Working Distance, Focal Length, and Perspective

<p>So I've been rolling a question around in my head. I've always understood it that the distance from the camera sensor/film to the subject dictates the perspective of the photograph. We often say that the focal length dictates perspective, but this is not exactly true, right? It's just that a specific focal length tends to put us at a distance from our subject. <br>

So at 1:1 macro working distances, perspective should be driven by working distance, right? The reason I ask is that I saw something about two macro lenses with significantly different focal lengths (Tamron 60mm vs. 90mm) but with similar working distances at 1:1. These should yield the same perspective for macro work, correct? This was not what the article stated, however. <br>

I'm interested in this because I'd like to get a less "flattened" perspective in my close-up work, but I'm not willing to sacrifice working distance to do so.</p>

<p>The perspective is solely controlled by the distance of the observer from the object, with or without a camera or lens. The farther you are away from the object the more flattened it will appear. The focal length of the lens controls how much the image of the object is enlarged on the film or sensor. For any given working distance the 90 mm enlarges more than the 60 mm but without changing the perspective. Your last sentence is asking for something that is physically impossible. To get a less flattened perspective you have to get closer. If you get closer to the object it would then appear larger on your sensor (in your picture). If you want to compensate for this, you would choose the shorter focal length.</p>

<p>1:1 working distance for the 60mm and 90mm Tamron lenses is the same (100mm). Therefore, at the same working distance, the enlargement is the same. I know this is not typical for macro lenses.<br>

What you're telling me is what I thought, that the articles I've read about these lenses are wrong. Even though they are different focal lengths, the fact that they both have the same 1:1 working distance means that they will yield the same perspective.</p>

<p>With our process camera; the lens is about symmetrical; thus a 600mm Apo Ronar at 1:1 is roughly 1200mm from the lens. If one shoots medical images with the old 200mm Nikkor medical lens; one is farther away than your Tamron lenses; so as not to disturb operation in progress.</p>

"Working distance" is the distance from the subject to the front of the lens. It's a physical measurement not an optical measurement. Optical measurements use optical reference points like entrance and exit pupils, principle points and focal planes.

 

"Perspective" insofar as it may be determined by distance doesn't use the front of the lens as it's reference point. When working distance is large the difference is small, but when working distance is small, the difference between the position of the front of the lens and the correct reference point (which I'm guessing may be the entrance pupil) may be significant.

<p>Correct, what I said assumed that the lenses have the same distance between sensor and front of the lens.</p>

<p>Dave,</p>

<p>It is like so many things, when you look closer the "rules" don't work very well. Lenses focal lengths are stated on them when they are focused at infinity. Now as you focus closer the focal length becomes shorter, the point of focus changes. This can be adjusted for though and at normal focus distances the effect is negligible.</p>

<p>However when you get down to very short focused distances it becomes more and more difficult to correct that change, so another technique is used by the lens manufacturers, built in extension tubes, notice how the lens bodies invariably extend a lot at closest focus? That is because you are basically using an extension tube built inside your lens. The effect of this is that all Xmm lenses are not equal, so to get 1:1 with maker A's 100mm macro does not give you the same working distance as maker B's 100mm macro because you don't know how far each went in correcting the focal length change before resorting to the built in tube method of getting the magnification. Is maker A's 100mm macro really an 85mm macro with shorter internal tubes but closer subject working distance at 1:1 and how does that change the working room to maker B's 100mm macro that is really a 95mm and longer internal tube at 1:1 (the second should give you a longer front element to subject distance).</p>

<p>In this instance the effective focal length at 1:1 is important it still cannot tell you how much glass is in the way but should be able to tell you the subject working distance from the front element. On macro lenses that have focus abilities nothing short of an optical engineering degree can tell you these figures, but testing can and is quicker and easier and you and I can do it. With macro lenses that don't have focus abilities the magnification (effectively extension), and focus (effectively lens to subject distance), is a function of focal length, extension and subject to lens distance.</p>

<p>Hope this helps, Scott.</p>

<blockquote>

<p>DaveL said,<br>

1:1 working distance for the 60mm and 90mm Tamron lenses is the same (100mm). Therefore, at the same working distance, the enlargement is the same. I know this is not typical for macro lenses.<br />What you're telling me is what I thought, that the articles I've read about these lenses are wrong. Even though they are different focal lengths, the fact that they both have the same 1:1 working distance means that they will yield the same perspective.</p>

</blockquote>

<p>The 90mm is the older model designed for 35mm full-frame, the 60mm is the newer model designed for APS-C sized cropped sensors. The 1:1 mag is with those factors in mind, so the 90mm is _not_ 1:1 at the same distance with an APS-C type DSLR, more like 1.6:1, larger than lifesize at the same distance, or you'll have to move further back.</p>

<p>Kin,</p><p>That is 100% wrong, sorry. 1:1 is 1:1, it does not matter if it is full frame or 1.6 crop. The 1.6 Crop camera is a CROP of the full frame one, not an additional magnification.</p><p>A lens that gives 1:1 does that at the same distance regardless of the sensor mounted behind it. If you put a Canon 100mm macro on a ff the working distance for 1:1 is around 8 inches, set that up on a tripod, now take off the ff camera and mount a 1.6 crop camera, theimage is still focused and it is still 1:1 but there is a lot less of the image to see because it has been cropped!</p><p> </p>

<p>"Now as you focus closer the focal length becomes shorter..."<br>

This is not true because rays entering a lens in close up situations are not parallel, they are diverging when striking the lens. This causes the focal point to be further back and necessitates the need for tubes. The focal length becomes longer and the focal ratio changes with it, requiring exposure adjustments.</p>

<p>Steve,</p>

<p>Next time just search google "macro lenses change in focal length". On the first page these two pop up, show me any macro lens, indeed any photographic lens, whose focal length increases as it is focused closer.</p>

<p>http://kmp.bdimitrov.de/technology/focalLength/index.html<br>

http://www.flickr.com/groups/macro123/discuss/112332/</p>

<p>There are hundreds of other articles on macro lenses and the way the focal length, tubes, working distance, depth of field, etc all play off against each other.</p>

<p>The short answer is that since your macro is a fixed focal length lens, and since when doing real macro shooting your distance from the subject is not going to change much at all (in the big picture), you will not experience much of a change in perspective. That's all there is to it.<br>

<br /> Now, if you switch from a 100mm macro to a 17mm with an extension tube - which is a lot of fun and opens up whole new worlds of macro - you will see a big change in perspective from the 100.</p>

<p>Brett,<br>

Your fixed focal length macro lens is not a fixed focal length lens. At the short working distances involved it does all change enough to make a difference to the perspective. Not as much as using a tube on a 17mm though but that needs very short tubes and tiny working distances.</p>