Hi Nick,
Thank you so much for this explanation and pictures! They are great, your examples are superb!
Thanks.
1. You said: it that means if I want to take picture of insects I would need more than one achropats attached to front of my lens?
No.
But it takes a bit of explaining. Please bear with me.
There are things called close-up filters. They typically come in sets of four lenses, designated +1, +2, +4 and +10. The bigger the number (known as the "diopters" of the lens) the more magnification it provides. You screw these on to the front of the camera lens, just like a filter. They let you focus closer to the subject than normal, so you see less of the subject/scene, giving you magnification. You can "stack" them screwing two or three of them together on the front of the camera lens, giving you even more magnification. They are usually very cheap, perhaps £20 or so for a set of 4.
So far so good. But the downside is that each of the lenses is made of a single piece of glass and will suffer from chromatic aberration, degrading image quality (see
3 - Chromatic aberration on this page - you might also want to look at
this page for some other introductory information about close-ups/macros). If you use two or more close-up filters at the same time the image degradation is worse. Some people are very happy with close-up filters, but other people say they give unacceptable results. I don't know, I've never used them, and it's a personal judgement anyway as to how good is good enough. However, if you aren't really sure if you will enjoy doing close-ups, then they are a very inexpensive way to dip your toes in the water, just to get the idea of what it's like being able to photograph small things.
Achromats are very similar to close-up filters, but they are made of two or more pieces of glass which act together to reduce chromatic aberration. You generally don't use more than one achromat at the same time; if you want more power you use a more powerful achromat. I have four achromats:
Canon 500D (not to be confused with the camera with the same name). This is a mild, +2 diopter achromat. It is one of the more expensive ones, around £85. I typically use this for flowers when I want to get a bit closer up than I can by just using the camera with no achromat attached, and for some larger invertebrates like dragonflies, small butterflies, large slugs or crane flies. I can use a working distance (between the subject and the front of the achromat) of between about 300mm to 500mm (this varies a bit between cameras/lenses, but not a lot, so the numbers I'm quoting here for my kit should be a fairly good guide). The 500D is very easy to use, because there is plenty of distance between you and the subject, and you don't have to be very precise about how far you are away from the subject.
Raynox 150. This is a stronger, +4.8 diopter achromat, costing about £40. I typically use this for things the size of wasps, large flies, snails and beetles. My working distance has to be between about 160mm to 210mm. It is fairly easy to use.
Raynox 250. This is an even stronger, +8 diopter achromat, costing about £40. It is very popular with people who want to get really close up to their subjects, getting a good close look at a spider's mouthparts or whatever. I typically use the 250 when invertebrates are too small for the 150. My working distance has to be between about 100mm to 120mm. Some people find it quite hard to use. I know I did when I started out - I almost gave up after a couple of days of not being able to get anything into focus. Other people pick it up and use it straight away with no problem.
Raynox MSN-202. This is a rather specialised piece of equipment, +25 diopters, costing about £55. It is not widely used. I use it for very small subjects such as the springtails I posted images of above, or other very small subjects such as fruit flies. My working distance has to be around 28mm to 31mm. It is extremely difficult to use. I have found that on a camera lens that extends, which most zoom lenses do, it is almost impossible to use out of doors, which is where I do my photography - it would be ok in a studio setting. I would not recommend anyone to try anything this powerful before mastering lower magnification close-ups/macros.
The only time I stack achromats is when I use the Raynox 150 and 250 together. This gives me magnification and working distance between that of the 250 and the 202, and is easier to use than the 202, although still rather difficult to use.
2. From your experience: what do I need to look at while choosing achromats?
It depends on money to some extent. A close-up filter set would be the cheapest way to start out. This would give you access to a wide range of magnifications to try out, as long as you accepted that image quality would not be stellar. And if you found out you did like doing close-ups/macros, then you would have to write off the cost of the close-up filter set when you bought something better.
The gentlest, but most expensive, way into this area would be to start with something like the Canon 500D, which is easiest to use, and then as you gain confidence work your way up to more powerful, but more difficult to use, achromats. Otherwise, and depending on what you would like to start off photographing, something like the Raynox 150 could be a good starting point.
There are alternatives to the lenses I have mentioned. For example, the Canon 250D is +4 diopters, the Marumi 330 is +3 diopters and the Marumi 200 is +5 diopters. These lenses are more expensive than the Raynox 150 and 250, but they have an advantage. The Raynox 150 and 250 have quite small diameters and can cause serious vignetting at lower focal lengths. On my 24x zoom bridge camera for example I have to get out to about 4x zoom or 7x zoom (depending on how I have it set up) before the vignetting goes away. On the other hand, on my 55-250 on my 70D I don't get any vignetting. I do have a Marumi 200, which I wanted to use instead of the Raynox 150, in order to get rid of the vignetting, but unfortunately my tests indicated that it was not as sharp as the Raynox 150. Here too, other people are very happy with their Marumi lenses.
You might want to do a comparison - buy a close-up filter set and an inexpensive set of extension tubes, and then compare the two approaches in practice and see which you feel more at home with. I generally find I have to try things before I'm really sure what works for me and what doesn't. It is a very personal thing as to what we variously feel comfortable with. This approach would also give you some practical experience to lean on, and to use as the basis for more probing questions from people who are into this sort of thing, before deciding which approach to commit more money on, one other approach of course being to use a prime macro lens.
I have Nikon D90 and some varius lenses:
35mm, 50mm, 24-55, 55-200. you mentioned that the best is to use achromats on 55-200?
I would have thought so.
You generally use achromats (and close-up filters) on zoom lenses. This is because they work differently from macro lenses. With a macro lens if you want to change the magnification you move towards or away from the subject. With an achromat you stay the same distance from the subject (well, you keep the working distance the same) and you zoom in and out to alter the magnification.
I have found longer zoom lenses better, as they avoid the vignetting issue, but I have used achromats on shorter zooms similar to your 24-55, but I couldn't use much of the zoom range because of the vignetting I got over most of the zoom range. But I have read of other people having different, and more successful, experiences with short focal length zoom lenses.
I thought until recently that achromats would work on any zoom lens. However, when I recently bought my 70D I also bought a 55-250. Autofocus was iffy with the 500D and 150, and wouldn't work at all with the more powerful 250 and 202. I upgraded to a more expensive 55-250 (Canon's STM version, which is especially good for video apparently) and that works a treat with all my achromats, including autofocus. I've never heard of anyone else having a problem like this, so perhaps my first 55-250 was defective, but I thought I ought to warn you all the same. (And most people don't use autofocus for close-ups/macros anyway, although I find it indispensible.)
Thank you for all your time for explainint all this to me
It is, as is presumably obvious by now, a pleasure (or an addiction - a bit of both I think).
[/quote]
My head is spinning now 
