scott_jorgenson

Hi Gordon,

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I'm emailing this to you, as well as posting it to the respective thread on the MFD Web site.

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I think there's been some confusion, and re-reading my post I see how it could have happened.

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Basically, I was comparing f/22 at 45mm with f/22 at 55mm, because 45mm in 6x4.5 format gives the same angle-of-view as 55mm in 6x7 format. So when I said:

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"But in 6x7 for this angle-of-view and circle-of-confusion, f/22

gives you DOF from only about 7.5 feet to infinity"

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I was referring to the 55mm lens.

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Your calculation, however, is for the 45mm lens. Apparently my comment was not clear enough and you thought I was claiming DOF at 45mm in 6x7 was 7.5 feet to infinity. But of course DOF at 45mm is the same in 6x7 as in 6x4.5, and there our calculations agree: 5 feet to infinity.

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In any case, my comparison of f/22 and 45mm for 6x4.5 versus f/22 and 55mm for 6x7 is the correct comparison, since these focal lengths are what give the same angle-of-view. Of course you can use a 45mm lens with 6x7 and at f/22 achieve DOF from 5 feet to infinity - but then you'll have a much wider angle-of-view. You're not taking the same picture, so its not really an apples-to-apples comparison.

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Incidentally, in 6x4.5, 35mm is the roughly-equivalent focal length for 45mm in 6x7. And at f/22, 35mm gives you DOF from about 3 feet to infinity - so here too 6x4.5 gives you about 1 extra stop of DOF at 6x7 for the same angle-of-view.

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Hope this clears it up,

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Scott

Regarding people's comments about the larger 6x7 negative versus 6x4.5 (or cropped 6x6):

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Keep in mind that frame size (ie image magnification) and depth-of-field are inversely related. As your frame size goes up, your available DOF declines for any given angle-of-view, aperture, and acceptable circle-of-confusion (ie, degree of sharpness/softness which you find acceptable at the limits of the DOF).

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You'll find this reciprocal relationship is most significant when comparing DOF in 35mm with DOF in any MF. But for the 6x7 versus 6x4.5 debate, what it means is that when you go to the larger frame, you lose about 1 stop of DOF. For example, consider f=45mm in 6x4.5, which offers a wide angle-of-view that is roughly equivalent to f=55mm in 6x7 (and f=28mm in the 35mm format). At f/22 for this angle-of-view, in 6x4.5 you can get everything from about 5 feet to infinity in focus (assuming a circle-of-confusion of about 0.03mm, which I find to be the limit for enlarging up to around 8X).

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But in 6x7 for this angle-of-view and circle-of-confusion, f/22 gives you DOF from only about 7.5 feet to infinity. For 5 feet to infinity, you would need f/32 (which isn't offered on many wide-angle MF lenses such as Pentax 67). I simply don't find this DOF to be adequate (remember, this is a wide angle lens we're talking about here).

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Of course, you'll be magnifying your 6x7 a little less than 6x4.5 for any given enlargement size, so you could settle for a larger circle-of-confusion if you're only interested in matching the 6x4.5 enlargement size, but if you're using 6x7 you probably want to be capable of a larger print size to begin with.

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So if your landscape style is to include foreground-background elements, I find that 6x4.5 (or cropped 6x6) is about the limit for a rigid-body camera (like most MF cameras).

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Hope this helps,

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Scott

(cross-posted from rec.photo.equipment.medium-format on UseNet)

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I'm looking into a Pentax 645 system. I've searched archives of the Pentax mailing list and the Medium Format Digest, and have found useful information on most aspects of this camera (plus, recently, some interesting speculation about a next-generation 645 that might be intro'ed by Pentax in the near future). However, I haven't seen much discussion of the 645 zooms and teleconverter, so would appreciate user comments on them.

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1) Pentax 645 45-85/4.5

Pentax 645 80-160/4.5

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How have users found these lenses to perform (sharpness, contrast,

mechanics) compared to the 645 prime lenses? I'm especially

interested about optical performance at the smaller apertures (for

the tripod-mounted landscape work I do, DOF is often a concern for

me, and I rarely shoot wider than f/8 or f/11, using f/16 and f/22

most commonly). I understand both of these lenses stop down to

f/32...what is performance like at this aperture?

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(The archives I've searched did include a couple of comments about

zoom creep on both lenses, and vignetting of filters on the 45-85.

Any other info, or further details on these problems - eg, is the

creep consistent or a flaw with that user's units; how much

vignetting are we talking about; etc - would be appreciated, too.)

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2) Pentax 645 2x teleconverter

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What about performance of this teleconverter when matched with the

80-160 zoom (assuming it matches with the zoom - does it?).

Again, I'm interested mostly in apertures at f/11 and smaller.

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From the B&H catalog, it looks like Kenco (sp?) makes a

Pentax-compatible 2x teleconverter, too. Does anyone know about

this?

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Thanks for the info,

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Scott