Putting some numbers on John's comment. The 105mm lens at 20 cm gives magnification of about 1.105. To get the same magnification withthe 35mm you need distance to subject of about 6.7 cm. Put that in the calculator and you get 0.06cm again - i.e just the same as the 105mm.

Peter

They also have the cheek to say "you represent that such use will not violate the rights of any third party, and you agree to take full responsibility for same".

That is, you let them use it for free, with no control over what they use it for, and if someone claims it violates their rights you have to defend it.

No thank you!

Thanks Algi, I'll have a look.

Meanwhile I had a closer look at the EXIF and there's colour space info in more than one place so I guess it may depend which field a particular program looks at.

P

If I set "color space" to adobeRGB in PDCU (version 4.31) and save an image as a JPEG, other software doesn't seem to recognise the adobeRGB, and the EXIF reads "No calibration". However if PDCU opens the JPEG it recognises it as adobeRGB.

If I save as sRGB in PDCU the EXIF reads "sRGB" and other programs recognise it as sRGB.

Can anyone explain what I am doing wrong?

Peter

Bernard

Sorry, calculations in my previous post were wrong. 217mm was "u" only. I think u+v would then be 290mm assuming 55mm. Also tried my 18-55 and filled the frame with about 65mm of ruler which is about 0.363 x magnification. Theoretically u+v would then be 281mm but I measured the object to image distance at about 245mm.

Probably most likely John is right that focal length varies with focussing (especially as zooms often do this by moving elements around rather than the whole lens).

I think I should stop now and go take some photographs

Just one final apology as I see you had thought about the internodal space

Peter

I mean on a pentax slr, where the lens mount is 46mm in front of the sensor/film plane, and the rear element of the lens can't be more than a mm or two closer than this.

Hi Bernard

You describe u and v as distances from the "centre of the lens". This is more or less OK for a simple "thin" lens but your 18-55 is not simple or thin.

For a compound or thick lens the object and image distances have to be measured from the "principal planes". These are usually different for the object and image sides. The principal planes don't have to be within the lens, in fact at 18mm the image principal plane must be at least 25mm behind the rear element, and for a telephoto it can (confusingly!) be in front of the front element.

To get the object to image distance you need to take u plus v plus the distance between the principal planes.

This isn't completely academic for macro work as it means you may need a bit more space than you might think, though I don't lose a lot of sleep over it!

For magnification of .34 with a 55mm lens I get u + v = 217mm. Assuming the quoted magnification and focal length are accurate but rounded, this could vary by 5mm either way. The 250mm probably isn't exact, so I guess the distance between the principal planes would have to be about 37mm plus or minus 10mm which doesn't seem unreasonable given that your 18-55 is probably about 60mm thick.

Peter

I think that's right about the Tessar. The ring can also loosen itself in use, which uncouples the focusing ring from the helix so you have no focusing action at all

I managed to retighten mine (with two screwdrivers!) but it's fiddly to get both this ring and the helix correctly positioned at the same time!

I bought an adaptor from SRB that had the same issue of the lens mounting "upside down" presumably because the starting point of the thread is not consistent. The customer service contact at SRB was very helpful and offered to exchange it for one he had checked to ensure it mounted with the index mark approximately at the top. In the end I bought this one as a second adaptor and kept the first to use on a bellows setup where the bellows mount can be rotated anyway.

Incidentally the SRB adaptor has a flange that actuates the aperture pin on automatic lenses which is useful if they don't have an auto/manual switch.