PDA

View Full Version : what focal lenght for macro?

Martin Miksch
6-Jul-2009, 10:00
For a project I attached a Nikon DSLR to the back of a Cambo 5x7, which extends to about 60cm. My goal is to get max. enlargment, so what is better, a short or a long lens or it just depends on the bellows extention?
Regards
Martin

Emmanuel BIGLER
6-Jul-2009, 10:57
Hello from France !

You need first to determine first what will be your magnification ratio M = (image size)/(object size).
Your detector covers either 18x24 mm of 24x36 mm for the full-frame series of DSLR.

Consider the 24x36 mm size as an example.

If your object is 1 cm in diameter, the maximum you can enlarge will fill 24mm in height so your magnification ratio will be M=2.4

From the value of M you'll find the needed bellows draw :

- additional bellows extension from the focal point : ext = M.f
- total bellows draw from the lens board, add one focal length ;
total bellows length approx. ext+f = (1+M).f

In this condition the multiplying factor for the exposure time will be
(1+G)^2 ((1+G) squared) but your DSLR will probably keep the shutter automatically open until the proper light dose is recorded !

Those simple formulae will tell you the amout of bellows draw required for your projet as a function of the focal length.

At magnifications above 1, since the object will be located at about one focal length in front of the camera, a focal length substantially longer than the standard 50mm is a good choice to keep a minimum comfortable distance to set-up your lights properly and avoid, for example, the shadow cast on the
object by the lens itself.

Regarding depth of field, general formulae exist as a function of the magnification factor, focal lengths and f-number, but at 1:1 ratio the depth of field is not directly dependant on the focal length, the value in this particular condition (object siez = image size) is plus or minus N.c where N is the f-number engraved on your lens and c the circle of confusion of acceptable sharpness.

Martin Miksch
6-Jul-2009, 12:01
Thanks for your kind answer; I am not that good in calculations; I have my lenses attached to other lensboards, so I have some work to try different lenses with my Cambo; beside the issue with light and shadow, is there a difference in magnification between say a 90mm lens and 360mm at given bellows extension?
Thanks
Martin

Skorzen
6-Jul-2009, 13:21
a shorter lens will require less extension, however your working distance will be less as well.

Dan Fromm
6-Jul-2009, 13:29
Martin, at the same extension, a 90 will give roughly 4x the magnification a 360 will.

If you want sensible advice that doesn't require you to calculate (you can't calculate? shame on you, sir!), tell us which lenses you have, including lenses for y'r Nikon and how much clearance you need between subject and lens.

Ken Lee
6-Jul-2009, 13:47
For purposes of discussion: If you had a setup with unlimited bellows extension, and a selection of good macro lenses of all reasonable lengths, you could achieve the same magnification with all of them.

What does change, is the distance from the camera to the subject, and thus the sense of perspective (http://media.photobucket.com/image/lens%20perspective/adorski/Perspective2.jpg).

When shot from a greater distance, with a longer lens (or with a short lens but given equivalent enlargement), the foreground and background will appear closer together. That is because from the greater distance, the separation between foreground and background is comparatively small.

When shot close-up, the foreground and background will appear to be far from one another, because with respect to the actual shooting distance, the separation between foreground and background is now substantial.

If your subjects are flat, then this isn't an issue. If they are 3-dimensional, then it does becomes an issue.

GPS
6-Jul-2009, 14:14
For a project I attached a Nikon DSLR to the back of a Cambo 5x7, which extends to about 60cm. My goal is to get max. enlargment, so what is better, a short or a long lens or it just depends on the bellows extention?
Regards
Martin

Better in what sense? For your grandma's cheese cake preparation? Before you start to ask imprecise questions why not to take two different lenses and experiment a little bit on the gg? You will then know better what you want to ask...

Martin Miksch
6-Jul-2009, 16:06
... My goal is to get max. enlargment, so what is better....
Hmm, my mistake, I thought defining a goal shows that "better" is meant as "better working to reach my goal" and as mentioned above my lenses are on boards for wooden field cameras and I have to attach them to Cambo boards, what of course were not a problem if I had all the boards with right drills.

I have no dedicated macro lens but I hope that this is not a problem, I just use a tiny bit from the center of the lense. My lenses are 90/8 SA, 120 Angulon, 135, 165, 180, 210, 270 and 360 Xenars.

Thanks and Kind Regards
Martin

Edit: my F-mount lenses are 24, 35, 50, 85, 135, 180 and 300mm,
clearance just what I need for light, maybe I try to use some glass-fibres, maybe someone can point me to a place where I can find some information about DIY glass fibre flashes or similar.

Skip Roessel
6-Jul-2009, 16:45
Quickly, if you want the same dimensions on your neg as the subject, the real meaning of "macro", your lens will be out twice it's stated (infinity) focal length, and you will have to open two stops from the stated iris indicators. Thus, to shoot with a 150mm, your lens is extended 300mm, and you set your iris at 5.6 if your meter suggests f/11.
s/

6-Jul-2009, 17:13
The 120 Angulon would allow you to get you to about 4:1. The 90/8 SA would get to 5.66:1. For better results and magnifications to 11:1 consider a 50mm macro or enlarging lens (either will perform better if mounted backwards at these magnifications significantly greater than 1:1). If you need bigger magnifications than that, then consider a shorter enlarging lens (backwards) or certain specialized short focal length macro lenses like the Zeiss Luminars. Luminars and the like are designed for magnifications greater than 1:1 and so do not need to be mounted backwards. A 16mm Luminar will let you get to about 36:1.

Bosaiya
7-Jul-2009, 06:51
A side note about long vs short lenses/bellows.

A longer lens allows for greater working distance between lens and subject at the expense of a longer working distance between the front and rear standards (longer bellows). Not too much of a problem until you get to the point where you can no longer reach the focusing dial while looking at the ground glass. Then the great shuffle begins.

Mark Sawyer
7-Jul-2009, 11:50
My goal is to get max. enlargment...

For "maximum enlargement", you want "minimum focal length" at "maximum extension". But, of course, pushing the limits too far gets ridiculous...

For better results and magnifications to 11:1 consider a 50mm macro or enlarging lens (either will perform better if mounted backwards at these magnifications significantly greater than 1:1). If you need bigger magnifications than that, then consider a shorter enlarging lens...

This is very good advice if you have an enlarger, especially one with several lenses. You may already have a good selection of macro lenses to try out.

Ernest Purdum
7-Jul-2009, 12:00
"Macro" describes quite a wide magnification range. If you are working with subjects that can't be lit from behind, and you don't need extreme magnification, the longer lenses mean more distance between your lens and your subject which greatly simplifies the placement of your lighting.

cowtown_eric
17-Jul-2009, 20:32
And when you get into macro, just extending the lens does a couple of things...

All the carefully designed corrections for a fixed range of focal distance from the film plane are now toast, yer film plane is a whack farther from yer lens than it's designer anticipated.

Further, yer subject is closer than they anticipated in their designs as well,

So, now you got exactly the inverse of what was designed for the lens, and that's why god invented 'REVERSING ADAPTOR" just so you could change yer lense end for end and maqke it's design characteristics work for you rather than aginst you.

I'm a newbie on this forum, and it's been 20ish years since I did anything professionally with glass and silver, (biomed) so maybe it's automattically assumed the lense was reversed....My apologies if I misspoke.

Eric in Calgary

Just my drive by input.