Stopping down with modified diaphragm (1 Viewer)

Here's a series of photos taken with a set of diaphragm blades out of an old lens. The assembly needs to be right up in the scope, pretty much at the end of the focuser and this insures no vignetting no matter how small you stop it down.

With the blades wide open at around 20mm I could shoot at the same shutter speed as with the scope that hasn't been stopped down. It's interesting to note though that when you compare the first two photos, the one with the 20mm has a nicer contrast over all. With the scope in standard mode the bright light starts to invade the edges of the photo but stopping down with the blades wide open you get a constant overall contrast.

At the other extreme, the 2mm hole was a bit unusable as the depth of field was so great that going from in focus to out of focus took so long that it was hard to tell when it was spot on. And at only 1/10 sec it's a bit slow in bright sunlight. Anything above 5mm was fine.

The subject was only 6m (20 feet) away but the background trees were about another 25m (82 feet) away or 30m (100 feet) overall.

Apart from depth of field being increased, the main things you get from this are better contrast and a way to control bokeh.

Without going to the lengths I've gone to with dismantling a lens, I'd say a good modification would be to reduce the hole at the end of the focuser to about 20mm to get the same effect I'm getting in the second photo and no loss in shutter speed.

Paul.

Nice test and show of DOF increase an iris can provide.
I tried that too in the past, and I found too the best place to put the iris was somewhere between the end of the focuser and the lens, anywhere closer to the camera and vignetting started to appear.

What I don't agree though is the 20mm giving the same exposure as the full aperture scope, in my tests 20mm shown a very noticeable drop in light. In your test, that's noticeable in the backgroung, altought the flower seems the same. If you don't mind can you repeat that test in an even lit target? Sky maybe? Or a grey card.

I've thougth about placing an iris on the scope for sometime now, not on the ED80 as it's DOF is most of the times enough, but at somepoint I'm going to replace it with something faster, F/6 maybe, and there I will definitely use an iris to control DOF.

Good one Paul. Did you mount only the diaphragm and can be controlled outside or fixed inside. This is similar to finding the sweet spot of the scope as similar to lenses.

Fernando, an iris control for the 80 would be very beneficial too as we can get focus faster and also for birds in flight.

Why would you get focus faster? You'd get more dof, that's all. IMO the more DOF you got the harder it get's to focus, also it get's darker too, so it's even harder to focus.
At least for me it is.

I haven't tried yet with a high magnification scope but when trying out stacking TCs and MF with a prime 100mm on a test card, I find the spot on focus quite hard to get and almost rely on the free play of the gears just to get it focused. So I thought that could be given more allowances for the spot on to stay longer before it get OOF again. Correct me if I were wrong Fernando, I always find it a challenge when using my 500mm MF lens. Maybe I just need more practice LOL.

Another thing, if we are to use the diaphragm, it's got to be the auto type similar to the auto M42 lens where we can press a button to focus then release to close the iris and shoot.

Cheers.

Here's a couple more examples. One of each are with the scope stopped down and one of each are with nothing at all. Each pair are at the same shutter speed and levels are not altered. I've written on two of them that they are stopped down to 20mm but I did an exact measurement of the blades when they are wide open and it is 22mm. In the brightly lit scene you get a slight polarizing type effect where some glare/shine is removed from the leaves. Depth of field at 22mm is about the same with no noticeable difference. In the stopped down ones they are slightly darker overall but not by a great deal. I found stopping down to 22mm helps control the brightest areas and stops them blowing out too soon.

Paul.

Here's a couple more, this time stopping down to around 8mm though it might have been smaller, maybe 6mm but no less. A much bigger depth of field and still decent shutter speeds, better overall contrast too.

Paul.

alphan said:

I haven't tried yet with a high magnification scope but when trying out stacking TCs and MF with a prime 100mm on a test card, I find the spot on focus quite hard to get and almost rely on the free play of the gears just to get it focused. So I thought that could be given more allowances for the spot on to stay longer before it get OOF again. Correct me if I were wrong Fernando, I always find it a challenge when using my 500mm MF lens. Maybe I just need more practice LOL.

Another thing, if we are to use the diaphragm, it's got to be the auto type similar to the auto M42 lens where we can press a button to focus then release to close the iris and shoot.

Cheers.

Click to expand...

If I understand you correctly, your saying that in your lens if stop down you get better focused images. Probably right, because you'll focus at full aperture and after you press the shuter the picture will be taken with the lens stopped down, giving you more DOF than you could see on the viewfinder.

But that's not what happens here, these iris wont allow you to shoot at full aperture, if you close it, you'll focus and shoot it closed. It's the same as shooting with the mask on the front of the scope. I used that a lot on my 80 F/5, it was always harder to focus, at least for me.

When I though on adding an iris to the scope my ideia was to go auto aperture (easier with Nikon), with a simple linkage inside it's doable. That said is not a feature I would use often so I'm not sure I'd ever go to that trouble.

Paul Corfield said:

Here's a couple more examples. One of each are with the scope stopped down and one of each are with nothing at all. Each pair are at the same shutter speed and levels are not altered. I've written on two of them that they are stopped down to 20mm but I did an exact measurement of the blades when they are wide open and it is 22mm. In the brightly lit scene you get a slight polarizing type effect where some glare/shine is removed from the leaves. Depth of field at 22mm is about the same with no noticeable difference. In the stopped down ones they are slightly darker overall but not by a great deal. I found stopping down to 22mm helps control the brightest areas and stops them blowing out too soon.

Paul.

Click to expand...

I think the drop on light is quite noticeable, don't know by how much, but you could see that easily by changing the shutter speed. I really wouldn't want to be giving any light away on my ED80, an iris is a nice add on, but for me it would have to allow me the scope's full aperture, the problem is an iris capable of that is a bit on the expensive side.

Paul, can you try your above test again. This time, get both in the same brightness, meaning a different SS, so that we can understand our SS sacrifice. The stepped down pics are much more contrasty but that could be aggravated by the darker pic.

Fernando, I once have a Takumar 50 M42 which have a pin to press for focusing and releasing for the shot. If this iris control is really good, then we can use and old prime lens, take out the optics machine the barrel to fit onto the focuser and become the aperture and extension. All this is again subject to another confirmation. When the diaphragm is fully open, what is the diameter and light loss. If not big enough, then we have to talk medium format lens...LOL getting more OOT.

alphan said:

Fernando, I once have a Takumar 50 M42 which have a pin to press for focusing and releasing for the shot. If this iris control is really good, then we can use and old prime lens, take out the optics machine the barrel to fit onto the focuser and become the aperture and extension. All this is again subject to another confirmation. When the diaphragm is fully open, what is the diameter and light loss. If not big enough, then we have to talk medium format lens...LOL getting more OOT.

Click to expand...

But that is one more thing to do at the shooting moment, there's situations where you can do it without issues, but most of the times bird wont give that much time, you have to focus and shoot as fast as you can.

I don't remember the exact iris size needed but I think it was more like 40mm or so to use the full ED80's aperture. But you can't put a 40mm iris at the end of the focuser because the outter diameter of the iris will be too big, and will hit the focuser barrel when you rack it out, at least the iris I've been looking at (edmund optics). So the best place to put a big iris is at the tube itself, right where the focuser ends, I don't know on what lens you'll find an iris this big.

alphan said:

Paul, can you try your above test again. This time, get both in the same brightness, meaning a different SS, so that we can understand our SS sacrifice. The stepped down pics are much more contrasty but that could be aggravated by the darker pic.

Fernando, I once have a Takumar 50 M42 which have a pin to press for focusing and releasing for the shot. If this iris control is really good, then we can use and old prime lens, take out the optics machine the barrel to fit onto the focuser and become the aperture and extension. All this is again subject to another confirmation. When the diaphragm is fully open, what is the diameter and light loss. If not big enough, then we have to talk medium format lens...LOL getting more OOT.

Click to expand...

The diaphragm that I have at the moment is 22mm fully open. I have a Pentax 50MM SMC and that diaphragm measures just over 30mm but it's not a lens I'm going to take apart, too valuable. I would say that when mounted in the right place you wouldn't lose any speed at 30mm. Judging by the histogram in Photoshop, on the ss sacrifice I'm only about one click off at 22mm. Can't see that you will need a diaphragm that can be opened and closed between shots plus it needs to be a good 6" up inside the focuser. Mines in a 2" barrel so I just slide it in and out and it's a nice snug fit. I don't find light loss in the viewfinder to be an issue until I get down to about a 5mm hole. On a 2mm hole it was quite dark and the depth of field was too big to know when you were in focus but I still got sharp hand held shots at 1/20 sec. A 10mm hole is very usable with a good gain in depth of field. Probably of more use for close up stuff where depth of field is very narrow anyway, dragonflies, butterflies, that sort of stuff.

Paul.

Just tried a 25mm hole and got no difference in shutter speed or light loss. With the 22mm diaphragm I had to go one click down, so from 1/640 it went to 1/500. So on the SW80ED the point of no light loss is somewhere between 22mm and 25mm. Tried it out in shade and sunlight just to be sure.

Paul.

Paul, that doesn't sound right to me. At least on my setup I can't get anywhere near what you've shown with a 25mm aperture. Are you sure your baffles are in the right place? That's the only explanation I can find. A 25mm aperture should really do a noticeable decrease in light on a ED80 and a DSLR. Or I'm not fully understanding what you're doing.

Here's a comparison between a 25mm baffle and no baffle, at the inside end of the focuser:
http://photos.smugmug.com/photos/987983943_pa48F-O.jpg

You can easily see that the 25mm baffle makes a drop of more than one stop if placed at the inside end of the focuser.

Here's another, this time at the outside end of the focuser:
http://photos.smugmug.com/photos/987983921_hkpQk-O.jpg

In this case the drop on light is not so severe, maybe 1/2 a stop or 2/3, but the vignetting is extremely noticeable, also this shot doesn't show but the increase in DOF is only in the vignetted areas, at least more on that area.

Mines about 2/3 the way up the focuser. Maybe by putting it at the end of the focuser you are cutting into the light cone too soon. I don't get anything like you are getting with a 25mm hole, in fact I get no difference.

Paul.

I can probably be stopping it down a bit too soon, but even if I move it backwards I can't get those results. Note that on the 2nd sample the baffle is at the other end of the focuser, right next to the camera. Even than I have more light drop than you have, and vignetting is already noticeable.

The only explanation I can find for this is one of your baffles being misplaced, too close to the lens, blocking a bit of light already and not making that much of a difference. I really can't see anything else for this difference.
In my scope that was the case for the 2nd baffle, it came twisted and misplaced, the original position was blocking 1/3 of a stop until I placed it correctly. But it was done at the factory since the paint marks showed it was painted in that position.

In any case, I don't think a scope like an ED80 could ever iluminate an APS sensor with a 25mm aperture at the focuser, that's even less than using a 1.25" focuser (31.75mm). Most of the scopes for AP these days are sold with 2.5" focusers for that issue exactly. For example the 600mm F/6.7 triplet from TS I linked in another post is sold with a 2" focuser for visual, but for AP with a DSLR it comes with the 2.5" moonlight, according to TS.

Here's mine. First is without and second is with baffle. I checked my factory fitted baffles and they are welded in firm and square. We'll have to take measurements on where the factory ones are located and see if they differ.

Paul.

The baffles are welded in ?

I removed both of mine

Musoman said:

The baffles are welded in ?

I removed both of mine

Click to expand...

I presumed they were spot welded in place as mine have never moved and I've certainly pushed stuff up against them, sometimes quite hard. I had a look just now and they do indeed move but they are a very tight fit, in my scope anyway. I can see though that their position could vary from scope to scope at the manufacturing process.

With your new focuser I guess you wont have any in the scope anyway?

Paul.

Dear all,

I hope it is OK with you all if I add a comment or two regarding the use of diaphragms, but I have been reading your very interesting technical conversation on the use of diaphragms etc and I would like to contribute my own findings from my own recent experiences, and hope they may be of use to you.

I use a Revelation FPL 53 ED telescope with object glass of 80 mm and f.l. of 500mm. I do not use this for photography at present as I have not got the camera I want yet, but I have been using it for some years with a binocular head from William Optics.

It achieves very fine images with pseudo 3 D effect, and using various Barlow's etc . But some six months ago I thought I would fit an iris diaphragm, with the purpose of excluding excess light (as used in a microscope sometimes ), and possibly achieve better resolution by removing glare.

I fitted an iris diaphragm obtained from an old bellows camera within the focusing tube and have allowed the operating lever to protrude through the side of the tube,,, see photos attached.

I have achieved excellent results from this, achieving more contrast, more resolution, less glare and better depth of field.

Whichever barlow I use, I start off at full aperture and then gradually reduce diameter to the best visual image position, of course this means a loss in brightness, but this is mostly beneficial because the major reduction of overbright areas, gives extra clarity and contrast of the whole of the image.

I have found that the best position for me to place the diaphragm is approximately 80 per cent of the focal length from the objective. At this position no vignette occurs, only an overall reduction in light, when the aperture is reduced.

The maximum opening of the diaphragm I think should be ( as long as Barlow is close behind it ) approximately the size of the field lens of your eye piece or Barlow, or slightly bigger… my own Barlow's are a maximum of 22 mm diameter and my diaphragm maximum opening is 20 mm and this seems fine. I have tested this because I have the ability to drop the diaphragm into its slot, as and when I wish, and I see no dimming of the image when dropped in fully open, and none of the original telescope fixed stops are too small or in wrong position.

I find that If the diaphragm is placed further forward than this then you will obviously need a bigger diaphragm to achieve the same light throughput, and any further away from the object glass would start to cause vignetting.

P.S. I hope I may achieve somewhere near the very high quality of photography that you guys have posted on this and connected forums, that is when I actually get started shooting.

Tom