Forgot Password
Pentax Camera Forums Home
 

Reply
Show Printable Version Search this Thread
09-20-2010, 12:14 PM   #16
Veteran Member




Join Date: Nov 2009
Location: Hong Kong
Posts: 1,352
Original Poster
Thanks guys, all very interesting and educational.

Being new to optics, I had presumed (perhaps like the majority), that an F2.8 would remain so at all focal distances.

Now that I know it's the nature of the beast, I don't have to concern myself when searching for a macro.

09-20-2010, 12:37 PM   #17
Veteran Member
Laurentiu Cristofor's Avatar

Join Date: Nov 2009
Location: WA
Photos: Albums
Posts: 3,044
QuoteOriginally posted by SOldBear Quote
Take a look at the aperture marking of this Mamiya 60mm macro F/2.8. At closest focusing distance (1:1 mag. ratio), the actual aperture is 2 stops slower than the aperture setting.
Nice markings on this lens - I wish they would have kept these on all macro lenses - very useful to quickly see the effective aperture.
09-20-2010, 12:45 PM   #18
Pentaxian
Lowell Goudge's Avatar

Join Date: Jan 2007
Location: Toronto
Photos: Gallery | Albums
Posts: 15,400
QuoteOriginally posted by Laurentiu Cristofor Quote
Nice markings on this lens - I wish they would have kept these on all macro lenses - very useful to quickly see the effective aperture.
blame technical progress. the markings became somewhat irrelevant with TTL flash, since the flash was metered directly off the film, eliminating the need to do manual calculations or corrections in Auto mode

It might have some interest today again, with respect to metering in general and errors that seem to be related to aperture, but perhaps that can be fed through the data pin on the lens mount.
09-20-2010, 12:47 PM   #19
Veteran Member




Join Date: Jun 2009
Photos: Albums
Posts: 1,679
QuoteOriginally posted by Unsinkable II Quote
Thanks guys, all very interesting and educational.

Being new to optics, I had presumed (perhaps like the majority), that an F2.8 would remain so at all focal distances.

Now that I know it's the nature of the beast, I don't have to concern myself when searching for a macro.
Don't feel badly, I am completely physics phobic, hence my degrees in math are NOT applied mathematics, just pure mathematics...so I too learned a LOT about the whole light fall-off and optics here. '

I would add a question in the face of these facts, is this a whole aperture shift or does the change only apply at MFD? So if i set the lense to f11 is it really f11? I ask because, well, it's the whole physics thing for me...not bad spatial ability only the "phear of physics" because it's just so....so....sooooo....UNATURAL!! hehehehe....

09-20-2010, 01:02 PM   #20
Pentaxian
Lowell Goudge's Avatar

Join Date: Jan 2007
Location: Toronto
Photos: Gallery | Albums
Posts: 15,400
QuoteOriginally posted by brecklundin Quote
Don't feel badly, I am completely physics phobic, hence my degrees in math are NOT applied mathematics, just pure mathematics...so I too learned a LOT about the whole light fall-off and optics here. '

I would add a question in the face of these facts, is this a whole aperture shift or does the change only apply at MFD? So if i set the lense to f11 is it really f11? I ask because, well, it's the whole physics thing for me...not bad spatial ability only the "phear of physics" because it's just so....so....sooooo....UNATURAL!! hehehehe....
It will change at all f stops I believe, because the aperture is reducing the light path diameter, somewhere in the lens, and since most magnification is achieved by extending the entire lens forward, you are only spreading the remaining light out to a larger image circle,

What might (and I can;t answer this because I don;t have one) change this, is if there are any internal focusing macro lenses. Internal focusing might alter not just lens extension but the actual flcal length of the entire lens. You can see this on some super zooms, where true focal length is only achieved at infinity, and close in you don't get the same magnification (or image size) that you would get with a prime lens at that distance.

Something to look into, and I'll bet if the lens data state the distance to the front element to the subject (working distance) for 1:1 you will be able to tell pretty quick, because 1:1 is when working distance = 2x focal length.
09-20-2010, 01:26 PM   #21
Veteran Member




Join Date: Jun 2009
Photos: Albums
Posts: 1,679
thanks Lowell...

I learned that about internally focusing lenses. My understanding was it applies to primes as well as zooms.

One thing I actually figured out about macros is it if you need 1:1 it's best to set the lense to the magnification ratio desired then position the camera to achieve focus...because as you focus a macro lense you are altering the mag ratio, this is one reason a MF macro lense is not a bad thing.

Oh, with my 35ltd I learned after the fact that the Pentax stated MFD is NOT the MWD (minimum working distance) which is what I assumed when originally buying it. So I am not sure about the 2x the FL as the MWD for 1:1 because the MFD of the 35ltd is about 5.5" which leads to that < 2" MWD at 1:1....

This did create some frustrations because a ~6" MWD is fine but a 6" MFD ends up at around less than 3" MWD, in fact it is under 2" which makes getting enough light to the subject at 1:1 a true challenge. Still it's possibably my favorite lense, even over my FA ltd's...
09-20-2010, 01:39 PM   #22
Pentaxian




Join Date: Feb 2009
Location: Alameda, CA
Posts: 3,198
QuoteOriginally posted by brecklundin Quote
...with my 35ltd I learned after the fact that the Pentax stated MFD is NOT the MWD (minimum working distance) which is what I assumed when originally buying it.
I've never seen any manufacturer publishing MWD (distance from the front element of the lens to the object). All I've ever seen are MFD (distance from the sensor/film plane to the object). This applies to all lenses, not just macro lenses.

BTW, I think the generally accepted correct spelling is "lens," not "lense."
09-20-2010, 01:57 PM   #23
Veteran Member




Join Date: Jun 2009
Photos: Albums
Posts: 1,679
first you are completely incorrect about the spelling of lense, in fact it is the traditional spelling dating back centuries...dictionaries are your friend but sorry if it so offends you.

second all my other macros have published the MWD not MFD...for example it was only recently that Sigma began publishing the MFD along side the MWD....
Macro Lenses - SigmaPhoto.com

09-20-2010, 02:09 PM   #24
Pentaxian
Lowell Goudge's Avatar

Join Date: Jan 2007
Location: Toronto
Photos: Gallery | Albums
Posts: 15,400
QuoteOriginally posted by brecklundin Quote
thanks Lowell...

I learned that about internally focusing lenses. My understanding was it applies to primes as well as zooms.
interesting
QuoteQuote:

One thing I actually figured out about macros is it if you need 1:1 it's best to set the lense to the magnification ratio desired then position the camera to achieve focus...because as you focus a macro lense you are altering the mag ratio, this is one reason a MF macro lense is not a bad thing.
this is why macro rails are so popular, me I use a bellows when I want preset magnification, I use macro lenses when I really don't care, but want to get something I like when framed
QuoteQuote:

Oh, with my 35ltd I learned after the fact that the Pentax stated MFD is NOT the MWD (minimum working distance) which is what I assumed when originally buying it. So I am not sure about the 2x the FL as the MWD for 1:1 because the MFD of the 35ltd is about 5.5" which leads to that < 2" MWD at 1:1....
that means at 1:1 the 2 inch distance implies focal length has dropped to 25mm
QuoteQuote:

This did create some frustrations because a ~6" MWD is fine but a 6" MFD ends up at around less than 3" MWD, in fact it is under 2" which makes getting enough light to the subject at 1:1 a true challenge. Still it's possibably my favorite lense, even over my FA ltd's...
This is wht I like A) Older MF Macros, and B) LONGER macros.

when you are working really close, it is a pain to get light in on the subject
09-20-2010, 03:26 PM   #25
Veteran Member




Join Date: Jun 2009
Photos: Albums
Posts: 1,679
QuoteOriginally posted by Lowell Goudge Quote
this is why macro rails are so popular, me I use a bellows when I want preset magnification, I use macro lenses when I really don't care, but want to get something I like when framed that means at 1:1
D'oh!! sure, that really makes complete sense...honestly I never made that connection before....sigh....I blame my poor toilet training!! hehehe...

QuoteQuote:
the 2 inch distance implies focal length has dropped to 25mm
Good point!! Actually there is a PF member with a blog that discusses the 35ltd and it's MWD...if I can dig up the link later I'll post it.

QuoteQuote:
This is wht I like A) Older MF Macros, and B) LONGER macros.

when you are working really close, it is a pain to get light in on the subject
Yeah, that was exactly the one fault I have with the 35ltd when shooting jewelry. The lighting is often a PITA so I end up with longer exposures and some really funky lighting positioning. Even under lighting is not always a solid solution for the smaller items. I have been pondering a swap out of my 77ltd for a 100WR or selling my 35ltd since I have the 43ltd which works great for most items I shoot. I just feel super comfy with the 35ltd but definitely need the added MWD of a 100mm macro. Right now selling the 35ltd is the most likely situation...if I sell just one lense, but I often need that wider angle...I guess the ideal setup would be a 28mm close focus and the 43ltd with a 100WR...
09-20-2010, 05:54 PM   #26
Veteran Member
RioRico's Avatar

Join Date: Jun 2008
Location: Limbo, California
Posts: 11,264
A couple quick points (yeah, I'll be brief for a change).

With any non-reversed lens, the closest focus (MWD) is the lens' focal length. This is easy to test. Put any lens on tubes (or bellows) of various lengths. See how close you can focus. My rough test is to hold a ruler against the screen of this laptop, then put my K20D in LiveView and slide it down the ruler until the screen is in focus. That gives me the MWD. With any reversed lens, the working distance (min/max) is the registration (mount to frame distance), which for Pentax lenses is ~45mm, under two inches.

There are SOME macro lenses whose maximum aperture does NOT change with magnification. I mentioned this here some time ago and was told that Nikon stole, er borrowed the design from somewhere. The iris blades open or close with focus, giving a constant f/4 or whatever. Apparently there hasn't been a great demand for this. Tsk.

Last edited by RioRico; 09-20-2010 at 06:10 PM.
09-20-2010, 06:25 PM   #27
Pentaxian
bdery's Avatar

Join Date: Mar 2008
Location: Quebec city, Canada
Photos: Gallery
Posts: 5,831
Something needs to be set straight. The pupil of the lens (the limiting aperture) does not change with focus distance. It's a fixed diameter. What changes is the RELATION between the pupil and the focal length. So no one is loosing any photon in the process, but the convenient ratio we photographers refer to as f-number does change.
09-20-2010, 06:36 PM   #28
Site Supporter
Nick Siebers's Avatar

Join Date: Jun 2008
Location: Madison, WI
Photos: Gallery | Albums
Posts: 2,152
Right, bdery, I was thinking the same thing. Like throwing a 50mm on a bellows - the size of the aperture is the same, but you need a lot more light or longer exposure to make up for the light lost to extension.

One of the positive aspects of the "close-up filters" is that they do not reduce light in this way. (The others are that they are easy to carry, and easy to put on. Downside - mostly they suck :-)

Reversing a lens is also useful for good macro with somewhat less light loss, but I don't know the math.
09-20-2010, 07:27 PM   #29
Veteran Member
Laurentiu Cristofor's Avatar

Join Date: Nov 2009
Location: WA
Photos: Albums
Posts: 3,044
QuoteOriginally posted by brecklundin Quote
first you are completely incorrect about the spelling of lense, in fact it is the traditional spelling dating back centuries...dictionaries are your friend but sorry if it so offends you.
Which dictionaries are you referencing? My Wordsworth doesn't have "lense" - only "lens".

Also, a quick search online produces this definition (lense - Wiktionary):
QuoteQuote:
Common misspelling of lens. (presumably a back-formation from the plural lenses)
I like etymology, so I'd like to find out more about this spelling form.
09-20-2010, 07:42 PM   #30
Inactive Account




Join Date: Mar 2009
Location: Australia
Posts: 207
QuoteOriginally posted by RioRico Quote
The formua for effective aperture: EA= NA*(M+1) where EA is effective aperture, NA is nominal aperture (the lens setting), and M is magnification. ...

I now remember reading about this somewhere, but it obviously didn't really register.

So, last week, I put a roll of film past a Macro lens I was using (Elicar 2.5/90 Macro), but accidentally forgot to reset the ISO on the camera (an OM2n) and had the ISO set to 200 for a 400 ISO film.

I didn't realize I had done this until about half-way through the film, when I reset it to 400.

I wondered how my shots would look overexposed by 1 stop (compared to my usual practice)

Well, actually, they came out very well.

Now, I see that -using the above formula- for a magnification of .25x (which is about what I was using, photographing flowers), I should be allowing a +1 stop exposure.

This is very interesting, and I will try to adopt this in my future technique.

Thanks

Last edited by Banjo; 09-20-2010 at 10:57 PM.
Reply

Bookmarks
  • 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!
distances, f2.8, k-mount, lenses, macro, pentax lens, slr lens
Thread Tools Search this Thread
Search this Thread:

Advanced Search


Similar Threads
Thread Thread Starter Forum Replies Last Post
1:1 macro lenses ismaelg Pentax SLR Lens Discussion 25 06-01-2010 04:37 AM
Macro lenses.. nikonnoah Pentax SLR Lens Discussion 10 09-19-2009 08:28 AM
Macro lenses -- AF or MF? K-9 Pentax SLR Lens Discussion 19 08-13-2009 08:02 PM
For Sale - Sold: Everything Macro: Bellows, Ext. Tubes, Macro Lenses MikeDubU Sold Items 12 03-14-2009 07:10 PM
D-FA 100/2.8 Macro vs other macro lenses? jankok Pentax SLR Lens Discussion 12 10-25-2007 09:01 AM



All times are GMT -7. The time now is 02:10 PM. | See also: NikonForums.com, CanonForums.com 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?
register
Creating a FREE ACCOUNT takes under a minute, removes ads, and lets you post! [Dismiss]
Top