How much extension is too much?

[Drake]

So I've been thinking about turning my M42 Helios 58mm f2 into a cheap macro lens. I figured that probably the best way of doing that is getting some extension tubes.

But I'd like to know the effect first. Is there a way of calculating what magnification I'll get? I've read somewhere that an extension tube of the same length as the lens should give you 1:1 magnification, eg. 50mm extension tube used with 50mm lens. On a crop body at least. I found it hard to believe, since all lenses have very different minimal focusing distances.

But I decided to give it a try anyway, removed the lens and started looking through the OVF with the 58mm lens roughly 50-60mm from the camera. And, to my surprise, it looks about right. Of course there's a huge light spill because the lens were hand held and not actually on a tube, but what's important is that the magnification was about 1:1. Am I missing something, or it is actually possible?

Now let's say I get a 50-60mm set of extension tubes to go with my 58mm prime. How will it be optically different from a Canon 60mm dedicated macro lens? How much light approximately will I lose because of the extension? 1EV? 3EV? More? What about the DOF? Will the DOF of my lens on let's say f8 be the same as of a real macro lens or smaller? And lastly, how far can you go with extension tubes? What if I put 2 sets of them between the camera and lens? Will I lose too much light this way to shoot? I know the DOF will be very, very small, but then, 2:1 magnification isn't something surprising, is it? The question is, will it be usable in terms of the amount of light required?

 

[StevePPhoto]

I made a cheap macro lens by taking an old film camera lens and flipping it around using the back as the front and the front as the back. I took my camera body cap and cut the center out then did the same to the lens cap glued them together and had myself a pretty decent macro lens for under 5 bucks lol. Heres what it looked like, this was done at low light so I might do another one for you later..

*EDIT* I shot a new pic for you and uploaded it as a pretty big pic so you can see the detail

 

[PASM]

But I'd like to know the effect first.

Experiment with a cut-down cardboard tube.

 

[StevePPhoto]

Here is what the lens looks like.

This is the front cap that I made using The lens and Body cap.

Here is the lens

 

[Drake]

I made a cheap macro lens by taking an old film camera lens and flipping it around using the back as the front and the front as the back. I took my camera body cap and cut the center out then did the same to the lens cap glued them together and had myself a pretty decent macro lens for under 5 bucks lol. Heres what it looked like, this was done at low light so I might do another one for you later..

Thanks for your sample photo. I've already tried it earlier, and my 58mm helios seems to be a bit too short, it gives an extreme vignette when reversed, both alone and coupled with the Canon 18-55mm IS.

Experiment with a cut-down cardboard tube.

Well, I guess that's a pretty good idea

 

[Overread]

First you are right the rough maths for magnification with extension tubes is:

length of tubes in mm divided by focal length of lens = :1

eg 50mm of tubes/50mm lens = 1:1 magnification
100mm of tubes/50mm lens = 2:1 magnification

However there is a more accurate mathmatical formula out there which gives a more exact answer, however the above guidline is close enough for most practical applications.

Crop factor of the camera has no effect on the magnification aspect because macro magnification is based on the size of the reflected image on the sensor.
1:1 means
Size of the reflected image on the camera sensor:actual size of the subject in real life

So it does not matter if the sensor is seeing more or less of the reflected image (eg crop compared to fullframe) because the size ratio of the reflected image to the real object will remain the same. Of course on crop sensor the angle of view will be different and when printing it will give the artificial feel of having more magnification.


As for the optical differences of a 50mm lens with tubes as compared to a dedicated macro lens of a similar focal length (warning canon's 50mm macro lens is only a 0.5:1 macro lens unless you also use the lifesize adaptor with it).

1) Working distance: this is the distance from the front of the lens to the subject (not to be confused with the minimum focusing distance which is from the subject to the sensor/film in the camera). A dedicated macro lens will have a longer working distance than a regular lens with extension tubes. This means more distance to work with which greatly helps with lighting and also helps with more skittish subjects such as insects.

2) Infinity focus: with extension tubes on any lens infinity focus is lost and overall you'll lose the ability to focus on further off subjects. How far you get depends on the lens and the amount of extension. This can be anything from meters to millimeters of distance.


Light loss through extension is a tricky area and I don't know specific values or comparison ideas. However I will point out that as you add more tube length your actual aperture reduces and this is also something that happens with all current market macro lenses. By 1:1 most f2.8 macro lenses are in fact closer to f5.6 in actual aperture (Nikon camera bodies actually report this change to the user whilst canon camera bodies do not. The actual aperture in both cases is still the same; however it explains why canon camera shooters tend to find f13 as their minimum ideal working aperture whilst nikon are at f16 (on crop sensor - add around 1 stop for fullframe).

Depth of field wise, as far as I know (and from what I can see in practical field terms) depth of field is pretty much universal to magnification. Any gain or loss between different approaches appears to be so marginal as to not be a major factor.
Note however that when one is using dedicated macro lenses, those with a longer focal length will render background areas in greater blurr. This is why some use long focal length lenses and even teleconverters with them to get more blurr to their backgrounds (and also why some will use a lens like a 10-20mm with a small amount of tube length to get a lens with more background focus).

On the subject of how much tube length you can add there is a limit. Each mm of tube length reduces your minimum focusing distance and your maximum focusing distance. Thus there comes a point where if you add any more tube length the focus range moves inside the lens body itself (thus you lose the ability to actually focus on anything).
In addition consider the lighting - more tubes means you've got to have the lighting held far further forward and shadowing by the lens becomes more dominant.
Typically most people use 1 set of kenko tubes (68mm of tube length) or 1 bellows unit and not much more. If you want to go more you have to look at other options such as reversing the lens onto another; microscope elements; dedicated macro lenses; diopters



Generally the cheapest approach for doing high magnification photography is either to use microscope elements, which is an area where you can get some serious magnification (10:1 kind); however its a market area where I've not done any work thus far so I can't go into specific details.
From there the next step is reverse mounting lenses onto each other. This is done via an adaptor (found on ebay) which has two filter screw threads - one for each lens. You typically mount a longer focal length lens (eg a 70-300mm) onto the camera body and then mount a lighter shorter focal range lens onto the front of that (eg a 28m; 50mm Note always mount the heavier lens onto the camera body.
This method works by the following rough math:
Focal length of lens on camera body divided by focal length of reversed lens = :1

eg 70-300mm lens at 300mm on the camera with 50mm reversed lens is:
300mm/50mm = 6:1

This is a very affordable way to get to some high magnifications as you can see.

The dedicated lens approach (kind of the market standard; but there are sharper options on the market if you research it) is the Canon MPE 65mm macro lens which "zooms" from 1:1 through to 5:1; though unlike all other dedicated macro lenses this lens has no auto focus motor at all and also has no infinity focus (its focusing range starts at 1:1)

 

[Pgeobc]

I am no expert on macro work, but you really need to get a book on it and study. The stuff that I have read over the years is much too much for discussion here. However, a some of observations:

1-You will know you have too much extension when the stuff falls off the camera or the pictures just plain look bad.

2-Because of the optical properties of lens design, many lenses give a better image when reversed and used with extension tubes. You will need a textbook to find out why, because I have forgotten most of that.

3-You can do pretty much anything you want to do, as long as the image quality is acceptable. Image quality is the standard. The amount of hassle one has to go through to get good images is often tied to one's pocketbook.

4-Generally, dedicated macro/micro lenses provide superior results and are much easier to use; they are corrected and designed for that work. However, that is not universally true of the image quality always; they are simply easier to focus and are well-integrated with the light metering functions of the camera.

5-Learn most of your craft the cheapest way possible. That way, when it comes time to buy good equipment, you have the money. You can learn the math and the practical aspects with simple equipment and good books. Then, with good technique, a good eye, and good equipment, you can make fame or money.

 

[Drake]

Thanks Overread, that's pretty much everything I wanted to know. I am not after such extreme magnifications as 10:1 or even 6:1. In the past I had been using a sigma 70-300 together with a Raynox macro converter, which at the longest setting delivered roughly 4:1 measured magnification. It was fun to play with, but my preferred magnification is somewhere around 1:1, 2:1 occasionally. That's why I am trying to get a setup capable of doing 1:1, maybe a little bit more, but nothing extreme.

 

[slate mike]

And my thanks to you, too, Overread. I learned more practical ideas from your reply than I have found in a week's search.