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02-14-2011, 04:09 AM   #1
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Metering with preset lenses

Yesterday I discovered in the attic a Pentax Bellows II with K-mount and a 100mm Bellows-Takumar mounted on it by means of a M42 thread-to-K adapter. I'd bought this kit some 30 years ago for my ME Super (which I still have) and forgotten all about it.

The Bellows-Takumar is a preset lens, two f4 to f22 rings.

I am having trouble getting both my ist DS and K20D to meter properly with it.

Here's what I do:

1. Yes, I set the camera to allow use of lenses with no A setting.

2. Focus at f4 with the inner ring stopped to f22.

3. Turn the outer ring to f22.

4. Set the camera to M.

5. Press shutter button halfway down.

6. Press either the green button or AE-L button to meter.

7. Press the shutter button all the way down.

This procedure isn't working; I'm getting almost black pictures. I'm doing something wrong or omitting a crucial step, but I'm too dense to figure out what.

Can anyone help?

02-14-2011, 04:56 AM   #2
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Aren't you targetting a bright area in the center of the frame ? You cannot use the 77-segments metering of the K20D. Only the center-weighted metering (default behaviour) and spot metering will work with manual lenses.
I do not recall the default settings of the K20, but I think pressing the shutter half-way before using green button will lock the exposure. Try skipping your step 5.
02-14-2011, 05:17 AM   #3
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QuoteOriginally posted by ZEPsikopat Quote
Aren't you targetting a bright area in the center of the frame ? You cannot use the 77-segments metering of the K20D. Only the center-weighted metering (default behaviour) and spot metering will work with manual lenses.
I do not recall the default settings of the K20, but I think pressing the shutter half-way before using green button will lock the exposure. Try skipping your step 5.
I would agree. No need to half press shutter
02-14-2011, 05:44 AM   #4
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If one or more of the electrical contacts on the mount is not short circuited the camera may not know a lens is mounted.

If the Bellows/camera mount is not shiny metal, maybe the mount's lowest pin (DATA) should be short circuited to get correct exposure? Put a piece of aluminum foil over it when mounting the bellows to test this hypothesis.

Dave

02-14-2011, 07:48 AM   #5
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I sometimes use my old Super-Takumar 50mm, f/1.4 lens on my K10D. This lens, on a dslr, might as well be completely manual.

I simply shoot in Av mode. I focus, half-press and stop down to my working aperture, and let the camera pick a shutter speed. Then, I full-press to take the picture.

It doesn't always meter perfectly, but its usually pretty close.
02-14-2011, 03:19 PM   #6
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Figured it out!

Thanks to all for your help.

Switching to spot metering AND putting some light on the subject solved the problem. Even at ISO 400 under a single 60 watt incandescent light in the ceiling there simply wasn't enough light getting through the long bellows at f22 to trigger auto metering. It worked fine at f4, which should have told me what was wrong, but I am a septuagenarian and getting kind of slow.

Daylight from the windows put enough light on the subject at f22 to make things work at ISO 400. (This was with a K20D and ist DS. A K-5 would probably handle the low light better.)

That preset Bellows-Takumar is nice and sharp from center to corner, by the way. Seems to be a match for my 90mm Tamron macro. The Tamron does 1:1, but the Tak does 1:1.25, I believe.
02-14-2011, 03:59 PM   #7
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QuoteOriginally posted by henryk Quote
....
That preset Bellows-Takumar is nice and sharp from center to corner, by the way. Seems to be a match for my 90mm Tamron macro. The Tamron does 1:1, but the Tak does 1:1.25, I believe....
The effective f-stop for exposure purposes for f:22 at a magnification of 1:1 is f:44 (ie. f(1+m))!

Not only does this decrease brightness a LOT it also put the lens well beyond its diffraction limit. Say a lens/camera combination has a diffraction limit at F* for normal photography, then for macro photography the effective limit is F*/(1+m). For a magnification of 1X, I'd expect diffraction softness to start at around f:10.

While high f-numbers will increase apparent Depth of Field, the gain beyond about F*/(1+m) is compromised by an accompanying loss in sharpness.

Dave

PS The 100mm Bellows Tak should approach 1:1 if the bellows extends to 150mm or so. At 150mm + 45.5mm for the camera, the mag would be (195.5/100 -1) = .96X
02-14-2011, 05:37 PM   #8
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Dave, thanks

. . . for that information. The bellows came with a somewhat cryptic manual, and I could not figure out the magnification ratios. Looks like the lens's sweet spot would be at about f11-f9.5, right?

02-15-2011, 03:32 AM   #9
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No matter the lens, the "sweet spot" on the DPS-C sensor is around f/9.5, the diffraction limit for that frame size. Depending on degree of enlargement, you might go to f/16 without noticing the loss of sharpness. And magnification is easy:

MAG = (TE-FL) / FL where TE is Total Extension and FL is Focal Length

For a 50mm lens without a focusing mechanism, like an enlarger or bellows-macro lens, the bellows extension is the TE. If the bellows is extended to 125mm, then MAG= (125-50)/50 = 1.5x. For a 50mm camera lens, like a SuperTak 50 focused to infinity, TE is the focal length of 50mm plus the bellows extension of 125mm, so MAG = 125/50 = 2.5x. Focusing the lens closer than infinity will stretch the TE and MAG a little bit.

High-mag macro is always tricky. No matter the (effective) aperture, DOF will be razor-thin. Some deal with this by focus-stacking: take a number of shots with slightly different focus, then combine them in software. Some only shoot fairly flat stuff, so DOF doesn't matter so much. Some just go with it, let a plane of interest be in-focus and to hell with everything else. Whatever.

As mentioned, macro eats light. Lots of magnification means a really tiny effective aperture:

EA = NA * (MAG+1) where EA is Effective Aperture and NA is Notional Aperture

So a lens with aperture set to f/11 and with magnification of 1.5x will have EA= 11*2.5 = f/27.5. Not quite Ansel Adams territory, but you need long exposures, or really bright light, or flash. And for extension without aperture linkage, like bellows, flash gets tricky. So, a tripod is your friend.
02-15-2011, 04:31 AM   #10
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QuoteOriginally posted by henryk Quote
. . . for that information. The bellows came with a somewhat cryptic manual, and I could not figure out the magnification ratios. Looks like the lens's sweet spot would be at about f11-f9.5, right?
I think it depends on the subject and is a gradual loss of sharpness, not a hard wall; if the subject has no detail at the smallest resolvable resolution then it likely does not matter much (and sometimes image softness is desirable - like a portrait of me.) But if you want the highest possible resolution keep the nominal aperture below F*/(1+m).

Dave

In principal one can go somewhat beyond this limit by post processing using focus correcting software like Focus Magic.
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