Thinking of getting a Macro lens for flowers etc...need plenty of depth of field..so I see macro lens's come in all sorts of lengths...can someone explain the benefits of various lengths and maybe the pros and cons of this idea.
Thanks
Eddie

Depth of field is a problem when it comes to close-up. Very often, the small point-and-shoot camera's have a greater dof when closing up because, to get the sensor filled up to the borders, the have to use wide-angle.

In my opinion, you need to consider four things:

1/ You have to be able to come as close as 1:1
2/ When shooting insects (even more when they are winged ones) you need not to frighten your subject. They see the lens as one big eye of a predator and will flee when they can. Also, more tele means more distance from your subject which can come in handy.
3/ Because at short distance even the lightest breeze causes unsharpness, you need a wide opening to compensate short shutter speed and often not too much light. This means less dof, but at that 1:1, there isn't much difference between 2.8 and 32
4/ Often, the autofocus does not focus on what you want but where there is a greater contrast in the neighbourhood. Which means that you need to be able to focus manually.

I have a Sigma 105mm f:2.8.

advantages:

• 1:1 (true size, meaning you get incredible "real" macro when looking at the full-size on your monitor.
• f:2.8
• medium distance from the subject (fartherr than 50mm, closer than a 150mm)
• very sharp lens
• useable as a short tele.

disadvantages:

• auto or manual focus has to be set with two steps, a ring and a button. Sigma warns that forgetting one of tose whilst changing the other one may damage the camera.
  This is not a real problem to me as I'm an old-fashioned aperture-priority and manual focus guy.

If you're interested, I can add a true-size detail of a flower/plant shot here.

Thanks for the info.I would appreciate a sample if possble.I am pretty much an aperture priorty guy anyway.Spent most of my life with a 6 x 6 Bronica and a light meter before going digital.
I will check out the lens you mentioned.I have a sigma 10 - 20 which I have found great for wide angle shots.I now use a Nikon D100  and I have a Nikon 70 - 300mm zoom and a Nikon 24 - 85 but the price of Nikon lens's is frightening compared to Sigma.
Thanks again.
Eddie

Any of the Prime Macro lenses on offer will fit the bill for you and I dont think I have heard much negative comments from any of them such as the Sigma , Tamron Tokina etc... Usually there is an advantage with going for a lens made my your camera.. Apart from Price!!  And 100mm will be ideal with the standard  digital sensor.. And also makes for a nice portrait lens..   Quick focus is one important consideration, and as has been mentioned insects can be skittish... The Sigma is a very fine lens but in Auto focus mode can be noisey, and moved in and out, again liable to scare of your subject.  

Longer focal lengths may help to get a shot  as you can be further away, and manual focus may then be ok as you may have a little more time..   But I am sure any Macro buff will tell you much of the skills are involved with the stalking and field craft..  Having a manual over-ride to the auto focus is to my mind one of the best aspects of the better brands..

I concur with Danny, above, there is little to choose between the different makes in optical quality; Sigma/Tamron/Tokina will give you results that are as good as a Nikon lens in this category. The difference is in the build quality.  Having said that the gap seems to be closing all the time. I use a Sigma 150mm which I consider a superb lens both optically and the build. 

You mentioned you need plenty of DOF and this could be a deciding factor in any decision. DOF alters with how close the subject is to the lens, as well as focal length and aperture.   Therefore macro is almost by definition shallow DOF.  To maximise the DOF you need to pick the shortest focal length that is suitable for your needs. With flowers in an indoor setting there is no problem increasing the shutter length and using a tripod so you can use a small aperture to maximise DOF. There would be a problem if you want to snap critters though, they will move and a tripod is at best awkward, so there comes the compromise - a focal length long enough to shoot before you get too close and scare them away,  you will need a high shutter speed to match the increased focal length and freeze the action or camera shake. 

Focusing I have never seen as a problem - most people use manual focus with a macro lens, because of DOF issues, set the focus to roughly the right place and then move the camera slowly back and forth to get the final adjustment is the method I use.  

OK, Eddie, I uploaded an image to my gallery as here it is even more limited.
I did search for a real close-up, not for atistic value here.

Nothing was changed or tweaked, only I had to use a 60% jpg compression to get it uploaded.
So there is  visible loss.

Seen its size, I don't think it'll stay in the gallery.

here's the direct link:

www.renderosity.com/mod/gallery/index.php

As always, some excellent advice from folks here so far!

You said...."need plenty of depth of field".....that's something all macro shooters wish for!  LOL!
The dof in most of the macros in my gallery is less than one mm!!

Richard accurately points out one of the key techniques for macro shooters...focus manually, then move the cam back and forth to get perfect focus (for bugs, usually on the eye)

Your lens search is really a difficult one to answer though....as different subjects often require different equipment and technique.  Skittish bugs require you to keep your distance....meaning a longer length lens....flowers/still objects allow you to set up, get close (allowing use of a shorter length lens) and a tripod.....if you want to maximize the dof, use a point and shoot cam as Tanchelyn mentions...if you want to get above 1:1 like some of my shots, that again requires different equipment..

As the emphasis of your requirements is DOF i thought you might be interested in this technique:

http://www.wonderfulphotos.com/articles/macro/focus_stacking/

Just google "focus stacking" and i'm fairly sure there are some programs out there which are free which will let you do this (i have never tried it - but it might be past time i did)

Another CHEAP option is REVERSE Macro Photography.  I've been wanting to try this, but haven't gotten around to it yet.

http://stephenelliot.com/2007/05/15/reverse-lens-macro-photography-tutorial/

http://www.dgrin.com/showthread.php?t=47347

DIY: reverse macro ring
http://flash.popphoto.com/blog/2007/07/tip-of-the-da-7.html

Joe

Good point Inshaala, focus stacking is a great way to "increase" dof....and it has been used manually in product photography for many many years.
It is ideally suited for static objects....unfortunately, using it handheld on moving bugs can be near impossible.

As mentioned by Tanchelyn,  depth of field, keep in mind at macro distances dof is quite shallow even with higher number aperture.  This will show the limited dof for both the 50mm and 105mm Sigmas at the mimimum focusing distance, where the 1:1 image size on sensor is realized.

MINIMUM FOCUS DISTANCE (1:1)
Sigma 50mm macro:   18.9cm/7.4 in
Sigma 105mm macro: 31.3cm/12.3 in.

Depth of Field
**50mm lens 
                **f/16   Distance 7.4 in:  7.19-7.62in.
                      f/2.8                           7.36-7.44in.

105mm lens
                     f/16 Distance 12.3 in:  12.18-12.42in.
                     f/2.8                          12.28-12.32in.    

I have both the Sigma 50mm macro and the 105mm.  Both are capable of fine images. I started with the 50mm then thought the added working distance of the 105 would help me.  I found out other reasons better than that one for using both lenses.

The 50mm works fine as a 50mm prime lens in people shots, wedding shots, any shots when the wider field of view is helpful.  In this case, the lens is quite capable as a 50mm prime when not used for macro shots.   This lens allows me to get a close macro on a single rose and then back off a bit to shoot the entire bush.   With the 105, the lens also works well as a prime when not used in macro shots. But for the roses, for the entire bush I would have to go across the street to let the bush fill the field of view, not so with the 50mm at half that distance for the same field of view, with me standing in the middle of the street. : ).

Then again, with the 105, the added distance is a hinderence in close quarters if not taking macros but a blessing if close up shots are wanted(even if not quite macro) and you need some distance from the subject. For instance, tomorrow I hope to watch and photograph some ladies repairing feathers on a bird or prey.  The 105 allows a close shot with me out of the way of the ladies.   For individual birds, I would have to go to a wider field of view and shorter mm lens or use a zoom.   There are benefits and drawbacks to either but I have found either of the Sigma macros mentioned excellent.

I hope this helps a bit in your choices.   The dof info should be helpful...bottom line, do not depend on aperture to get it all in focus with so little difference in my examples of f/2.8 and f/16.

By all means on macros, go manual focus.  You need to focus on the most interesting part of the composition, not the particular part auto focus happens to choose.

Best wishes with a macro lens.       Tom.   

I would like to thank all you guys for the most informative input....still cant decide which way to go but there is plenty of food for thought.
I will have to re read everything several times for it all to sink in...it happens when you get old :-)