X100 & digiscoping problem

HI all,

From time to time I stick my X100 onto the eyepiece of a birding telescope, and it works pretty well (around the equivalent of an 850mm lens, at the minimum scope zoom setting). But I've always had to work around a weird problem:

At f2 (wide open on the X100), when using Aperture Priority, the exposure seems accurate and what I see in the EVF matches the final image. If I stop down to anything smaller than f2 however, then as soon as I half-depress the shutter release, the EVF flares into overexposure and this is matched in the final image too. I can use the exposure compensation dial to correct it, provided I don't go smaller than f4. As soon as I take the camera away from the scope, it behaves perfectly normally again.

If any of you have seen this behaviour, or can figure out what might be going on, I'd be very grateful for any advice.

In case anyone hasn't heard of digiscoping before, here's an example of the kind of magnification that can be achieved: if you view the image below at 100%, you'll see a mosquito about half-way up the left-hand side of the image - it's a fluke that it happened to be in the field of focus at the time, but you can see its legs, from about 30m away!

Anyway - thanks again for any pointers about the exposure behaviour.





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Flickr photos: https://flic.kr/s/aHsjXbgGu9

So this reply is a couple of years too late but I find it via Google and felt it deserved a reply (even if I can't give you the explanation you need).

1. Why does the picture change exposure only when you press the shutter?

All cameras use a large size aperture when you are not pressing any buttons - even if your settings are on a high /f number. This gives you a nice bright image to look at while you compose. Once you press the shutter button the aperture closes down to your chosen setting to take the image.

DSLRs have a depth of field preview button so you can see the actual view through your chosen aperture. It always makes the image much darker. Fuji x100 cameras also have this function but it's hidden in the settings. You could hold one of your fn buttons and temporarily change it to depth of field preview. Apparently in aperture priority mode the x100 does a kind of preview anyway, which may be what you are seeing before you take your image.

2. How do lenses react when they are partially covered?

If you slightly cover a lens (with a finger for example) at a close distance you won't see the shape of the object, the overall image will get darker. I guess this is the interaction we are seeing, somehow.

3. Why does the image get brighter instead of darker?

I don't know! An interesting puzzle! I don't have enough experience with binoculars or scopes to be able to observe how lenses react together. I would have expected the image to get darker when the shutter is pressed - because of how the smaller size aperture would interact with the binocular image.

4. Can we fix the problem?

My first idea would be to try using the depth of field preview button on your x100. The key would be to align your telescope/binocular with your camera while holding the depth of field preview button. Only while holding this can you see the true interaction between the two objects and the result on the sensor. Then you can move your camera back/forwards/sideways to find the best result.

Good luck to anyone trying this! Have a nice day.

It's never too late! Thank you very much for taking the time to respond. To be honest, I haven't done much digiscoping recently so I've ignored the problem, but I'll have another opportunity fairly soon, and will try the DOF Preview thing.

Thanks again, and best regards,

Al

Very cool! Can you show the pieces of kit you need for this to work?

I'm expecting a scope and some kind of adapter to connect the two, but I'm not 100% on the particulars.

Hi Christian,

The right equipment varies according to the type of scope/camera/lens you have, but I used this gadget to attach the camera to my Swarovski scope:

https://www.uttings.co.uk/p101379-s...a-base-inc-balance-rail-straight-swdcbadcb-s/

If you Google 'digiscoping' and your camera model, you'll see loads of tips.

The technique has become more popular, and there are many really skilled practitioners - have a look at Tara Tanaka's gallery:

https://www.flickr.com/photos/focused-on-birds/

It's challenging! But don't give up. When you get your first pin-sharp results and mega-magnifications, it'll all be worth it!

Cheers,
Al