www.photomacrography.net :: View topic - Diffraction effects and magnification
www.photomacrography.net Forum Index
An online community dedicated to the practices of photomacrography, close-up and macro photography, and photomicrography.
Photomacrography Front Page Amateurmicrography Front Page
Old Forums/Galleries
 
 FAQFAQ   SearchSearch   MemberlistMemberlist   UsergroupsUsergroups   RegisterRegister 
 ProfileProfile   Log in to check your private messagesLog in to check your private messages   Log inLog in 
Diffraction effects and magnification
Goto page 1, 2  Next
 
Post new topic   Reply to topic    www.photomacrography.net Forum Index -> Macro and Micro Technique and Technical Discussions
View previous topic :: View next topic  
Author Message
LordV



Joined: 22 Nov 2007
Posts: 1558
Location: UK

PostPosted: Sat Nov 14, 2009 1:54 am    Post subject: Diffraction effects and magnification Reply with quote

Am I correct in thinking that rather like DOF effects in macro, diffraction is approximately only dependant on the set aperture and the magnification of the system and not on the method of achieving that magnfication ?

ie would you expect different levels of diffraction from setups all yielding say 2:1 magnification but using say a macro lens plus 2X TC, a macro lens plus extension tubes, a macro lens plus a dioptre or reversed lens on the front assuming the main lens aperture is the same in each case ?

Brian v.
_________________
www.flickr.com/photos/lordv
canon20D,350D,40D,5Dmk2, sigma 105mm EX, Tamron 90mm, canon MPE-65
Back to top
View user's profile Send private message Send e-mail
lauriek
Site Admin


Joined: 25 Nov 2007
Posts: 2404
Location: South East UK

PostPosted: Sat Nov 14, 2009 5:43 am    Post subject: Reply with quote

I'm certainly under the impression that what you say in the first paragraph is true.
_________________
Flickr | www.laurieknight.net | Blog
Back to top
View user's profile Send private message Visit poster's website
LordV



Joined: 22 Nov 2007
Posts: 1558
Location: UK

PostPosted: Sat Nov 14, 2009 9:58 am    Post subject: Reply with quote

lauriek wrote:
I'm certainly under the impression that what you say in the first paragraph is true.

Thanks laurie Smile
Brian v.
_________________
www.flickr.com/photos/lordv
canon20D,350D,40D,5Dmk2, sigma 105mm EX, Tamron 90mm, canon MPE-65
Back to top
View user's profile Send private message Send e-mail
AndrewC



Joined: 14 Feb 2008
Posts: 1436
Location: Belgium

PostPosted: Sat Nov 14, 2009 10:13 am    Post subject: Reply with quote

You guys are gluttons for punishment - wait until Rik reads this and composes a treatise on the subject Laughing
_________________
rgds, Andrew

"Is that an accurate dictionary ? Charlie Eppes
Back to top
View user's profile Send private message Visit poster's website
rjlittlefield
Site Admin


Joined: 01 Aug 2006
Posts: 17614
Location: Richland, Washington State, USA

PostPosted: Sat Nov 14, 2009 10:35 am    Post subject: Re: Diffraction effects and magnification Reply with quote

LordV wrote:
diffraction is approximately only dependant on the set aperture and the magnification of the system and not on the method of achieving that magnfication ?

AndrewC wrote:
wait until Rik reads this and composes a treatise on the subject

It's a short treatise. Delete "approximately", change "set aperture" into "effective aperture", and you have an even better statement.

--Rik
Back to top
View user's profile Send private message Visit poster's website
AndrewC



Joined: 14 Feb 2008
Posts: 1436
Location: Belgium

PostPosted: Sat Nov 14, 2009 10:44 am    Post subject: Re: Diffraction effects and magnification Reply with quote

rjlittlefield wrote:
LordV wrote:
diffraction is approximately only dependant on the set aperture and the magnification of the system and not on the method of achieving that magnfication ?

AndrewC wrote:
wait until Rik reads this and composes a treatise on the subject

It's a short treatise. Delete "approximately", change "set aperture" into "effective aperture", and you have an even better statement.

--Rik


Smile
_________________
rgds, Andrew

"Is that an accurate dictionary ? Charlie Eppes
Back to top
View user's profile Send private message Visit poster's website
LordV



Joined: 22 Nov 2007
Posts: 1558
Location: UK

PostPosted: Sat Nov 14, 2009 10:59 am    Post subject: Reply with quote

Ok perhaps I should rephrase the question, do any of the magnification methods affect effective aperture more than others ? with a given set aperture on the main lens ?

Brian v.
_________________
www.flickr.com/photos/lordv
canon20D,350D,40D,5Dmk2, sigma 105mm EX, Tamron 90mm, canon MPE-65
Back to top
View user's profile Send private message Send e-mail
rjlittlefield
Site Admin


Joined: 01 Aug 2006
Posts: 17614
Location: Richland, Washington State, USA

PostPosted: Sat Nov 14, 2009 2:09 pm    Post subject: Reply with quote

Sorry, that's a bit longer answer.

Tubes increase the effective f-number as effective = nominal * (magnification+1).

Teleconverters increase the effective f-number by an additional factor of whatever their power is. If you had effective f/11, then adding a 2X teleconverter would push you to a new effective f/22.

Adding an unstopped lens in front does not increase the effective f-number -- it stays at what it would have been without the added lens. (The extra magnification in this case comes from reducing the overall focal length, so that the f-number set on the aperture ring is no longer an f-number in the classic sense of diameter/FL.)

Stopping down the front lens changes the game completely and you have to redo the calculation in terms of the front lens's aperture.

In all cases, you get the same relationships between DOF vs diffraction softening and exposure time vs scene brightness, so there's no gain or loss on those fronts.

In theory, adding an extra lens in front lets you run at a larger effective f-number, which would imply less diffraction softening. But in practice, the added lens is also likely to add aberrations that force you to stop down to restore good image quality. Note that adding a strong lens in front, but stopping down in the rear, is liable to introduce bad aberrations in addition to strange perspective. See Stopping down a lens combo for more discussion about these effects.

--Rik
Back to top
View user's profile Send private message Visit poster's website
LordV



Joined: 22 Nov 2007
Posts: 1558
Location: UK

PostPosted: Sat Nov 14, 2009 2:36 pm    Post subject: Reply with quote

Thanks Rik for the detailed explanation.

Can I assume from this then that if I want to shoot at say 2:1 with a macro lens with an aperture setting suitable to give me a set level of DOF, then the solution likely to give the highest IQ is extension tubes simply because I am not adding more glass ?

Brian v.
_________________
www.flickr.com/photos/lordv
canon20D,350D,40D,5Dmk2, sigma 105mm EX, Tamron 90mm, canon MPE-65
Back to top
View user's profile Send private message Send e-mail
rjlittlefield
Site Admin


Joined: 01 Aug 2006
Posts: 17614
Location: Richland, Washington State, USA

PostPosted: Sat Nov 14, 2009 5:24 pm    Post subject: Reply with quote

Yes, but the key word is "likely". There are exceptions.

Some more background...

There's a common belief that "no added glass" means "no added aberrations". But that's not correct. Whenever you add extension to a lens, you take that lens outside the focus relationships that it was designed for. The extension alone is sufficient to introduce aberrations, notably spherical aberration, that degrade resolution.

In some cases, you can get a better result by combining two lenses than by using either of them alone.

Suppose, for example, you have an ordinary 50 mm lens that is designed for shooting landscapes and portraits, say 1 meter and farther. If you just extend that lens to give 1:1 magnification at 0.1 meter, then the image quality will plummet because of spherical aberration. In this case, you would do better to take two copies of the same lens and assemble them as a combo, one reversed in front of the other, so that both lenses would be working at their usual distance of 1 focal length from the mounting flange.

This is a contrived example, but hopefully it makes the point. Adding glass can be a good thing if that glass corrects for aberrations that would be present without it. Most of the time that's not the case, but it certainly can be.

From the standpoint of image quality, I think the best approach is always to use a single high quality lens in the focus arrangement that it was designed for. The MP-E 65, with its floating elements, is specifically designed to cover the magnification range of 1x to 5x. The Olympus 38 mm bellows lens is designed to cover a similar range, 1.8x to 6x. For people who don't have something like those, a good substitute is to use a high quality enlarging lens, which is designed for minimum aberrations at close focus, and reverse that onto the end of tubes or bellows.

I know that you do have an MP-E 65, so I am curious about why you are asking these questions. Are you thinking about your own setups, or wondering how to advise other people?

--Rik
Back to top
View user's profile Send private message Visit poster's website
PaulFurman



Joined: 24 Oct 2009
Posts: 595
Location: SF, CA, USA

PostPosted: Sat Nov 14, 2009 10:56 pm    Post subject: Reply with quote

I just wanted to emphasize what Rik said about how putting another lens on the front increases magnification without reducing light or shrinking the aperture. The common implementation of that idea is a 'closeup lens' attached to the front filter threads. The two-element versions of these are not so cheap as the common sets of a few with different diopter strengths that can be stacked. I have a Canon 500D which is two-element, pretty well corrected +2 diopter and it's handy for getting closeups from longish telephoto lenses. It's designed for I think around 200mm and has almost no effect on a short lens like 24mm. I've mounted it on a 300mm f/2.8 and it works fine even though it's only 77mm on the 110mm front element of the 300. At close focus, I guess only the center of the lens is being used.

PS I ran into that backward perspective the other day using a 1.4x teleconverter on the camera, then 38mm of extension tubes and a 35mm 5x Canon Macro-photo on the end. I guess the light-reducing properties of the teleconverter amounted to stopping down at the rear. Image quality seemed quite good considering it was almost 4x and really compact. The front lens was stopped down a bit.
Back to top
View user's profile Send private message Visit poster's website
PaulFurman



Joined: 24 Oct 2009
Posts: 595
Location: SF, CA, USA

PostPosted: Sat Nov 14, 2009 11:01 pm    Post subject: Reply with quote

PaulFurman wrote:
...it works fine even though it's only 77mm on the 110mm front element of the 300. At close focus, I guess only the center of the lens is being used.


Or more likely it isn't f/2.8 any more but there's no vignetting, it works fine.
Back to top
View user's profile Send private message Visit poster's website
LordV



Joined: 22 Nov 2007
Posts: 1558
Location: UK

PostPosted: Sat Nov 14, 2009 11:20 pm    Post subject: Reply with quote

Thanks Rik again
This is mostly a case of trying to understand my own observations on lens systems and to advise people.
For example when I first started macro, I was just using a sigma 105mm Ex and was trying to get some resolution of fly's eyes and only just about getting any. The lens developed an aperture fault and I had to send it away. As I was addicted to macro by then I cobbled together some old stuff I had from pentax slr days - I had a set of pentax fit ext tubes that I'd never used and a pentax 50mm lens, I cobbled a canon Tmount onto the smallest tube and used the 50mm lens with the ext tubes and much to my amazement started getting excellent resolution on fly eyes. It was only after a while that I realised I was shooting at 1.5:1 - the total tube length was about 75mm. I quickly bought a set of canon fit extension tubes and started using them with the repaired sigma macro lens and never looked back.

Like wise I did some tests with a reasonable quality 1.4X TC on the macro lens and came to the conclusion I was just as well off cropping the image from an IQ POV for a 10X4 print. I have also done test shots of a bee with the MPE-65 at 1.5:1 and the sigma at 1.5:1 with ext tubes and could not see any quality differences.

Before I got the MPE-65 I used the sigma 105 with a full set of ext tubes and a reversed 50mm lens to get to 4:1 and the only difference I noticed in image quality between that and the MPE-65 was really in the bokeh not in the subject itself. I was actually rather disappointed with the MPE-65 at high mags but it was then I did some aperture tests and found out about diffraction Smile

So I'm carrying my own biases based at least on some comparisons but do like to know some of the more technical background behind the observations both for my own information and to be able to give better or justify advice given to others.

Brian V.
_________________
www.flickr.com/photos/lordv
canon20D,350D,40D,5Dmk2, sigma 105mm EX, Tamron 90mm, canon MPE-65
Back to top
View user's profile Send private message Send e-mail
LordV



Joined: 22 Nov 2007
Posts: 1558
Location: UK

PostPosted: Sun Nov 15, 2009 12:32 am    Post subject: Reply with quote

Thanks Paul for the added info.

One thing I do not quite understand (I like to simplify things as much as possible).

If DOF is only dependant on the magnification and "aperture" and not on the magnification system actually used and the DOF/diffraction relationship is independant of the magnification system used. Then surely as far as diffraction is concerned the effect of extension tubes on apparent aperture must be balanced by another effect in the lens plus dioptre system giving the same magnification and that indeed the light loss would be the same ?

Should add I realise that diffraction is not the only parameter affecting IQ at macro magnifications.

Brian v.
_________________
www.flickr.com/photos/lordv
canon20D,350D,40D,5Dmk2, sigma 105mm EX, Tamron 90mm, canon MPE-65
Back to top
View user's profile Send private message Send e-mail
PaulFurman



Joined: 24 Oct 2009
Posts: 595
Location: SF, CA, USA

PostPosted: Sun Nov 15, 2009 1:48 am    Post subject: Reply with quote

I'm prone to speculation and fairly new at this so take the following with a big grain of salt.

If the lens on the front has a large enough opening to not limit the lens opening nearer the camera, there is no light loss. Extension limits light by simple math, it's exactly like cropping. When you crop, you have less light, the edges are gone and their light is gone. When you have a large enough front magnifier, that's gathering a wider field of light so there is no loss. Hmm, if I'm right, that means the front adapter scenario is actually making the lens faster, at least for closeup work. The new front element is larger than the effective aperture of the bare lens so it is gathering more light.

When I mentioned the smaller closeup filter on the 300/2.8, that's confusing because it's a smaller front element but if I'm following the logic correctly, at close focus, most of the 300/2.8 was wasted. It could just as well be a 300mm f/4.

OK I just tested that idea and the meter gives the following exposures:

Code:
1/1000  at f/2.8 with the closeup lens
1/320   at f/2.8 with bellows extended 175mm to match
1/160   at f/4   with bellows extended 175mm to match
1/100   at f/5.6 with bellows extended 175mm to match
1/40    at f/8   with bellows extended 175mm to match


It's too late for me to make any conclusions other than the closeup lens setup is a lot shorter, lighter & faster than the bellows. Even though it's smaller than the front element. I'd guess the closeup lens has compromised resolution but maybe not by much if I stopped down to match exposure.

On a related note, I was amazed to learn that a 1.4x teleconverter gives exactly the same image as a 1.5 crop sensor (minus the compromise from extra glass). DOF, shutter speed and noise level (after compensating with ISO) are exactly the same. The lesson there is a full frame DSLR plus 1.4x teleconverter costs a lot more than a DX camera with no converter and the loss from adding glass is probably not much. As usual, the gains diminish as the cost goes up. But back to the original question, the cost of a good closeup lens seems to provide a lot of benefit over the lesser cost of extension tubes, so maybe that's an exception to the rule.
Back to top
View user's profile Send private message Visit poster's website
Display posts from previous:   
Post new topic   Reply to topic    www.photomacrography.net Forum Index -> Macro and Micro Technique and Technical Discussions All times are GMT - 7 Hours
Goto page 1, 2  Next
Page 1 of 2

 
Jump to:  
You cannot post new topics in this forum
You cannot reply to topics in this forum
You cannot edit your posts in this forum
You cannot delete your posts in this forum
You cannot vote in polls in this forum


Powered by phpBB © 2001, 2005 phpBB Group