25mm vs 12.5mm vs 8mm for 10x magnification

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merxx
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25mm vs 12.5mm vs 8mm for 10x magnification

Post by merxx »

Hi all

I have been a long time macrophotography user but not a long time since I have discovered your forum :)

I wanted to do a comparison between my lenses for 10x magnification. First I did a series of tests to find out which is the sharpest aperture of the lens at x5. Let's say I found out that F/2.8 is the sharpest aperture for all lenses...

Well I have been using lenses reversed for over 5 years and this was the first time I noticed this...

The 8mm lens at f/2.8 is darker at 10x compared to the 12.5mm at 10x and the 12.5/2.8 was darker at 10x compared to the 25/2.8. I am not really sure if this should be the case, I thought it should be the other way around because with longer lenses you need longer bellows thus more light lost..... or maybe it is a lighting issue ( but doesn't seem ) ? Is this how it should behave or my conclusions are wrong ?

Well the 12.5mm F/1.9 is a Wollensak raptar D-mount cine lens and it gives a little worse results to the olympus 20mm macro ( paid 5$ ). Quite useable at f/2.8 but tests show F/4 to be even better.

The 25mm f/1.4 at f/2.8 is as good as the olympus 20mm macro ( very little difference ) . At x10-x15 F/2.8 looks much better than F/4 , contrast is there , all in all amazing lens. It is a Pentax ( cosmicar rebranded ) TV lens .. VERY sharp BUT the 12.5/1.9 cine lens gives more detail when you go beyond X15

The 8mm F/1.3 is a wide angle TV lens. It is sharp at f/2.8 but looks sharper at F/4 , I'll test it a little more..

I used to have the Tominon 17mm f/4 but was quite dissapointed with the contrast and the f/4 when going beyond x10

enricosavazzi
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Re: 25mm vs 12.5mm vs 8mm for 10x magnification

Post by enricosavazzi »

merxx wrote:Hi all

The 8mm lens at f/2.8 is darker at 10x compared to the 12.5mm at 10x and the 12.5/2.8 was darker at 10x compared to the 25/2.8. I am not really sure if this should be the case, I thought it should be the other way around because with longer lenses you need longer bellows thus more light lost..... or maybe it is a lighting issue ( but doesn't seem ) ? Is this how it should behave or my conclusions are wrong ?
Hello, and welcome.

What you are seeing should be the effect of a different pupil ratio of these lenses. Assuming that magnification, illumination and nominal aperture are the same, a different pupil ratio is the only factor I can think about that could give the difference you observed. Focal length is not a factor in this case, nor the bellows extension in itself.

The effects of pupil ratio have been discussed in a few threads, and I believe there is a relatively recent FAQ about it.
--ES

mgoodm3
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Post by mgoodm3 »

My first thought on the brightness issues is the working distance, or lack thereof with the shorter focal length lenses.

Charles Krebs
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Post by Charles Krebs »

Greetings merxx,

As Enrico said, the "lighter/darker" effect you are seeing at the same magnification and "marked" aperture is almost certainly due to differing "pupillary magnification factors" of the lenses. As a result, when extended for higher magnifications, the actual "effective" apertures will be different.

(With some lenses, if you hold the lens up at arms length, and look through the front, then the back, you will likely notice that the size of the aperture appears different depending on which side you look through). The PPM is the ratio of the exit pupil size divided by the entrance pupil size. For "on paper" calculations it can get messy because these values are seldom provided by the manufacturer and can be tricky to measure accurately.

But the main thing is to realize that diffraction effects and depth-of-field are related to the effective aperture, not the "nominal" marked aperture. So if you wish to accurately compare lenses it's best to try and do it at the same effective aperture.
(And there's no "free lunch". You don't gain DOF, or more light using a lens with a PPM different than "1"... you just end up with a larger or smaller effective aperture than the equations (that assume a PPM=1) provide.

rjlittlefield
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Post by rjlittlefield »

merxx, welcome aboard!

I wrote the FAQ on pupil factor, and I have some doubts that your problem lies there. The reason is that you are using lenses reversed at high magnification. In that combination (reversed, high mag), even lenses with large pupil factors go back to acting close to their rated apertures. At 10X, it is unlikely that pupil factor can explain more than a few percent difference between lenses.

How much difference in exposure are you seeing? A good way to quantify this is to measure the exposure time needed to match the histograms. What is the ratio of exposure times when you do that?

--Rik

merxx
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Post by merxx »

I'll do some tests and I'll post them here :)

merxx
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Post by merxx »

8mm at F/2.8 at around 10x

Image

8mm at F/2.8 at around 10x , exposure compensated to match 25mm ( about +1.80 EV )

Image

25mm at F/2.8 at around 10x , original exposure

Image


I did the test very quickly... and it seems, comparing the dots, the 8mm has a little more magnification ... Also maybe the 8mm image needed a +2EV exposure compensation

Also there is something more, the 8mm lens seems to have kinda dark elements if that makes sense, not really dark like an ND filter but they do seem a little darker to what I am used.. ? So maybe the light transmittance is not really good and this accounts for the exposure difference ? I'll try the 12.5/1.9 as well..

Also I cannot guarantee the shorter working distance isn't the factor but doesn't seem to ..

So I guess I have to go back to the drawing board and redo the tests...

rjlittlefield
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Post by rjlittlefield »

1.80 EV (3.5X longer exposure time) is a huge amount, much bigger than could be explained by pupil factor.

As you say, the darker image has a little higher magnification, but I measure only about 13% difference based on spacing of the red dots. That would account for only about 1.27X difference in exposure time.

Illumination is a possibility, but it seems like you would notice the shadowing.

One way to eliminate the issue of possible shadowing is to use a backlit subject.

--Rik

merxx
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Post by merxx »

HI rjlittlefield

just wanted to tell you that the 8mm lens is a wide angle design (tv lens) . i just hold it in front of me and looked through both sides and the aperture looks something like 4-6x bigger compared to the other side (highly asymmetrical design ) . Maybe this accounts for what I am experiencing ?

thanks !

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Post by rjlittlefield »

Certainly that is a bigger number than I was considering, but I still can't make the calculations explain what you're seeing.

While I'm thinking more about this, let me ask to be sure: Does the pupil look smaller from the front or from the camera side, when the lens is in its normal position, not reversed?

--Rik

merxx
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Post by merxx »

Normal position ( not reversed ) . Small hole front ( looking at the subject) , big hole rear ( looking at the camera ) .

rjlittlefield
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Post by rjlittlefield »

merxx wrote:Normal position ( not reversed ) . Small hole front ( looking at the subject) , big hole rear ( looking at the camera ) .
OK, that's what I expected.

I definitely cannot make sense of the exposures you're getting. Lenses that have big rear pupils in normal orientation (P>1) have apertures that are wider than symmetric lenses when reversed, at the same magnification.

At 10X the differences are small. A symmetric lens with marked f/2.8 will be effective f/30.8; a lens with marked f/2.8 and pupil factor 6, reversed, will be effective f/28.5.

This problem is driving me a little nuts. I keep rechecking the theory and revisiting old experiments. They keep coming out consistent with each other, which of course is good. But that leaves your observation unexplained, which of course is bad. I look forward to eventually understanding what's going on.

--Rik

ChrisLilley
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Post by ChrisLilley »

merxx wrote: Also there is something more, the 8mm lens seems to have kinda dark elements if that makes sense, not really dark like an ND filter but they do seem a little darker to what I am used.. ? So maybe the light transmittance is not really good and this accounts for the exposure difference ? I'll try the 12.5/1.9 as well..
Would it be possible to get a photo *of* the lenses, such that you are looking through the 8mm lens and one of the others, with some brightly illuminated white paper behind them?

I wonder if the glass has yellowed, or been covered with something which has oxidised, or something like that. "kinda dark elements" rings warning bells.

merxx
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Post by merxx »

Ok

I looked through the lenses against my monitor (white background) and my 25mm lens looks quite brighter like 20-30% brighter than the 8mm one, also it seems that it depends on the angle you view through it but it is consistent dark/brownish when viewed at a 90 degrees angle. I took a photo but I think the difference seems more pronounced when seen with my eyes. It seems some elements have gone brownish ?! I can guess this lens was made in sometime in the 80's :roll: To be honest I didn't expect the difference to be that obvious, when compared against my other lenses it seems it is the darkest and the others are consistent between them. This and the higher magnification of the 8mm test image (compared to the 25mm ) could explain what I am encountering ?

Image
Last edited by merxx on Sat May 15, 2010 2:52 pm, edited 2 times in total.

ChrisLilley
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Post by ChrisLilley »

Are any macro lenses or microscope objectives known to be made with thoriated glass?

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