Stopping down the scope with a fixed diaphragm (1 Viewer)

Hi, after reading this old thread:

http://www.birdforum.net/showthread.php?t=179595

I tried a simplified version, and tested some fixed diaphragm and looked for the best place to locate it. By the way, I have tried to reply in that thread but I get an error message telling me that it is too old to reply in it.

After some checking, I found that in my Televue 76 scope the best place (and also the easier) is putting the foam diaphragm just in front of the 2'' barrel of the first extension tube that I use. So it is placed inside the focuser, 4 or 5 cms. inside it.

I cutted some foam circles of different diameters: 25mm, 14mm and 11mm. You can see attached how badly I made the cutting, it was just a quick test.

Also you have attached how the 14mm diaphragm looks like n front of the 2'' barrel of the tube extension (it's the one that came with the GSO 2'' 2x barlow).

I have also attached a test of vignetting with the smaller one, the 11mm aperture. You can see no noticeable vignetting even with the smaller one.

Then I mounted a little scene with three plastic animals, giving a total size similar to the birds that I use to photograph. I wanted to increase the depth of field for getting in focus the whole body of the birds, not only the face and some parts of the body.

I made these shots near the fall, but with a similar light in all of them. I tried to get more or less the same exposure in all of the pictures, so I was playing manually with speed and ISO trying to compensate each shot to the same global exposure.

After that, I studied the speed and ISO data obtained with each of them, and that way I calculated that the equivalent apertures for each one of them were:

- f6.3 with the open scope. This was the reference value used for calculating the others.

- f7.2 with the 25mm opening.

- f9.0 with the 14mm opening.

- f13.0 with the 11mm opening.

These are aproximated values due to the fact of exposure being not the same for each shot (contrast changes so much that makes difficult to achieve equivalent exposure), and also due to the badly cutted apertures, not perfectly round.

But anyway this gave me some useful data for determining which of them use. The 25mm opening is so slightly stopping the scope down that you could use it anytime, bettering slightly contrast and DOF of my prime focus arrangement.

The 14mm opening gives a big improvement, and can be much of the time, being not recommended only in heavy shadowing.

The 11mm opening (aprox. f13) can only be used with direct sun light, and gives a magic DOF.

You can see clearly the effect of DOF in the little cheetahs.

All the photos are made with the Canon 1000D and with no barlow, directly coupling at prime focus the camera to the Televue 76 wih two extension tubes

The last unlabeled picture is the one taken with the 11mm f13.0 equivalent aperture. ISO200, 1/200s.

To my surprise, not only contrast and DOF were clearly better stopping the scope down, but also fine detail obtained was clearly richer and better.

Last pictures are showed uncropped, only reescaled, and here I attach cuts of each original images, showed at real points, with no processing, just the jpeg obtained by the camera (no raw post processing, no edition of any kind).

The change in detail is so high that you ask yourself am I going to miss so much detail anymore? Well, each scene and picture needs different parameters, and sometimes you could want to work with narrow DOF, but generally speaking I would want to shoot most of the time with f9.0 or f13.0.

Very good. Very helpful. Gracias!

At 11mm you seem to be getting a little loss of detail to go along with the extended dof. Refraction? Usually starts around f/11 on long telephoto lenses but becomes really noticeable around f/16. The f/9 (13mm) seems to have the sharpest eye.

cruedag said:

ut generally speaking I would want to shoot most of the time with f9.0 or f13.0.

Click to expand...

Yeah, one can see your are from Sunny Spain
Shooting f13 in Stockholm will get you... no joy ;-)

(Just joking )

/Carlos

Well, I really don't know if it's a matter of diffraction or shaking because of the 1/200s and not having locked the mirror. It was my fault, in my experience below 1/320s you have to deal with some degree of shaking due to mirror movement.

Could be. I always use mirror lock up and a shutter delay of 5 seconds to rule that out. I did a bunch of tests on some telephoto lenses this summer and was surprised at how quickly the sharpness goes down the tube when refraction begins, and how really unsharp an otherwise very sharp lens can become if stopped down too far.