Vignetting

A forum to ask questions, post setups, and generally discuss anything having to do with photomacrography and photomicroscopy.

Moderators: Pau, rjlittlefield, ChrisR, Chris S.

stevekale
Posts: 172
Joined: Wed May 11, 2011 2:40 pm
Location: London, UK

Vignetting

Post by stevekale »

I finally got back to playing with this after a long spell away. I was using a 10x Nikon CFI Plan Achromat lens on Canon 135mm and 70-200mm L series lens. One thing that immediately struck me was vignetting unless the lens was stopped wide open. This surprised me given how small the aperture in the Nikon lens must be. With the Canon lens open to f2.8 it obviously makes for extremely small depth of field. Is there a relationship between lens focal length, f-stop, the Nikon lens and vignetting governing how much I can stop each down or do they just have to be wide open all the time?

rjlittlefield
Site Admin
Posts: 21036
Joined: Tue Aug 01, 2006 8:34 am
Location: Richland, Washington State, USA
Contact:

Post by rjlittlefield »

The tube lens should be wide open. The shallow DOF is what comes along with using a microscope objective with its high resolution and correspondingly wide aperture. If you stop down the tube lens, you can get a little more DOF at image center, but then the two apertures start fighting with each other and you get darkening and potentially lose image quality toward the edges.

It is possible to stop down a microscope objective without vignetting by adding an aperture very close to the objective. However, this will also degrade sharpness due to diffraction. It's a good technique for improving the OOF background of a deep stack, but not for shooting the bulk of the stack.

--Rik

stevekale
Posts: 172
Joined: Wed May 11, 2011 2:40 pm
Location: London, UK

Post by stevekale »

Wow thanks for the fast response. Ok got it. Is there a limit to how short I can go with the "tube lens" (i.e. could I use a 28-70mm L series lens) if I want less magnification?

PS: is there a quick way to manage all the file -> save -> tiff etc etc. I tried making an action but it didn't work!

Pau
Site Admin
Posts: 5168
Joined: Wed Jan 20, 2010 8:57 am
Location: Valencia, Spain

Post by Pau »

What camera, FF or aps-c?
Because you have the lenses try them. 70mm will be too short, the shortest I remember for another forum member post is with a micro-Nikkor 105mm.

With my 70-200 f4L IS and a 7D it's OK up to 140mm, approx. 7X before vignette become a problem.
Pau

stevekale
Posts: 172
Joined: Wed May 11, 2011 2:40 pm
Location: London, UK

Post by stevekale »

Canon 1Ds II (full frame)

I was hoping for a magnification solution between my 100mm 2.8L Macro lens with Kenko extension tubes and using the Nikon lens.

Pau
Site Admin
Posts: 5168
Joined: Wed Jan 20, 2010 8:57 am
Location: Valencia, Spain

Post by Pau »

MP-E 65?
Reversed enlarger lens or classic macro lens on bellows?...
Pau

stevekale
Posts: 172
Joined: Wed May 11, 2011 2:40 pm
Location: London, UK

Post by stevekale »

I think bellows. Unfortunately the used bellows I bought don't allow my camera to move along them (not deep enough). I need to look out for medium format set.

Out of interest, what magnification am I getting with the 100mm Macro set at 1:1 attached to 78mm of extension tubes?

Pau
Site Admin
Posts: 5168
Joined: Wed Jan 20, 2010 8:57 am
Location: Valencia, Spain

Post by Pau »

Magnification is easier to measure than to calculate:
Take a photo of a ruler in landscape orientation. If, for exemple, you capture 18mm in your 36mm wide sensor, your magnification is 36/18= 2X
Pau

rjlittlefield
Site Admin
Posts: 21036
Joined: Tue Aug 01, 2006 8:34 am
Location: Richland, Washington State, USA
Contact:

Post by rjlittlefield »

In my tests, the Nikon CFI Plan Achromat 10x NA 0.25 WD 10.5mm only covers about a 5 mm diameter field on the subject. Outside that the quality plummets. So on a full-frame camera, with sensor diagonal = 43 mm, pushing below 8.5X will give bad corners even if they don't darken. (This may still be acceptable, depending on the subject.)
PS: is there a quick way to manage all the file -> save -> tiff etc etc. I tried making an action but it didn't work!
I'm reading between the lines here. It sounds like you're shooting raw and using Photoshop to make TIFF to feed into Zerene Stacker. If that's correct, then I'm confused. Using Photoshop CS5 I just now created an action named ConvertRawToTIFF that consisted of three steps: Open (a .CR2 file), Save (a .tif file), Close. Then I used File > Automate to apply that action to a folder of .CR2 files, producing a folder of .tif files. That all worked fine. What is it that you're doing, and what's going wrong?

--Rik

stevekale
Posts: 172
Joined: Wed May 11, 2011 2:40 pm
Location: London, UK

Post by stevekale »

Ok thanks re the Plan Achromat.

I am opening all the RAW files together, making "global" adjustments for colour processing, sharpening, crop (sometimes), white point etc as a batch, opening them all into PS at once. I then need to save them all as TIFFs.

Actually I think you've just pointed me to my answer. Don't "open images" but "save images" from ACR with settings set to TIFF. Duh... I have always gone on to work on images in PS and so have never used Save Image from ACR. Thanks! Not even a need for File -> Automate.

BTW I notice vignetting on the 70-200mm set at 200mm/f2.8. Using the 135mm lens seems to offer much more latitude (but of course less magnification). I can see this sport is going to require an immense amount of experimentation.

stevekale
Posts: 172
Joined: Wed May 11, 2011 2:40 pm
Location: London, UK

Post by stevekale »

BTW here is the vignetting I get with the 70-200mm set wide open (f2.8 ) at 200mm (I should have shot a blank image rather than using one to hand but it shows the issue in any event)

Image

and here is the clean 135mm at f2.0

Image

I'm still having trouble getting my head around why there is vignetting on such a large aperture lens (of the required 200mm focal length) with a tiny objective on the front....
Last edited by stevekale on Mon Oct 17, 2011 2:05 pm, edited 2 times in total.

rjlittlefield
Site Admin
Posts: 21036
Joined: Tue Aug 01, 2006 8:34 am
Location: Richland, Washington State, USA
Contact:

Post by rjlittlefield »

stevekale wrote:Hmmm...why don't the images appear in the post?
You're missing the .jpg extension, which makes the forum software think that these are not images and thus should not be displayed inline. It's a subtle problem -- took me quite a while to see also.

--Rik

stevekale
Posts: 172
Joined: Wed May 11, 2011 2:40 pm
Location: London, UK

Post by stevekale »

rjlittlefield wrote: You're missing the .jpg extension, which makes the forum software think that these are not images and thus should not be displayed inline. It's a subtle problem -- took me quite a while to see also.

--Rik
Hmm the Sugarsync function to get the public link doesn't deliver the .jpg to the clipboard. I simply didn't notice! Thanks.

rjlittlefield
Site Admin
Posts: 21036
Joined: Tue Aug 01, 2006 8:34 am
Location: Richland, Washington State, USA
Contact:

Post by rjlittlefield »

stevekale wrote:I'm still having trouble getting my head around why there is vignetting on such a large aperture lens (of the required 200mm focal length) with a tiny objective on the front....
Here's an experiment that may provide some intuition:

Remove the object and its adapter. Point the eyepiece of the camera at a bright wall and look through the lens so that you can see the focusing screen. Close one eye and slide your head around until you can see one corner of the screen. You should have no trouble doing this.

Now for the interesting part. Tape a piece of black paper to your adapter, with a centered hole in it that's the same size as the exit pupil of the objective. Put the adapter back on the lens and repeat the previous exercise of looking through the front of the lens at the focusing screen. With a vignetting lens, what you'll now find is that you won't be able to see the corner of the focusing screen through all portions of the hole in the black paper. You'll only be able to see it through some of them, or if the vignetting is bad enough, maybe not any.

What's happening is that you now have two potentially limiting apertures: the entrance pupil of the tube lens and the exit pupil of the objective (modeled here by the hole in the black paper). If those two apertures are sized and located so that in combination they block access to the corners of the field, you get vignetting.

The typical problem with a zoom telephoto is that its entrance pupil is located far back, where its edges have a good opportunity to shadow the corners of the sensor from the standpoint of the objective's small and centered aperture. The vignetting does not occur when the telephoto is used by itself, because then the corner of the sensor gets its light from off-center portions of the lens's front element.

In contrast, a fixed length lens typically has its entrance pupil farther forward, so that light coming through the small and centered aperture of the objective can get through the tube lens's aperture even though it may not be very big.

I hope this helps. The concept of vignetting seems simple enough as it's usually presented in textbooks, but as it manifests in these combos, it's definitely hard to get a head wrapped around.

--Rik

stevekale
Posts: 172
Joined: Wed May 11, 2011 2:40 pm
Location: London, UK

Post by stevekale »

Thanks Rik. I'll play around this weekend.

I was reading the thread here

http://www.photomacrography.net/forum/v ... 3c7cb85711

I presume I am being penalised by the full-frame sensor. (I have no idea whether the larger objective of the 70-200mm versus the 135mm is a factor; 77mm vs 72mm.)

I'm a bit disappointed to find the vignetting problem. I had thought from conversations here that picking up the infinity objective would lead to useful combinations with these two lens. Now I find heavy vignetting with the 70-200mm in pursuit of 10x with a "right sized" focal length (makeshift) tube lens and your comments that pushing below 8.5x (135mm => 6.75x) will give bad corners. :(

Post Reply Previous topicNext topic