[Marc Sabatella]

I wish I could find the reference again, but according to the specs that Raynox formerly displayed, the DCR-150 focusses at 211 mm and the DCR-250 at 109 mm. From my reading you will get 1:1 magnification when the FL of the lens matches focus distance.

OK, that does sound logical. I hadn't realized you had those numbers to go off. But then, I'm still a bit confused as to why a DOF calculator would yield "0" for DOF. Less than whatever unit of measurement it uses, I guess? Seems not right - empirically, DOF with my 50-200 seems at least 1mm at all apertures, depending on how you measure it (actual DOF does depend on print/viewing size).

Very interesting effect, BTW, regarding the 18-250 giving more magnification at infinity than minimum focus!

Besides, with a Raynox mounted I always focus at infinity.

FWIW, I often take advantage of the range in magnifications and working distances I can get by actually using the focus ring. But it's normally at one end or the other - too hard to finesse the middle even with the best focusing rings.

I've found that DOF is highly dependent on FL and that's what the calculator says too. The other day I photographed a rose bud with a 18-250 set at 250mm and then the 16-45 set at 45mm. I was surprised to see slightly better magnification from the 16-45 as compared to the very long 18-250 (especially since Tamron calls the 18-250 a "macro" LOL). The other thing that struck me was the DOF. The 16-45 had an out-of-focus background, but the 18-250 was pure blur, no detail whatsoever.

Right, that much is the phenomenon I already referred to. Independent of whether the *size* of the *in-focus* area is the same or not, it does seem to be true that the *appearance* of the *out-of-focus* areas will depend on focal length for a given magnification.

[newarts]

Right, that much is the phenomenon I already referred to. Independent of whether the *size* of the *in-focus* area is the same or not, it does seem to be true that the *appearance* of the *out-of-focus* areas will depend on focal length for a given magnification.

That's true; an area out-of-focus at the same distance behind the subject looks *much* blurrier with a long focal length lens than for a short focal length lens at the same magnification.

Actually, the apparent background blurrieness is in the ratio of the focal lengths for the same f-number and magnification.

Dave

EDIT: According to my estimates based on thin lens theory an out-of-focus point a real distance Z behind the plane of focus is a disk of diameter d given by:

d=(Z/N)(h/ho) [m/(1+1/m)]

where N is F-number, the h/ho ratio is the size ratio behind the focal plane due to perspective, and m is magnification at the focal plane.

While this is not the most compact way to write the equation, it embodies simple parameters important to the photographer:
1) The out-of-focus spot size tends to increase in proportion to distance behind the focal plane.
2) The out-of-focus spot size decreases as F-number increases.
3) The out-of-focus spot size behind the focal plane is decreased by the perspective effect (countering effect #1).

The net effect is that long lenses tend to create fuzzy backgrounds because the perspective effect does not greatly decrease the size of out-of-focus points. These results are consistent with observations.

[newarts]

From my reading you will get 1:1 magnification when the FL of the lens matches focus distance.

The magnification is 1:1 when the focal length of the diopter lens equals that of the main lens.

magnification_minimum = main_lens_focal_length/added_lens_focal_length

Dave

[newarts]

Here's my final demonstration of DoF relationships in this series.

Theory says:

Dof/Wof = C*F-number(1+1/m)

I tilted my laptop screen sideways about 45 degrees with respect to the optic axis & took 1:1 macros of it at different f-numbers (magnification and width of field constant - only the f-number was changed between shots). Here's the results:



Clearly, the results are consistent with DoF being proportional to F-number*.

Dave

* we may rightly argue about exactly where to put the line, but can probably agree that a straight line represents reality pretty well.

---

Last edited by newarts; 07-12-2009 at 06:55 AM.

[newarts]

This online DOF calculator says you're wrong. Online Depth of Field Calculator
....

DCR-150: Subject distance 21 cm

Depth of field
Near limit 21 cm
Far limit 21 cm
Total 0 cm
In front of subject 0 cm (NaN%)
Behind subject 0 cm (NaN%)

So Sheesh right back at ya.

Dan,

I apologize for Sheeshing.

Dave

PS probably the way to use the DOF calculator with the Raynox is to adjust the primary lens' focal length due to putting the Raynox on it:

1/f=1/f-old +1/f-raynox

so using a 190mm original lens and a 211 mm raynox, the combination gives;

f=100mm.

For a 1:1 magnification,the distance from the lens to the subject is 2 times the focal length (20cm).

Putting these numbers into the DoF calculator gives the following:
Subject distance 20 cm

Depth of field
Near limit 20 cm
Far limit 20 cm
Total 0.02 cm

In front of subject 0.01 cm (50%)
Behind subject 0.01 cm (50%)

This is about 10 times smaller than my estimates I think....I'll check further.

[audiobomber]

The magnification is 1:1 when the focal length of the diopter lens equals that of the main lens.
Dave

Yes, that's what I was trying to say.

[audiobomber]

I apologize for Sheeshing.

Thank you.

PS probably the way to use the DOF calculator with the Raynox is to adjust the primary lens' focal length due to putting the Raynox on it:

1/f=1/f-old +1/f-raynox

so using a 190mm original lens and a 211 mm raynox, the combination gives;

f=100mm.

For a 1:1 magnification,the distance from the lens to the subject is 2 times the focal length (20cm).

Putting these numbers into the DoF calculator gives the following:
Subject distance 20 cm

Depth of field
Near limit 20 cm
Far limit 20 cm
Total 0.02 cm

In front of subject 0.01 cm (50%)
Behind subject 0.01 cm (50%)

This is about 10 times smaller than my estimates I think....I'll check further.

This looks much better. The one thing I wonder about though is the 190mm lens. I don't think you can get to 1:1 using a DCR-150 on a 190mm lens and still maintain focus. I thought we agreed that you need a 210mm lens to get to 1:1. If you use a shorter lens and move in closer, you lose focus. With a 190mm lens, you would only get about 1:1.1

[newarts]

Thank you.



This looks much better. The one thing I wonder about though is the 190mm lens. I don't think you can get to 1:1 using a DCR-150 on a 190mm lens and still maintain focus. I thought we agreed that you need a 210mm lens to get to 1:1. If you use a shorter lens and move in closer, you lose focus. With a 190mm lens, you would only get about 1:1.1

I'm pretty sure the calculation I used allows for the lens' focus ring to be adjusted: You are right that if the 190 lens is focused at infinity and the 211 Raynox is added, the magnification will be 1:1.1, but if you increase the distance between the original lens and the sensor (turn the focus ring) the mag will increase (at least for a non-IF lens.)

I used 190mm by mistake; If I re-do it with 250 mm & 211 mm, I get:

The effective focal length of the pair is 114mm. The mag with the 250mm lens focused at infinity is about: 250/211=1.18 & the DoF calculator gives:
Subject distance 21 cm

Depth of field
Near limit 21 cm
Far limit 21 cm
Total 0.05 cm

In front of subject 0.03 cm (50%)
Behind subject 0.03 cm (50%)


Dave

[res3567]

Thanks, Andi, Lee and Marc.

I would be using this on the Sigma 70-300mm, and probably on the MZ-7 which I have been using lately more than the MZ-5. The 70-300mm has a filter ring of 58mm. So that adapter that they say comes with it at B&H, should be work. And if it doesn't work I can return it.

Check it:

https://www.pentaxforums.com/forums/pentax-slr-lens-discussion/61939-raynox-flexability.html

[res3567]

I had a cool pic of a tiny spider with 5 legs taken with my Raynox 150 and I accidentally DELETED it!!! Boohooboohooboohoo!!!! !*&^%#$^!!!!!!!!

[audiobomber]

I'm pretty sure the calculation I used allows for the lens' focus ring to be adjusted: You are right that if the 190 lens is focused at infinity and the 211 Raynox is added, the magnification will be 1:1.1, but if you increase the distance between the original lens and the sensor (turn the focus ring) the mag will increase (at least for a non-IF lens.)

That's what was tripping me up. I only ever use a Raynox at infinity focus, because i read in a photography book that maximum sharpness will be at infinity, plus Raynox says "Raynox macro conversion lenses were designed to use at maximum Telephoto position of camera's zoom lens unless specified." Plus I only ever use the diopters on long zooms, so I change the magnification using the zoom ring instead of focus.

I tried a few photos using the DCR-150 and 16-45 at its minimum focus distance and it worked fine, so I will remember this is an option from now on.

[newarts]

That's what was tripping me up. I only ever use a Raynox at infinity focus, because i read in a photography book that maximum sharpness will be at infinity, plus Raynox says "Raynox macro conversion lenses were designed to use at maximum Telephoto position of camera's zoom lens unless specified." Plus I only ever use the diopters on long zooms, so I change the magnification using the zoom ring instead of focus.

I tried a few photos using the DCR-150 and 16-45 at its minimum focus distance and it worked fine, so I will remember this is an option from now on.

Great to hear of your successful tests at minimum focus distance. From a theory standpoint image quality should not depend much on small changes in focal distance (at least for perfect lenses.)

The hard thing to predict is just how much change in focal distance is "too much" for an imperfect lens. Experiments are called for.... Raynox is being properly conservative in their guidance.

Dave

[audiobomber]

The hard thing to predict is just how much change in focal distance is "too much" for an imperfect lens. Experiments are called for.... Raynox is being properly conservative in their guidance.

Dave

I just realized on re-reading that I misinterpreted the Raynox statement. Somehow I was thinking focus distance. What it actually says is that I should not be adjusting the focal length of the zoom with a Raynox mounted. I assume this is due to vignetting, so it will be pretty obvious with any given lens whether one should use shorter FL's with a particular zoom or not. The 55-300 with a DCR-150 attached does not vignette to any significant degree at different focal lengths, but the 18-250 does.

[Marc Sabatella]

This looks much better. The one thing I wonder about though is the 190mm lens. I don't think you can get to 1:1 using a DCR-150 on a 190mm lens and still maintain focus. I thought we agreed that you need a 210mm lens to get to 1:1.

As has already been noted, this is true only if you insist on leaving the focus ring on infinity. Maybe it's theoretically sharper at infinity, but I have no complaints at minimum focus distance - any difference is inconsequential.

If you use your lens at minimum focus distance, then you get 1:1 at a *much* lower focal length. I have measured and found I'm within a couple percent of 1:1 with my M135/3.5 at minimum focus. With my M200/4 or DA50-200 at 200, I'm *way* over 1:1.

[audiobomber]

I know how to calculate the resulting macro ratio with a Raynox on a lens when focussed at infinity. Is there a way to tell what kind of magnification adding a diopter will bring at minimum focus? (Obviously not valid for an internal-focus lens).