Lens focal length versus magnification - in an indoor setting

[Takizawa]

I've heard that you have to take the focal length of a lens and times it by two to reach a 1:1 magnification. So I gather a 90mm lens would need 180mm bellows length. In terms of varying focal lengths is the practical difference the distance of the camera to the subject? I'm most curious if a wide angle lens forces you so close that you obscure the light source / create a shadow.

Also, if my max bellows is 330mm, any thoughts on a good lens for flowers? I know that a process lens is optimized for a flat surface.. but would something like that still work? and in a short enough focal length to make this doable?

Thanks!

[Mark Woods]

I shoot a lot of flowers 1:1 with a 210mm Repro Claron. I often shoot at a f/22. I also use my 90mm Super Angulon for the same subjects. The 210mm might be too long for your bellows.

[Dan Fromm]

The magic formula for the distance from the lens' front node to the subject given the lens' focal length and magnification is:

distance = focal length * (magnification + 1)/magnification

For magnification = 1, this gives front node-to-subject distance = 2*focal length

For most of the lenses used on LF cameras (not true telephotos!) the front node is near the diaphragm.

The corresponding magic formula for rear node to film distance given focal length and magnification is distance = focal length * (1 + magnification) The front node is usually near the diaphragm.

If you think about that a little, you'll see that the film-to-subject is shortest at 1:1. At all other magnifications it is greater than at 1:1 and at all other distances but 4f + the lens' internodal distance there are two magnifications that will give good focus.

[ic-racer]

I've heard that you have to take the focal length of a lens and times it by two to reach a 1:1 magnification. So I gather a 90mm lens would need 180mm bellows length. In terms of varying focal lengths is the practical difference the distance of the camera to the subject? I'm most curious if a wide angle lens forces you so close that you obscure the light source / create a shadow.

Also, if my max bellows is 330mm, any thoughts on a good lens for flowers? I know that a process lens is optimized for a flat surface.. but would something like that still work? and in a short enough focal length to make this doable?

Thanks!

At 1:1 both the bellows extension and lens-to-subject distance will be equal to each other. Each distance will be 2 times the focal length.

[Takizawa]

Very interesting. Thanks, I have a much better feel for this now.

[C. D. Keth]

A 150mm g-claron would be great for that purpose. With your amount of bellows you could just surpass 1:1.

[Leigh]

At 1:1 both the bellows extension and lens-to-subject distance will be equal to each other. Each distance will be 2 times the focal length.

That's only an approximation. The error increases as the focal length becomes longer or shorter than "normal".

The actual distances are measured from the front and rear nodes, which can be anyplace inside or outside of the lens, depending on the design.

At 1:1, the distance from the front node to the subject = 2 * the optical focal length, and from the rear node to the film = the same distance. The location of the front and rear nodes and their separation will be found on the datasheet for the particular lens in use. A problem arises in that even if you know the location of the nodes, determining their true physical location may be difficult.

The easiest way to set the camera is to focus at infinity, then move the lens forward by a distance equal to its focal length. Place the subject approximately 2 * the focal length in front of the lensboard, then move the subject or camera until the subject is properly focused (which should be the correct size).

Make any final focus adjustment by moving the back of the camera.

- Leigh

[drew.saunders]

I've heard that you have to take the focal length of a lens and times it by two to reach a 1:1 magnification. So I gather a 90mm lens would need 180mm bellows length.

For 1:1, you'll need to extend the bellows by the flange-back measurement (the distance from the flange to the film plane at infinity focus) + the focal length. Many 90mm lenses have a flange back measurement greater than 90 (see http://www.ebonycamera.com/articles/lenses.html for this measurement for most every modern lens), so you might need 183mm or 190mm or something like that depending on your lens.

In terms of varying focal lengths is the practical difference the distance of the camera to the subject? I'm most curious if a wide angle lens forces you so close that you obscure the light source / create a shadow.

That's a definite problem with using wide angle lenses for macro work, although you'll need less bellows draw, you'll be very close to your subject, so lighting can get tricky.

Also, if my max bellows is 330mm, any thoughts on a good lens for flowers? I know that a process lens is optimized for a flat surface.. but would something like that still work? and in a short enough focal length to make this doable?

Thanks!

Do you care about perfect edge to edge and corner to corner sharpness? For flower photography, many people don't worry about critical flat-field sharpness, so you might be happy with a regular 135mm or 150mm plasmat racked out. You might even prefer the slightly softer edges that you'd get with a general purpose lens used in macro. Plus, you probably already have one and/or could use it for general purpose work. I have 365mm of bellows extension and do most of my tabletop flower work with a 165mm or 200mm lens, both are Tessar designs, neither are designed for closeup work, and I'm happy with the results.

Drew

[Emmanuel BIGLER]

A few additional ideas related to macro work:

- in macro work the depth of field does not directly depend on the focal length in use, only on the magnification and the f-number; hence, "longer that normal" focal lengths are better for macro since they allow more space between the camera and the subject for a given magnification ratio, without any loss of depth of field;

- a real issue is vibrations and overall stability of the camera + subject altogether ... Dan Fromm can comment better than me on this aspect;

- the problem of properly lighting the subjet when the lens and camera are really close the subject is a real issue that I encountered frequenty when I had small-size objects (about 1 - 2 cm in size) to photograph.
A possible solution is to use a thin semi-transparent glass held at 45° in front of the lens so that you can direct your light from the side without any obstruction from the lens and camera. Thin glasses are very fragile and usually held in a kind of cubic box, the semi transparent glass being held along the diagonal of the cube. I used a translucent box surrounding the object and I used a small flash unit partly illuminating the diffusing box and partly directed to the subject through semi-transparent glass plate at 45°.
The semi-transparent mirrors I used in the past were designed for 35 mm photography, size about 5x5 cm (2"x2") but you can find them in sizes of about 100x100 mm (4"x4") from suppliers of optical components. There was one on the Arca Swiss (and also Sinar) catalogue up to the beginning of the 2000's, hence for sure large format photographers have used this accessory in the past.
Not very practical to be used in the field for wild flowers, though, but indoors, you have in principle enough time and comfort to set-up your mirror and lighting equipement properly

Since depth of field is shallow, possible dust deposited on the semi-transparent glass was not a problem for me, since their surface is far out of focus. Reasonably clean is enough, not "absolutely dust-free' like a silicon sensor