Standard macro with extension tube worse than crop and magnify?

5 or 6 queries down from this one an interesting assertion was made

re. the deterioration effect of the "air" in extension tubes on

image quality.

 

"Adding a little air between camera and lens (via an extension tube)

cannot deteriorate the image, if we assume that air is totally

transparent. Right? [i had said]

 

[someone answered:]

Wrong. Lenses are designed for optimum performance at certain

distances (equivalently, magnifications). Adding extra extension can

degrade the image somewhat."

 

Let me take this to another level of understanding, rather than leave

it misunderstood:

 

If I take a standard 50mm lens and add a 50mm extension tube to its

back, i have a macro set-up for 1:1 magnification. If I compare the

image or picture of a newspaper tacked to the wall and shot from about

10 cm away at 1:1 with the extension tube plus lens, how will that

image compare quality-wise to the one I get with my standard 50mm

lens, extended to its max and the picture shot from 45 cm away, after

the center is blown up so that I see the same inch by inch and a half

portion of the newspaper around 4 times lifesize on a 4 by 6 print?

 

Note that my two hypothetical pictures show exactly the same part of

the newspaper; one is taken in 1:1 magnification via "air extension",

the other picture is an enlargement of a center crop of the picture

taken from 45 cm away, using the same lens both times.

 

I am not sure of the answer, but I am certain the air is not to blame

in any way.

 

My hypothetical lens is a standard 50mm lens (90$), not a macro or

process lens (400$ with flat field of focus). In both 4 by 6 prints

we see the same lens' deviation from flat field of focus. The picture

taken with the tube will be much sharper, though showing a bit of

field curvature; the one taken from 1 1/2 feet away will be subject to

exactly the SAME curvature of field, though, but rather fuzzy around

the edges of letters on the newspaper due to my crop and enlargement.

 

Of course, a macro (flat field) lens would be a better choice, but in

essence, extension tubes DO NOT add, cannot add image deterioration

that is not already inherent in the lens used.

 

Am I right or wrong? That is the question.

 

If I am wrong, then the use of extension tubes is plain useless!

 

And barring me owning a macro lens, there is no better macro image

from that given non flat field lens than by adding extension.

 

Sorry, ocean physics.

I believe you are right.

 

The space between the lenses is inside the camera. This is where the R&D is done. Angel of lens how far apart etc... From the end of the lens to the film/sensor plane should not have any effect if the air is clean.

 

There is no real way to test this. The elements in a 50mm lens and a 50mm macro will be different. There is no way to do an actual test on one lens because ratios will never match up.

 

I've seen some tac sharp images taken with tubes.

 

I too would like to hear more about this IF someone can come up with FACTS to prove tubes degregate an image.

With added extension, and your capture medium uses only a central expanded crop of the image circle. How sharp this image will be will depend on the actual lens resolution (often, there is plenty of resolution in good lenses). Any lens flaws including flares, deep scratches, distortion AND field curvature will be magnified. With a perfectly good non-macro lens the image thus produced will be comparable to macro lenses as long as the curvature is not too much and diffraction situations are avoided. Note that in most macros much of the magnification is due to built-in extension in the focus helical.

I guess comparing with a crop isn't quite fair because, grain etc will come into play. To do a test one might take two impeccably sharp lenses of comparable focal length - one macro and the other not. A feasable setup might be the 200/f4 macro and the 180/2.8+PN-11. Maybe if Arnab has both he might be able to test them and say if there is any visible loss of quality due to extension.

Depends on the lens and whether or not it is corrected (not just field flatness, there are a number of aberrations) for the magnifications with extensions.

 

All the proposed experiments assume a lot of factors to be constant. So, it would not produce any "stantardized" result.

 

What Ocean said is generally true. Serious macro setups (Multiphot, Ultraphot, Aristophot and the like) had dedicated condensors made for a particular macro lens. These setups also had a multitiude of accessories for different types of lighting- a key factor.

Frank, its not the air that does it, although some still believe that exposure to bad air, not Plasmodium in the blooe, causes ague.

 

It is well known (= look it up for yourself) that a lens can be optimized for only one magnification (often expressed as "one pair of conjugates"). Now think of two lenses, one optimized for far distant subjects, the other optimized for near subjects. The first one can be made to focus a near subject, but won't give as good image quality in that situation as the second. That's all that's going on, there's no need to get all mystical.

If you are into hiking trails, the $90 (50mm) lens and a 3-tube extension tube set will work on flowers and what-ever-else you want to photograph. 'Air' has no bearing on the sharpness of the image.

 

 

 

 

If you want to invest in and haul along a 60mm or 90mm or 105mm Micro-Nikkor lens, either one will work pretty well on flowers and such.

 

 

 

 

 

There is no 'right' answer.

Nice straw man, Frank.

 

There are other ways to increase magnification that don't involve cropping or adding extension. Close-up filters and teleconverters come to mind. They may or may not be better or worse than tubes; you can't say a priori.

When going real close it is often advised or was advised to reverse the lens. A 50mm lens reversed sans extensiontube should give a ratio around 1:1 with very good imagequality. 28mm reversed = aprox 2,6:1.

Regards

Well, I can't see the air in a paltry 10mm or 20mm or 50mm having a huge effect on picture quality. There may be some slight detrimental effect on ultimate quality, but nothing very noticeable. And it won't be due to the air in the tube. The slight gap the tube adds is a far cry from trying to shoot through the haze and moisture in the air, at a distant object a mile away. That's my OPINION and I'm sticking with it.

 

I suppose one way to settle it is for somebody to shoot a resolution chart without an extension tube, and then crop and resize the picture, then shoot it again with the extension tube, then stare at the resulting photos at 400% magnification making comparisons. In other words, a somewhat scientific test to discover the facts. But, I'm more than happy with my results using extension tubes to even worry about the slight differences that may or may not appear in such a test.

 

I would also suspect that an extension tube would give a cleaner result than adding more glass elements to the front of the lens via a close-up lens. But again, the only way to settle it would be to perform a direct comparison test. Even then, I don't know if you could draw a blanket conclusion as some close-up lenses are better than others in quality. Again it's only my opinion, but I can't see how adding an extension tube would degrade quality more than attaching a thick heavy piece of glass in front of the lens.

There is no optical advantage in reversing the optics until the reproduction ratio (magnification) is significantly greater than 1X! For best imaging, the "rear" of the lens should be pointed towards either the object or image whichever is closer! At 1:1 either configuration is equally valid, but neither is particularly good. Reversal excells at about 5:1 magnification, or greater.

 

Extension tubes are fine for copy work, though they can be difficult at times. In a controlled environment bellows are often the most practical. For what it is worth, I have macro lenses, extension tubes, a bellows, close-up lenses (diopters), and a TC. They all have their places.