Macro on the cheap (and suggestions for focus rail, please)

No. Now I have confused things!

David was talking about lenses of the same focal length, so that is what I was commenting on.

The only purpose of tubes is to enable you to focus closer to the subject. Macro lenses are simply sharp lenses, generally with a fairly flat plane of focus, which have more travel in the focusing mechanism (than ordinary lenses), so can focus closer.

The angle of view remains the same, and a longer focal length will enable you to photograph things that are further away.

G

What I was meaning was that a lens which focusses closer without a tube will also focus closer with a tube. So, assuming you have two lenses and one focusses closer than the other, using an identical extension tube length, the closer focussing lens will give you the biggest magnification!

Hope this is clear! George is also correct that you can increase the magnification with more tubes (because this will allow closer focus), but you get to a point where this becomes impractical.

George also makes a good point in that longer lenses allow more working distance between the camera and the subject, which is why I sometimes use my 135mm and 200mm lenses for macro work. Less risk of disturbing the insects this way!

Regards
David

If the creature is venomous don't poke it with a stick. From the book "There are more things that can kill you in Austrailia than anywhere else on the planet"

So, in really simple terms that even I can understand, because I'm still struggling.

I've tried the 50mm tube with the 135mm lens and, whilst it allowed closer focussing and some magnification, it was nowhere near as much magnification as the 50mm lens with the 50mm tube.

What length of tubes would I need to put on a 135mm lens that would give me the same magnification as the 50mm tube with the 50mm lens or is it not as simple as that.

What I'd like is the same magnification and potential quality as I got from the 50mm lens and tube, but from a little further away so I don't poke the subject in the eye......or is this simply not possible on a budget? Is this what a macro teleconverter is for?

All the help appreciated - apologies if I'm being thick.

I have no idea how many tubes you would need with your 135mm lens. Picking upon David's point, it all depends how close the lens will focus to start with.

If you don't already have one, you should buy a set of tubes. They come in an assortment of lengths so that you can choose a combination that suits your requirements at the time.

Here is an example: Tubes

There are three tubes in the set, of differing widths, so you can experiment to see what works best.

Good luck.

G

If both the 50mm & 135mm are set at infinity then adding 135mm extension to the 135 will give the same magnification as 50mm extension on the 50. (life size reproduction)
When the lenses focusing is adjusted that (effectively) includes some extra internal extension, and so complicates matters. In most cases adding extension is exactly what the focusing mechanism does, but 'internal focusing' lenses are different hence the 'effectively'.

Basically the calculations (based on simple lens theory) are :-

1/U + 1/V = 1/F

M = U/V

where
F = focal length of lens
U = combined distance of extension + focal length (assuming lens is focused on infinity)
V = object distance

so for a 50mm lens focused on infinity the inmage distance = 50mm
the same lens + 50mm extension would focus on 100mm and have a magnification of 1

a 135mm lens + 50mm extension would focus on an object 499.5mm away and have a magnification of 0.27.

If you do the maths yourself to calculate the tube length for 1:1 you should find a length of 135mm

regards
Bernard

Is that just a perception or is it real? I would have thought that any 50mm lens in an identical set up to take a photograph would have the same depth of field given the same f-number, format size, and subject magnification where the film/sensor plane is the same distance from the subject. Or is something else in play to affect the image?

John

PS Great ladybird capture by the way. Well done.

It sounds like you are making for yourself a convincing case to buy yourself a dedicated 100mm Macro lens! This is probably the only way you will get the working distance you need with the correct magnification. Another alternative is a teleconverter as you have mentioned. This increases the magnification without altering the focussing distance. Therefore, with a 2x teleconverter you could get twice as far away and still get the same magnification. A macro focussing teleconverter is one step further and magnifies the image whilst also allowing you to focus even closer. However, as with most things in life there is a downside, or rather with this combination several downsides!!

1. You will lose some image quality and sharpness
2. You will lose some light (2 stops with a 2x teleconverter)
3. You will find it harder to focus accurately
4. Camera-shake will become even more of a problem

I would recommend the approach that George suggested. Buy yourself a set of decent quality tubes (dont buy the very cheap Amazon ones as they are rubbish) and experiment with them. FFordes photographic (www.ffordes.com) often have a good selection of quality tubes by makers such as Vivitar for between £10 and £20. Only when you have tried out different approaches will you be able to make a decision about what piece of equipment you should buy next.

Regards
David

Regards

Having gone back & read the posts I'd missed at the bottom of the first page...

Yes, longer focal lengths give you a longer working distance, though require more work to get the same magnification.

If the macro teleconverter is similar to my two, (I have both 2x PK & 3x PKA versions) it's a useful addition to the macro amoury. It's basically a variable extension built into a teleconverter, giving a second focusing ring. It can be used in conjunction with your current extension tubes giving different effects depending which side of the TC you put them.

I've no idea why but when mixing extension tubes and x2 converters I found that :-

Camera - x2 - tube - lens gave a different distance from camera to subject than
Camera - tube - x2 - lens

I've not been able to rationalise the maths though


Incidentally in my previous comment I forgot to mention that rather than setting the focus on infinity you focus on a closer object you are effectively introducing an imaginary tube


regards
Bernard

Thank you all. I think I'm beginning to understand - it's obviously not as straightforward as simply adding a set number of tubes.

It sounds like the best bet is simply to play around with different combinations of tubes and lenses which is hardly a chore.....

I just have one tube at the moment. It's a genuine Pentax 50mm auto tube which was a bargain on Ebay. I've been looking for more Pentax ones, but can't find any more, apart from a set in the US for £120, so will probably go with Vivitar or similar. I know the Pentax ones probably aren't any better, but they do go well with the M 50mm lens!

Sadly I'm not in a position to get a dedicated macro lens at present. The only way would be selling my Pentax 18-250mm which I'd be reluctant to do.

Are there any particular macro teleconverters anyone would recommend? There is a 3X Clubman one for sale on the forum, but I'm not sure I need 3X. Presumably a smaller magnification would be better for image quality as with normal teleconverters?

Thanks again for taking the time and trouble to help. It's much appreciated.

The two set ups are actually quite different. The first will give higher magnifications, as the tube is working with a shorter focal length. (It's approximately twice as effective)

This is my contribution Will probably be shot down in flames

[IMG]http://i1100.photobucket.com/albums/g406/tyronet2000/DoFtable1.jpg[/IMG]

Don't see why - that's really helpful. Shows just how hard it is to focus even when playing with a generous 1.07mm as I was yesterday!

Thanks