Minimum actual Depth of Field - DPChallenge Forums

03/24/2006 10:40:29 AM · #1

PLEASE NOTE: the original question for this thread has fallen apart as it was based on some flawed math. There might be something interesting here for big zoom P&S users and/or macro fans though. Sorry for any confusion that might have been caused.

Ok, here is a technical question for all y'all. (sorry, spent too much time reading the Jeff Foxworthy Challenge suggestion)

So I've been toying with some ideas of Macro work and I was just thinking about adding a 50mm lens to my list before I get my DSLR because it looks like there might yet be a few months before I get to that point (availability and sane pricing on the 30D being the issues at this point). It might be fun to play with the 50mm reversed on my S2 IS which has a rather interesting 0CM min Focal distance, which I've heard should give me a 1:1 macro ratio (maybe? not really sure about this).

The question is though, DOF is sharply reduced when adding a reversed lens. I only really need to check this out, so I'm not too worried about it.

On the other hand, I already have an 80-200mm f/2.8 lens.

Can this too be reversed?

If it can, I would imagine that the DOF will be reduced to some pretty staggeringly shallow plane.

So the question arises, Can DOF get so small that it disappears?

Obviously, it can get so small that usability is severly impaired, but is it just like a fraction which keeps getting smaller and smaller as it approaches 1/infinity, or is there an absolute threshold here?

The lens on my S2 is 6-72mm. Super macro forces the lens to wide, or 6mm. reverse a 200mm lens on the end of that and you get uhhhhh 33.3:1

Dang!

Might have some uses... maybe... More like a really big microscope at that point.

Would I also be surpassing the resolution capability of the on-camera lens at this point or is that a non-issue?

What would happen to my working focal length?

Message edited by author 2006-03-24 11:55:30.

Canon EF 24-105mm f/4.0L IS

03/24/2006 10:46:14 AM · #2

not sure you figured your magnification correctly there.

yes the 200mm is 33.3 times as long as 6mm but i don't think that's how you figure macro magnification.

../

Message edited by author 2006-03-24 10:57:34.

Artifacts

Canon EOS-5D Mark II

03/24/2006 10:46:56 AM · #3

On my film SLR I used to reverse the normal lense a lot for macro shots. That worked very well. I could not reverse the wide angled or telephoto lenses because there was no way to hold them properly to the camera when reversed. If you could I would imagine the focal point in front of the reversed lense would be further for a wide angle. The telephoto would be much closer to the lense, so close that it might be impractical.

Message edited by author 2006-03-24 10:55:57.

Canon EF 24-105mm f/4.0L IS

03/24/2006 10:49:19 AM · #4

this might be of interest

//www.oncloserinspection.com/Photomacrography/Articles/Reverse_Lens/reverse_lens.htm

fotomann_forever

03/24/2006 10:52:18 AM · #5

I don't thin' DoF can "dissappear" so to say, but, as you say, it reaches 1/infinity, it could get so thin that it's virtually unusuable.

Message edited by author 2006-03-24 10:52:36.

Artifacts

Canon EOS-5D Mark II

03/24/2006 10:58:25 AM · #6

Originally posted by soup:

this might be of interest

//www.oncloserinspection.com/Photomacrography/Articles/Reverse_Lens/reverse_lens.htm

Very interesting article and an idea worth looking into if you are into super closeups. :)

Message edited by author 2006-03-24 10:58:36.

03/24/2006 10:59:23 AM · #7

Soup, I'm also not entirely sure that that's the right way to measure macro magnification. I thought I read that it was done by dividing the focal length of the reversed lens by the focal length of the forward lens... but it's late and I've got a headache...

If anyone feels like correcting with accurate numbers, please feel free.

Reading the link now.

EDIT: Eureka

Originally posted by www.oncloserinspection.com:

You can easily calculate image magnification using the following formula:

If we were to reverse mount a 50mm focal length lens to a 100mm lens, focused at infinity focus, attached to the camera body the resulting image magnification would be 2:1 (2x life size). It should be readily apparent that image magnification is gained by reverse-mounting shorter focal length lenses onto longer focal length lenses. If we were to reverse mount a 35mm, focused at infinity focus, to a 100mm lens attached to a camera body, the resulting image magnification would be 2.9:1 (2.9x life size). The working distance would be an incredibly short 1.4", however. We could reverse mount the 50mm lens on a 200mm lens, focused at infinity focus, attached to the camera and obtain an image magnification of 4:1 (4x life size) but still maintain a working distance of 2".

So it looks like I've got this backwards.

However, it also looks like if I focus the lens at max magnification NOT in macro mode, and reverse the 50mm on there, I might get some interesting results...

Sounds like an interesting experiment.

Message edited by author 2006-03-24 11:05:35.

03/24/2006 11:03:48 AM · #8

you have to use the 35mm equivelant focal length of your fixed lens camera. your wide end isn't really 6mm or you'd have a round fish eye image ;} at least that's how i read it.

Message edited by author 2006-03-24 11:05:10.

03/24/2006 11:08:20 AM · #9

Yes, I just realized that, just before you posted that.

I would need to use the 35mm equivalent to figure out magnification size (because we are talking about crop factor which is also an effective "magnifier" at least where the math is concerned).

Interestingly, with an effective 36-432mm, my full telephoto could pull some really interesting results! That's 8:1 magnification! Still using a 2" working distance! Woah! That beats the stuffin' out of my normal macro mode. I don't know if my focus will work though...

I would still need to use the actual figures for my lens when figuring DOF stuff though.

Another vital clue:

Originally posted by www.oncloserinspection.com:

Just like simple supplementary lenses, camera lenses mounted in reverse on other camera lenses do not increase magnification. They simply allow camera lenses to focus closer.

Message edited by author 2006-03-24 11:12:07.

Sony 24-70mm F4 Vario-Tessar T* FE OSS

03/24/2006 11:44:51 AM · #10

Ok, So I finished reading that and boy did it do wonders for my headache :)

The forumula again for magnification is Camera lens length/reversed lens length.

The clincher:

Originally posted by www.oncloserinspection.com:

Many of these photographers claim that a fixed-lens digital camera with a reverse-mounted camera lens is a superior instrument for making macro images. They will also further proclaim that higher image magnifications can be obtained using fixed-lens digital cameras. I say, "Hogwash!". Here's why...

First of all, we need to consider the actual focal length of lenses that are installed on fixed-lens digital cameras not the "equivalent" focal lengths when compared to 35mm film cameras.

This is supplemented with 3 images at the bottom of that article.

There is one image that shows the ACTUAL 35mm (one would assume that he's using either a scan or a FF digital camera, or simply a calculated crop from a typical DSLR).

Naturally, this is going to be the image with the best detail and best light etc.

Then, he shows the picture that would be CAST BY THE LENSES, or the actual image circle. Obviously this is minus the edges sliced off by every actual camera. (Ok, maybe I just haven't heard of any circular film based cameras)

This shows that the lenses are actually magnifying far less in the P&S model than in the DSLR model.

His statement is true that this therefore means that the P&S model uses DIGITAL MAGNIFICATION to go beyond this.

In the case of reversing a 50mm on my 6-72mm lens, at full telephoto(72/50=1.44x), this means that I would have LESS magnification than a 100mm (2x).

What really gets wild though is that, as he said, a CROP FACTOR must be applied and the so called DIGITAL MAGNIFICATION of the CROP FACTOR ends up in the mix. This means that my EFFECTIVE magnification, be it by crop of the field of view or by actual glass-based magnification, still works into the final image. Whether they are high quality pixels or not, the miniscule area of that frame that is covered by the sensor of my camera STILL contains 5 million pixels. If those pixels can be fed adequate light information, they can be blown up by means of a print.

There is NO extra information here, but there's a LOT of it that is discarded by a P&S camera. This is generally bad, but there's a catch.

Where his argument MAY fall apart though is this simple point. EVERY picture that I take with this camera is subject to this same CROP FACTOR. ANY image that comes out of this camera is going to be experiencing these issues.

The plus side here is that the lens has been designed around this and CAN provide some rather decent images... Sometimes.

Here we are now dealing with resolving power of lenses and this goes over my head quite quickly.

What is interesting though is that the resolving power of lenses is pretty likely to be well beyond the abilities of the sensor.

By pushing the magnification here on my P&S, I will probably be moving well into a range of magnification that actually does surpass the resolving power of my lens. I will be able to see this pretty quickly by zooming in and out. That is IF I can get my camera to zoom to infinity...

So now that the original post has crumbled into ashes, a new question arises.

How far into the zoom range will I hit this physical resolution limit in my glass?

Indeed, if the bulk of the magnification is done by the reversed 50mm lens, is this likely to occur at all?

The DOF issue appears somewhat moot now as DOF would be no worse than experienced by anyone using a 100mm macro lens with a 50mm reversed.

I'm sorry if this seems like thinking out loud, and indeed it somewhat is, but I've learned a lot in the last 35 minutes that I couldn't have learned without that link provided by soup.

Thanks.

Does anyone actually have the equipment lying around that they could have a gander at what results at the telephoto end of any Big zoom camera are like with a reversed 50mm?

Bear_Music

Samsung Galaxy S24 Ultra

03/24/2006 11:45:18 AM · #11

Originally posted by eschelar:

This isn't quite true. All that's important is the actual focal lengths and their relationship to each other. Crop factor doesn't enter into it. The lenses are doing exactly what they're doing regardless of the sensor upon which they project their images.

If you want to call your 6mm lens a 36mm lens (35mm equivalent) then to get the magnification factor you have to multiply the focal length of the other lens by 6 as well, and the ratio remains the same.

Robt.

03/24/2006 11:51:50 AM · #12

Right. Poor choice of words here in calling it magnification size. This is why in the bracket area I used the "" marks.

Would it be more accurate to say this?:

I would need to use the 35mm equivalent to give myself an idea of the Effective magnification is. After finding out what the optical magnification is, I still need to apply my crop factor to give an idea of how much of the magnified area/field of view I will actually capture in camera.

to anyone who might be reading, this is not entirely accurate, see below
Hence:
Magnification = Optical Magnification.
Effective Magnification = Optical Magnification x Crop Factor

The lenses take care of the magnification, and the crop factor will slice the image circle down to my P&S Field of view, or what I actually see in the picture I take.

Oh yeah, and thanks for your patience as all this newbie-ite drivel pours out of me :)

Message edited by author 2006-03-24 12:43:14.

fotomann_forever

03/24/2006 11:54:57 AM · #13

All this math :-P Just shoot with the darn thing and post some pics :-)

Sony 24-70mm F4 Vario-Tessar T* FE OSS

03/24/2006 11:59:12 AM · #14

Only problem there is that I haven't got any of the equipment needed for this and my budget is locked until probably around the 10th of next month.

I need a:

50mm lens
reverse mount adaptor
step up/down ring
bayonet thread adaptor for the S2

to do this properly.

I might also be able to muck about with it just the 50mm, but the $$$ problem still isn't fixed.

In the meantime, it's mathematical dreams for me! It's how I while away time on the light rail and bus system that I spend 40 minutes to 2 hours on every day when I forget to bring my PDA (with reading material).

Bear_Music

Samsung Galaxy S24 Ultra

03/24/2006 12:15:04 PM · #15

Originally posted by eschelar:

No, this isn't true. 1:1 is 1:1 regardless of the size of the sensor. The only thing that changes is the amount of the object you see with a larger (or smaller) sensor. The magnification is constant. That 1:1 (or whatever) ratio refers to the mapping of size of object in reality to size of object on sensor.

If the sensor were film, you could lay the film over the object and confirm the 1:1 mapping. If you shot a quarter at 1:1, on 35mm film it would fill the frame top to bottom, on 4x5 film it would be just a round circle in the middle of the film, but the magnification would be the same; the actual image would be the exact same diameter as a quarter.

Robt.

03/24/2006 12:40:31 PM · #16

Gotcha! I really have got to start doing this sort of thing earlier in the day... I confuse myself sometimes after 1:00 am...

1:1 is a reference of the size of the subject relative to the size of the image it casts on the other side of the lens. The sensor cannot influence the actual size of the subject. Neither can the glass.

Not actually a ratio of glass at all. I really did know this. :) Switching from a P&S camera to a DSLR does not change the size of the people.

I think we are actually saying the same thing here, but I am just using the wrong words. It was actually with Medium format/larger film types that I used the words Crop Factor in the definition of "effective Magnification".

Hence Magnification is a function of the glass, but the ratio number itself is not.

So how's this looking?:

Magnification = Optical Magnification (a function of the lenses and a reference of relative size of subject and the image cast INSIDE the camera)
Effective Magnification = Optical Magnification x Crop factor (merely an inclusion of the relationship of your film size to the size of the image, which itself is a ratio, see above)

Magnification of a subject affects how big the subject appears in relation to its actual size.

Effective Magnification is only really relevant when considering the results captured by a camera.

Again, thanks for helping me out as I roll this one around in the old noggin'.

Message edited by author 2006-03-24 12:41:51.

03/24/2006 01:55:47 PM · #17

you sure about this?

Originally posted by Bear_Music:

If you want to call your 6mm lens a 36mm lens (35mm equivalent) then to get the magnification factor you have to multiply the focal length of the other lens by 6 as well, and the ratio remains the same.

Robt.

03/25/2006 07:05:10 AM · #18

Yes, this is correct.

The ratio doesn't change by the sensor.

Look above and you will see a quote above that mentions that adding a reversed lens does not actually change magnification, it just allows a certain focal length of lens on the camera to focus closer.

So the math is actually being applied from glass to glass. The size of the sensor is irrelevant in the magnification equation.

Therefore, just as in math, when you have an equation like x=y, you cannot apply a change to one side that you do not apply to the other side. 2x=2y, but 2x != y.

Remember that the magnification factor is a number which refers to the size of the physical object in relation to the image cast by it on the sensor side of the glass. This is optics only, not sensor.

My addition of the word "effective" adds in the crop factor, but just as the 1.6x crop factor does not actually change the focal length of a lens, the "effective" magnification does not have any bearing on the ACTUAL magnification.

EDIT to add a cross-reference link to see actual results of me playing around with an 80-200 and the S2 IS.

Message edited by author 2006-04-14 22:05:49.