Help: Pentax-M 100mm f/2.8 vs. 100mm f/4 Differences?


I’m using telephoto lenses in an extreme macro photography setup for documenting fossil amber inclusions; I currently have a Pentax-A 645 200mm f/4 telephoto lens (manual focus), but I’m hoping to also find a 100mm lens to halve the resulting magnification when used with a Mitutuyo 5x super-long working distance infinity-corrected objective (≈20x w/ the 200mm, ≈10x w/ the 100mm). It would be far more cost-effective to get a lens with a smaller focal length, than to purchase an additional 2x infinity-corrected objective.

I’ve been told that macro lenses are not ideal for use in these kinds of setups with microscope objectives, and I’m unsure as to the critical differences between the “f/2.8” (telephoto) and “f/4” (macro) versions of the Pentax 100mm lens: knowing the differences would help me decide which would be a more appropriate choice.

I’d also appreciate it if someone might explain to me why dedicated macro lenses are not suitable for extreme macro photography (i.e., when combined with microscope objectives); I was told macro lenses are not a good choice, but I had never received an explanation when I was told this. Also, would the image format of a lens make much of any difference in a captured digital image’s quality (Pentax-A 200mm is for 645 film, and the two Pentax-M 100mm are for Full-frame or 35mm film)?

Thank you, I appreciate the assistance anyone may be able or willing to provide.

I’m not too experienced with this approach but if I’m not mistaken the tube lens should be corrected for infinity, and used at the infinity setting. Apparently there is some sample variation with the Pentax M series lenses but my copy of the M 100mm f/2.8 performs very well at infinity, so it might work well in your setting. The f/4 Macro lens is a rather simple (Heliar type) construction and performs well in the intended close range but is reputed to be less brilliant at infinity.

How do you like the results you are getting with the A 645 200mm f/4? Which camera are you using?

Hi, thank you!

I currently use a Canon EOS 500D, APS-C camera; I’m thinking I might need a higher-resolution camera in the future, maybe a 750D. Being a hobbyist on a budget, the 500D is probably good enough for my needs though. :sweat_smile:

Your details on the Pentax lenses and experience with them is most helpful; I have yet to get the 100mm f/2.8, but in the future I might continue the route of using a tube lens and microscope objective: at the moment, I’m planning on getting a Laowa 25mm f/2.8 2.5-5x Ultra Macro lens; it costs about the same as a 5x Mitutoyo objective, but hopefully it will work well.

I haven’t used the 645 200mm f/4 lens yet, but hopefully I’ll eventually get a good Mitutoyo and an M 100mm f/2.8 lens; the paired-lens route might give better results than the Laowa, but I’ll have to see.


I haven´t done super microscopic macro photography, but I have owned and used the Pentax-M 100mm f4 Macro and it is a wonderful lens. Focus has a very long throw so you can get very precise. It is sharp at f4 and has wonderful colors and bokeh. The Pentax-A 100mm f2.8 Macro is supposed to be better but it I haven´t used it.

Thank you!

You’re absolutely right, the M 100mm f/4 is a great macro lens; actually, I had considered using it with a Mitutoyo objective (paired lens setup), but I found out that dedicated macro lenses aren’t very suitable for such use, since the front element of the lens is distanced relatively far back (

I do use a Canon 60mm f/2.8 Macro occasionally by itself, and it also performs quite well, although not so much with attached Hoya 2x and 4x circular magnifiers (I’ll attach a pic of two 3mm Pseudomyrmex sp. ants in Sumatra amber, taken using the diopters on the Canon 60mm).

Terrible quality photos, but it’ll be much better with a proper lens setup. : )

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