[kristoffon]

for macro purposes (close to 1/1 magnification), the magnification of the subject and the aperture will determine the dof

no they don`t, just look at the two flower pictures just posted

[newarts]

Theory says that depth of field for macros does not depend on focal length, only on f-stop and magnification:

DoF/WoF=2C*f-stop*(1+1/magnification)

where C is constant for a particular camera & display; WoF is width of field.

Dave in Iowa

[Ben_Edict]

The 50mm would be of more use for someone photographing inanimate objects - stamps, jewelry, etc. The depth of field is roughly double at the same magnification and aperture, and the perspective is different as well.

The 100mm is a better choice for taking pictures of creepy crawly things that fly away when you get too close.

The FA* 200mm f/4 Macro would be even better, but my wife won't let me mortgage the house to buy one.

The depth of field at the same magnification is roughly the same for the 50mm and 100mm lens, as you would be at about half the shooting distance with the 50mm lens, compared to the 100mm.

The only real difference is (as Dave illustrated so nicely) is the perspective, which is different at the same magnification, as you would go nearer with the 50mm for that. And perspective only changes with shooting distance.

Other than that, I fully agree, that longer fl macro lenses are very useful for anything biting, stinging or shy. I would love the A 200, but have to stick with my Voigtlander 180mm, instead...

Ben

[Blue]

. . .

Other than that, I fully agree, that longer fl macro lenses are very useful for anything biting, stinging or shy. I would love the A 200, but have to stick with my Voigtlander 180mm, instead...

Ben

I am having to make do with my Tammy 180mm (63B) as well.

[Ben_Edict]

I am having to make do with my Tammy 180mm (63B) as well.

Is that an Adaptall modell or AF? (Sorry, I am not Tamron lens designation conventions literate, though I have a 60B). The three Tamrons I have (the 90mm macro, the 300/2.8 of old and a 18-200 for the K-m of my partner) are all very competent performers, especially the primes. So I would expect even better from the 180.

Ben

[Marc Sabatella]

no they don`t, just look at the two flower pictures just posted

I'm no expert on this, but my impression is that, when comparing lenses of different focal lengths at the same magnification, the area in acceptable focus isn't much different, but the *appearance* of the OOF areas is. The longer focal lengths will be more blurry in the OOF areas, but the size of the in-focus is comparable. At least, that's how things seem to me in my limited experience here.

[jem]

I am having to make do with my Tammy 180mm (63B) as well.

I didn't realize that existed - thought it was only C/N. I did a quick Google search and found one vendor stocking the P/K but wonder if it is real. What are your impressions? I have and like the Tamron AF SP DI 90mm for reference...

[Blue]

Is that an Adaptall modell or AF? (Sorry, I am not Tamron lens designation conventions literate, though I have a 60B). The three Tamrons I have (the 90mm macro, the 300/2.8 of old and a 18-200 for the K-m of my partner) are all very competent performers, especially the primes. So I would expect even better from the 180.

Ben

Yes it is the SP adaptall 2 lens.

I didn't realize that existed - thought it was only C/N. I did a quick Google search and found one vendor stocking the P/K but wonder if it is real. What are your impressions? I have and like the Tamron AF SP DI 90mm for reference...

Its the SP 180mm f2.5 adaptall 2 lens. Only 3000 of them were made and are dispersed among all camera makes.

[jem]

Its the SP 180mm f2.5 adaptall 2 lens. Only 3000 of them were made and are dispersed among all camera makes.

Argh! Another rare lens... I was looking at the wrong one. I see it here - I remember the Nikkor 180mm very well - you have a very nice long macro!

And my search continues...

[bdery]

Not to barge in, but being a phycisist specialized in optics I feel that maybe I should provide some input on DOF calculations...

A macro lens is just a lens, and it's important to remember this. Its only special capability is to allow closer focusing than a regular lens. As with any lens, the standard equation applies for magnification. If O is the distance between lens and subject, and I is the distance from the lens to the image (the sensor or film, in the case of a camera), then magnification is M = I / O.

Focal length is linked to those parameters by the so-called "lens equation":

1 / f = 1 / O + 1 / I

I'm neglecting signs, for the sake of simplicity.

Depth of field is mildly more complex. It involves the circle of confusion (a measure of resolution) among other things. There is an aproximation of the real DOf value that is often used which is only valid if the subject is far from the lens. This approximation shows that DOF is inversely proportional to focal length, but proportionnal to subject distance.

There is another approximation that is used when the subject is close, as with macro shots. This is the interesting part. I'll quote wikipedia because it's very well expresed there:

for a given magnification, DOF is independent of focal length. Stated otherwise, for the same subject magnification, all focal lengths give approximately the same DOF. This statement is true only when the subject distance is small in comparison with the hyperfocal distance, however.

The reason for this is that although the 50 mm lens has a shorter focal length, it also has a shorter minimum focus distance, and those two parameters cancel each other.

Hyperfocal distance for a 50 mm f2,8 lens is 44 meters, roughly. For a 100 mm lens at f2,8, hyperfocal is at 178 meters.

To summarize, if two lenses are used at 1:1 magnification, AND if they are used very very far from the hyperfocal distance (as in any macro situation) then they will both yield similar DOF.

I hope this helps.

[gofour3]

ps: why would you ever reverse a macro lens? reversing for macro only makes sense for "normal" lenses, as far as i understand, to try to get around the disadvantages of the "1 to many" optical design of a regular lens (turning it into many to one, which somehow seems to work better for macro)? or am i wrong?

Reversing a K50/4 Macro will give you higher magnification, due to the deeply recessed front lens element. Reversing acts like an extension tube in this case and image quality is also improved when magnification is greater than 1.0x.

[GeneV]

That's an amazingly effective demonstration - thanks!

BTW, regarding the usefulness of the focal lengths for non-macro purposes - I actually find 100mm far more useful than 50mm. But that's because I also have a 28 and a 40 for use as "normal" lenses - I got these because I have a 50 but find it too long for most purposes. Whereas 50mm is also too short for portraits for me (not that I do portraits often). 100mm is on the long side for portraits (but still usable), but I find it *perfect* for concert photography, and long enough for general telephoto use (not wildlife) to replace a 135 in one's bag.

To each his own. I just found that looking over a lot of my zoom photos from early in my purchase of a Pentax digital SLR, far more of them were taken at around 50mm than 100mm.

[Marc Sabatella]

To each his own. I just found that looking over a lot of my zoom photos from early in my purchase of a Pentax digital SLR, far more of them were taken at around 50mm than 100mm.

Oh, it is indeed a personal preference thing. And like I said, my feelings are strongly influenced by the fact that I already have a 28 and 40 - if I had no primes at all, a 50 might make more sense to start with.

Still, there are a number of other factors here that are worth considering.

First, I wonder, what lens(es) were you using? If it was 18-55 and 50-200, then you get a way disproportionate number of images taken at 55 and 50 - basically, the images where you should have changed lenses but settled for what you had on. Also, if you tended to leave the 18-55 on because it was more generally useful, then who knows how many photos you simply didn't take, because the 18-55 wouldn't have cut it and it wasn't worth the bother to change. While you might still find more shots arondd 50 than 100, it might not be quite as big a discrepancy as the numbers initially suggest.

Now, if you were primarily using the 18-250 or something similar that includes both both 50mm and 100mm, then on the surface, more shots around 50 than around 100mm might seem significant.

I'd argue that's still a bit misleading, though. For one thing, you should consider how many of those shots at around 50mm really would have benefited that much from being shot with a prime, compared to the shots at 100mm. My 100mm-ish shots are almost all low light concert concert photos, or similar "performances" - something fast like a prime is practically a necessity. Whereas a lot of my shots around 50mm are landscapes.

But more importantly, as I alluded too before if I include 30-40mm as being "near 50", then I too would have lots of shots "near 50" - but so many more would be in that 30-40 range than actually *at* 50, that I'm way better off with a 35mm or 40mm prime than a 50mm. And given that one has a 35 or 40 (or the 31, or anything in that ballpark), the need for a 50 prime diminishes greatly.

So again, if you have no other other primes, then I would agree most people would be better off with a 50 than a 100. But I think most people are better off with a 35-ish prime than a 50, and if you've got that, then 100 makes a lot of sense as one's *second* prime. And if you do decide you want a 50, manual focus 50's that are a stop or faster than the 50 macro make more sense for many people.

Anyhow, again, everyone's needs and tastes are different, and it's certainly possible that many people would be better off with a 50mm prime than a 100mm prime for general use. But the issue *is* a bit more complicated than simply looking at the focal lengths at which you shoot with your zooms.

[pentaxman]

Reversing a K50/4 Macro will give you higher magnification, due to the deeply recessed front lens element. Reversing acts like an extension tube in this case and image quality is also improved when magnification is greater than 1.0x.

When reversing a lens, the more wide angle the lens, the more magnification you have. I have reversed a 24mm and ended up with over 3:1 magnification.

https://www.pentaxforums.com/forums/pentax-slr-lens-discussion/43370-ultra-cl...ngle-lens.html

[gofour3]

Yep reversed wide angles are the best for extreme magnification.

A Pentax Bellows K unit fully extended and a reversed K28/3.5 will give you close to 7:1
magnification.