Macro Magnification

Am I correct in thinking that a 100mm lens will have a smaller DOF than a 50mm at the same aperture, also something to consider.
I have a cosina 100mm af 3.5 its very poorly made but not a bad buy for £35.

A 28mm, 50mm or 100mm macro lens is the same as any other 28mm, 50mm or 100mm lens except that they allow much closer focusing allowing --- if they are "true" macro lenses --- a 1:1 ratio between the size of the object and the size of the image on the film/sensor.* As to the "best" it depends what you are using it for. If you want to photograph bugs, a longer lens is better. If you are taking pictures of small objects on a copy stand (I do a lot of this) then a longer lens is a pain as you end up having to stand on a stool or put the stand on the floor.

*Note to all, yes I know they also have a flatter plane of focus etc. etc. but let's keep it simple for now.

Best wishes, Kris.

No.

At the same magnification, the depth of field is dependant on the aperture, not the focal length. So whether you're using a 100mm lens or a 35mm one, at any given aperture, at 1:1 magnification, the depth of field will be the same.

What will be different is the working distance (you'll be a lot closer to your subject with the 35mm) and the perspective. So if for instance you were in some woodland taking a pic of a toadstool, the 35mm lens would show more of the trees in the background, and the 100mm lens would show less, perhaps making it easier to compose your photo and reduce background 'clutter'.

Dan

A greater working distance also allows extra light to reach the subject when the camera would otherwise obscure it.