a "fast" lens wide open. In theory, the viewfinder should how you what you are getting - since you are wide open, DOF preview is not needed. But the viewfinder may show 20 lines of text in focus, and the picture may show only 10, because the viewfinder is showing too much DOF. Thus some of the text that appeared in focus in the viewfinder won't be in focus in the shot, and it's not the fault of the lens or your eyes or the camera per se - just the focus screen.

One solution is to install a different focusing screen - the KatzEye suppsedly works well. But really, with practice, it is possible to get decent at figuring out how to place your subject toward the *back* of the apparently-in-focus area in the viewfinder, and then when you take the picture, you'll find it more centered. I can't say I'm perfect at it, but I'm much better than I was. Most people's natural tendency is to focus such that the intended subject is toward the *back* of the "zone of acceptable sharpness" rather than the front - we subconsciously expect things closer to us to be *easier* to see (hence more in focus) than things further way. You just need to turn that part of your brain off. I now tend to focus by finding something just in front of my target, and focusing so that it is *not* in focus but my subject is.

Lensbaby, of course, adds a whole new wrinkle, and I've never messed with that!
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