[tubadad]

But you could just go with extension tubes for the 50mm.

Thank you
Russell

True, but they are out of stock everywhere I looked - at least the Kenko ones.

There are some KPR ones available, but I understand that the Ricoh pin can casue problems. B&H has them for under $90.

Be nice if Pentax made some.

[8540tomg]

I'm a bit late to this thread but never at a loss for an opinion. I have both the Pentax M 50mm f4 macro and the A 100mm f 2.8 macro. Both are excellent lenses in either film or digital. The M 50mm is a great little lens and can be found fairly cheaply (under $150??) on Ebay. If you are looking for an inexpensive entry level macro lens this might do the trick. It will only provide 1/2 life size images without extension tubes. The A 100mm is much more expensive but provides that extra length for additional lighting and sometimes safety (snakes etc). It also will give you a 1 to 1 reproduction ration without extension. I use both but always reach for the 100mm first as I like the extra length and addition 2.8 speed.

Tom G

[Light_Horseman]

+1 for the Tamron 90mm. You will not regret it.

[Wheatfield]

funny how many answers given so far, when the original poster was so completely vague about what they wanted to shoot with it, or how they wanted to use it or what other lenses they had...

Perhaps that's because he was asking what people used and why.
Just a though.

[MikePerham]

A lot of folks have advocated 100mm as their choice. I used a 100mm for years when shooting 35mm but now with my K10D I find it a bit long in many cases. Ideally if I could chose only one focal length I would like about a 60mm with an APS sensor. If Pentax don't come out with an APS specific macro (like the 35 limited) at around that focal length, I would likly get the 50mm. I really liked the idea of Nikons 70-180 macro zoom, a real macro lens unlike a lot lenses that purport to be macros but can't get more that 1:4 ratio's. Perhaps Pentax can be unique again with a true 60-120 macro zoom.

[tscip22]

Most of the time I will be shooting outdoors, mainly flowers and bugs. I would like to thank everyone for their input. I am leaning towards the 100mm.

The 100mm is also a good portrait length when you have room to use it.

[Sean Nelson]

As mentioned elsewhere, subject distance is an important thing to consider. Longer lenses mean you can work further from the subject.

Depth of field is pretty much a non-issue, assuming that you would frame the subject similarly (ie, have the subject fill the frame by moving closer with the 35mm or further away with the 100mm). Under those conditions depth of field will be virtually the same with the 35mm, 50mm or 100mm lenses.

You should also consider whether you can or need to use the lens as a normal, non-macro prime. I chose the DA 35mm because it doubles as quite a nice "normal" lens for general-purpose photography.

Something I haven't seen mentioned so far - field of view. A benefit of a longer lens for macro work is that it's easier to position the lens to exclude unwanted stuff from the background of the shot. A shorter lens has a wider field of view, so it's often difficult or impossible to get an angle that eliminates some distraction or the other from the background.

[Gooneybird]

depends what you want.. for animals you need at least 100mm.. for flowers lower is also good, because you can get close.. I wish there was a 150mm for Pentax

Since no one else has mentioned it, I'm going to recommend what I feel is one of the best Macro lenses out there, but it is costly. That's the Sigma 180 mm Macro. It will focus down to within about 17 inches, and is the sharpest lens I've ever seen. Just my 2 cents.

Olin McDaniel

[shownoff]

I just took up a serious interest in macro photography this spring with the local photography club. I did not own any true macro lenses so I picked up a set of extension tubes to start with. I got some decent images with it, but started my search for a good macro lens. I found a SMC Pentax-FA 50mm f/2.8 Macro lens on eBay that was a new lens that was a demo at a camera shop to show people. I picked it up for about $275. I love this lens! I have gotten some fantastic shots with it. I continued to look at macro lenses and found a SMC Pentax-FA 100mm f/2.8 Macro lens in mint shape on ebay that I picked up for less than $350. I just got this lens last Saturday and have had a chance to play with it. It is a fantastic lens as well. I will say that it lets you get farther away from insects, and butterflies are one of my favorites to shoot. Right now my butterfly bushes are a little lean on blooms, but I did manage to get a number of subjects to shoot. I would be hard pressed to choose which one I would choose as my favorite though. I am impressed with the image quality of both these lenses.

[Wheatfield]

Most of the time I will be shooting outdoors, mainly flowers and bugs. I would like to thank everyone for their input. I am leaning towards the 100mm.

I don't think you can go wrong with that focal length for macro.
I shot this a little while back with the A100/2.8 macro on the K20.
The mushrooms were perhaps a couple of inches tall.

[brownargus]

Depth of field is pretty much a non-issue, assuming that you would frame the subject similarly (ie, have the subject fill the frame by moving closer with the 35mm or further away with the 100mm). Under those conditions depth of field will be virtually the same with the 35mm, 50mm or 100mm lenses.

Sean, I think you are right. I remember somewhere in the distant past reading that DOF was directly related to reproduction ratio, but have not been able to rediscover where I read it. Consequently, it didn't matter what the focal length the lens was, if the ultimate image on the film/sensor was the same size, the DOF would be the same. Can anyone confirm this?

John

[Wheatfield]

Sean, I think you are right. I remember somewhere in the distant past reading that DOF was directly related to reproduction ratio, but have not been able to rediscover where I read it. Consequently, it didn't matter what the focal length the lens was, if the ultimate image on the film/sensor was the same size, the DOF would be the same. Can anyone confirm this?

John

Consider it confirmed.

[chirpy]

Since no one else has mentioned it, I'm going to recommend what I feel is one of the best Macro lenses out there, but it is costly. That's the Sigma 180 mm Macro. It will focus down to within about 17 inches, and is the sharpest lens I've ever seen. Just my 2 cents.

Olin McDaniel

I'll second that. I have the Sigma 180mm Macro and the DFA-100 Pentax and almost always use the Sigma for instect macro work. It's a stunning lens.

I've got the Tamron 90mm and absolutely love it as an all around macro lens. But if you're going to get serious about shooting macro there's more to consider than merely how close you are from the subject.

DOF plays a huge part in macro photography. For example at f/16 and 6 inches from subject:

50mm yields a DOF of 0.16 inches
100mm yields a DOF of 0.02 inches

So the subject matter and light can drive your focal length requirements quite a bit. Flash and/or ND filters can even things out but they are not always available or feasible for the shot.

You are right about depth of field and distance, BUT to have the same object at the same size with a 35mm lens or a 100mm lens, the depth of field will be the same because you are going to be farther away from the subject with the longer lens, so the depth of field shouldn't be part of the equation when selecting a macro lens. The subject you intend to photograph is the most important criteria.

I agree with flyer. Your (venturi) calculations are correct. But when you switch from 50mm to 100mm you have to move back to twice the distance, for the same angle of view, and there you have the very same DOF, not more not less.