Macro Lens Questions

[Martyn]

I have a Nikon 18-105 zoom lens which has a minimum focusing distance of 45cm (43cm in practice at 105mm). At 105mm the image of my ruler measures 100mm wide through the viewfinder, which makes the image 1/4 size (roughly) on my 24mm wide sensor.

 

The Sigma 105 macro lens has a minimum focus distance of 31cm yet produces a 1:1 image.

 

If I move forwards with my Nikon lens at 105mm to a focus distance of 31cm (ok, the image is out of focus) the subject is about 1/3 size and nowhere near the 1:1 of the macro lens.

 

Can someone please explain the difference in image size between the two lenses at similar focal lengths and focusing distances? I know I must have missed something fairly basic here, but I woke up this morning thinking about this and my brain is starting to hurt!

 

 

 

 

 

[Paul]

The official definition of a macro lens is that it should be able to reproduce a life-sized image of an object on the recording medium – in this case the image sensor. One feature that’s shared by all macro lenses is that they’re ‘flat field’ lenses. General-purpose lenses typically suffer from field curvature, so the point of focus will be at a slightly different distance towards the corners of the frame compared to the centre.

 

Additionally, macro lenses are optically optimized for very close focusing distances, although most also perform very well at “normal” shooting distances, too. Macro lenses generally employ floating designs, with additional groups of elements that move during focusing to counter aberrations at close focusing distances. Floating elements and internal focusing alter the focal length, decreasing it as focusing distance is reduced, but for practical purposes, this doesn’t matter: A macro lens still will provide 1:1 magnification at its minimum focusing distance. A macro lens achieves great magnification by moving the optical elements away from the camera body (same way as extension tubes work) at the same time as controlling the focus. At great magnification (close focus), the lens becomes significantly longer than at infinity focus. Basically, the closest focusing distance standard is written from the back of the lens (that is, where the lens meets the camera body). So if you have a closest focusing distance of 50cm but your lens is 15cm long, then in theory you should be able to focus on something at 35cm away from the FRONT of your lens.

 

A macro lens is specially designed to be able to focus CLOSE - which will allow you to get LARGER images of (usually) small things like insects, flowers, coins, blades of grass, your finger, etc. Because you know from life, that the closer your finger is to your eye the bigger it "appears". However unless you know how to use the camera (e.g. it needs to be REALLY steady with macro photography) and lighting can be tricky at close working distances, you might not often need to be "as close as your lens can get you" - e.g. with your 105mm f2.8 lens. Furthermore if you use f2.8 at the closest focusing distance on your 105mm macro lens you will have a VERY thin depth of field "dof". That means only a millimetre or two will be in focus, everything else will be out of focus.

 

All very complicated.

 

Does that help any?

 

Paul.

Edited April 13, 2014 by Paul

[Martyn]

..... A macro lens achieves great magnification by moving the optical elements away from the camera body (same way as extension tubes work) at the same time as controlling the focus. At great magnification (close focus), the lens becomes significantly longer than at infinity focus. Basically .....

 

Cheers Paul, it was the moving elements thing that I hadn't taken into account. It would be interesting to hear what someone like Black Pearl has to say about this.

 

Seriously considering a macro lens!

[Paul]

Some people just can't get on with macro. Some find it very difficult and lose the will. Patience is a virtue.

 

If you do get a macro lens, you've got to give it, and yourself, a chance.

 

 

Paul.

[Martyn]

Another question:

 

How would extension tubes affect  the magnification factor of a macro lens? I currently have a 69mm set.

[Paul]

Extension tubes allow you to make macro (1:1 or greater magnification) photos with non-macro lenses as well. If for example you have a 50mm lens with a 1:2 magnification factor and you attach 50mm of extension tube to it you will achieve 1:1 magnification.

The basic formula for calculating extension tube magnification is this:
M = (F + X) / F

Example:
Lens Native Magnification (M): 1:2
Lens Focal Length (F): 50mm
Extension Tube Length (X): 50mm

M = (50mm + 50mm) / 50mm = 2:1 magnification factor applied by the extension tube.

To calculate the total magnification simply multiply the extension tube magnification factor by the lens' native magnification factor:

Total Magnification = 2:1 * 1:2 = 2:2, or 1:1 ("pure" macro).

My understanding of what really makes a true "macro lens" special has more to do with the optical properties of the lens itself. True macro lenses generally have a very flat (planar) field of focus across the breadth of the lens - more so than most "regular" lenses at least.

When doing macro work your depth of field even stopped down is extremely thin, so it is very desirable that the focus distance be precise from edge to edge on the lens.
Most regular lenses have an at least slightly convex field of focus; the focal point at the edges is slightly closer than at the center of the lens.

While it is rarely noticeable at "normal" shooting distances, in the macro world 0.05mm can make a huge difference.

 

 

Paul.

Edited April 13, 2014 by Paul

[fuzzyedges]

Extension tubes just move the lens a fixed distance from the sensor like a macro lens does

Using extension tubes with a macro means you start at a diffrent point of magnification then magnify again with the lens

but a macro lens has better close up quality machined in than a standard lens like Paul says

[Martyn]

Thank you guys! At the end of the day it's the picture that counts, but the engineer in me can't help but think about the technical things. I may well be better off sticking with my current setup of a reversed wide angle zoom, the only downside being a dark viewfinder when stopped down (the engineer in me is working on that).

[FUJI]

You gotta Love engineers! and Techies.......all those numbers and formulae, they all leave me completely baffled, and in a state of ....Ignorance is Bliss.......

My neighbour is a pro engineer, he goes into raptures about anything and everything mechanical or technical, numbers, formulae and numbers pour from him, he is a whizz with tools, but hasn't an artistic or creative spark in him.....

My son in law, a top accountant, can calculate profit, loss, gains and tax returns at the drop of a hat, his Excel sheets are a wonder to behold, but he too has zilch creativity.......neither can he do any kind of DIY.

Me?........Numbers, formulas and the remembering of strings of words or numbers have always been and always will be a complete mystery to me.........I just don't get them.......only in a very basic fashion, even as a pro carpenter joiner for 25 years, I measured and calculated in my own sweet way, but my hand crafted doors, windows and cabinets all fitted and looked good.

But, give me.....any Art materials.....a blank wall, canvas or paper, and I'm off.........the larger the blank surface the better, I designed and painted stage Sets in my spare time for years, engraved glass, made prints etc.

Only in retirement did I turn to Photography, I almost gave up because, all the numbers, and lens combinations literally.....Did my head in.....but, thank to the patience of BP and other knowledgable friends, I ( like Eliza Doolittle) .....Got it!

But only in my own way........I see it, if it looks right through the viewfinder........I SHOOT!

We humans are so different......thank goodness.

FUJI