DA 20-40mm Limited f-stop markings - which f.l. are they for?

gavinhw · 09-22-2019, 09:33 AM

Hi

Forgive my ignorance, but the DA 20-40mm has fixed f-stop markings just below the distance scale to allow for hyperfocal shooting.

I'm confused as to which focal length they apply to?

The width of a given pair of markings for a given f-stop should widen or narrow based on focal length, no? But they are fixed here.

Educate me!

lytrytyr · 09-22-2019, 12:51 PM

The wide-apart markings give the depth of field at 20mm.
The close-together markings give the depth of field at 40mm.

gavinhw · 09-22-2019, 01:33 PM

Originally posted by lytrytyr

The wide-apart markings give the depth of field at 20mm.
The close-together markings give the depth of field at 40mm.

Thanks! I thought all along the wider-apart markings were just there to allow room for the display of the numerical f-stop values.... face palm.

parinya-ekparinya · 09-22-2019, 01:42 PM - **1 Like**

Although I have never owned this lens (I wish I could...), let me guess based on my knowledge.

Your understanding is correct; the depth-of-field scale should be varied with different focal length and lens design. In the case of the HD DA 20-40mm lens, which is a zoom lens, I believe its depth-of-field scale applies to the longest focal length, which is 40mm.

Let me provide a few reasons to support my belief.
First, we can see that the scale starting from f/4 to f/32; this range applies to the long-end, obviously.
Second, the longer the focal length the narrower the depth-of-field. Therefore, applying the depth-of-field scale of the long-end shall also ensure enough depth-of-field for any shorter focal length because the depth-of-field of a shorter focal length is wider anyway. This leads me to believe that the scale may not be precise for the short-end, however, it should be good enough to be used in practice.

I hope that helps!!

**yucatanPentax** · 09-23-2019, 02:27 PM

Originally posted by lytrytyr

The wide-apart markings give the depth of field at 20mm.
The close-together markings give the depth of field at 40mm.

? ? ?

**JensE** · 09-23-2019, 02:49 PM

Checking with DOFMaster, using 1000m focus distance (close enough to infinity) and the default 0.02mm circle of confusion, I get the near point for 40mm and f/32 at 2.49m - consistent with the narrow scale. For 20mm, it's 0.62m at f/32 and 1.25m at f/16 - both consistent with the wide scale.

Given the fuzzy nature of the DoF-transition, using the narrow and wide scales at 40mm and 20mm respectively will work as indicated by lytrytyr, within the limits of the wide distance scale spacing and short focus throw.

**c.a.m** · 09-23-2019, 02:58 PM - **1 Like**

Originally posted by yucatanPentax

? ? ?

Following the post by @lytrytyr, the scale is further explained here: DOF scale on DA 20-40 - PentaxForums.com

Last edited by c.a.m; 09-23-2019 at 03:10 PM. Reason: typo

parinya-ekparinya · 09-24-2019, 04:08 AM

Originally posted by c.a.m

Following the post by @lytrytyr, the scale is further explained here: DOF scale on DA 20-40 - PentaxForums.com

Wow!! I have never thought of that before; it is such a simple idea yet practical design. Thank you for the information.

**JensE** · 09-24-2019, 11:57 AM - **1 Like**

Originally posted by parinya-ekparinya

Wow!! I have never thought of that before; it is such a simple idea yet practical design. Thank you for the information.

A lot of push-pull zoom lenses, like the SMC Penatx-A 35-135mm, have elaborate curves showing the reciprocal DoF curves. Showing the DoF for the minimum and maximum focal lengths on the 20-40mm while at the same time using it to space out the labels is indeed very clever use of the constraint space.

09-22-2019, 09:33 AM	#1
gavinhw Senior Member Join Date: Oct 2007 Posts: 145	DA 20-40mm Limited f-stop markings - which f.l. are they for? Hi Forgive my ignorance, but the DA 20-40mm has fixed f-stop markings just below the distance scale to allow for hyperfocal shooting. I'm confused as to which focal length they apply to? The width of a given pair of markings for a given f-stop should widen or narrow based on focal length, no? But they are fixed here. Educate me!

09-23-2019, 02:27 PM	#5
yucatanPentax Loyal Site Supporter Join Date: Dec 2009 Location: Dallas / Yucatan Posts: 1,840	Originally posted by lytrytyr The wide-apart markings give the depth of field at 20mm. The close-together markings give the depth of field at 40mm. ? ? ? Attached Images

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09-22-2019, 12:51 PM	#2
lytrytyr Pentaxian Join Date: Nov 2011 Photos: Gallery \| Albums Posts: 4,310	The wide-apart markings give the depth of field at 20mm. The close-together markings give the depth of field at 40mm.

09-22-2019, 01:33 PM	#3
gavinhw Senior Member Join Date: Oct 2007 Posts: 145 Original Poster	Originally posted by lytrytyr The wide-apart markings give the depth of field at 20mm. The close-together markings give the depth of field at 40mm. Thanks! I thought all along the wider-apart markings were just there to allow room for the display of the numerical f-stop values.... face palm.

09-22-2019, 01:42 PM - 1 Like	#4
parinya-ekparinya Senior Member Join Date: Nov 2017 Location: Bangkok Photos: Gallery Posts: 114	Although I have never owned this lens (I wish I could...), let me guess based on my knowledge. Your understanding is correct; the depth-of-field scale should be varied with different focal length and lens design. In the case of the HD DA 20-40mm lens, which is a zoom lens, I believe its depth-of-field scale applies to the longest focal length, which is 40mm. Let me provide a few reasons to support my belief. First, we can see that the scale starting from f/4 to f/32; this range applies to the long-end, obviously. Second, the longer the focal length the narrower the depth-of-field. Therefore, applying the depth-of-field scale of the long-end shall also ensure enough depth-of-field for any shorter focal length because the depth-of-field of a shorter focal length is wider anyway. This leads me to believe that the scale may not be precise for the short-end, however, it should be good enough to be used in practice. I hope that helps!!

09-23-2019, 02:49 PM	#6
JensE Site Supporter Join Date: Dec 2015 Location: Leipzig Photos: Gallery \| Albums Posts: 1,969	Checking with DOFMaster, using 1000m focus distance (close enough to infinity) and the default 0.02mm circle of confusion, I get the near point for 40mm and f/32 at 2.49m - consistent with the narrow scale. For 20mm, it's 0.62m at f/32 and 1.25m at f/16 - both consistent with the wide scale. Given the fuzzy nature of the DoF-transition, using the narrow and wide scales at 40mm and 20mm respectively will work as indicated by lytrytyr, within the limits of the wide distance scale spacing and short focus throw.

09-23-2019, 02:58 PM - 1 Like	#7
c.a.m Site Supporter Join Date: Jan 2013 Location: Ottawa, Canada Photos: Gallery \| Albums Posts: 4,189	Originally posted by yucatanPentax ? ? ? Following the post by @lytrytyr, the scale is further explained here: DOF scale on DA 20-40 - PentaxForums.com Last edited by c.a.m; 09-23-2019 at 03:10 PM. Reason: typo

09-24-2019, 04:08 AM	#8
parinya-ekparinya Senior Member Join Date: Nov 2017 Location: Bangkok Photos: Gallery Posts: 114	Originally posted by c.a.m Following the post by @lytrytyr, the scale is further explained here: DOF scale on DA 20-40 - PentaxForums.com Wow!! I have never thought of that before; it is such a simple idea yet practical design. Thank you for the information.

09-24-2019, 11:57 AM - 1 Like	#9
JensE Site Supporter Join Date: Dec 2015 Location: Leipzig Photos: Gallery \| Albums Posts: 1,969	Originally posted by parinya-ekparinya Wow!! I have never thought of that before; it is such a simple idea yet practical design. Thank you for the information. A lot of push-pull zoom lenses, like the SMC Penatx-A 35-135mm, have elaborate curves showing the reciprocal DoF curves. Showing the DoF for the minimum and maximum focal lengths on the 20-40mm while at the same time using it to space out the labels is indeed very clever use of the constraint space.