macro lens, what's difference does the focal length/ratio make?

I am new to the DSLR world having just purchased the Nikon D50 and I

am interested in macro photography. My previous camera was a compact

that allowed me to get as close as 3cm from the subject. I know this

close range was only possible because of the small size of the sensor.

When inspecting a compact's macro ability, one only had to look at

how close the camera was able to focus on a subject. However, macro

lenses have a ratio attribute which confuses me a bit. I appreciate

any insight on any of the following questions that would help me

understand how to interpret this ratio factor in macro lenses.

 

Assuming the two lenses have the same minimum focusing distance, is a

50mm 1:1 ratio lens roughly equivalent to a 100mm 1:2 ratio lens since

the 100mm would bring you twice as close to the subject?

 

What is the formula/equation for calculating a lens?s ratio?

the ratio is the size of the original image to the size of the projected image. so with 1:1, if you take a picture of an acorn it will almost fill the frame. (the projected image will be the same size as the veiwed image).

 

the lens lenths really only get you working distance. a 100mm 1:1 will project the same size image as a 50mm 1:1 except you can be farther back from the subject, making good conditions to shoot pictures of bees and such.

 

there is a formula that is good to use if you have a bellows system or an extension tubes set, but for just the lens you don't need it.

the equation btw is l/f were l is the length you protrude the lens (ie on a bellows or an extension tube) and f is the focal length of the lens. this also assumes focus set at infinity.... also you will have to compensate for extending the lens (on a bellows or extension tube) and this will be your magnification+1 so if you have your 50mm macro lens on a bellows set at 150mm then the magnification will be 3 and your compensation for exposure will be 4 stops.

 

but this will not apply to just the lens. lenses usually have all this built in.

While longer focal length gives you more working distance and a narrower angle of view, it has nothing to do with magnification. 1:1 is 1:1 - period! Magnification is the ratio of the image size on the sensor/film to the size of the object. So for 1:2 an object that is 6mm in real life will render as 3 mm on the sensor. Now for a small sensor that 3mm might be frame-filling while with a medium format camera with 6cm x 6cm film the object would fill a miniscule portion of the frame. But magnification, DOF etc are the same.

 

Any macro book will give more details.

 

-A

Lens focal length divided by focus distance will give the magnification ratio for any one picture. The ratio shown for a macro lens tells what you can get without any other focus aids, just racking the lens all the way out.

 

If a 50mm lens is focused on a subject 50mm away, the ratio is 1:1, which is life size.

 

If a 100mm lens is focused on a subject 50mm away, the ratio is 2:1, twice life size.

 

If a 50mm lens is focused on a subject 100mm away, the ratio is 1:2, which is 1/2 life size.

 

If a 50mm lens and a 100mm lens focus to the same distance, the 100mm lens will capture an image twice as big as the 50mm lens.

 

The focus distance is measured from the plane of the sensor or film, not the front of the lens. As I recall, there is a line with a circle through it on the top right of your D50 that marks the focus plane, if it is designed like my D70.

 

Because of differences in the designs of lenses, you may not be able to double your distance from the front of the lens to the subject by switching to a lens of double the focal length. You will double the distance from the focus mark to the subject, generally.

 

At close distances the focal length of the lens changes so that the formulas do not work perfectly. This is the Voodoo part of optics. I do not have the specifics on this. I do recall(not with my lenses right now) that my Micro-Nikkors do not give distance numbers but magnification ratios at close distances. I assume that Nikon marks the lenses to take into account these focal length changes so I do not have to figure it out.

 

I expect that more knowledgeable macro guys may post formulas or manufacturer's specs for determining the true ratios for any particular lens.

 

The minimum focus distance for any lens may be shortened by the use of extension tubes. Macro lenses are corrected for better results at close distances compared to other lenses.

 

Bill

<I>If a 50mm lens and a 100mm lens focus to the same distance, the 100mm lens will

capture an image twice as big as the 50mm lens.</i><P>

 

To be a little more precise: in this scenario, the image of a given object (not the total image

size) will be twice as long in linear dimensions with the 100 mm lens as with the 50 mm lens,

and hence will be four times larger in terms of the area it covers on the sensor.

Usually, people want longer-focal-length macros so they can get farther from the subject. If you're trying to photograph bugs or snakes or whatever, that's helpful.

 

I've got a 50mm Sigma macro that focuses to 1:1. With the lens hood on, you have problems with the lens itself making a shadow on the subject. Solution: longer focal length (at some indefinite point down the road).

If you can only have 1 macro lens, then by virtue of its versatility the 100mm range macro is the way to go. 50s don't have the working distance most of the time and 180s are incredibly expensive if you're not doing really serious close work.

Thank you all for your responses. There seems to be some contradiction with the responses though.

 

Referencing: "Bill B , sep 14, 2005; 05:47 p.m"

 

If I understand correctly, the answer to my first question would be "yes" since you stated the following:

 

"If a 50mm lens is focused on a subject 50mm away, the ratio is 1:1, which is life size.

If a 100mm lens is focused on a subject 50mm away, the ratio is 2:1, twice life size."

 

Assuming the 50mm lens is 1:1 and the 100mmm lens is 1:2, then if I am focused on a subject 50mm away with the 100mm 1:2 lens, my magnification ratio is 2:1 with the 100mm lens which will bring me to a 1:1 (1:2 & 2:1 = 1:1) right on par with the 50mm (ignoring angles of view).

 

---- However ----

 

Referencing: "Anupam Basu , sep 14, 2005; 04:06 p.m"

 

"While longer focal length gives you more working distance and a narrower angle of view, it has nothing to do with magnification. 1:1 is 1:1 - period!"

 

---- Conclusion (2 options) ----

 

1) Anupam Basu is correct and my math with ratios is incorrect.

 

2) Bill B is correct. My math with ratios is good. Anupam Basu has been misled.

 

Anyone have more insight?

Bill B wrote "If a 100mm lens is focused on a subject 50mm away, the ratio is 2:1, twice life size."

 

Not possible. A lens can't be focused on an object nearer than one focal length to its front principal plane. And when front principal plane-to-subject distance is one focal length, magnification will be, um, very large. Do the arithmetic, and try it out with a real lens.

Jay,

 

I believe Anupam and I are both correct. There is no disagreement regarding ratios that I see. Where do you see a difference?

 

You can get to life size(1:1) on both a 50mm and a 100mm lens. To do so on the 50mm, you will be focusing at a distance of 50mm. To do so on a 100mm lens, you will be focusing at a distance of 100mm. As long as the ratio of focus distance and focal length is 1:1, you are at life size, regardless of focal length of the lens. The longer lens will allow a larger working distance.

 

Dan says a 100mm lens cannot focus to 50mm. I believe it can because extension tubes and bellows have allowed photographers to go larger than 1:1 for ages. As have microscopes. For example, the Nikon PB-6 bellows is promoted as allowing reproduction ratios up to 11x, depending upon lens used. There will be a practical limit because of the subject bumping up against the front element of the lens and definitely of the lens shade. I am willing to be corrected, however. Are Dan and I talking about different concepts but using the same words? Now is the time for all good optics-knowledgeable photographers to come to the aid of their associates.

 

There is no question that at large repro ratios lighting is a problem for front-lighted subjects. The blasted lens gets in the way because of lack of working distance, unless you use long focal length lenses.

 

There is another possible difference in pictures taken with different focal length lenses. That is the perspective. A longer lens will increase the size of objects in the background compared to a shorter lens if the subjects are in the foreground and at the same magnification ratio. I do not know if the differences are meaningful at 1:1 distances. Also, the depth of field at large ratios is so shallow, the background is often just a blur of color.

 

Mark made a good comment about my use of the word "big." It is ambiguous. I am used to thinking of big as relating to linear dimensions. Big can obviously be used for area, also, a squared number, just as well. That is what we use in comparing aperture sizes. Thank you Mark for reminding me to use more descriptive words.

 

Bill

Thanks Dann,

 

I forgot a third conclusion option:

 

3) Anupam Basu is correct. My math with ratios doesn't matter. Bill B has been misled.

 

I guess the math for the ratio lies within the optics of the lens and it is what it is. I would imagine there is probably at least a few ways of tweaking the ratio after the lens has been constructed via adaptors and close-up filters which makes me wonder. Would a 1x close-up filter change a 1:2 ratio lens to a 2:2 which would, of course, make it a 1:1?

Bill, you are dead wrong. The formula for front principal plane to subject distance is d = f*(m+1)/m, where f = focal length and m = magnification. At 1:1, m = 1 and d = 2f. Go read a book.

hey,, you know it really doesn't matter what the math is unless you are using a bellows or extension tubes. if you just have a lens attached and it states that it can focus up to 1:1 ratio.. then that is it. get as close as you can and still be in focus.. the lens length will affect how close you can get, but yes if you have a 1x attachment on a 1:2 lens then that will give 1:1 capability

 

also as a note to a statement above. - the lengths from the lens plain to the image projection plane (the digital sensor in this case) is the focal lenth of the lens.. 50mm 100mm etc. focusing at 1:1 means the lens plain is 50mm from the subject matter with a 100mm lens it is the same 100mm and 100mm. focal lenths are based on a focus set at infinity (I point this out because some lenses actually move in and out from the image plane to acheive focus while others move internal elements).

 

so to get more than 1:1 capablility you actually have to move the lens out from the camera body (unless you use a 1x attachment), which is what bellows and extension tubes do, but in doing this you loose focus at infinity. to measure the new ratio once you have extended the lens from the body, you measure the distance you have extended the lens.. say a 50mm extension tube. this gives you a 2:1 Magnification ratio with a 50mm lens and a 1.5:1 magnification ratio with a 100mm lens (L/F = M,, 50mm(extension)/50mm(lens focal length)=1x).

 

to your very first question.. "is a 50mm 1:1 ratio lens roughly equivalent to a 100mm 1:2 ratio lens since the 100mm would bring you twice as close to the subject?" no, the 50mm lens (assuming your lens plain is 100mm from the subject) will be 1:2 and the 100mm lens will be 1:1. the image on the sensor will be actual size with the 100mm lens and half sized with the 50mm lens.

 

to get the ratio correct in your mind the closer you get to 1:1 the bigger your image can get a 1:2 lens can only get half sized images.

 

to exceed a magnification factor you will need either an extension tube, or bellows, or the attachment you mention.. also I think some lenses actually will act as a 'bellow' of sorts and these can focus and magnify objects more than 1:1 ratios.

I agree with Bryan's explanation.

 

I took Dan's advice to do some reading. The photo.net lens tutorial at http://www.photo.net/learn/optics/lensTutorial has a good explanation of why formulas do not necessarily work with photo equipment:

"In lens formulas it is convenient to measure distances from a set of points called "principal points". There are two of them, one for the front of the lens and one for the rear, more properly called the primary principal point and the secondary principal point. While most lens formulas expect the object distance to be measured from the front principal point, most focusing scales are calibrated to read the distance from the object to the film plane. So you can't use the distance on your focusing scale in most calculations, unless you only need an approximate distance."

The same words can have different definitions in different contexts. "Approximate" does not work well in the macro world of small distances.

 

The whole tutorial is great and includes several of the formulas for measuring magnification. Mr. Jacobson's work appears in various places on the Internet for a good reason.

 

I also recommend the Philip Greenspun article on macro at http://www.photo.net/learn/macro/. He recommends the classic John Shaw's Close-Ups In Nature as a good book for those going beyond 1:1.

 

If I gave a physically impossible example, sorry, pick another set of numbers. It was just for illustrative purposes of ratios. Remember, though, that photographers do not use simple lenses. A true telephoto is not the same as a long focal length lens, nor is a short focal length lens the same as a retro formula wide angle. They do not act the same way

 

Now where did I place that optics text? Or did I sell it after the semester.

 

Bill

The correct formula for a lens being in focus is:

<p>

1/f = 1/s + 1/d

<p>

where f is focal length, s is distance from front nodal point of lens to subject and d is distance from rear nodal point to film. when focused at infinity, the equation is invalid but if you take 1/s to be 0 then it works fine.

<p>

By the geometry of similar triangles, magnification M is given by:

<p>

M = d/s

<p>

From those two equations, you can compute focusing distance at a given magnification or magnification at a given focusing distance.

<p>

Light loss to bellows extension leads to an effective aperture that is smaller than the normal aperture on the lens. If the setting aperture/f-stop is S, and Se is the effective one, then:

<p>

Se = S * (1 + M)

<p>

If your magnification is 3x ie 3, 1 + M = 4 and you do indeed lose 4 stops, but this example is a bit misleading. You only lose 1+M stops when M = 1 (ie 1x magnification) or M = 3 (3x magnification). For instance, if M = 0.4, that is 1:2.5, then 1+M = 1.4, but you only lost 1 stop as multiplying an fstop by 1.4 loses 1 stop.

<p>

Light loss to bellows factor is not just an issue for macro lenses. In fact, the aperture set on a lens only really applies to infinity focus. Multiplying by 1+M at other points still applies, but if magnification is say 1/10 or less, it is negligible (Se = 1.1 * S at 1:10). For extremely critical work, you might take it into account at 1:10.