Hasselblad 120mm f/4 Makro-Planar

[steve_barrett]

<p>When using a Hasselblad 120mm f/4 Makro-Planar lens, is it necessary to increase exposure time as the lens is extended toward its close focus limit? If so, by how much?</p>

[jpthurston]

<p>Not that I have ever been aware of in my use of that lens.<br>

Only until you add extension tubes will you have to start adding 1/2, then full stops for Exposure Compensation.<br>

Check out the Hasselblad site or other materials for amount of additional exposure for additional extension beyond 120mm.</p>

[leighb]

<p>Not within its normal focusing range.</p>

<p>If you use extension tubes then you have to compensate, as shown in the manual for the extension tubes or on the close-up calculator.</p>

<p>I love mine. The attached photo was taken from about three feet. Note the hair on the logo, snaking up across the seconds dial at 10, and another small hair above the bird, and another tiny one just below the 9.</p>

<p>- Leigh</p><div></div>

[q.g._de_bakker]

<p>Absolutely! Yes, you do.</p>

<p>The two above apparently have been saved by using a film with wide enough latitude, but you lose 0.5 stops (a tiny bit more, in fact) when the lens is set to its close focus limit.</p>

<p>Use <a href="http://www.hasselbladhistorical.eu/HT/HTCuC.aspx">the online Close-Up Calculator</a> to see when you need to apply how much correction.</p>

[leighb]

<blockquote>

<p>Use <a rel="nofollow" href="http://www.hasselbladhistorical.eu/HT/HTCuC.aspx" target="_blank">the online Close-Up Calculator</a> to see when you need to apply how much correction.</p>

</blockquote>

<p>When I click on the Calculate button I get a full-page error dump beginning with<br>

"Server Error in '/' Application."</p>

<p>The URL of the error message page is<br>

http://www.hasselbladhistorical.eu/HT/HTCuC.aspx</p>

<p>- Leigh</p>

 

[q.g._de_bakker]

<p>Hello Leigh,</p>

<p>Yes, it sometimes does that. Sorry!<br>

The URL is of the page itself, not the errorpage. What you get to see (the error dump) is what the server produces when it doesn't manage the session state properly. It should build and show the page with the calculator.<br>

The thing to do when that happens is to surf away, to some different page alltogether, so the server closes the session and forgets about you. When you then return to the page, it should work.<br>

The thing i should do is find another, better host for the site. But i'm afraid it's expensive enough as it is ...</p>

[leighb]

<p>Hi Q.C.,</p>

<p>When I first visit the page, the calculator does build and display properly. I can select lenses from the drop-downs, and the field entries appear to update properly.</p>

<p>I then change the distance entry and click on Calculate. That's when the error displays. It seems consistent, though I only tried it a few times.</p>

<p>Very nice calculator. It's the first one I've found that does exposure compensation as well as DoF. Must have taken considerable effort. </p>

<p>Well done. Thanks.</p>

<p>- Leigh</p>

 

[q.g._de_bakker]

<p>Leigh,</p>

<p>Thanks.</p>

<p>A quick note though: since i'm not one of the too many believers in that mystical entity called DoF, my calculator does not calculate DoF ;-). </p>

[leighb]

<p>Hi Q.G.,</p>

<p>In fact, it does not do DoF. I was busy looking at other things and just __assumed__ that DoF was in there somewhere. I agree with you in the general case; I'm a precision-focus freak. However I do use DoF and hyperfocal settings in situations like rapidly-changing street scenes when I don't have time to focus properly.<br>

<br />A couple of questions regarding the calculator, if I may (using the 120mm CF lens).</p>

<p>Total Extension = Lens Extension + Extra Extension. What is Extra Extension? Is it constant for a given lens?</p>

<p>It appears that Subject Distance and Image Distance are from the first and second principle planes (H and H'), respectively. Correct? Focusing distance is film plane to subject?</p>

<p>The entrance and exit pupil diameters agree with the Zeiss data sheet, but their positions do not. What are the references for these?</p>

<p>This is an interesting lens in that the first principle plane (H) is closer to the film than the second principle plane (H') by almost 10mm. Perhaps I should have chosen a different lens for the analysis.</p>

<p>Thanks.</p>

<p>- Leigh</p>

<p> </p>

[q.g._de_bakker]

<p>Leigh,</p>

<p>Lens extension is how far the lens mount will take the lens away from the film. Extra extension is the extension you add by using extension tubes or bellows.<br />You sometimes get to see both boxes displaying a non-zero value when you select a lens, because the calculator fills in the boxes with the values needed to get to the nominal close-focus limit of the lenses.</p>

<p>Yes, subject and object distances are measured to the principal planes. Focussing distance (the thing that is printed on the lens) from subject to film, i.e. includes both subject and object distances, and the internodal distance.</p>

<p>The pupil sizes do agree with Zeiss's data, because Zeiss were kind enough to give me a CD full of Zeiss' data. ;-)</p>

<p>The pupil positions on the Zeiss data sheets are measured to/from the first or last lens vertex. Informative, but not immediately useful.<br />The distances the calculator shows are measured to the film plane. That way you get to know immediately where to position the center of rotation if you want to stitch panoramas, and it provides the distance to use instead of the focal length to calculate the correct exposure compensation numbers.</p>

<p>It's not uncommon to have the principle planes 'switch position'.<br />What were you analysing, and how? And what lens were you looking at?</p>

<p>P.S.</p>

<p>I see that the server is acting up quite badly today. Perhaps time to find the funds to move away from cheap shared hosting and to a dedicated server after all...</p>

 

[leighb]

<blockquote>

<p>Lens extension is how far the lens mount will take the lens away from the film. Extra extension is the extension you add by using extension tubes or bellows.</p>

</blockquote>

<p>In this case (using the default data for the 120mm CF), the Extra Extension is shown as 0.46mm???</p>

<blockquote>

<p>The pupil positions on the Zeiss data sheets are measured to/from the first or last lens vertex. Informative, but not immediately useful. The distances the calculator shows are measured to the film plane.</p>

</blockquote>

<p>The film plane reference makes much more sense, providing useful information as opposed to the lens vertex references.</p>

<p>Is that your site?</p>

<p>- Leigh</p>

 

[Ed_Ingold]

<p>At its closest focusing distance, the built-in extension (focusing helix) of a CF120 is 26mm, decreasing the effective aperture by 0.5 stops.</p>

<p>Calculating the effective aperture is complicated by the fact that it is determined by the position of the exit pupil, not the optical "center" of the lens (i.e., focal length at infinity) nor the diaphram itself. This affects the ratios imposed by the built-in and external extensions.</p>

[q.g._de_bakker]

<p>Leigh,</p>

<blockquote>

<p>In this case (using the default data for the 120mm CF), the Extra Extension is shown as 0.46mm???</p>

</blockquote>

<p>Yes, because:</p>

<blockquote>

<p>[...] the calculator fills in the boxes with the values needed to get to the nominal close-focus limit of the lenses.</p>

</blockquote>

<p>The lens alone will not provide enough extension, so it would need that (small) amount of help, to be provided by a tube.</p>

<p>Edward,</p>

<blockquote>

<p>Calculating the effective aperture is complicated by the fact that it is determined by the position of the exit pupil, not the optical "center" of the lens [...]</p>

</blockquote>

<p>Indeed, though i wouldn't say it is complicated.<br />Calculating exposure compensation (and understanding why it is needed) is a matter of applying the simple inverse square law.<br />The intensity at the film plane changes in that proportion to the change of the distance between it and the place the light is coming from, i.e. the exit pupil.<br />The 'usual' formulae use the focal length (assuming a flat lens in which everything is in the same position) because the position of the exit pupil rarely is known. But that will lead to rather large errors.<br /><br />There is a trick though to get an approximate (and quite close too) figure for the exit pupil position when the lens maker does not supply that number. You simply measure (using a straight ruler) the pupil sizes from both the front and rear of the lens, and divide the one measured from the rear by the other. <br />That will give you the pupil magnification factor. Apply that to the focal length, and the result will be the position of the exit pupil.<br />Example: the entrance pupil of the 120 mm lens is 29.7 mm (i take the numbers form the lens data sheet, because i'm too lazy to actually measue something that is already known. ;-) But it works!), that of the exit pupil 33.5 mm. 33.5 / 29.7 = 1.128 (approx.). 1.13 * 120 = 135.6. Use that in your exposure compensation calculations instead of the focal length, and you get correct answers.<br />(But don't use that figure in calculations of magnification or scale, or anything that involves something expressed in mm or inches, like field of view. Use the true focal length for that.)</p>

<p>Sounds a bit complicated, perhaps. But it really isn't.</p>

[Ed_Ingold]

<p>By "complicated", I meant that the distance of the exit pupil to the film plane, rather than the nominal focal length, is used when calculating the effective aperture. That's not a widely known and seldom identified physical property of a lens. Zeiss specifications are a notable exception.</p>

<p>I think Leigh was confused, thinking that the distance between the optical center and the exit pupil constitutes the "built-in extension". The extension is actually the difference between the position of the exit pupil (or optical center) focused at infinity and when focused at some closer distance.</p>

<p>The means you describe to estimate that factor is very helpful. Another new trick for an old dog (me).</p>

[q.g._de_bakker]

<p>You're absolutely right, Edward.<br>

I was caught up in a "whatever we do, let's not make it sound like it could be (too) difficult"-mode. A personal thing, but i would like people to know what they are doing and use the grey matter instead of relying on (and repeating) those tired old things over and over and over again. (Start a discussion about DoF, and you'll quickly see what i mean ;-) ).<br>

Yet something i'd perhaps better give up on, in these days of wikis.<br>

Never has so much nonsense been spread so quickly. Never too has so much nonsense gained Revealed Truth status so quickly. Completely off-topic here, but Wikis are the worst thing we have seen in a long time!</p>

<p>Leigh,</p>

<p>If you want to know what the Calculator does (in the background), <a href="http://www.hasselbladhistorical.eu/HT/HTComp.aspx">this How To page</a> says it (almost) all.</p>

 

[leighb]

<blockquote>

<p>...the calculator fills in the boxes with the values needed to get to the nominal close-focus limit of the lenses.</p>

</blockquote>

<p>Perhaps you're using a different definition of "close focus limit" than I do.</p>

<p>To my mind, the definition is:<br>

"The closest subject that can be focused when the helicoid is racked out to maximum."<br>

By this definition, the "extra" extension is, and must be, zero.</p>

<p>The only way the "extra" extension could be non-zero would be if the helicoid offset shown was less than its true maximum.</p>

<p>I'm an engineer, and pretty familiar with optical design. I use Zemax usually, but some other programs when the spirit moves me.</p>

<p>I think your problem with DoF stems from a rejection of the concept of "circle of confusion". It's nice to think of focus as being a single point on the optical path. That can be achieved under very limited circumstances, i.e. for a monochromatic light source focused on an infinitesimally thin plane through a lens exhibiting zero aberrations. A potentially interesting academic exercise, but of no practical value.</p>

<p>Such a point, if one could create it, would be of no use in photography because the sensors (film grains or digital sensors) are of finite size, and would not respond to a point-focused stimulus. Couple this with the fact that any sensor has a finite thickness, and you have a cone of light which stimulates the sensor, always.</p>

<p>- Leigh</p>

 

[q.g._de_bakker]

<p>Leigh,</p>

<p>As an engineer, you must be familiar with the concept of nominal values.<br>

If Zeiss say the lens focusses down to 80 cm, that's a nominal value. The lens itself needs that extra bit of extension to really do so, i.e by your definition too, the extra extension must not be zero to make Zeiss' 'nominal promise' true.<br>

I had explained that before, i'm sure.</p>

<p>I don't reject the concept of the circle of confusion. I have no "problem" with DoF either. On the contrary: i know what, and how, it is.<br>

Perhaps it's being an engineer that makes you assume things about what other people do and do not think. I think you got a bit confused here. ;-)</p>

<p>That confusion extends, by the way, into your assertion that film nor digital sensors "would not respond to a point-focused stimulus". Of course they would.</p>

 

[WAn]

<p>Gentlemen, I found that it is much easier to make the close-up exposure adjustment basing on magnification only. <br />The approximate formula that works very well (the error is less than 0.1 stop!) in the range from m=0 (infinity) to m=1 (close-up 1:1) is<br>

<br />h1 [ev] = 2*m + 0.1<br>

Here m = G/L is the magnification (G=size of the our object on ground glass and L=size of that object in real life) and 'h1' is the needed adjustment.<br>

The formula is simple and it is very easy to use without any calculator.<br>

For portraits there is even simpler form: the size of adult face is ca 20cm (from chin to start of hairs), so let L=20cm and we get<br />h2 [ev] = 0.1*G[cm] + 0.1<br />-- any schoolboy can do the calculation mentally ;)<br>

This is true for any lens, any focal length, any focusing distance, and any format from SM to ULF, the 'm' is the single combination that the adjustment depends on.<br>

For shots closer than 1:1 these both approximate formulas are not true anymore, use the precise one instead:<br />h = 2*log (1 + m)<br />where log is base 2 logarithm. Of course here the calculator is a must.</p>