Max f-stop value when reversing a lens?

[paul_ong1]

<p>Hi,<br /><br />I am trying to figure out the change in effective or initial f-stop when reversing a lens for macro photography. For both my Nikon 28mm f/2.8 and Nikon 50mm f/1.4, the rear element is smaller than the front element, so I assume that there is less light gathering ability. I know that the f-stop needs to be estimated taking into account the magnification. What I want to know is what is the initial max f-stop value on a reversed lens. Thanks.</p>

[leicaglow]

<p>Paul, I don't believe it should make any difference. The aperture is a ratio of the diameter of the iris (aperture) to that of the length of the lens.</p>

[bjørn rørslett]

<p>Any optical system works in reverse. The light rays don't know in which direction they travel. So as such nothing has changed. The main difference now is that it is the former exit pupil which plays the role of the entrance pupil, and vice versa. Thus, if the lens is not symmetrical, you have to factor in the (inverse of) pupil magnification plus image magnification in order to arrive at the effective aperture.</p>

[akira]

<p>When 28/2.8 is mounted on the body via BR-2(A), it gives 2:1 magnification, according to the user manual accompanying BR-2A. In this case, the exposure magnifying factor is (1+2)^2=9 which corresponds to 3+ stop. So the opening aperture should be f8+, if I'm not missing something.</p>

[benoit_sauzier1]

<p>Let's start with the basics: f-stop = focal length/ D, where D is the diameter of the front element. So, for a Nikon 28mm f2.8, the front element diameter is 28/2.8=10mm. You can verify that for yourself. So, if you want to know what the initial max f-stop is when the lens is mounted in reverse, then just measure the diameter of the rear element, and divide the focal length by that number, in millimeters of course!</p>

[bjørn rørslett]

<p>Sorry but this is entirely incorrect. The relevant parameter is not size of the front (or rear) elements, but the size of the entrance pupil in effect. The pupils are virtual quantities and bear no direct relationship to the physical size of the front/rear glass. In fact, it is possible for the pupil to be bigger than the lens diametre. The entrance pupil acts as the collector or light on behalf on the lens, the exit pupil likewise illuminates the recording media (film or digital sensor).</p>

<p>The 28/2.8 reversed will give at least a 3 stop drop in effective aperture at 2X magnification. Probably at least 1 additional stop is lost through the asymmetrical design (pupil factor < 1 in normal orientation, > 1 for the reversed lens).</p>

[benoit_sauzier1]

<p>The question was: "I want to know is what is the initial max f-stop value on a reversed lens." So, the answer is the one I have provided in my previous post. Now, to figure out what the effective aperture is at a given working distance, magnification has to be taken into acount.</p>

[bjørn rørslett]

<p>Still incorrect. You need to use the size of the entrance pupil, not the front element of the lens. Have you seen a 28/2.8 Nikkor? The front element is much bigger than 10 mm .... But the entrance pupil is 10mm. Just look into the lens to observe that simple fact.</p>

[benoit_sauzier1]

<p>Well, maybe it does not work for wide angles, but my 55 f2.8 micro Nikkor front element measures approx 19mm, 105mm f2.5 measures 43mm, 180mm f 2.8 measures 66mm... Are you telling me that the front element of a 600 mm f4 does not measure 150mm? Or that the front element of a 300 f2.8 does not measure 107mm? </p>

[Michael R Freeman]

<blockquote>

<p><em>Well, maybe it does not work for wide angles...</em></p>

</blockquote>

<p>It doesn't work for normals or telephotos either. Take the humble (and I presume close to symmetrical) Ai 50mm f/1.8 ... front element diameter = 35mm. Do the math. :)</p>

<p>The front element of my AF-S 24~85mm is approximately 53mm, yet for some reason Nikon is telling me that my maximum aperture at 85mm focal length is only f/4.5 rather than f/1.6. I've been cheated! ;)</p>

[frank uhlig]

<p>Yes, Bjorn is right. Unless the 300/2.8 vignettes since its front lens is too small to support Benoit's false idea. My 300/4 lens has in fact a front lens diameter of slightly under 75mm and it does not vignette at 2.8.</p>

[steve_levine]

<p>I know that with my Nikon reversing ring, a 50mm will give me approximately, 1:1 magnification. And a wide angle, say a 28mm gives around 3:1. And yes , the same quantity of light is available in either direction. Light loss only occurs as you add extensions, or any added length to the optical system.</p>

[paul_ong1]

<p>Thanks everyone. I think I am starting to understand in terms of light equivalent. Good for setting exposure, but not a major issue with behind-the-lens metering.</p>

<p>I guess I should have stated that my main interest in knowing the f-stop is avoiding diffraction, and also aiming for optimal aperture opening for sharpness. Of course, I could test, but too many lens options and magnifications with extension tube(s) or bellows. So a general rule of thumb is helpful.</p>

<p>So, when I estimate the equivalent f-stop to account for magnification and other factors (e.g., light lost from reversing a lens not designed for that), how is that calculated f-stop related to the general rule of avoiding small settings (f/11 and smaller) or to what is consider the optimal f-stop (generally one or two stops down from wide open)? </p>

<p>I am asking because it seems like diffraction sets in earlier if I just look at the nominal f-stop setting.</p>

 

[Andrew Garrard]

<blockquote>In fact, it is possible for the pupil to be bigger than the lens diametre.</blockquote>

 

<p>Hi Bjørn. Would you mind elaborating on that, please? I can possibly understand it for a lens that doesn't focus to infinity (the cone of light contributing to the image is never infinitely long); otherwise I'm confused. Just curious, and I've yet to order my copy of A History of the Photographic Lens to teach myself the appropriate optics. Cheers.</p>

[jose_angel]

<blockquote>

<p><em>"The front element is much bigger than 10 mm .... But the entrance pupil is 10mm. Just look into the lens to observe that simple fact."</em></p>

</blockquote>

<p>This is the way to check the entrance pupil, isn`t it? Just look throught the front of the lens.<br /> What I can`t figure is how to measure it. The only way I can imagine is to divide the focal lenght by its aperture (f- number).</p>

[francisco_disilvestro]

<p>The <a href="http://homepage2.nifty.com/akiyanroom/redbook-e/repro/repro1pon.html">85 mm Repro Nikkor f 1.0</a> has a diameter of less than 60 mm</p>

 

[scott_murphy_photography]

<p>There is no change in the effective aperture whether the lens is mounted conventionally or reversed. If you want to prove this to you, take your camera and point it at an evenly lit surface, that fills the frame. A clear sky is a good choice. Meter the exposure. Now, remove the lens, reverse it and point it at the same place and re-meter. You will find the two exposures to be the same.</p>

<p>Magnification differences will not affect exposure either. Where you mount a lens conventionally or reversed, the distance from the node to the film/sensor plane does not change. The reason you lose exposure is due to something called the inverse square law. As you double the distance between the node and the film/sensor plane, the amount of light reaching it is halved. Double it again, and now the light is halved again, or is now 1/4 of what it was when you started.</p>

<p>I did a pretty extensive article on macro photography you might find useful. You can find it <a href="http://www.scottmurphyphotography.org/macrophotography.htm">here</a></p>

[Andrew Garrard]

<p>Scott - cool article! Thanks for the link.</p>

[Michael R Freeman]

<blockquote>

<p><em>Now, remove the lens, reverse it and point it at the same place and re-meter. You will find the two exposures <strong>to be the same</strong>.</em></p>

</blockquote>

<p>I think you will find that will not apply with a reverse mounted *wide angle* retrofocus lens. There is significant light loss.</p>

<p>AiS 24mm f/2.8, conventional mount, f/5.6 @ 1/1000, frosted glass backlit by sky ...<br /> <img src="http://i170.photobucket.com/albums/u255/mikerfns/photonet%20stuff/24f28conventional.jpg" alt="" /></p>

<p>-<br>

AiS 24mm f/2.8, reverse mount, f/5.6 @ 1/1000, same frosted glass backlit by same sky ...<br /> <img src="http://i170.photobucket.com/albums/u255/mikerfns/photonet%20stuff/24f28reversed.jpg" alt="" /></p>

<p> </p>

[bjørn rørslett]

<p>The 85/1 Repro-Nikkor has an entrance pupil of 87 mm (actually, its focal length is close to 87 mm as well) and this is much bigger than the filter thread which is 53mm (external), 49mm (internal). When you look into the lens it is strange, but illustrative, to observe the large pupil extending beyond the lens barrel.</p>

<p>As to the claim of not changing the effective aperture when the lens is reversed, it rests on a false assumption. The reversed lens is not *focused* on the sky, it has its focus 46.5mm from the bayonet mount. So what you meter is light coming from "beyond infinity" and which cannot form an image. Repeat with the subject in focus in either case and it is easy to see that in fact the effective aperture is changed.</p>

[paul_ong1]

<p>Hi,</p>

<p>Again, thank you for the information and discussion. There is certainly lots to digest. For example, I do accept that it is possible to have an entrance pupil larger than the front lens. However, I wonder about the light gathering ability of two lenses of the same focal length and identical maximizing entrance pupil but (significantly) different front lens element sizes. Would the light gathering ability be identical? Won't the one with the larger front lens have more potential? I just do not know enough about lens design.</p>

<p>But back to my main concern, which is about where to set the nominal f-stop to avoid diffraction. Say that I am using a reversed lens (nominally a f/2.8) that after accounting for magnification (of say 2x), the effective maximum effective f-stop is about f/8. Does that mean that I will start experiencing diffraction if I close it down a step or two? What if the optimal (sharpest) setting for that lens is a nominal f/5.6? Is there, then, a trade off between moving to the "sweet" stop and introducing diffraction?</p>

<p>Thanks in advance.</p>

<p>(Edit, additional comments). I realized that the formula I used to calculate an effective f-stop is nominal f-stop times (1+magnification) squared. I notice that some people use nominal f-stop times (1+magnification). Not sure which is correct, but the concern about diffraction remains the same. If the effective f-stop is f/8, does closing the lens down introduce difrraction?</p>

[joseph_wisniewski]

<p>Yes, you will experience diffraction. That is based entirely on "effective aperture". That is why "focus stacking" has taken the macro world by storm, it allows us to shoot at wide "effective apertures".<br>

<br />There is no hard and fast rule in macro about best apertures. There isn't even one in non-macro photography, the "two stops down" thing is more "old wives' tale" than "rule of thumb". It's right up there with DOF being distributed 1/3 in front of the subject, 2/3 behind.</p>

<p>You really, truly do have to test ever single flarking lens. And you have to test it multiple times, at different magnifications, and by using extension tubes or coupling it to another lens, because some lenses perform much better extended, while others perform better coupled. Short extensions (someone mentioned reversing a lens directly in front of a camera) cause lenses to perform totally unpredictably. Lenses only perform predictably when their "near side" and "far side" are near the "design distance" for the lens. You have to extend the typical reversed lens about a foot before it settles down and becomes predictable. Since that involves effective apertures that make lenses basically useless (a 20mm f2.8 extended 300mm is somewhere around f45) you pretty much never get to experience a predictable, well behaved reversed lens.</p>

<p>It's a wild ride ride, hang on tight and enjoy it.</p>

[joseph_wisniewski]

<blockquote>

<p>Jose Nagel - This is the way to check the entrance pupil, isn`t it? Just look throught the front of the lens.</p>

</blockquote>

<p>Nope. That may result in an occlusion, where you don't actually see the entrance pupil. And, for a reversed lens, where you're looking at the exit pupil, it will almost definitely result in an occlusion.</p>

<blockquote>

<p>What I can`t figure is how to measure it. The only way I can imagine is to divide the focal lenght by its aperture (f- number).</p>

</blockquote>

<p>You do it by triangulation. Mount the lens on a device that lets you revolve it around an axis while racking it in and out, sight the opt or bottom of the pupil through the lens using a bench telescope, and revolve and rack until you hit a point where there's no parallax as the lens revolves.</p>

<p>Well, you asked.</p>

[bjørn rørslett]

<p>Effective aperture is f-number * (1 + magnification/pupil factor). The squared term is for exposure time. Note pupil factor=exit/entrance pupil so for a telephoto design it is < 1, for a retrofocus wide-angle lens > 1. This for the normal orientation in which subject and image distances correspond to the design values. When you reverse the lens, the inverse factor applies.</p>

<p>A practical consequence of the above is that using a typical wide-angle lens with lots of extension is not a good idea, since the effective aperture becomes very small. Joe W. already pointed that out. He also made the very relevant, yet often overlooked, point that for close-up and photomacrography, testing at the intended shooting conditions needs to be conducted, since it is never entirely certain whether the setup works well. Some lenses yield fully acceptable results even at horribly small effective apertures, most don't. Some double-lens combinations deliver first-class results, many are mediocre, and most will vignette unless extra extension is added. Only testing will show what's going on.</p>

[paul_ong1]

<p>Joseph and Bjorn, many thanks for your explanations. I have learned much, and I will try to do some of my own testing.</p>

<p>Wonder if you can give me some starting point regarding some of your preferred combination of stacked lenses. I try not to spend too much time "reinventing the wheel." (Bjorn, I have used your incredibly useful web site as a guide to help me select and buy lenses over the last few years.)</p>