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07-27-2016, 05:48 AM   #1
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Question about the D FA 100mm F/2.8 Macro

When the D FA 100 mm F/2.8 Macro is mounted on a digital crop sensor Pentax body is the F/2.8 really F/4.2 since this a full frame lens?
1.5 x F/2.8=F/4.2 or does that not apply to this lens?
Any assistance is appreciated!

07-27-2016, 05:55 AM   #2
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the crop factor (which should honestly be ignored, IMHO) only applies to the focal length (field of view), not the aperture....

Last edited by pepperberry farm; 07-27-2016 at 06:06 AM.
07-27-2016, 05:56 AM   #3
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QuoteOriginally posted by Joe M Quote
When the D FA 100 mm F/2.8 Macro is mounted on a digital crop sensor Pentax body is the F/2.8 really F/4.2 since this a full frame lens?
1.5 x F/2.8=F/4.2 or does that not apply to this lens?
Any assistance is appreciated!
No it is always 2.8 regardless of the camera you put it on. The crop factor only affects the field of view. So 100mm on a 1.5 crop factor would be like you are looking through camera with a 150mm lens.
07-27-2016, 06:00 AM   #4
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What pepperberry said. The aperture is a physical attribute of the lens (as is the focal length), and will be the same no matter which camera you put it on.



07-27-2016, 07:13 AM   #5
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In my experience chasing insects you're not going to use f2.8 anyway. You'll have it stopped well down, I've recently been shooting f16 with a Sunpak ring flash and been VERY happy with the results. In sun without the flash I'd stay in the f11-f13 range.
07-27-2016, 07:28 AM   #6
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If you mean how much depth of field you get then be ready for endless debate. What I'd say is that you get more depth of field with any lens when mounted on a smaller sensor than on a bigger one. But the exposure should be essentially the same. In practice you can argue that higher ISO options on larger sensors allow different exposures and you can argue subject distance and framing equivalence / but in the end none of this cares if the lens is full frame or cropped. A 100mm lens on APSC at f2.8 is going to work the same even if it was a massive 6x7 lens on an adapter.
07-27-2016, 08:07 AM   #7
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QuoteOriginally posted by TER-OR Quote
In my experience chasing insects you're not going to use f2.8 anyway.
For most people who buy this lens for its intended purpose on anything, not just insects, they're in close and DOF concerns will drive you far away from maximum aperture. OTOH if you are also using it as a general short tele, f/2.8 is nice to have in a lens of this focal length, especially (as was the case with me for a while) if all your other glass which reaches that focal length is zoom and at least a stop slower (e.g. the D-FA 28-105 is TWO stops slower at 100; the DA tele zooms are probably down to f/4.5ish by this stage so a stop and a half; the 60-250 is f/4 all the way, so one stop; only the 50-135/2.8 keeps up in speed terms), and this one is WR and full frame to boot.
07-27-2016, 10:24 AM   #8
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The lens does not become dimmer if you put it on a smaller camera. Ignore all equivalence unless you actually own both formats of cameras. In that case, equivalence can help you imagine what a lens rendering (FoV, DoF) might look like on the other format.

Also, the 100mm macro gives 1:1 magnification on FF, so on APSC it appears to give more than 1:1 magnification. If you go into the equivalence vortex you will just get confused over useless things. The DFA 100mm is a great lens, on ASPC and on FF. You will enjoy it on either camera, or both.

QuoteOriginally posted by Joe M Quote
1.5 x F/2.8=F/4.2 or does that not apply to this lens?
You cannot just multiply 2.8 by 1.5x. Even hardcore equivalists only multiply the EV, not just the aperture number.

07-27-2016, 10:45 AM   #9
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QuoteOriginally posted by Na Horuk Quote
The lens does not become dimmer if you put it on a smaller camera. Ignore all equivalence unless you actually own both formats of cameras. In that case, equivalence can help you imagine what a lens rendering (FoV, DoF) might look like on the other format.

Also, the 100mm macro gives 1:1 magnification on FF, so on APSC it appears to give more than 1:1 magnification. If you go into the equivalence vortex you will just get confused over useless things. The DFA 100mm is a great lens, on ASPC and on FF. You will enjoy it on either camera, or both.


You cannot just multiply 2.8 by 1.5x. Even hardcore equivalists only multiply the EV, not just the aperture number.
Thanks, I am seeing it really only matters if you have both format cameras then it will perform differently on one camera to the other! Thanks again!

---------- Post added 07-27-16 at 11:46 AM ----------

QuoteOriginally posted by Na Horuk Quote
The lens does not become dimmer if you put it on a smaller camera. Ignore all equivalence unless you actually own both formats of cameras. In that case, equivalence can help you imagine what a lens rendering (FoV, DoF) might look like on the other format.

Also, the 100mm macro gives 1:1 magnification on FF, so on APSC it appears to give more than 1:1 magnification. If you go into the equivalence vortex you will just get confused over useless things. The DFA 100mm is a great lens, on ASPC and on FF. You will enjoy it on either camera, or both.


You cannot just multiply 2.8 by 1.5x. Even hardcore equivalists only multiply the EV, not just the aperture number.
Thanks!

---------- Post added 07-27-16 at 11:47 AM ----------

QuoteOriginally posted by pathdoc Quote
For most people who buy this lens for its intended purpose on anything, not just insects, they're in close and DOF concerns will drive you far away from maximum aperture. OTOH if you are also using it as a general short tele, f/2.8 is nice to have in a lens of this focal length, especially (as was the case with me for a while) if all your other glass which reaches that focal length is zoom and at least a stop slower (e.g. the D-FA 28-105 is TWO stops slower at 100; the DA tele zooms are probably down to f/4.5ish by this stage so a stop and a half; the 60-250 is f/4 all the way, so one stop; only the 50-135/2.8 keeps up in speed terms), and this one is WR and full frame to boot.
Thanks!

---------- Post added 07-27-16 at 11:47 AM ----------

QuoteOriginally posted by UncleVanya Quote
If you mean how much depth of field you get then be ready for endless debate. What I'd say is that you get more depth of field with any lens when mounted on a smaller sensor than on a bigger one. But the exposure should be essentially the same. In practice you can argue that higher ISO options on larger sensors allow different exposures and you can argue subject distance and framing equivalence / but in the end none of this cares if the lens is full frame or cropped. A 100mm lens on APSC at f2.8 is going to work the same even if it was a massive 6x7 lens on an adapter.
Thanks!

---------- Post added 07-27-16 at 11:48 AM ----------

QuoteOriginally posted by TER-OR Quote
In my experience chasing insects you're not going to use f2.8 anyway. You'll have it stopped well down, I've recently been shooting f16 with a Sunpak ring flash and been VERY happy with the results. In sun without the flash I'd stay in the f11-f13 range.
Thanks!

---------- Post added 07-27-16 at 11:48 AM ----------

QuoteOriginally posted by savoche Quote
What pepperberry said. The aperture is a physical attribute of the lens (as is the focal length), and will be the same no matter which camera you put it on.
Thanks!

---------- Post added 07-27-16 at 11:48 AM ----------

QuoteOriginally posted by pearsaab Quote
No it is always 2.8 regardless of the camera you put it on. The crop factor only affects the field of view. So 100mm on a 1.5 crop factor would be like you are looking through camera with a 150mm lens.
Thanks!

---------- Post added 07-27-16 at 11:49 AM ----------

QuoteOriginally posted by pepperberry farm Quote
the crop factor (which should honestly be ignored, IMHO) only applies to the focal length (field of view), not the aperture....
Thanks!
07-27-2016, 11:17 AM   #10
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Welcome!
07-27-2016, 11:33 AM   #11
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Just a warning if you're considering buying it - if you miss focus, usually because the focus point is parked on the edge of the object or on a very thin object, or if you are changing from near to far focus or vice versa, it will rack from one end of its focal range to the other before it settles, and it is one of the louder screwdrive lenses in the Pentax range. However, it is blessed with Quick Shift - get close manually, almost there, then touch up with AF to get it perfect.
07-27-2016, 01:58 PM   #12
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QuoteOriginally posted by Joe M Quote
Question about the D FA 100mm F/2.8 Macro
Many beat me to it, I'll just say welcome.
07-28-2016, 02:19 PM   #13
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QuoteOriginally posted by UncleVanya Quote
Welcome!
Thanks!

---------- Post added 07-28-16 at 03:20 PM ----------

QuoteOriginally posted by Kerrowdown Quote
Many beat me to it, I'll just say welcome.
Thanks!
01-16-2017, 05:46 PM   #14
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QuoteOriginally posted by UncleVanya Quote
Welcome!
Thanks!

---------- Post added 01-16-17 at 06:47 PM ----------

QuoteOriginally posted by pathdoc Quote
Just a warning if you're considering buying it - if you miss focus, usually because the focus point is parked on the edge of the object or on a very thin object, or if you are changing from near to far focus or vice versa, it will rack from one end of its focal range to the other before it settles, and it is one of the louder screwdrive lenses in the Pentax range. However, it is blessed with Quick Shift - get close manually, almost there, then touch up with AF to get it perfect.
Thanks!

---------- Post added 01-16-17 at 06:47 PM ----------

QuoteOriginally posted by Kerrowdown Quote
Many beat me to it, I'll just say welcome.
Thanks!
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