Field of view pictures for Rik

Cyclops · Post by **Cyclops** » Fri Mar 13, 2009 12:19 pm

Just done a definitive test on both the 28 and 50mm lenses reversed and got some interesting results!
I did two tests for each lens, one with the macro lens set to infinity and a second at its closests focus, as theres quite a lot of travel in the 100mm lens and I imagined that must have an effect on magnification. Not as much as I thought as it turned out, and it is useful as a focussing mechanism when working this close, and not having a movable table.

So, the results.
First the 28mm reversed. FOV at infinity7mm,
http://img.photobucket.com/albums/v11/D ... finity.jpg
at closest FOV=5.5mm
http://img.photobucket.com/albums/v11/D ... losest.jpg

Next the 50mm
At infinity FOV=11mm
http://img.photobucket.com/albums/v11/D ... finity.jpg

at closest FOV=8mm
http://img.photobucket.com/albums/v11/D ... losest.jpg

Here's a pictorial representation i just put together, if I may:

Larger version here:
http://img.photobucket.com/albums/v11/D ... ovtest.jpg

rjlittlefield · Post by **rjlittlefield** » Fri Mar 13, 2009 1:39 pm

Cyclops, nice job -- we'll make you into a scientist yet!

Let's see, for the 28 reversed on the 100 at infinity, simple calculation suggests m=100/28=3.57X, FOV = 22.7/3.57 = 6.35mm. The image shows 7.0mm, so you're getting a little less magnification. By the numbers, m=22.7/7.0=3.25X. This isn't exact, but it's very nicely in the right ballpark.

For the 50 reversed on the 100, simple calculation suggests m = 100/50=2X, FOV = 22.7/2.0 = 11.35, and the image shows almost exactly that.

For those same combos with the 100 at closest focus, I don't even want to try calculating what the magnification "ought to be". Those macro lenses both extend their physical length and shorten their focal length as they focus closer, and the manufacturers don't publish the specs that would be needed to calculate from.

But the experiment gives us direct answers right off. At closest focus, the reversed 28 gives m=22.7/5.5 = 4.1X, and the reversed 50 gives 2.8X. No muss, no fuss, no bother, and not much math. Pictures of rulers are good!

--Rik

Cyclops · Post by **Cyclops** » Fri Mar 13, 2009 2:25 pm

Cheers Rik, hopefully this will come in handy for other macro photographers on here.

I got another technique I#m happy with too! I found that with just the macro lens I can hold a cardboard tube over the pop up flash and get well lit shots at lifesize. this means I am not tied to a tripod when chasing bugs! I can now hunt spiders and other bugs in the shed and certain areas indoors, and do it one handed!
It wont work with the reversed lenses, the tube just isnt long enough to get in there.

mgoodm3 · Post by **mgoodm3** » Fri Mar 13, 2009 4:31 pm

The problem with putting the focus on a reversed lens at anything but infinity is that you will lose working distance with very little increase in mag.

You lose working distance because the lenses typically extend into the lens when you focus close. For a reversed lens that means that the end of the lens extends without the lens, eradicating some of your working distance.

Edit: That means if the lensfocused predominantly by extension you won't get much if any extra mag. If the lens changes focal length you may get some imcrease mag.

rjlittlefield · Post by **rjlittlefield** » Fri Mar 13, 2009 4:51 pm

mgoodm3, I think you're talking about something different. Cyclops and I are talking about changing focus on the rear lens, not on the reversed lens. I think you're talking about changing focus on the reversed lens. I agree that changing focus on the reversed lens isn't much use, for the reasons you've outlined. But changing focus on the rear lens both gives more magnification and can be helpful for focusing without a table, as Cyclops has illustrated.

Have I misunderstood your comment?

--Rik

mgoodm3 · Post by **mgoodm3** » Fri Mar 13, 2009 4:52 pm

The rear lens does do something. I thought that we were talking about the front.

Opps.

rjlittlefield · Post by **rjlittlefield** » Fri Mar 13, 2009 4:54 pm

OK.

We could just delete these postings, might save some confusion for other readers...

--Rik

Cyclops · Post by **Cyclops** » Fri Mar 13, 2009 4:58 pm

Interestingly,when using a reversed lens i do tend to pull the focus out to its closest point. The reason I do this is that the lens element is then recessed and the part of the reversed lens mount thus protruding acts as a little lens shade. But better would be to get a spare rear lens mount and cut the centre out,so when its mounted onto the lens you have a shade.

rjlittlefield · Post by **rjlittlefield** » Fri Mar 13, 2009 5:27 pm

Ah, built-in lens shade -- I didn't think about that. Good idea!

BTW, forget that "delete the postings" suggestion I made -- this is interesting & productive discussion after all.

--Rik