After getting necessary film to cover the whole aperture of the lens, I decided to try out a few shots in the morning. Starting off with the highest shutter speed of 1/4000, I realised that even that gives a VERY bright picture (as opposed to the 1/80-1/40 shutter speed which gave a "reasonably ok" view yesterday).
Now here's a bit of math. The "brightness" of the image is dependent on both the expose time and the aperture. The expose time has a linear effect (i.e. you expose for 2 seconds, you get twice more light than when you keep it open for 1 second). What sort of an effect does aperture have?
Assuming aperture is measured as a diameter of the lens/opening (and that it is circular), the area that is open is given by pi*(d/2)^2 where d is the diameter of the aperture. The area it self has a linear effect on the brightness as well. I.e. you let twice as much as light in for the same period of time, you get an image that is bright twice as more. If the two apertures you need to compare are d1 and d2, then the brightness increase/decrease with d2 compared to d1 is given by d2^2/d1^2. This is a very important factor. It means that if you make the aperture twice as large, you increase the amount of light four times.
When I tried yesterday, I had an effective aperture of about 1.25". With today's mod, I have an aperture of about 2.2" (i.e. the full aperture of the lens). What this means is that under the same settings, the camera now gathers 3 times more light.
But these numbers don't tally at all. If it is 3 times more light, then the shutter speed should get to about 1/120-1/150, not 1/4000. So what changed? Two things;
1. The film changed.
I am not using the same film as the day before.
2. Cloud cover.
When I took the first set of photos, it was during an evening and there was a little bit of haze. This seems to have dramatically reduced the amount of light available.
I am still not sure if I want to shoot using the telescope. It has a diameter of 4.4", i.e. the light is going to be four times more than the 75-300mm tele-lens that I have. The maximum shutter speed I can get on the camera is 1/4000. An option is to stack multiple filters on top of each other. Let's see how it goes. In the meantime, here is a photo with 1/4000 ISO100 with the brightness completely reduced to -2.
Now here's a bit of math. The "brightness" of the image is dependent on both the expose time and the aperture. The expose time has a linear effect (i.e. you expose for 2 seconds, you get twice more light than when you keep it open for 1 second). What sort of an effect does aperture have?
Assuming aperture is measured as a diameter of the lens/opening (and that it is circular), the area that is open is given by pi*(d/2)^2 where d is the diameter of the aperture. The area it self has a linear effect on the brightness as well. I.e. you let twice as much as light in for the same period of time, you get an image that is bright twice as more. If the two apertures you need to compare are d1 and d2, then the brightness increase/decrease with d2 compared to d1 is given by d2^2/d1^2. This is a very important factor. It means that if you make the aperture twice as large, you increase the amount of light four times.
When I tried yesterday, I had an effective aperture of about 1.25". With today's mod, I have an aperture of about 2.2" (i.e. the full aperture of the lens). What this means is that under the same settings, the camera now gathers 3 times more light.
But these numbers don't tally at all. If it is 3 times more light, then the shutter speed should get to about 1/120-1/150, not 1/4000. So what changed? Two things;
1. The film changed.
I am not using the same film as the day before.
2. Cloud cover.
When I took the first set of photos, it was during an evening and there was a little bit of haze. This seems to have dramatically reduced the amount of light available.
I am still not sure if I want to shoot using the telescope. It has a diameter of 4.4", i.e. the light is going to be four times more than the 75-300mm tele-lens that I have. The maximum shutter speed I can get on the camera is 1/4000. An option is to stack multiple filters on top of each other. Let's see how it goes. In the meantime, here is a photo with 1/4000 ISO100 with the brightness completely reduced to -2.
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| Sun Spots - Very visible in this image. |
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