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Old Oct 22, 2003, 7:49 AM   #11
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Default Re. reverse mounting

Thanks again Shene, Richard. Thanks for the A40 pics Richard, I have a good idea of what to expect now. You may already have answed this question and it probably is a stupid one, but, is it possible to reverse mount a lens to make it a telephoto lens?
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Old Oct 22, 2003, 2:26 PM   #12
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Default Re: Re. reverse mounting

Quote:
Originally Posted by Terapixel
is it possible to reverse mount a lens to make it a telephoto lens?
No, because the depth of field (the distance in front of and behind the subject that is in focus) is very narrow or tight. You can see on some of the above pictures where the top surface of the coin is in focus, but just a few millimeters below is out of focus. That is quite common in closeup photography...you might take a picture of a flower where the centre is sharp, but the outer rim of the flower is soft, or out of focus.

To see a variety of lenses, check out http://www.raynox.co.jp/english/egindex.htm where you can select various camera models and see examples of the lenses mounted on the camera, and from there see actual pictures taken with that camera/lens combination and compare them with the bare camera.
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Old Oct 23, 2003, 5:16 PM   #13
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other than a physical test, does anyone know how to calculate minimum focus distance and magnification of a a reversed lens?
i just ordered a 55mm - 55mm ring to reverse my 50mm f/1.4 lens to my fuji s602z. i also have a 200mm f/2.8 canon lens to try also.

dennis
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Old Oct 23, 2003, 5:41 PM   #14
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Default Re: Re. reverse mounting

Quote:
Originally Posted by Terapixel
You may already have answed this question and it probably is a stupid one, but, is it possible to reverse mount a lens to make it a telephoto lens?
I indeed answered your question; however, the answer is hidden in the explanation. If you only wish to know the answer, it is NO. Otherwise, please continue.

The power of a lens of focal length F measured in millimeter is 1000/F in the unit of diopter. Thus, a 50mm lens has a power of 1000/50=200 diopter, written as +20. A 1000mm lens has a diopter of +1. This diopter value is exactly the same as the number marked on a close-up lens. On the other hand, given a diopter value D, the focal length of the lens is simply 1000/D. Thus, a close-up lens of diopter +3 has a focal length of 1000/3=333mm. Since a lens is reversible meaning light rays can enter the lens in either direction, the power value does not change even the lens is reversed. The power of a lens indicates the "strength" of bending/refracting the incoming light.

When two lenses are putting together very closely, the combo has a diopter value very close to the sum of the individual diopter values. This is the reason that many close-up books would tell you that putting two close-up lenses of diopters +a and +b together would yield a close-up lens of diopter +(a+b). Thus, if the on-camera lens has focal length 50mm and a +4 close-up lens is used, the diopter of the combo is 4+(1000/50)=24. Thus, the combo has a diopter of +24.

Once we know the diopter of the combo, we certainly can compute its focal length. In the above example, the combo has a diopter +24, its focal length is 1000/24=41.7mm, which is shorter than the focal length of the camera lens (i.e., 50mm). Since a +4 close-up lens has a focal length of 1000/4=250mm, the focal length of the combo (i.e., 41.7mm) is also shorter than that of the close-up lens. But, is this always true? The answer is YES.

Let the focal lengths of the camera lens and the attached lens (e.g., a close-up lens or a reversed SLR lens) be F1 and F2, respectively. Then, the camera and the attached lenses have diopters 1000/F1 and 1000/F2, respectively, and the combo will have a diopter of (1000/F1 + 1000/F2). Its focal length is 1000/(1000/F1 + 1000/F2), which is equal to (F1*F2)/(F1+F2). Since (F1*F2)/(F1+F2) < F1 (and F2, too), we learn that the combo always has a shorter focal length than those of the camera lens and the attached lens. Consequently, reverse mounting a SLR lens only makes the combined focal length shorter, and the combo can never be a telephoto lens!

CK
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Old Oct 23, 2003, 5:50 PM   #15
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Quote:
Originally Posted by djb
other than a physical test, does anyone know how to calculate minimum focus distance and magnification of a a reversed lens?
It is very difficult to calculate the minimum focusing distance because there are too many factors involved. In general, it is very close to and maybe slightly larger than the distance from the rear element to the focal plane. The focusing ring of the reversed lens may help, but not by much.

The magnification, as mentioned in my first post, is computed as Y/X, where Y is the on-camera lens focal length (the real one rather than its 35mm equivalent) and X is the focal length of the reversed lens.

Quote:
Originally Posted by djb
i just ordered a 55mm - 55mm ring to reverse my 50mm f/1.4 lens to my fuji s602z. i also have a 200mm f/2.8 canon lens to try also.
You might want to stop down your 50mm F1.4 a little. But, your 200 F2.8 may have a large thread and "see" too much. Thus, vignetting may be a problem. If you have a smaller aperture 200mm Canon (e.g., F4), it may have a 55mm thread. Then ,you will have a 200/50=4X lens. I used Nikon's manual 200mm F4 with Nikon 50mm F1.8 to get 4X and Nikon 24mm F2.8 to get 8X. Results are really good.

CK
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Old Oct 23, 2003, 5:55 PM   #16
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thanx shene!!!

dennis
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Old Oct 27, 2003, 12:36 AM   #17
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Default Re: Reverse mounting ?

Here are more examples. I forgot to set the white balance mode correctly, and, as a result, white balance is not right. However, it does not matter because what I really want to show is the magnification. The camera used was a Nikon D100.

The following was taken using Nikon's AFD 60mm F2.8 micro. This is a reference shot.


My favorite reverse mounting configuration is the old Nikon MF 200mm F4. This lens is excellent even shooting wide open at F4. The following shows a AFD 50mm F1.8 reverse-mounted on the MF 200mm F4.


This one was shot with the above setup. In theory, the magnification is 200/50=4X.


This one was shot by reverse mount a AFD 24mm F2.8 on the MF 200mm F4. Now, the magnification is 200/24=8.3X!


Note that since the dollar bill was not treated properly, it is not completely flat and some edge/corner blur is unavoidable. With a magnification as high as 4X and 8X, the DOF is so shallow. Sometimes, 1 millimeter is a big difference.

Hope the above illustrate the reverse mounting technique and magnification well.

CK
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Old Nov 26, 2003, 6:49 AM   #18
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Default I finally tried it too...

Recently I bought a nice used Oly 2100 UZI and started to explore it's features, which are really amazing but I always look for info and accessories to expand them to new dimensions.
First I decided to improve the close-up capabilities. Although this topis is mostly Nikon (and short zoom camera) related, it helped me much - thanks Shene and others. However there are still some problems and questions to ask.
So first I bought a Rowi low-end +4 close-up lens. In general the results are something about what I wanted to achieve concerning magnification but the imeage quality suffers. Most of the picture is soft, worst by the edges. Moreover there is too much red and blue aberation, again most in the border.
My first question is: can this be due to a poor quality of the lens or is it a light vigneting? There is no black/gray corner, just softness. The lens is 49 mm, same as the UZI's filter thread. I removed the Skylight filter (used for lens protection) before attaching this lens.
Should I generally look for a bigger diameter lens or simply a better quality lens??? No need for a step-up ring would be convenient.

Second step was reverse mounting of a Russian 2/55 SLR lens. Use of a reversing ring which was manufactured by a local photo retailer was a breeze and the results are satisfactory. The magnification is extreme as I can shoot an area of about 5x3,75 mm, quite matching the Sheenes formula, where Magnification=70/55=1,27 and UZI's CCD is 6,4x4,8 mm.
Again there are a few problems:
1) Slight vigneting; againg the thread of the SLR lens is 49, no step up; would a higher aperture help? Especially when I zoom out a bit to reduce the magnification, vigneting starts to make results "worse usable".
2) Very very low DOF; yes I knew this would be a problem but many results of shooting 1:1 (or so) posted on the net seem to me of somewhat deeper focus. Any suggestions?

Any general suggestions about using UZI as microscope?

I wanted to post samples but I don't have them yet hosted on the net and it seems it's not possible to embedd them directly to the post. I'll try to add them later.
Thanks, Martin[/img]
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Old Nov 26, 2003, 6:52 AM   #19
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Default I finally tried it too...

Recently I bought a nice used Oly 2100 UZI and started to explore it's features, which are really amazing but I always look for info and accessories to expand them to new dimensions.
First I decided to improve the close-up capabilities. Although this topis is mostly Nikon (and short zoom camera) related, it helped me much - thanks Shene and others. However there are still some problems and questions to ask.
So first I bought a Rowi low-end +4 close-up lens. In general the results are something about what I wanted to achieve concerning magnification but the imeage quality suffers. Most of the picture is soft, worst by the edges. Moreover there is too much red and blue aberation, again most in the border.
My first question is: can this be due to a poor quality of the lens or is it a light vigneting? There is no black/gray corner, just softness. The lens is 49 mm, same as the UZI's filter thread. I removed the Skylight filter (used for lens protection) before attaching this lens.
Should I generally look for a bigger diameter lens or simply a better quality lens??? No need for a step-up ring would be convenient.

Second step was reverse mounting of a Russian 2/55 SLR lens. Use of a reversing ring which was manufactured by a local photo retailer was a breeze and the results are satisfactory. The magnification is extreme as I can shoot an area of about 5x3,75 mm, quite matching the Sheenes formula, where Magnification=70/55=1,27 and UZI's CCD is 6,4x4,8 mm.
Again there are a few problems:
1) Slight vigneting; againg the thread of the SLR lens is 49, no step up; would a higher aperture help? Especially when I zoom out a bit to reduce the magnification, vigneting starts to make results "worse usable".
2) Very very low DOF; yes I knew this would be a problem but many results of shooting 1:1 (or so) posted on the net seem to me of somewhat deeper focus. Any suggestions?

Any general suggestions about using UZI as microscope?

I wanted to post samples but I don't have them yet hosted on the net and it seems it's not possible to embedd them directly to the post. I'll try to add them later.
Thanks, Martin[/img]
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Old Nov 26, 2003, 9:01 PM   #20
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Default Re: I finally tried it too...

Quote:
Originally Posted by MartinCZ
So first I bought a Rowi low-end +4 close-up lens. In general the results are something about what I wanted to achieve concerning magnification but the imeage quality suffers. Most of the picture is soft, worst by the edges. Moreover there is too much red and blue aberation, again most in the border. My first question is: can this be due to a poor quality of the lens or is it a light vigneting?
This is likely due to BOTH. The +4 close-up is perhaps a single element one which always suffers from chromatic aberration. Then, its poor quality will be magnified my the camera lens and hence becomes worse. If you do not use the longest zoom end, the situation may improve somewhat but not by very much. To get better results, you could try some doublet close-up lenses. Of course, they are more expensive. Nikon and Pentex both offer good quality doublet close-up lenses. The thread size may not be 49mm, and, hence, a step-up ring may be needed. Since most zoom lenses are soft to very soft at their longer end (in fact, the longer the zoom the worse the quality at the longest end), I would not suggest to use the longest end for close-up, although in doing so you will get higher magnification. Use the lower 3/4 or 2/3.


Quote:
Originally Posted by MartinCZ
Should I generally look for a bigger diameter lens or simply a better quality lens??? No need for a step-up ring would be convenient.
Larger diameter close-up lenses may not help with image quality. Good quality doublet close-up lenses do. Minolta offer some 49mm close-up lenses; but, I don't know about their quality. Nikon have two sets, one for 52mm and the other 62mm. The 52mm ones are very good; but, they need a 49-52mm step-up ring.

Quote:
Originally Posted by MartinCZ
1) Slight vigneting; againg the thread of the SLR lens is 49, no step up; would a higher aperture help? Especially when I zoom out a bit to reduce the magnification, vigneting starts to make results "worse usable".
The reversed lens should be wide open for reducing vignetting. But, closing down the aperture of the camera lens may cause light fall-off because in doing so some light near the corners will be blocked. If a wide open lens still causes vignetting, it could be (1) that lens aperture is not large enough, (2) the gap between the first elements of both lenses are to wide so that the camera lens "sees" too much, and/or (3) the front element of the reversed lens is deeply recessed which creates a situation like (2). Zooming out cannot reduce vignetting but create more because the camera lens can "see" more.

Quote:
Originally Posted by MartinCZ
2) Very very low DOF; yes I knew this would be a problem but many results of shooting 1:1 (or so) posted on the net seem to me of somewhat deeper focus. Any suggestions?
The camera lens may have a very small aperture, the subject being photographed may be positioned carefully so that most important areas are all in focus, some of the not-so-out-of-focus areas may be sharpened locally, and/or better equipment may be used.

CK
http://www.cs.mtu.edu/~shene/DigiCam
Nikon Coolpix 950/990/995/2500/4500 User Guide
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