Strange smear

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scitch
Posts: 463
Joined: Sat May 29, 2010 12:35 am

Strange smear

Post by scitch »

What could have caused this? It looks like it was painted and somebody wiped a brush across it. I looked at the "as adjusted" pictures and there didn't appear to be any movement. Here was the technique:
I used the light demonstrated here: http://www.photomacrography.net/forum/v ... highlight=

I set my Raynox on top of the light and then pointed the Tamron 90mm down through it. Should have been a magnification of about 2.5X.

Camera settings were: F/5.6, ISO-200, 1/200 sec, ZS PMax stack of 67 images.

Image

The legs seem to be OK, but the body doesn't. I didn't do any PS editing except resizing.

Mike

Barney64
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Location: Leatherhead, Surrey, UK

Post by Barney64 »

I think you have three problems here, not one.

(1) The SP90 hates any form of supplimentary lens. Try extension tubes instead.

(2) The smearing looks to me like a form of internal reflection within the lens, between the lenses or from the base of the mount (most likely). Check everything for shiny surfaces and matt them over with a black felt tip. If this reduces the effect then it's time to get the matt black paint out and do a permanent job.

This sort of smearing can also result if the stacking software is unable completely to reconcile small movements between frames.

(3) Presumeably you are using mirror lock-up. But even so there is still shutter movement to take care of - there is definitely also some movement there. 1/200th is nowhere near quick enough to freeze it. I initially had shutter induced movement problems and only cured them entirely by changing technique. I long ago abandoned all use of continuous illumination for macro, except for setting up the shot.

I use the on-camera flash to trigger two remote flashes. I set the camera to manual, 2 seconds exposure and forced flash on at the minimum flash output. Most importantly; second curtain. The camera's flash then triggers a Metz (CS-28 digital) remote flash that is programmed to ignore the camera's pre-flashes.

The Metz then triggers a very old Vivitar auto-flash that I nearly threw away 20 years ago. I relish digging out long obsolete equipment and finding a new use for it. This is set to auto (its like a flame-thrower on manual) and is fired by a remote flash-activated hot shoe (from China - £7 on Ebay) that is itself shielded from the camera flashes.

The sequence of events is then (1) camera pre-flashes (2) shutter opens (3) two second delay while everything settles down (4) final camera flash triggers Metz, Metz triggers Vivitar (5) shutter closes.

Exposure is adjusted by moving the flashes nearer or further away as required. The Metz also has a 5 stop adjustment and is usually set at the minimum output. The SP90 is sharpest at f5.6 so that never changes. All ambient lighting off.

I also use any number of mirrors to fill-in the shadows and foil-lined kitchen towel tubes to direct the camera flashes to the Metz (and sometimes the subject) but away from the Vivtar. The bench ends up looking like something from Blue Peter but it all seems to work.

The important point is that everything is absolutely stock still and there is no physical movement at all when the shot is actually taken.

rjlittlefield
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Re: Strange smear

Post by rjlittlefield »

scitch wrote:I set my Raynox on top of the light and then pointed the Tamron 90mm down through it.
Sorry, I missed this detail the first time I read this thread a couple of weeks ago.

In general, it's not a good idea to move the Raynox off the main lens. Even if you can keep them properly aligned (not easy), the Raynox's are computed to be well corrected when mounted on the main lens. Making the gap larger will mess that up.

In this particular case, I suspect the Raynox was tipped and/or off center with respect to the optical axis of the Tamron. This is liable to introduce astigmatism, which smears the image in two perpendicular directions depending on focus. In some frame, the image will be smeared in direction A; in a different frame, in direction B perpendicular to A. The stacking process then combines the two smears to produce a cross- or checkerboard pattern like we see on the carapace.

To check for astigmatism, play through the stack like it's a film strip. If you have astigmatism, then as you focus through the subject you'll see details go from completely blurred, to sharp but smeared, to symmetric but blurred, to sharp but smeared in the perpendicular direction, to completely blurred again.

--Rik

scitch
Posts: 463
Joined: Sat May 29, 2010 12:35 am

Post by scitch »

Thanks to both of you. I know I'll never use this technique again, I just wondered what caused it.

Barney, unfortunately my camera doesn't have second curtain, silent, or mirror lock. It also has a problem that I assume is a defect that if I try to shoot faster than 1/250 second, the top half of the picture gets cut off, presumably by the mirror. That's very disappointing. I'm typing on the phone right now. I can upload an example later. Is it possible that this is caused by using the camera vertically? Maybe it has a harder time moving the camera quickly in that configuration?

Rik, that all makes sense. My eyes have astigmatism and this smear is exactly how I see bright lights at night. I have extension tubes, they just didn't give the magnification that I needed.

Mike

rjlittlefield
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Post by rjlittlefield »

scitch wrote:It also has a problem that I assume is a defect that if I try to shoot faster than 1/250 second, the top half of the picture gets cut off, presumably by the mirror.
Most likely this only happens when you're shooting with flash. If so, then it's not a defect, it's just that you're using the camera outside the range where its shutter can possibly work with flash.

The key thing to recognize is that a focal plane shutter has two curtains. A frame is exposed by "opening" one of the curtains, then "closing" the other curtain one exposure time later. Flash only works when the exposure time is long enough that both curtains are fully open when the flash goes off. For shutter speeds faster than 1/250 second, your shutter begins to close the second curtain before the first curtain is fully open. The cut-off portion of the picture corresponds to the portion of the sensor that was behind the second curtain when the flash went off.

Some flashes provide a mechanism called "high speed sync" or some such, that essentially changes the flash output from a single short bright pulse to a longer non-so-bright extended output that lasts long enough for the shutter to complete its action (several milliseconds). This is mainly designed for scenes that are brightly lit with continuous illumination that requires an exposure time less than 1/250 second, and for which you'd like to use flash as fill-in.

--Rik

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