[ChrisPMR]

Hi everyone,
I have a sigma 50mm macro lens. I can focus within a few centimeters (around an inch-ish) from the front of the lens to the subject.
When I was looking up other macro lenses I saw that the pentax 100mm macro has a minimum focusing distance of 30cm (12inches). I don't understand how it can be a macro lens if that's the closest focusing distance.
I tried taking photos (albeit out of focus) with my 18-135 and my 55-100 @ 100mm from around 30cm away and it didn't look close to 1:1
I feel like I'm missing something obvious! Help please :-)
Chris

[Lowell Goudge]

Note that 1:1 reproduction ratio (For a simple lens) is with the lens to subject distance =lens to fills or sensor distance = 2 x focal length. So for a nominal 100 mm lens, you would be 400 mm from the subject with the sensor. This is how many lenses state the close focus distance.

If you are at less than this, then likely the lens is internally focusing and reducing focal length to achieve close focus

[photoptimist]

First, the minimum focusing distance (MFD) in the specification or on the lens barrel is measured from the sensor or film plane to the in-focus subject, not from the lens to the subject.

Second, the "macro" designation refers to lenses that have a high reproduction ratio with most "true macro" lenses reaching 1:1 (like both your Sigma and the Pentax 100mm macro do). For a simple lens (e.g., a single-element lens), the MFD at 1:1 is four times the focal length. Most macro lenses are complex, making the MFD longer or shorter than the "simple lens" formula would suggest.

Third, if you compare the Pentax 100mm macro to the Sigma50 mm macro both at their MFD, you'll see that both magnify the subject by the same amount. Both images of the subject will be "life size" on the sensor. However, the front of Pentax 100 will be much farther from the subject than the Sigma 50 is which can be useful for capturing cautious critters (a 190 or 200 mm macro is even better for this). The image from the Pentax 100 will also show less of the background than the image from the Sigma 50 -- the Sigma being a wider-angle lens. The DoF at 1:1 will be identical for both lenses because the general rule that wide angle lenses have greater DoF only applies to images of distant subjects.

[Apet-Sure]

If you look at the specs for the Pentax-F and FA 50mm f2.8 Macro, the minimum focus distance is 19cm, which gets you the 1:1 reproduction ratio. For the D FA it's 20cm. If your Sigma 50 can focus much closer than that, I wonder if you are getting more than 1:1. I own a Sigma 17-70mm f2.8 DC macro. While reading user reviews, one person commented that they could focus with the subject almost touching the front element. Don't know what focal length they were using for that result. Though I haven't kept records, the closest I get is probably 3-5 inches.

When I was looking up other macro lenses I saw that the pentax 100mm macro has a minimum focusing distance of 30cm (12inches). I don't understand how it can be a macro lens if that's the closest focusing distance.

My Pentax-F 100mm f2.8 1:1 Macro focuses down to 31cm. The term 'Macro', when used properly, refers to a lens's ability to achieve 1:1 reproduction, regardless of what the minimum focus distance is.

[swanlefitte]

Camera terminolgy. The sigma 50mm is 66.xxmm long, the penrax 100mm is 80.xxmm long.

The sigma minimum is the registration distance ( sensor to lens) + lens length+ distance to subject.
The 50mm means distance from sensor to the lens.
The lens is actually multiple lenses working different ways and the "lens" is the place where these lenses effects average/cancel each other out.

It gets weirder. My 70mm sigma lens is about 35mm at infinity and 60mm when it telescopes out for macro.

If i read or calculate the distance my 70mm needs for macro then read how long the lens is, add that to the registration distance and subtract that from the total distance to see how close my lens is to the subject i will be wrong because it wont take into account the telescoping.
End of rant.

[stevebrot]

When I was looking up other macro lenses I saw that the pentax 100mm macro has a minimum focusing distance of 30cm (12inches). I don't understand how it can be a macro lens if that's the closest focusing distance.

Minimum focus distance is measured from the subject to the focal plane. With the 100mm macro, that is 1:1 reproduction or 1x magnification. Working distance is measured from the subject to the front of the lens. A 100mm lens on a macro bellows requires 100mm extension from the lens mount to accomplish 1:1 and most macro lenses will require at least that much extension at 1:1.


Steve

[stevebrot]

the front of Pentax 100 will be much farther from the subject than the Sigma 50

My Sigma 50/2.8 EX DG Macro at 1:1 (18.9cm MFD) ...




Steve

[Lowell Goudge]

It’s a shame they never quote working distance

[normhead]

It’s a shame they never quote working distance

For butterflies etc. my old 70-300 macro was great, even though it was only 1:2. (For some reason Sigma calls 1:2 lenses macros.) you were far enough away you had much less chance of spooking an insect. But as pointed out, 1:1 is 1:1 one. You don't get more magnification because it focuses closer.

And not having the lens close enough to block your light source is a good thing unless you use a macro-ring light.

If memory serves me well, my both my Sigma 70 and DFA 100 need to have their lens hoods removed to focus on sometime at the minimum. But I could be wrong. The FA 50 is recessed from the front of the barrel and doesn't need a lens hood.

[stevebrot]

It’s a shame they never quote working distance

Yep...the Irix 150mm Macro has an MFD of 34cm, but due to its being internal focus, working distance is about the same as the Pentax D FA 100/2.8 WD Macro.


Steve

[aslyfox]

perhaps this article and video might be of interest to the OP

The Advantages of a Dedicated Macro Lens
Achieving lift-size magnification
By PF Staff in Tutorial Videos on Apr 4, 2016

Read more at: The Advantages of a Dedicated Macro Lens - Tutorial Videos | PentaxForums.com

[ChrisPMR]

Thank you all for your helpful replies :-)

[aslyfox]

Thank you all for your helpful replies :-)

oh

the replies were supposed to be helpful

oops

[mikesbike]

Apparently it boils down to how close does one want to get to the subject, and what kind of perspective would be desired for the effect one is looking for.

---------- Post added 05-07-20 at 06:51 PM ----------

Yep...the Irix 150mm Macro has an MFD of 34cm, but due to its being internal focus, working distance is about the same as the Pentax D FA 100/2.8 WD Macro.


Steve

Right! I had not considered that factor- "focus breathing" which shortens effective FL when getting closer, could be a big factor for lenses designed for closeup work!

[swanlefitte]

I am curious if dof is measured from the sensor, the lens center, or front of the lens. With macro that matters alot.