50mm and 100mm macro lenses difference at the minimum focus distance?

Question

So I bought a Kiron 105mm f2.8 1:1 lens recently to be used with my Sony a6000. I know that the difference between a 50mm macro and a 100mm macro is how close you can get to your subject, that is with 100mm you can back up a little bit and still get the same magnification.

But when I tried getting some pics with my Kiron lens at 1:1 setting I still have to get pretty close to the subject (probably about 15cm or so from the front end of the lens) to get it in focus. Although I have heard people say 100mm macro is good for insect/bug photography as you can get pics without scaring them off, my working distance is still pretty small at 1:1, which probably isn't a practical distance.

Can someone explain what I'm not getting here? At 1:1 magnification ratio is there a real difference in using a 50mm or a 100mm in terms of the working distance?

Answer 1

When a macro lens has a reproduction ratio of 1:1 an object with a given size will be reproduced at the image plane at the same size. This is irrespective of the focal length. The only difference is that a longer focal length will afford you the ability to achieve that reproduction ratio at a greater distance than the shorter focal length.

The precise working distance (distance from the front of the lens to the subject) is dependent on more than just the focal length. The specifics of the optics used in a particular lens will dictate the working distance. Focal length is the distance from the rear nodal point of the lens to the image sensor. The location of the nodal points shift depending on the optical formula and can in some cases be in front on the front element or behind the rear element. Generally, however, a shorter lens of a particular focal length will have a greater working distance than a longer one at the same reproduction ratio.

Answer 2

As agf1997 says, minimum focus distance for macro lenses varies somewhat with lens design, however as a rule of thumb you can expect the minimum focus distance (i.e. minimum working distance) to be roughly the same as the focal length of the lens (assuming 1:1 magnification here). Hence a 300mm lens with a whole lot of extension tubes (as a silly example to illustrate the point) should give you 1:1 magnification and a working distance of about 300mm from your subject, compared with approx. 50mm working distance for a 50mm macro lens at 1:1 magnification. In my experience, longer macro lenses are much more useful for most subjects unless you are copying flat smallish documents on a copy stand that will only accommodate a shorter focal length. Another practical example is one of perspective; if you fill the frame with a person's face using a 50mm lens, the resulting 'peephole' perspective is quite distorted (because the subject>focal plane distance is short), whereas framing the face the same way with a 105mm lens will give the face a much more realistic appearance. This example isn't quite macro photography per se, but the concept applies equally.