9 replies to this topic

[fschuindt]

Hello there,

 

I never shot film before, recently I bought a Canon EOS 500 SLR to match with my set of Canon EF lenses. I'm planning to shot using my GEM and a 40mm EF lens. However, I'm struggling to plan how I'm going to perform the focusing. I was reading some material online, saw some methods like the knife-edge focuser, but I think this maybe too much for me right now. Unless I'm missing something.

 

I also have a Canon T3i DSLR, that uses the same EF lenses. So I thought about focusing on the DSLR with computer-aid and maybe a Bathinov mask, then carefully swap the lens back to the SLR body without touching the focusing ring. I know there are some mechanical tolerances and maybe the focal plane is some millimeters off, but I'm wondering, is this reliable? Have some people used this method before? Is it OK? Or, actually, any tips for me here?

 

I have tried this same approach on a daylight subject and it seems to work. It's hard to know 'cause I think pretty difficult to judge the quality of a focus by looking on the viewfinder. But it looked OK.

 

BTW, this is the plan so far:

• Canon EOS 500 SLR
• Kodak Portra 160
• Ilford HP5 Plus 400
• Canon EF 40mm f/2.8 STM
• Meade LX85 GEM
• guide-scope and guide-camera on PHD2

 

Any tips are welcome. Thank you!

[Avgvstvs]

You just look and turn the focus dial till it looks sharp to your eye. Stopper down to 5.6 to 8 for the gererally sharpest image. Just like looking through a telescope and focusing. 

[Avgvstvs]

And yeah, you don't know till it's processed and in your hot little hands, thats how the real fun world of film camera's works.

Time for another roll and 2 weeks of processing till ...........

 

Next add a step called 'hypering' ........ life is so easy with digital  .. enjoy the journey 

Edited by Avgvstvs, 18 September 2023 - 06:38 AM.

[fschuindt]

Thanks for the advices, I'm going to try that. I personally find it hard to judge by eye, as I mentioned. BTW, I'm farsighted, should I be using glasses? That's another question I have.

I also have some viewfinder magnifiers, but it also has its focusing, so I'm leaving it out of the equation for now.

 

Also, are there some generalized formulas for calculating exposure time giving Bortle, ISO, f-Stop, something like that? Just out of curiosity, these usually serves a starting point.

 

Anyway, thank you!

[UrsaMaatRa]

I have used a process similar it’s easier to do the switch on a fixed telescope so In theory this should work. I would choose slower film for sure. Ektar 100 pushing 1 to 2 stops works very well for color and acros II for BW. Im waiting for a clear night to give this a go again.

[UrsaMaatRa]

45mins with a f5.6 scope

Attached Thumbnails

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[Michal1]

Hello fschuindt!

 

Focusing a film camera using a digital camera: I'd try that at daylight on a ground object. I assume that you are able to use the standard focusing elements on your film camera's focusing screen. Then switch the camera to digital and check if the focus is correct.

 

Exposure time calculators: I am afraid that they don't exist. We have to test this by ourselves. The time depends a lot on your film (not just ISO, but also reciprocity failure characteristics), local light pollution conditions, etc., as you write. It is the easiest to look for astrophotos of someone else using the same film and get inspired by their exposure times. But this is just a first guess. You will have to experiment anyway. In my experience, for a given f-ratio widefield lenses require longer exposure times.

 

Stopping the lens down: For film, generally I wouldn't go below f/6. I have no experience with your lens, but is usually necessary to stop down just by 1-2 stops. The film medium itself can't produce images that are as sharp as digital ones, even if you use the same lens. Therefore focusing is not that critical and the lenses can be less sharp. However, are you sure that this lens can produce (relatively) sharp images of the full format? I can see that it was intended for smaller digital sensors.

 

Note that different films can have different reciprocity failure (reduced sensitivity to faint light), even if they have the same ISO. Therefore not all films are suitable for astrophotography. Generally, fast films have worse reciprocity failure. Paradoxically, you can thus capture fainter objects faster with a slow film!

 

Good luck!

[Todd N]

One of my film cameras is a Canon EOS Rebel 2000. It has a clear focusing screen that makes it way easier to manually set focus along with a magnifying right-angle finder. This works for telephoto lens and scopes with greater focal lengths. But with smaller focal length lenses it's extremely difficult; I tried with an EOS 28-75 zoom lens. I don't think knife edge is going to be easy or even possible at 40mm fl. I have used a variation of knife edge with a ronchi screen and I can only get it to work on newtonians. You should do test shots and note a series of focus positions around the infinity mark and then permanently mark that position. Wide-field Astrophotography(Reeves) and Astrophotography for the Amateur(Covington) note this procedure in detail.

[fschuindt]

@UrsaMaatRa Thanks! For 40mm lenses, what will be fixed is the camera body, but I'm planning some tricks to help me. I'm going to post some pictures here of the setup when ready. I'm also going to try on my Askar FRA400.

 

Your M42 picture is beautiful! Nice job.

 

@Michal1 Thank you for your response.
 

However, are you sure that this lens can produce (relatively) sharp images of the full format? I can see that it was intended for smaller digital sensors.

I've checked some documentation online and saw some videos of other people using this same lens on 35mm film, everything seems to be ok. 

But yeah, I think you're right. There's a lot for me to experiment here. Thank you!

 

@Todd N Thank you! I'm definitely going to mark the positions. Does stopping down the aperture in also helps with focusing in AP? BTW, nice references!

Edited by fschuindt, 19 September 2023 - 11:25 AM.

[Todd N]

 

@Todd N Thank you! I'm definitely going to mark the positions. Does stopping down the aperture in also helps with focusing in AP? BTW, nice references!

I achieve focus with the f-ratio I intend to shoot at for my Canon camera. On the other hand, my Pentax K cameras and lenses(Pentax & Vivitar) are properly set so, I just have to turn the lens focus all the way.  The range of precise focus is narrower at faster f-ratios which makes it technically harder from what I recall. But with the limits of film resolution, I don't think it's much of an issue. For example, my Vivitar SLR is just a bit off from proper infinity focus judging by viewing through the focus screen's fresnel lens at a distant object. But the film images are sharp.