Forgot Password
Pentax Camera Forums Home

Show Printable Version Search this Thread
01-13-2013, 09:26 PM   #106
New Member
cak's Avatar

Join Date: Jan 2009
Location: San Diego, California USA
Posts: 18
QuoteOriginally posted by Anvh Quote
The Pentax 35mm macro has a wider DOF as for example the 100mm or the 50mm macro at 1:1 magnification
This can easily be explained by focal length alone.

I found an interesting article on DOF and pupil magnification here:

The DOF equations

a quote from the above article:
"Apart from the DOF, the pupil magnification affects quantities such as the depth of focus, the effective aperture (in relation to exposure) and the field of view, For faraway subjects the pupil magnification has no significant influence on these quantities; it becomes important for image magnifications greater than, say, 0.1. In the very macro regime the impact of a nonunitary P is substantial."

in other words, when you start attaching extension rings, bellows or reverse mounting a lens, pupil magnification becomes relevant.

Unless you do a lot of close macro work it's not likely to play any significance.

my 2 cents

01-13-2013, 11:17 PM   #107
Veteran Member

Join Date: Feb 2011
Posts: 4,893
QuoteOriginally posted by cak Quote
This can easily be explained by focal length alone.
Towards the macro-end of things the DOF is often approximated to be independent of focal length.
01-14-2013, 04:21 AM   #108
Junior Member

Join Date: Jul 2008
Location: Milton, Ontario
Photos: Gallery | Albums
Posts: 42
Is this it?

"Specialist accessory manufacturer Metabones and optics company Caldwell Photographics have jointly announced the 'Speed Booster', a lens adapter for APS-C and Micro Four Thirds mirrorless cameras that reduces the focal length by a factor of 0.71x and increases the maximum aperture by 1 stop."

Metabones announces 'Speed Booster' lens adapter for mirrorless cameras: Digital Photography Review
01-14-2013, 05:07 AM   #109
Veteran Member
Anvh's Avatar

Join Date: Sep 2011
Posts: 4,616
QuoteOriginally posted by ElJamoquio Quote
Towards the macro-end of things the DOF is often approximated to be independent of focal length.
Yep but i actually said it wrong, the with retrofocus lenses so that accounts most lenses wider then register distance the pupil magnification is higer then 1 so that mens that the pentax 35mm has less DOF actually then the 100mm at the same magnification.
And DOF is actually calculated by effective aperture and with the magnification... some like to use focal length and focus distance instead, but that's the same as magnification though
And with the same magnification and the same effective aperture you get the same DOF off course, also with normal photography

@cak if you have actually read the page that you link you will also read that focal length doesnt matter for DOF but the magnification does, that's not only for macro photography but all photography.
Beside that i never said that it's relevant with all photography except if you want to be 100% correct, then you must include it
But anyway he says that a magnification of 0,1 is enough, you know most lenses can focus twice as close, the 18-55 kit lens goes to 0,34 even
So where you get the idea from that you need extension tubes or such is beyond me...

01-15-2013, 01:05 PM   #110
Veteran Member
Ben_Edict's Avatar

Join Date: Jul 2007
Location: SouthWest "Regio"
Photos: Albums
Posts: 3,303
QuoteOriginally posted by Fogel70 Quote
Yes, I know this. but wasn't it just what I wrote?

A smaller sensor capture less light because it is smaller, not because intensity of light is lower. "Area of sensor" x "intensity of light" = total amount of light captured by the sensor.
Yes, I agree, that the "total amount of light captured" is different - because the collecting area is different. Just as with photovoltaics on your rooftop. But what is the message? If you break that down to the image formation question, there is no message. Because Lowell is right. In photography we are talking about the illumination level over a certain sensor area. A lens will form an image on the sensor and collect the exact same amount of light in the same are on the sensor - whatever be the size of the sensor. That is the important message. Because that constitutes the basic principles using aperture values.


  • Submit Thread to Facebook Facebook
  • Submit Thread to Twitter Twitter
  • Submit Thread to Digg Digg
Tags - Make this thread easier to find by adding keywords to it!
aperture, crop, f/5.6, factor, ff, formats, fov, k-mount, lens, lenses, pentax lens, slr lens
Thread Tools Search this Thread
Search this Thread:

Advanced Search

Similar Threads
Thread Thread Starter Forum Replies Last Post
lens aperture and focal length markings sylvanite Pentax SLR Lens Discussion 3 11-13-2011 12:45 PM
FFF question - fisheye, focal length and FOV d.bradley Pentax SLR Lens Discussion 10 09-11-2011 11:21 PM
Variation in true focal length between copies of the same lens dgaies Pentax SLR Lens Discussion 2 05-22-2011 10:46 AM
What is the True Focal Length of your IF lens? Lowell Goudge Pentax SLR Lens Discussion 28 03-22-2011 04:04 PM
possible for 2 lenses to be lighter/darker at the same focal length and aperture? paperbag846 Pentax SLR Lens Discussion 6 08-27-2010 03:28 PM

All times are GMT -7. The time now is 02:34 AM. | See also:, part of our network of photo forums!
  • Red (Default)
  • Green
  • Gray
  • Dark
  • Dark Yellow
  • Dark Blue
  • Old Red
  • Old Green
  • Old Gray
  • Dial-Up Style
Hello! It's great to see you back on the forum! Have you considered joining the community?
Creating a FREE ACCOUNT takes under a minute, removes ads, and lets you post! [Dismiss]