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04-12-2010, 07:53 AM   #16
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as an overall summary for TCs and extension tubes.

Note:

A teleconverter doubles the image size by adding a 2x (for example) magnification lens into the optical path. As a result, it effectively doubles the focal length of the lens, but does not impact the minimum focusing distance. This is all done at the expense of 2 stops in effective aperture because doubling the size of the image spreads it out over 4 x the area,

An extension tube increases the magnification of the lens by allowing closer focus, at the expense of infiinity focus.

You achieve 1=1 magnification when the lens extension is equal to the focal length,, and the working distance to the subject then becomes 2 x the focal length.

04-12-2010, 08:05 AM   #17
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QuoteOriginally posted by RioRico Quote
Hi Marc. I just checked my authority on this subject, FIELD PHOTOGRAPHY by Alfred Blaker, which deals extensively with usable math for extreme closeups. He gives the same formula, with M (magnification), F (focal length of lens) and E (camera extension - LENS TO FILM DISTANCE) (my emphasis). When you've added extension, you CAN'T focus to infinity! So magnification is ONLY dependent on focal length, and total extension.
The point is, that if a lens is NOT focused to infinty, it is already extended slightly! So you need to add the focusing tube extension into account and add it to the length of the extension tube. As this varies from lens to lens and also leaves modern IF lenses completely out, all formulae are given for the infinity position of the lens. On top the basic formuly applies to all kinds of lenses, even for large format, which do not have own focusing tubes.

Ben
04-12-2010, 08:07 AM   #18
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QuoteOriginally posted by Lowell Goudge Quote
as an overall summary for TCs and extension tubes.

Note:

A teleconverter doubles the image size by adding a 2x (for example) magnification lens into the optical path. As a result, it effectively doubles the focal length of the lens, but does not impact the minimum focusing distance. This is all done at the expense of 2 stops in effective aperture because doubling the size of the image spreads it out over 4 x the area,

An extension tube increases the magnification of the lens by allowing closer focus, at the expense of infiinity focus.

You achieve 1=1 magnification when the lens extension is equal to the focal length,, and the working distance to the subject then becomes 2 x the focal length.
Exactly.

One thing about adding a tc to extension tubes:
if you add the tc to the lens and then mount the complete combo onto extension tubes (or bellows), you will actually reduce(!) the magnification, as the focal length of the lens is doubled (for a 2x tc).

if you add the tc directly to the camera body, then moun the extension tube and finally the lens, the tc will add magnification. But htta far from the lens, the tc will not work best and degrade image quality more than usual.

Ben
04-12-2010, 11:43 AM   #19
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QuoteOriginally posted by Ben_Edict Quote
Exactly.

One thing about adding a tc to extension tubes:
if you add the tc to the lens and then mount the complete combo onto extension tubes (or bellows), you will actually reduce(!) the magnification, as the focal length of the lens is doubled (for a 2x tc).

if you add the tc directly to the camera body, then moun the extension tube and finally the lens, the tc will add magnification. But htta far from the lens, the tc will not work best and degrade image quality more than usual.

Ben
Ben, if you follow exactly the logic I wrote, you are not correct in your first comment. You add an extension tube equal to the lens focal length and focus at 2x focal length to get 1:1 , providing when you add the TC you double the extension tube as well, then you are back to square 1, but at 2x the working distance,

04-12-2010, 01:41 PM   #20
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QuoteOriginally posted by Lowell Goudge Quote
Ben, if you follow exactly the logic I wrote, you are not correct in your first comment. You add an extension tube equal to the lens focal length and focus at 2x focal length to get 1:1 , providing when you add the TC you double the extension tube as well, then you are back to square 1, but at 2x the working distance,
My remark was simply aimed at previous posts, which left the impression, that adding a tc would automatically increase magnification - which is not the case, if one works with additional extension. I only used your quote, because you were summarizing the facts.

Ben
04-13-2010, 07:36 PM   #21
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Hi folks,

Thank you for all of the discussion; I did get extension I was looking at and my first few pictures came out well after some Picasso touch up.

The following was taken with a fixed 50mm lens:
http://dl.dropbox.com/u/1491001/macro/IMGP0759.PEF.jpg
http://dl.dropbox.com/u/1491001/macro/IMGP0776.PEF.jpg
http://dl.dropbox.com/u/1491001/macro/IMGP0777.PEF.jpg

I did notice how difficult it was to keep even a tiny area of the photo focused.


Question: what is 'Catch-in focus' aka 'trap focus'?

Most of my lenses are completely manual outside of one zoom lens which has the A setting but focus is still manual.

also: not sure what that black spot is on the top left of each photo is. I seen them in many, but not all, of the pictures i have taken thus far. I originally thought it was dust on one of the lenses but for the photos linked I used a 50mm lens I have not used yet; i guess it must be dust on the ccd?

Last edited by Capslock118; 04-13-2010 at 07:42 PM.
04-14-2010, 06:42 PM   #22
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QuoteOriginally posted by Capslock118 Quote
I did notice how difficult it was to keep even a tiny area of the photo focused.
Welcome to the macro world.

QuoteQuote:
Question: what is 'Catch-in focus' aka 'trap focus'?
Most of our modern Pentax dSLRs have a Custom Menu option to Enable Catch-In Focus. When the AF switch is at AF-S and the right lens (see below) is used, you point at something and hold the shutter down. When you bring the subject into focus, or the subject moves into focus, the shutter snaps. You've caught or trapped the subject in focus; the poor-person's autofocus.

A good use: set drive mode to Continuous, set camera on tripod, insert wired remote with latch, aim and focus somewhere you expect something to happen, and latch the remote. Whenever something comes into focus there, the trigger fires. This is commonly used when shooting wildlife, birds, burglars, nudists, etc.

There's an advantage over plain manual focus: Often, we aim at something, slowly bring it into focus, get the green-hex confirmation, then shoot. There's a brief interval in which we or the subject may have moved. Blur. Oops. Trap-focus / CIF responds much quicker.

And there's a disadvantage: at small apertures, CIF might not kick in. It needs enough light and contrast to trigger the green-hex confirmation. Another problem: if we don't focus slowly enough, we might swing past the optimal focus point. Blur. Oops. But within its limitations, it's great.

What's the right lens? A manual lens with a shiny mount that (safely) shorts-out the lens mount contacts. A black mount might need paint scraped away, or metal tape added -- thin metal adhesive tape used for furnace repair, sold at most hardware stores. The alternative is a bit of tinfoil, but that blows away easily. Some M42 and M39 lenses have very narrow bases that won't hold metal tape in place, so tinfoil might be needed. Also, AF lenses CAN be used, with tinfoil to short those contacts.

Another alternative for M42: a cheap safe flanged no-infinity-focus M42-PK adapter, which also shorts out those contacts. I use this for close-up and portrait shooting where I don't WANT infinity focus.

QuoteQuote:
Most of my lenses are completely manual outside of one zoom lens which has the A setting but focus is still manual.
Then with a bit of metal tape, and setting the menu option and AF-S switch, you're all set for trap-focus.

QuoteQuote:
also: not sure what that black spot is on the top left of each photo is. I seen them in many, but not all, of the pictures i have taken thus far. I originally thought it was dust on one of the lenses but for the photos linked I used a 50mm lens I have not used yet; i guess it must be dust on the ccd?
Look in your manual for Sensor Cleaning, and follow the procedure for Dust Alert.

Last edited by RioRico; 04-14-2010 at 06:50 PM.
05-21-2010, 01:06 PM   #23
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New question that relates to my original thread.

This quote is in reference to the extension tubes I purhased, it has no aperature linkage or the sort; very very basic metal tubes:

QuoteQuote:
It also has no aperture linkages, so you'll have to have a way to stop the lens down yourself, though the adapter on the lens end may hold down the aperture pin for you. Therefore, you need an aperture ring on the lens.

I recently purchased this lense as it was dirt cheap on ebay and I needed some sort of wide angle lense which I didnt have; just primes and zooms over 100mm.

Unfortunately, there is no aperature ring on this lens; making it a love-hate relationship with this lense. Its the only lens I have that can take advantage of all my k100d features (since all my others lenses are manuals) but...well, this isnt the point.

The point is...is there a trick I can do to manually stop down the lense somehow so I can use this lense with the extension tubes?

05-21-2010, 01:09 PM   #24
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Not really. Simpler to just spend $20 on a manual lens (50s and 28's tend to be very cheap).
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