I know that the closer the distance to your subject is, the smaller the DOF is. This is true for macro lenses, diopters, extension tubes, reverse lenses etc. But suppose I have a magnification of 1:1, either with a macro lens, diopters, extension tubes or a reverse lens: how is DOF between these methods? Do they all have same DOF using the same aperture?
TL;DR For the same (effective) aperture value with all macro methods, you'll get the same DoF, e.g. with effective f/16 and a "sharp spot" defined as 0.02mm, you'll get +/-0.32mm. Read on for a more detailed explanation plus a few caveats.
With a 1:1 scale, you also get a 1:1 relation between the object displacement and the picture displacement, meaning that an object 1mm closer to the lens will get projected onto a point 1mm behind your sensor. The light that focusses there comes through a cone formed by the lens opening and this focus point. Only there in the focus point, all the light concentrates in one point and gives the sharpest picture. If the sensor isn't placed there, it instead sees a circular spot of light.
The bigger the displacement between the focus point and the sensor, the bigger the light spot gets, depending on the opening angle of that cone, and that angle directly corresponds to the aperture value.
E.g. f/1 is a 60° cone, meaning that the light spots grows by 1mm for every millimeter displacement ahead of or behind the sensor plane. With f/11 you get a 5° cone, meaning an 0.09mm spot for 1mm displacement, and with f/32 it's 0.03mm per mm.
With your desired 1:1 scale, the displacements also translate 1:1, meaning that a 1mm displacement at the sensor side correlates to 1mm displacement on the object side as well. (That's true no matter how you achieve the 1:1 ratio.)
Now then, how big is the depth of field?
That depends on two parameters:
- How big a spot of light do you accept as still being "sharp"? In the good old analog 35mm times, this was typically assumed as 0.03mm, being roughly equivalent to a 1-megapixel effective resolution. If you're using a crop-sensor body, then 0.03mm will be closer to 0.5 megapixel, and that sounds like rather poor quality. So, I'd go for a smaller spot size, maybe 0.02mm or even 0.01mm.
- What's the effective aperture value you'll use? Multiply your maximum spot size with this aperture value, and you get the depth of field (then multiply by two if you want both halves added, the part in front of the focal plane plus the part behind it).
Caveat 1 (aperture value):
Maybe you noticed that I used the term "effective aperture value", as that's not the nominal value you see on the display, or what you adjust the aperture ring to:
- With an extension tube, the effective value will become twice the nominal one. E.g. when setting to f/16, you'll effectively get f/32.
- The same factor two applies to the lens in retro position (with extension tubes or bellows to reach the correct position for 1:1).
- With a close-up lens (a secondary lens mounted in front of your base lens), the effective aperture value roughly stays true to the nominal value.
- With a specialized macro lens, it's can be anything between the nominal value and the factor two, depending on the inner focussing mechanics.
- With a tele-converter (one possibility you didn't mention), you have a factor of two, multiplied by an unknown (smaller) factor for the 2:1-capable macro base lens.
Caveat 2 (diffraction):
One nasty property of light is its nature of being an electromagnetic wave, resulting in an effect called "diffraction". The narrower you make the cone of light by closing the aperture, the blurrier the focus point gets. A rule of thumb is to multiply the effective aperture value with the light wavelength (roughly 0.0005mm), and then you get the size of the diffraction spot.
So, if you try to increase the DoF by closing the aperture maybe to an effective f/64, then not a single object detail will be sharp, you'll always have blurs of 0.032mm, even for details exactly in the focal plane.
There are potential differences.
The relative sharpness of details is dictated by the subject distance (relative size of the details), and the size of the entrance pupil (diffraction)... magnification doesn't change this.
The depth of field is then determined by the magnification of that relative sharpness; regardless of where/how that magnification occurs.
So, adding a diopter which does not significantly change the subject distance but does significantly change (increase) the size of the entrance pupil can cause a different result than using a reversed lens for example.
thanks for sharing your thoughts with me.
I guess I'll have to do some tests. But with images on 1:1 or 2:1 there is really a very thin DOF. I'll be difficult to measure (for me). But maybe it gives me an impression. I need to find lenses + accessories to get the same magnification in order to compare.
Field flatness is something to think about. My subjects are small flowers (pistils and stamens), insects, moss etc. These are rarely flat and rarely parallel to my sensor.
In house I often use a manual operated macro rail in combination with extension tubes or bellows and a reversed EL-NIKKOR 50 mm f/2.8 enlarger lens. I also use focus stacking, but I'm struggling with figuring out the individual focus increments. The required focus increments depends on the DOF. Depending on the scale (1:1, 2:1, 4:1) I use focus increments of about 2..0.5 mm, with f/8, but it's just guessing. Maybe I need smaller or larger focus increments, I don't know. My focus stacking results are not always what I expected.
Inhouse I can easily change lenses and add/remove extension tubes without worrying to much about dust. But outdoors, while hiking, I prefer to avoid that as much as possible. And, while hiking, I try to limit the number of stuff and weight to carry with me.
Making images outdoor (windy) for focus stacking is a challenge anyway and so is focus stacking with live insects as subjects.
I do not have a 1:1 macro lens at the moment.
Outdoors most of time I use a Marumi DHG Macro Achromat 200 +5 diopter. I can simply screw the Marumi on/off as I want... This gives me most of time acceptable images, but with small DOF. Sometimes that's nice, but most of times I wanted more DOF.
So I wondered if a 1:1 macro lens would have more DOF (with a comparable aperture). If I'm sure this would be the case, then I could consider to invest for such a lens. And to take it with me while hiking and change lens from the one I use for landscape photography. And indoors, maybe less images are required while focus stacking.
Some of you made remarks about a budget, but I do not know how that influences DOF. I have seen many DOF calculations, but none has budget as parameter.
Another aspect to think about is the working distance, the distance between the frontlens and the subject. For sure the working distance is greater for a 100 mm 1:1 macro lens compared to my reversed EL-NIKKOR 50 mm. This is an advantage outdoors while taking photo's of insects. But this also has nothing to do with DOF.
Hmmm, I have someting to think about. I also try to do some tests with the stuff I do have.
Thanks all of you for sharing your thoughts.
DOF would not effectively change. DOF, even if it did change due to different lenses w/ different designs being compared, it would be so minor that you wouldn't be able to see it at 1:1. By this I mean to say that lens aperture declarations aren't very accurate and diffraction makes it difficult to judge accurately in the actual image. In addition, you will not achieve an exact 1:1 ratio with any system except for a quality bellows on a rail with a flat field lens.
As a rule of thumb depth of field span extends 1/3 back toward the camera and 2/3 down stream from the subject. Actually the span is a variable based on subject distance. You should look for your self using an online depth of field calculator.
As the subject distance decreased from infinity, the span of depth changes. when you achieve magnification 1 (unity 1:1), span of depth of field becomes equal fore and aft and practically zero.
Addendum: Per Mike Sowsun 2 - I didn't answer the question "different methods of Macro" I don't consider myself an expert on this subject. Unity or 1:1 is achieved if the subject is 2X the focal length forward of the front nodal and the film/digital sensor is 2X behind the rear nodal. Thus unity can be achieved using a variety of lenses. I personally don't believe depth-of-field tables, charts, on-line calculators yield data this is engraved in stone. Their are too many variables that these tables take for granted. Factors are acuity of vision of observer, contrast of displayed image, level of illumination of displayed image and distance observer to displayed image, to name just a few. Using 0.03 circle of confusion, various focal lengths, a format of 24mm by 36mm, I rigorously calculated DOF for some focal lengths. 20mm focus point 40mm 39.1mm thru 41.0mm 25mm focus point 50mm 49.1mm thru 51.0mm 50mm focus point 100mm 199.1mm thru 201.0mm 100mm focus point 2000mm 199.o thru 201.1mm My conclusion, the usual criteria come to play i.e. subject distance and focal length. Let me add, we reverse a lens mainly to achieve better flat field. This is because the rear ray trace is expected to project on a flat such as film or digital image sensor. The front ray trace is expected to image a curved world. In Macro work, the subject has shallow height, thus it's better is the lens has a flat field. When we reverse the lens, the two cardinal nodal points are reversed. The result is likely a focal length change that affords a tiny bit more magnification. It is unlikely that the focal length will be know exactly in this configuration. Bottom line - DOF tables - charts etc. become more confusing.