lens aperture and dSLR digiscoping

[mmdnje]

From a theoretical point of view, if I ichange the f stop say from 2.8 to 4, would that result in "better lens preformace" or sharper image. This practice is common in general dSLR but I do not know if it would help in digiscoping.
I`m using a dSLR with it lens attached to an scope with eyepiece. Thanks in advance. jose

[Feathered one]

Hello Jose

Changing the f stop to a higher number will certainly give you more depth of field, so more chance of getting the subject in focus.
From digiscoping tests I did with the Nikon P5000 this certainly shows up. Have you looked at my digiscoping pages, there the test results are shown.

Click on this link then you will see a link to the test

http://lynandmholidays.mysite.wanadoo-members.co.uk/p5000pictures.htm

Malc

[mmdnje]

That is great and ceratinly answre my question. thanks alot. Jose

[jourdaj]

Nice job, Malc! Thanks, also.

[dahyon]

Unfortunately the DOF of a ps-camera digiscoping rig cannot represent that of a Dslr digiscoping rig. As an example : a Dslr 50mm lens at f# 2.8 has a camera aperture diameter of 17.86mm; my Zeiss85mm scope with a 30x eyepiece has an exit pupil diameter of 2.83mm; therefore the source of the limiting aperture is the telescope exit pupil and remains so until f#18 at which point the camera aperture becomes limiting. Distance from the object is another factor to consider when considering DOF. I have attached a chart which shows the efect of varying f# and D(distance) on DOF for a Dslr rig described above.

Cheers Dave

[iporali]

Unfortunately the DOF of a ps-camera digiscoping rig cannot represent that of a Dslr digiscoping rig.

Dave,

I notice you beat me to it ;)

Well, I'll post this anyway since there are some example calculations that might be useful for some.

f-number = focal length/aperture diameter

DSLR:
f2.8 on a 50mm means 17.9mm "aperture"
f4 on a 50mm means 5mm

Corresponding field-of-view on a compact camera is about 15mm
f2.8 on the 15 mm means 5.4 mm "aperture"
f4 on the 15mm means 3.75 mm

Feathered One has probably used the widest FL of the P5000 (7.5mm)
f2.7 on the 7.5mm means 2.78 mm aperture

Now, a 82mm scope + 30x eyepiece produce an exit pupil of 2.73 mm

What does all this mean?

With a DSLR the aperture "hole" is a lot larger than the exit pupil, and therefore the scope determines the aperture.
With the ED82 the P5000 lens determines the aperture and therefore you immediately see the effect of stopping down at wide end of the zoom.

In Feathered One's case: Yes, stopping down increases depth of field.
In Jose's case: No, stopping down does not increase depth of field - until beyond useless f18 - but it may cause vignetting a lot earlier.

HTH,

Ilkka :t:

[mmdnje]

That is fascinating. I was actually interested in "lens performance". Is the lens sharper when stopped down and would that mean a sharper digiscoped image??

[iporali]

That is fascinating. I was actually interested in "lens performance". Is the lens sharper when stopped down and would that mean a sharper digiscoped image??

In theory, yes (the "sweet point" being usually somewhere at f5.6 - f8) - but in practice your scope is probably so much softer than the camera lens at f2.8 that I would be slightly surprised if there were a noticeable difference. But, if your afocal coupling between the eyepiece and the camera is optimal, and stopping down does not increase vignetting - why not give it a try. It should not slow down shutter speeds (until at f18) or make viewfinder any darker (unless you have a very old lens), just the lens *a little* sharper.

Best regards,

Ilkka

[Feathered one]

Sorry for misleading you with the results of my test with the P5000. You did say you were using a dslr.

To be honest, I had no idea about camera aperture "hole" values with slr's and different lenses.

Using the 82mm scope with 30x eyepiece, which gives a exit pupil of 2.73mm.
On the point and shoot camera, if possible should one then try to match the exit pupil with the f stop and camera zoom used.
For instance if using 20mm zoom on camera use f7.0 which gives 2.86 aperture.
And if using 15mm zoom on camera use f5.6 which gives 2.67 aperture.

Also what effect will engaging macro have on these figures.

Sorry I am going off the dslr track that Jose first posted for, but I am eager to learn from the experts here.

Malc